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Welcome to the mid-ocean ridges
Here we tell the story of the BRIDGE project, a major research initiative of NERC, the UK's Natural Environment Research Council, which explored the mid-ocean ridges.
Most people know that the outer skin of the Earth is made up of a jigsaw puzzle of plates, all moving relative to one another like ice floes on a frozen sea. As the plates move, about 3 square kilometres of new Earth surface are formed each year, and the same amount is destroyed. Creation of new earth happens at teh mid-ocean ridges that run down the centres of the oceans and around the Earth like a seam on a tennis ball; destruction happens in the deep ocean trenches, where old ocean floor slides back down into the interior. At the mid-ocean ridges, plates are moving apart at 2-20 centimetres per year, and the six-kilometre thick ocean crust is formed in a string of long, thin volcanoes erupting magma from deep inside the earth. It is a world of submarine volcanoes, hot springs and exotic animal communities. Read on.
Volcanoes of the deep earth
Investigating Iceland and the Reykjanes Ridge
Slices of ridge
Bending and breaking the ocean floor
Smoking out metals
Twist and spout
Beyond the black smokers
Sniffing out plumes
A bug's life
The creatures that live at the vents
The strange tale of the vent shrimp
Machines for the abyss
BRIDGE to the future
What is BRIDGE?
British researchers have been watching, measuring and even sniffing the deep Atlantic for the past six years, working together to understand the strange deep-sea environments and how the ocean floor itself came into being. The story starts here.
BRIDGE is a major British research project that has been exploring the creation of new ocean floor in the deep ocean basins. The project has brought together oceanographers, geologists, biologists and geophysicists in a unique multidisciplinary collaboration, and has resulted in fundamental new discoveries in these remote and challenging places. It was funded by the Natural Environment Research Council (NERC) as part of its task of understanding the complex environment in which we live.
What are mid-ocean ridges? As the tectonic plates that make up the shell of the Earth move apart, hot, soft, partly molten rock wells up from the Earth's interior and fills the space between. The melt, at 1200 íC, rises from the solid residue to feed a belt of submarine volcanoes that girdles the Earth. This belt of volcanoes makes up the mid-ocean ridges and it is here that new ocean floor is created. The mid-ocean ridge volcanoes, though invisible beneath two to four kilometres of ocean water, pour out much more lava than the rest of the Earth's volcanoes put together. From this lava, and from melt intruded deep below the ocean floor, comes new ocean crust. About 3 square kilometres form each year, from 20 cubic kilometres of magma.
Hot springs under the seas
But making crust is not all that volcanoes do, especially under the sea. Molten rock in the roots of the volcanoes heats seawater that percolates down through cracks in the rock, and the hot water spurts up and out onto the sea floor at temperatures close to 400 íC. While it is being heated, the seawater reacts with the rock and strips metals and sulphur from it. The water returns to the ocean transformed into a fluid from which metal sulphide ores can form. Some sulphides precipitate at the seafloor to make massive mineral deposits rich in copper, zinc and gold, while the rest forms a plume of black smoke that drifts away in the deep oceans, leaving a trail from which the hot springs can be tracked.
In this hostile environment of hot springs, where toxic fluids pour out like smoke, oases of life prosper. Microbes thrive, depending on the chemical energy from the reaction between sulphide in the hot spring water and oxygen dissolved in the deep ocean. The microbes are equipped to deal with environments that are deadly to most other forms of life, and have great potential for application in biotechnology. They form the base of a food chain which includes exotic animals of kinds found nowhere else on Earth. The animal communities, isolated in the cold, dark ocean, struggle to survive, and also to find any new hot springs that arise. Organisms have developed complex strategies to deal with this hostile corner of the world.
Several lines of evidence suggest that it was in similar hot spring environments, sheltered at the bottom of the ocean, that life originated close to 4000 million years ago. So the mid-ocean ridges are not simply the places where new Earth is being forged today, but may also be the hearth at which our most remote ancestors first lived. They are strange and fascinating places, totally invisible and still mostly unknown to us above the ocean surface.
BRIDGE is part of the total international scientific effort looking at mid-ocean ridges. BRIDGE was planned at the same time as the US RIDGE project, and ran in parallel with similar projects in France, Germany and Japan. Our scientists sailed often on British ships, but also dived to the ocean floor in submersibles from the US, France, Japan and Russia. Scientists from many countries participated in turn on British expeditions. Together we were able to achieve much that we could not have done alone, but BRIDGE also had very much its own flavour.
Achievements of BRIDGE
The achievements of BRIDGE are set out in this site. Every area had success. Here are a few examples. We found new pools of molten rock below the ocean floor where none was expected. We discovered large fields of hot springs too, where the wisdom of the time said there should be none. We followed the strange lifecycle of the blind shrimp that live around hot springs in the Atlantic. We made sonar images of the first of a family of enormous faults that slice through the ocean floor, bringing deep rock to the surface. We showed how the flow of one of the big, hot spring fields was affected by scientific drilling. We traced the relationships between animals in hot spring communities up and down the Atlantic.
We built new instruments, too, that can operate in these hostile regions; a deep-towed hot-spring-sniffer, a seafloor device that can monitor the activity of hot spring fields and a drill that can take oriented cores of rock from outcrops on the ocean floor. And in parallel with this we ran a major programme of research on land, including pioneering models of how the plumes from the hot springs organize themselves into spirals in the deep ocean waters.
All of these and more you can read about in the pages that follow. All of these are achievements of which I, as BRIDGE Chief Scientist, feel very proud. I am proud too of the way in which BRIDGE has helped build teams of scientists that were not there before, has brought new people in to this strange world of the deep oceans, and has fostered multidisciplinary collaboration that was truly unexpected. Many people have said that new areas in science emerge at the boundaries where different fields meet, or when scientists from different disciplines come together to look at a single problem. BRIDGE is a fine demonstration of the truth of that idea. The support of the Natural Environment Research Council helped us to reach targets that we could not have achieved otherwise, but the teamwork that developed added a great deal more than that. Click through the pages and find out what we did.
Joe Cann , University of Leeds
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Volcanoes of the deep ocean
Mid-ocean ridges build the ocean floor. A wild and rugged landscape of high mountains, deep rift valleys, cliffs and escarpments arises from faulting and earthquakes. At the centre of the ridge, volcanoes spew out lava, constantly reshaping the sea floor. But what lies beneath this shifting terrain.
At the heart of the belt of mountains that form the mid-ocean ridges is a chain of active volcanoes, spaced tens of kilometres apart and linked to each other along the split between the plates. For several years we have known the basic features of these volcanoes. Molten rock, magma, forms deep in the Earth as the soft mantle of the Earth rises to fill the gap between the diverging plates and partly melts as it rises. The magma seeps up through the hot mantle to feed the roots of the volcanoes from which the 6 km thick oceanic crust is created, and lava is erupted from these volcanoes to cover the young ocean floor. What we did not know were the answers to some fundamental questions about these volcanoes. How does the magma rise into the volcanoes? How is it stored there, and what is the internal magmatic plumbing like? How is the lava erupted from the volcanoes? BRIDGE helped make substantial inroads into these questions, and found some unexpected answers.
The upper part of the ocean crust is made of lava flows fed from dykes, which are thin vertical sheets of magma that rise from the magma chamber and solidify within the crust. We can see these lavas and dykes in great scarps that slice through the ocean floor - like many volcanoes on land. But do the oceanic volcanoes erupt in the same way?
Fantastical lava flows
The answer that Joe Cann and Debbie Smith came up with during BRIDGE is that they are very different indeed. The new TOBI high-resolution side-scan sonar system, produced sonar images and bathymetric maps of parts of the Mid-Atlantic Ridge that showed a fantastical range of shapes of seafloor lava flows, nothing like those seen on land. Instead of spreading out in thin sheets, as the lava flows do in Hawaii or in Iceland, the ocean floor lavas form thick narrow curving flows, broad flat-topped terraces, fields of hummocks, narrow ridges like caterpillars, all part of the picture above. All the lava from a single eruption is focused into a few large flows - often only one - through a tube that runs down away from the lava vent. Recognizing this has transformed the way we think about the construction of the upper crust of the Atlantic Ocean.
Another new insight came from measuring how fast the molten rock rises into the volcanoes. The magma forms deep in the Earth's mantle and must rise to feed the volcanoes from a pool of molten rock known as a magma chamber. Does it take geological ages to reach the volcano or does it bubble up quickly? We measured this speed with a clock based on minuscule amounts of radioactive isotopes in newly-erupted seafloor lavas. These isotopes act as chemical messengers from the deep mantle. Peter van Calsteren made such measurements in BRIDGE on very young lavas from the Mid-Atlantic Ridge. He confirmed what had been seen elsewhere: that the magma had moved very fast indeed. A handful of his samples had formed just a few years before - so they had risen astonishingly quickly through tens of kilometres of mantle, then spent little or no time in a magma chamber. Other samples show greater ages - up to tens of thousands of years - but still far short of geological timespans.
Puddles, not lakes, of magma
But what about the magma chamber itself? Joe Cann suggested many years ago that at the heart of the mid-ocean ridge volcanoes should be a magma chamber. Though it took some years for this idea to be accepted, the idea of a subterranean pool of magma in a giant tube lying below the sea floor gradually took hold. Such a tube, if it is as big as was thought then, several kilometres wide and deep, would be easy to detect by seismic methods such as are used to find oil. The first seismic experiments showed some signs of magma chambers, but as the experiments became more sophisticated, the possible magma chamber became smaller and smaller. Recently a series of observations has suggested that the magma chamber is a thin sheet of liquid rock lying on top of a zone of partly-molten, superhot mush, mostly soft rock.
During her BRIDGE fellowship, Jenny Collier, with Satish Singh, applied the most exacting tests to this idea yet, and concluded that the zone of liquid could only be a few tens of metres thick, no more, and a kilometre or so wide, an astonishing shrinkage since the early ideas. This conclusion causes some fundamental problems with other ideas about the ocean crust, which are only now being tackled.
A hell of a problem?
Not all mid-ocean ridges are equal. One of the major variations is in how fast the plates are moving apart - their spreading rate - which ranges from about 140 mm/yr to less than 10 mm/yr. The ridges in the Atlantic Ocean spread at between 20 and 30 mm/yr - towards the slower end of the range. Given the spreading rate, we can calculate how much heat is carried upwards by the magma into the crustal magma chamber. We can also calculate how quickly heat is lost from the top of the magma chamber - mostly through the hot springs. At fast spreading centres we can show that, with a magma chamber in the crust, the heat coming in can balance the heat going out. But the same models show that at slow spreading rates - such as in the Atlantic - too little heat comes in to keep the magma chamber molten within the crust. Based on these calculations, if hell were beneath the Mid-Atlantic Ridge, it would soon freeze over.
Ramesses finds a way out
Yet there was still geological evidence for magma chambers in the Atlantic. This problem led to a fierce debate among ridge scientists about the roots of volcanoes beneath slow spreading ridges. Do they resemble the fast spreading ridges, but with short-lived magma chambers that form and freeze, form and freeze again? Or are they totally different, with tiny pockets of magma oozing upwards through the crust but always freezing quickly and never coalescing to form a large magma chamber? In one of the largest and most complex marine geophysical studies of the decade, a team of BRIDGE scientists led by Martin Sinha and Christine Peirce went to the Mid-Atlantic Ridge west of Scotland in search of a definitive answer. The Ramesses project combined two techniques to investigate the roots of the ridge, down to more than 100 km. One was seismic, using vibrations from small explosions to make images of the rocks below the ocean floor, and the other electromagnetic, using receivers on the ocean floor to detect electromagnetic waves sent out from a deep-towed transmitter.
If large but short-lived magma chambers can exist at slow spreading ridges, then the key to finding them is to look in the right place. Sinha and Peirce selected their target using a model of cyclic evolution that Jenny Collier had developed to explain observations from previous BRIDGE work at an intermediate spreading rate ridge in the Lau Basin (southwestern Pacific). This model predicts what a seafloor volcano would look like if it had a large magma chamber; the team identified a volcano that fitted the description, and set to work.
The results of Ramesses were spectacular. The BRIDGE team found the first large magma chamber to be discovered beneath a slow-spreading mid-ocean ridge. This structure is very similar to those found beneath fast-spreading ridges - with the principal difference being the lifetime of the magma chambers. A second important finding was that they were able to quantify the total amount of magma present in the thin melt pool and the mush zone beneath the Ramesses volcano. This turned out to be many times larger than that found beneath fast spreading ridges - providing more evidence for the highly episodic nature of the magma chamber.
A new cyclic picture emerges
The volume of magma now present represents at least 20 000 years' worth of crustal growth - but calculations show that the entire magma chamber will probably freeze over in less than 2000 years. Thus Ramesses also provides evidence that at slow spreading ridges, a magma chamber is likely to exist for no more than 10% of the time. For the other 90% of the time or more, the crust beneath the axis is magmatically dormant. In other words, at any one time, at least 90% of volcanoes on the Mid-Atlantic Ridge are extinct, while just 10% or less have a magma chamber at their root.
This would explain why previous attempts to find magma chambers beneath the Atlantic had failed. The same line of evidence shows that the magmatic cycles at slow spreading ridges probably have a period of at least 20 000 years - in other words, when a ridge volcano becomes extinct as its magma chamber finally cools and freezes, you would need to wait 20 000 years or more before another active volcano forms in the same place.
The Ramesses results make exciting links with features and patterns seen in sonar images collected by Lindsay Parson and Roger Searle of the same area of mid-ocean ridge. The seafloor sonar images show an astonishing diversity of volcanic structures. Some parts of the ridge are covered by very fresh, young lava flows and pillow eruptions. Others show little evidence of recent volcanism, but instead are dominated by signs of tectonic activity, stretching of the sea floor by earthquakes, faulting and fissuring. Still others show all stages in between.
These links gave scientists a picture of how the crust grows: an intermixed cycle of magma movement, volcanic eruptions and faulting. In this concept of tectono-magmatic cycles at slow spreading ridges, variations in the rate at which magma is delivered from the mantle melting region into the crust result in profound changes in the physical and thermal structure of the crust through time. They also bring cycles of vigorous volcanism followed by long periods of magmatic dormancy and tectonic dismemberment of the seafloor volcanoes.
BRIDGE scientists: Martin Sinha Southampton Oceanography Centre; Christine Peirce, Roger Searle and Debbie Navin, University of Durham; Joe Cann University of Leeds; Peter van Calsteren Open University, Jenny Collier Imperial College, Lindsay Parson Southampton Oceanography Centre; Lucy MacGregor, Satish Singh University of Cambridge. International colleagues: Steven Constable (California), Debbie Smith (Woods Hole), Anthony White (Australia), Graham Heinson (Australia).
Making magma: how and where does rock melt?
Beneath the plates, at depths of 100-200 km, the mantle is at temperatures of 1250-1300íC, close to its melting point, and very hot and soft, but still solid because the pressure is so high. As the mantle rises below the mid-ocean ridges, it cools slightly, because it is under less pressure, but its melting point falls faster as the pressure drops. So if rising mantle is hot enough, and rises far enough, it will start to melt. Once melting starts, about 3% of the rock melts for each 10 km that the rock rises. More melt will form if the mantle beneath the ridge is hotter than normal, because it will start to melt deeper down, and less melt will form if the mantle is colder.
The melt forms within the rock as thin films along grain boundaries, and then starts to percolate upwards, joining other films of melt, growing in size as it flows up through the mantle. It rises into a shallow magmatic plumbing system, where the melt starts to be cooled by the ocean above. As more melt percolates into this plumbing system, the pressure inside it rises, until at last the top of the shallow plumbing splits apart to give a narrow crack, about a metre wide, up which the pressurized melt flows and erupts onto the sea floor. The melt in the crack solidifies to form a dyke, and the magma that erupts makes a lava flow. As the plumbing system cools, the melt within it solidifies, and this solid becomes the lower layers of the ocean crust. The total thickness of the crust reflects the amount of melting that has happened in the rising mantle. At normal mantle temperatures, enough melt forms to create ocean crust that is 7 km thick. Under Iceland, on the other hand, the mantle is much hotter in the rising Iceland plume, so there is much more melting, and the crust is some 20 km thick.
The international view
As the BRIDGE Initiative draws to a close after six years of intensive scientific study of the mid-ocean ridge system and creative technological development of new methods to collect data and samples, the United Kingdom should look with pride at the significant accomplishments of BRIDGE during its short lifetime. In a brief comment such as this, I cannot begin to describe the contributions BRIDGE has made in all its areas of focus. However, I would like to highlight three areas where significant advances by BRIDGE are paving the way for the future.
The first of these is the extensive multidisciplinary work that BRIDGE scientists have conducted on hydrothermal systems on the Mid-Atlantic Ridge. These studies have not only led to the discovery and characterization of new sites of hydrothermal activity, but have also contributed to our understanding of the styles of fluid flow, mineralization processes, mass and energy fluxes between the ocean and the lithosphere, and the ecology of the vent communities. I think it is fair to say that BRIDGE scientists have played a major role in this effort, and through the productive international collaborations that have been fostered, they will continue to be at the forefront of research in this area.
The second area I wish to highlight is one of the four BRIDGE geographical areas. Since its beginning, BRIDGE has funded detailed bathymetric, geological, geophysical, and hydrothermal plume surveys of the Reykjanes Ridge, making it one of the best mapped parts of the mid-ocean ridge system. In 1993, a non-BRIDGE cruise collected geophysical evidence for a low velocity zone, possibly a crustal melt body, beneath the ridge axis. The last cruise to be funded by BRIDGE conducted a multichannel seismic reflection experiment in the summer of 1998 to further define the geometry, nature, and physical properties of the low velocity zone, and investigate its relation to the observed tectonic features and accretionary processes. We eagerly await the results of this exciting research!
Finally, BRIDGE has given a considerable boost not only to oceanographic technological development, but also to the creation of productive collaborations between scientists and engineers. The symbiosis between science and technology nurtured by BRIDGE has resulted in improved methods to image the sea floor, both optically and acoustically, that will have far-reaching impacts on mid-ocean ridge research for many years to come. Studies of hydrothermal systems will continue to benefit from the development of Medusa, a package to measure and sample diffuse fluid flow, and Bridget, a multisensor, deep-towed vehicle for studying hydrothermal plumes. In addition, the newly proven capability to take and retrieve short, oriented rock cores in deep water opens up many new avenues of research. BRIDGE activities have made a significant impact on our understanding of the global mid-ocean ridge systems; future projects will be able to build on the legacy of scientific ideas and equipment that BRIDGE leaves.
Susan Humphris, WHOI
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Slices of ridge
The mid-ocean ridges are made up of many segments, each with their own pattern of faulting and volcanoes, divided by faults. But just how different are they?
Along the length of the mid-ocean ridges, the continuity of the ridge axis is broken every few tens of kilometres by a jog, a step, that throws the axis sideways one way or the other, perhaps by only one or two kilometres, perhaps by hundreds of kilometres. The larger jogs are marked by long, straight faults that slice through the ridge, called transform faults. The smaller ones are more irregular and subdued; they are the non-transform discontinuities (NTDs). Between the jogs are straight segments of ridge from 20 to 100 km long. This pattern shows up well in the coloured map, which shows part of the Mid-Atlantic Ridge including the Kane and Atlantis Transform Faults, and the 18 segments that lie between them, an area where much BRIDGE work was concentrated.
Two dimensions or three?
Until a few years ago, scientists discussed mid-ocean ridges in two dimensions, looking at slices and choosing for these cross-sections places in the middle of segments. BRIDGE began at a time when the third dimension, along the ridge, became recognized as important; some of the most distinctive contributions during the BRIDGE programme concern segments and how they work. For this, BRIDGE scientists used our sophisticated multibeam echo-sounder to collect hundreds of seafloor soundings every few seconds, and the UK's TOBI deep-towed sidescan sonar to make sonar images in broad stripes across and along the ridge axis.
The first TOBI images that Lindsay Parson, Bramley Murton, Roger Searle and their team collected on the Reykjanes Ridge showed that within each segment, lavas accumulate in flows or hummocks tens of metres high and wide. These hummocks merge and coalesce to form larger mounds and ridges - some up to hundreds of metres across and kilometres in length. At the same time, intermingled with these constructs, TOBI images picked out sheet flows, huge flat-lying lava blankets covering square kilometres at a time, which must have resurfaced the sea floor in a geological instant. Most dramatically, individual seamounts were picked out as ragged conical forms, or as thick, pancake-shaped mounds. Chemical analysis of the lavas shows how the Earth system delivers magma to the plate boundaries, how long it is trapped in the shallow crust before eruption, and how it focuses along certain sections of the ridge. This newly discovered plethora of volcanic shapes has since been found in every BRIDGE survey of the Mid-Atlantic Ridge.
In an equally spectacular demonstration of the scale of activity at the ridge system, our TOBI images show how the stretching of the crust as the plates move can destroy these freshly built basalt mountains and flows. Fissuring, fracturing and faulting are almost ubiquitous - only the youngest terrains appear to escape some sort of deformation. The relentlessness of the plate motion is translated into wildly different structures, in one part a field of intensely fissured sheet flows, and in another a volcanic ridge dissected by faults as cleanly as if it had been cut by a gigantic egg-slicer. The power to destroy as well as to build is evident in every image we now record from the ridge axis.
The heart of the matter
The most striking conclusion of our work is that each segment has an individual character. One may have recently been flooded by a huge volcanic eruption, another may be dissected deeply by faults. A third may have a large volcanic ridge down its axis, shaped like an upturned boat hundreds of metres high, studded with small lava piles; another may have abundant signs of small eruptions, but no large axial ridge. Adjacent segments can look very different from each other.
The same variety of segments was seen in the north along the Reykjanes Ridge as in the south, around 20-30N. Each segment must be a separate volcanic system, an individual volcano, long and thin, extending parallel to the spreading axis. Each segment evolves as a unit, but quite differently from its neighbours, so that one segment may be receiving copious supplies of magma from the mantle, while the next one may bestarved of magma. This vision is critical for understanding how mid-ocean ridges function.
The end of the affair
The interactions between segments at their ends are intriguing. The spreading of the mid-ocean ridges goes on inexorably. Magma fills the cracks that form and spills over onto the sea floor; where there is not enough magma the rocks crack and split. Where two segments meet there seems to be strife between their magmatic systems. The one that is, for the moment, most abundantly supplied, expands and pushes into the territory of the other, which may then fight back. Over thousands or millions of years the battle swings to and fro, so that at times a whole segment may vanish, conquered by its neighbours, or a new one may emerge. All of this is recorded in the topography of the seafloor. The result, as Sara Spencer, Joe Cann, Simon Allerton, Roger Searle and colleagues have shown, is that the places where segments join are unstable, constantly changing as the battle rages.
What goes in, must come out
An intriguing experiment, led by Bramley Murton, set out to make an inventory of inputs and outputs to one whole segment. This segment contains the Broken Spur hot spring system, at 29N, with its black smokers and animal communities. It happens that this segment is almost entirely enclosed within a deep valley, and so is an ideal candidate.
First, they made a complete survey of the water within the basin, measuring its temperature, salinity and chemistry, as well as the distribution of black smoke particles and of larvae within the water column. Vertical chains of instruments were moored in the deep water across the gap, and within the segment. They recorded temperature, flow rate and direction of flow of seawater within the valley for a year. The aim was to determine the energy input into the segment by hot spring activity or by lava flows, the chemical contribution from the hot springs to the water column and the biological flux from the system towards other parts of the ridge. It turned out to be difficult, but the conclusion was an estimate of all of these fluxes through the year. No volcanoes erupted during the monitoring: it would be useful to repeat the experiment when the volcanoes are active, for example, because this and other changes would alter the flows.
Working at the segment scale poses considerable problems, but has great rewards if one of the segment volcanoes can be probed from the mantle to the water column. There are plans to continue this research at the international level, with a major experiment that may take place within one of the areas studied in BRIDGE, and will certainly build on the science that emerged from our work.
BRIDGE scientists Lindsay Parson, Bramley Murton Southampton Oceanography Centre; Roger Searle, Christine Peirce,University of Durham; Simon Allerton, University of Edinburgh; Martin Sinha, University of Cambridge, Sara Spencer, American University, Beirut; Joe Cann, University of Leeds. Roy Livermore British Antarctic Survey.
Although most ocean crust is born in the open ocean basins, new crust also forms above subduction zones. These generally destroy ocean floor, but as the old, cold sea floor descends into the Earth's interior again, it pulls on the overlying plate, and can rip it apart to make a small ocean basin.
BRIDGE studied two of these, the Lau Basin, Tonga in the South-west Pacific, and the East Scotia Sea, in the South Atlantic near the Falkland Islands. There are two reasons for being scientifically interested in these places; one is that, because they form in a very different environment to mid-ocean ridges, they test how generally applicable are ideas developed at the ridges; the other is that they are often isolated from other spreading centres, and thus could hold more exotic forms of life around their hydrothermal vents.
A team led by Martin Sinha and Christine Peirce went seeking magma chambers in spreading segments in the Lau Basin. They found a magma chamber but were surprised to notice that it was largest where two segments overlap, not at the segment centres. This flies in the face of much of what scientists think they know from the mid-ocean ridges. Is this difference accidental or systematic? It certainly suggests that current ideas need another look.
In the East Scotia Sea, a team from the British Antarctic Survey led by Roy Livermore mapped for the first time the spreading centre there. This forms the western boundary of a strange little plate that is nearly isolated from the rest of the tectonic system of the world. The mapping showed a pattern of segmentation very like normal mid-ocean ridges, but the chemistry of the lavas has revealed a very complex pattern of mantle flow underneath, a pattern that we would not be able to see in normal mid-ocean ridges because the mantle is so uniform there. The latest surveys have discovered plumes from active black smokers. The team plans to return with a remotely-operated vehicle to see if the isolation has produced distinctive animal communities around the vents. We will have to wait and see.
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Investigating Iceland and the Reykjanes Ridge
The Reykjanes Ridge is not only the nearest mid-ocean ridge to Britain, it is also one of the most unusual. BRIDGE scientists have delved beneath the sea floor with gravity maps, echosounding and earthquakes
Every ocean basin on the Earth contains a mid-ocean ridge - a range of faulted volcanic mountains under the sea, where the Earth's tectonic plates separate. The nearest to Britain is the Reykjanes Ridge, which runs southwest from Iceland. It is one of the geographical areas chosen as a focus by BRIDGE - but not because it is nearby. The Reykjanes Ridge is decidedly unusual.
There are four major ways in which the Reykjanes Ridge is odd. The most obvious is that the ridge forms the island of Iceland. Usually all ocean floor lies well below sea-level because the oceanic crust is much thinner and denser than that of the continents. But in Iceland it rises to over 1.8 km above sea level - 4.5 km higher than normal. Something is holding Iceland up: scientists believe that it is a mantle plume. This is a narrow column of rock that is even hotter and less dense than the sheet of rock rising in the normal way under mid-ocean ridges, and may originate very deep in the Earth's mantle - possibly even at the surface of the core, 3000 km down. Radioactive heating of the interior of the Earth is the ultimate cause of such plumes, which well up as convecting columns. The buoyancy of the plume helps to raise the surface above it, but most of the additional elevation arises because the plume, being unusually hot, produces more melt and thus a thicker low-density crust. And the thicker this crust is, the higher it floats.
The Reykjanes Ridge also has an unexpected shape in cross-section. The shape of most mid-ocean ridges is related to the rate at which the tectonic plates are moving apart. Those in the North Atlantic are fairly slow; less than about 2.5 metres per century. Such ridges usually have a wide, deep median valley at their crests, which is produced by necking as the plates pull apart - rather like the way that a bar of toffee gets thinner in its centre when you pull on the ends. But on the Reykjanes Ridge there is almost no median valley for many hundreds of kilometres either side of Iceland. Instead, the Reykjanes Ridge rises continuously to a crest - a shape typical of ridges such as the East Pacific Rise, where plates diverge over twice as fast.
The reason is again thought to be heat, in particular, the heat distribution. Where ridges spread quickly, deep, hot rock comes to the surface much faster and more heat is put into the plates. But the heat cannot escape equally fast, because it is limited by conduction through the rock. So the lithosphere - the rigid outer layer of the Earth that constitutes the plates - is on average hotter and therefore it is weaker. As a result, it breaks at a lower stress, with less thinning. The lack of a median valley on the Reykjanes Ridge might mean that its lithosphere is hotter than usual, probably because of the hotter mantle associated with the Iceland Plume.
The third unusual feature of the ridge is it orientation and continuity. Most ridges are broken up into series of segments, offset on faults (see Slices of ridge). Within each segment, the line of the plate boundary is almost perpendicular to the spreading direction. By stepping sideways on faults every few tens of kilometres, ridges keep an overall orientation oblique to the spreading direction, perhaps along a line inherited from the original continental rift that formed the oceans. But the Reykjanes Ridge appears to be continuous and free of such offsets over hundreds of kilometres, despite lying at nearly 30 degrees to the orientation expected from the spreading direction.
The last anomaly is subtle and is best seen in the very detailed images of the Earth's gravity field that have been made by satellite altimetry. The North Atlantic shows a remarkable set of nested chevron-shaped ridges, all pointing away from Iceland down the Reykjanes Ridge. With such a collection of unusual features to study, the Reykjanes Ridge was a fine challenge to BRIDGE investigators.
Gill Foulger and colleagues are conducting the most detailed study ever of a mantle plume, by observing how seismic energy from distant earthquakes passes through the mantle beneath Iceland. They set up an array of 35 state-of-the-art seismometers to supplement the 40 stations of the Icelandic national network, and recorded for over a year. Analyses so far reveal a narrow, hot plume with a diameter of about 200 km, in which waves from earthquakes travel about 12% more slowly than usual, indicating that the mantle there is hotter than normal. They have detected signs of the plume as deep as 1600 km (well below plate movements), still with a diameter of only 250 km. A tomographic calculation, like a medical CAT scan, reveals more detail of the structure of the plume, including variations in its strength with depth, which suggest that the plume may pulse in time.
Simrad and the sailors
A major marine effort was focused on the Reykjanes Ridge. Roger Searle, Barry Parsons, Bob White and collaborators conducted a major survey cruise on RRS Charles Darwin, using the new Simrad multi-narrow-beam echosounder (funded by BRIDGE) and other instruments to measure the shape of the ridge in detail, together with minute variations in its gravity and magnetic fields. This produced a topographic map of a 600 km length of the ridge, extending 100 km away from the axis and representing seafloor up to 10 million years old, with resolution comparable to the best Ordnance Survey maps of Britain. With it we can detect, for example, volcanic cones a few hundred metres across and 20 metres high: you could easily see the Millennium Dome! The gravity and magnetic maps, while not quite so detailed, nevertheless had a spatial resolution of about a kilometre.
The topographic image reveals a remarkable pattern on the ridge. Its axis is marked by a series of linear volcanic ridges (called axial volcanic ridges, AVRs), which are arranged parallel to each other, but offset and slightly overlapping. In a study at the beginning of the BRIDGE programme, Lindsay Parson, Bramley Murton, Roger Searle and colleagues had deployed TOBI, the deep-towed sidescan sonar, over a few AVRs and revealed how they grow by volcanic activity and are then broken up by faulting as the young sea floor evolves. We believe that these ridges build up over cracks or fissures that open as the plates separate, allowing lava to flow out onto the sea floor. The ridges run almost at right angles to the spreading direction, as you would expect for cracks formed by tension.
Similar ridges can be seen over the plate boundary in Iceland, and have also been recognized on other parts of the Mid-Atlantic Ridge. This study has made the connection between the two. We have shown that AVRs are a ubiquitous feature of slow spreading ridges, and reflect an important aspect of the way in which oceanic crust is formed.Traces of the plume
The importance of the lithosphere in the formation of the crust is also evident from subtle variations in the chemical composition of the ocean floor rocks. The proportions of certain trace elements in the rocks act as signatures of both the particular melting process and the composition of the mantle below. Bramley Murton and colleagues have found chemical variations on scales from 100 to 1000 km. The rocks of the Iceland plume, for example, are different from the usual mantle beneath the ridge. So the shallow parts of the plume can be traced. Its influence spreads 1000 km south of Iceland. On a smaller scale, the chemistry of the ridge rocks shows regions about 100 km apart where there has been more melting, where the mantle has risen locally. No process in the deep, viscous Earth can alone account for variations at such close spacing, so the lithosphere must also play a part in when and where the mantle begins to melt.
It was on the Reykjanes Ridge that Martin Sinha and Christine Pierce, together with their colleagues, conducted the Ramesses experiment. This geophysical assault on the ridge showed the existence of a magma chamber under one of the AVRs (the Ramesses volcano), making sense of the differences between this slow-spreading ridge and other faster ones. Their work is described in more detail in Volcanoes of the deep ocean. The picture that emerged from it was of many short-lived volcanoes, with considerable variation through time. In the rock record, these temporal variations become differences in the rocks preserved at different places along the ridge.
The pulsing of the plume
Our detailed topographic maps confirmed that most of the Reykjanes Ridge lacks a median valley, but also showed that the transition from median valley to axial high is abrupt. More interestingly, the maps (particularly the gravity image) suggest that a small median valley reappears in the north - a complete surprise.
The detailed topography revealed large faults (producing cliffs tens of kilometres long and tens to hundreds of metres high) as elsewhere on mid-ocean ridges. But the faults here are not perpendicular to the spreading direction like the volcanic ridges, so they are not controlled simply by extension at the ridge axis; the broader structure of the plate boundary counts too.A dearth of faults
Interestingly, faults on the Reykjanes Ridge become rarer and smaller northwards, reaching a minimum near 60N and then increasing slightly to the north. The opposite happens with seafloor volcanoes - they are more common and bigger near 60N.
We also found that the sound reflection recorded by the echosounder is strongest over the AVRs. This is to be expected if they represent the youngest parts of the crust, with surfaces of rough basaltic rock with little sediment cover, making very good acoustic reflectors. Not all AVRs reflect equally well and we found more-highly reflecting - and therefore very young - AVRs around 60N.
The magnetic field tells the same story. Seafloor basaltic rock is strongly magnetic - it contains the naturally magnetic mineral magnetite. Over time magnetite oxidizes and loses some of its magnetization. We found that the AVRs are the most strongly magnetized parts of the ridge, confirming their youth, but that in the 60N region they are less magnetic. This could mean they are older there - but that contradicts the evidence of the sound reflection. Alternatively, the crust is hotter around 60N (as suggested by the higher incidence of volcanoes) - hot material is less strongly magnetic - or the chemistry of the lavas is slightly different. In summary, there are many signs that something special happens near 60N - but what?What's going on?
The significant clue comes from the chevron-shaped ridges seen in the gravity field. Two of these intersect the area of our detailed survey. Our gravity studies suggest that these ridges have crust that is 1-2 km thicker than their surroundings. Bob White has shown that this could arise if the underlying mantle were as little as 30 degrees C warmer than usual: more mantle would melt as the plates formed. One of these chevrons currently intersects the Reykjanes Ridge axis at 60N, so the idea of an unusually hot mantle here fits all our observations: it would produce an unusually shallow ridge axis, a weak crust that would not support much faulting or a median valley, extra volcanism, rapid production of young lavas in AVRs and - because of subtly different chemistry - lower magnetization.
As the plume 1600 km beneath Iceland pulses, the mantle temperature waxes and wanes. The plume spreads out as it reaches the Earth's surface, carrying the temperature fluctuations along the Reykjanes Ridge away from Iceland. When the plume is especially hot, there is more melting, a thicker crust, chevron ridges and numerous volcanoes, robust axial volcanic ridges, reduced faulting and no median valley.
Jules Verne's Journey to the Centre of the Earth began on Iceland and explored a ridge volcano. BRIDGE's scientific journey of discovery followed a similar route: we have deduced how the rocks of the ocean floor formed deep in the interior, a process central to the evolution of the Earth.
BRIDGE scientists: Roger Searle, Gill Foulger, Christine Peirce, University of Durham; Bramley Murton, Lindsay Parson, Martin Sinha, Rex Taylor Southampton Oceanography Centre; Barry Parsons Oxford University; Bob White University of Cambridge.
The Towed Ocean Bottom Instrument (TOBI) is an echosounder, which works by sending out pulses of sound waves that bounce back like echoes from the seafloor. TOBI records the time and strength at which they return, information that can be used to build up a picture of the shape of the seafloor below. Much of TOBI's time has been spent on sidescan sonar, in which the instrument directs sound waves across the sea floor. The images show bright areas that reflect back a lot of the energy, often steep slopes facing the instrument, shadows where there is no information, and varied brightness and textures in response to terrain in between.
One of the things that makes TOBI special is that it works very deep in the oceans, usually around 300 m above the seafloor. The detail that such an instrument can see depends on how far it is from the sea floor, so TOBI gains great advantage from its depth. The disadvantage of TOBI's working depth is the difficulty of controlling and locating it.
One of TOBI's successes came in the summer of 1994 on the cruise that discovered the Rainbow plume, south of the Azores where the TOBI team imaged 200 km of ridge-crest. The ridge here is unusual in that it is not aligned with the spreading direction between the North America and Africa plates. The ridge is organized into a staircase of short sections of volcanic ridge, typically 50-80 km long, separated by faults which step the ridge sideways to the east as you move north along the ridge. Each strike-slip fault displaces the ridge by some 30-50 km, giving a zig-zag pattern.
Chris German, collecting geochemical data from an instrument added to TOBI, takes up the story: "To image this section of ridge crest with TOBI, therefore, required some highly skilled navigation and shipboard operations because it was not enough simply to tow the vehicle straight down the ridge. Instead, we had to slalom down the axis taking 90 degree bends, right, then left, every 50 km or so. An easy enough task in a ship moving at just a few miles an hour you might think - but complicated when you consider that the instrument being towed behind you is like a 3 tonne wrecking ball on a 6000 m length of cable which is streamed 1-2 km behind you and passes your ship's position between half an hour and an hour later. And as a final complication, it is the instrument's position when it passes over the ground that dictates whether you collect the precise data you require, not the ship's position."
TOBI was successfully modified during the BRIDGE programme to perform swath bathymetry. Its working position, 300 m or so above the seabed, made it an ideal platform for all sorts of measurement, so that TOBI even played a key role in the discovery of the Rainbow Plume, defining the terrain and tracking the plume at the same time: truly a versatile machine
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Bending and Breaking the sea floor
As the oceans grow, they bend, break and slide. The rocks tear to form huge faults. BRIDGE scientists revealed the pattern, and even saw fault surfaces exposed on the ocean floor.
One of the most intriguing topics explored by BRIDGE scientists is how the ocean crust breaks. The continents are full of ancient fractures. In Scotland, ancient fractures created 400 million years ago still shudder and split from time to time. Pull on the continents, or squeeze them, and old wounds re-open. But new ocean floor is created in a pristine condition, without such half-healed scars. In the oceans we can watch as new fractures form, and discover the factors that control their evolution as nowhere else on Earth. When rock fractures, it makes faults: splits in the rock where the beds on either side are displaced. During BRIDGE we discovered much more about how faults work, and also saw for the first time a type of gigantic fault, pulled out from the inside of the Earth, and lying flat on the seafloor.
One way to get an idea about faults is to break rock in the laboratory. A team at University College London, led by Phil Meredith, built a BRIDGE rock breaker in which they could study rock fracture at temperatures up to 450 íC, pressures equivalent to depths of two kilometres below the seafloor, and with the rocks saturated with brine. They could show that, for example, the presence of salty water weakens the rock at high temperatures, so that above a critical temperature of about 300 íC the rock below the ocean floor must be much weaker than when it is cooler. Their work is still underway, together with parallel projects that sought to understand the faults out there in the oceans.
There are three types of fault, depending on how the rocks have been deformed. Normal faults (the most usual kind) form when rock is stretched, and result in the crust becoming longer. Reverse faults (the opposite of normal faults) arise from compression, and produce shortening. Strike-slip faults are the odd ones out. They are faults, such as the San Andreas Fault in California, where rock slips sideways. Reverse faults are not important in mid-ocean ridges, but the other two are.
Faults are worth thinking about. For example, they form the edges of oil reservoirs, helping to trap the oil against sealing rocks. Here it is critical to know how and when the faults have formed, which faults formed at the same time, and which at different times, and whether the faults have allowed the oil or gas to escape, or whether they hold it tight. Faults and fractures are critical in defining the safety limits of future nuclear waste repositories, and in controlling the creation of ore deposits.
Faults instead of magma
At mid-ocean ridges, almost all the space that opens up is filled with magma, with lava flows at the seafloor, with dykes (sheets of magma that solidify underground) in the mid-crust, and with the remains of magma chambers in the lower crust. But at times there is not enough magma to fill the space. The crust breaks instead, stretching to form normal faults; these are the critical cases that BRIDGE scientists studied.
TOBI turned out to be the perfect tool for studying faults. It maps a stripe of the sea floor 6 km wide with a resolution of about 10 m, ideal for imaging the precise structure of faults on the ocean floor. In BRIDGE we used TOBI to survey a number of segments of the Mid-Atlantic Ridge between 23 and 30N with the specific aim of looking at the patterns of faulting as well as the volcanology. Eddie McAllister was able to trace these normal faults from the moment they formed, within the zone of lava accumulation very close to the spreading axis, to the time they stop growing a few kilometres away, and then out to older and older ages until the earthquakes on them had stopped for good. The growth of the faults strongly supported a model of Patience Cowie's, that many tiny faults nucleate to start with, as the crust begins to fracture, and then link up to form larger fault arrays, with a complicated pattern in plan that reflects this complex origin.
Building on this work, Roger Searle, Javier Escartin, Patience Cowie, Neil Mitchell and Simon Allerton set out to map the complete fault pattern of asingle segment of the Mid-Atlantic Ridge 70 km long, out to 50 km from the axi s, using TOBI and the Simrad multibeam echosounder on the RRS Charles Darwin. Their remarkable map is extraordinarily instructive. It not only shows the variation of the fault pattern with time in the segment (time is equivalent to distance from the spreading axis), but also the way that the faults change towards the ends of the segment, reinforcing the earlier conclusions of Javier Escartin and Jian Lin that faults become larger and are spaced further apart in the inside corner close to the offset that separates one segment from the next. They were able to show that between 5 and 10% of the extension was by faulting, and that the faults have grown to full size within 15 km of the axis, which corresponds to about a million years.
On the very next expedition of the Charles Darwin after Roger Searle's, there was electrical trouble with TOBI, and after a number of frustrations, Joe Cann and Donna Blackman decided to give everyone a rest by doing a bathymetric survey. They mapped the sea floor with the multibeam echosounder in strips, one by one, diagonally across the Atlantis Transform Fault, a strike-slip fault that separates one spreading segment from the next. Eddie McAllister took the survey and played with illuminating the topography to give a more sophisticated image. Suddenly an extraordinary feature showed up north of the transform fault: a very gently arched smooth surface 8 km across, crossed by corrugations with a wavelength of a kilometre and a height of tens of metres. A similar surface lay south of the transform.
Tows with TOBI showed that both surfaces are striated just like fault surfaces on land where blocks of rock have slid past each other, but on a much larger scale, with striations 100 m across and several kilometres long. These are single faults, far larger than any that had been seen before.
Faults similar to this had been predicted from work further south in the Atlantic, but only fragments had been seen. Other such faults had been detected in the deserts of eastern California, but they were only found after painstaking geological mapping. Here at the Atlantis Transform the faults were laid bare on the ocean floor. How could such faults form? It is most unlikely that a single fault could be active along all of its length at once, especially at such a low angle. The laws of mechanics seem to forbid it. But if the fault was extruded from the Earth so that only part of it was active at any one time, and if the fault, where it was active, was steep, and flattened out as it bent under gravity during extrusion, it might be possible.
A workable model
There was a model constructed by Brian Tucholke and Jian Lin that had these features. Despite his initial distrust, Joe Cann was persuaded that something like this had to be happening. It was the only thing that seemed to fit. It seems that, at times, close to the end of a spreading segment, and especially close to a major transform fault, there is very little magma indeed. So a large part of the spreading happens just by stretching of the crust, pulling it so thin that in places no crust remains at all, only rocks of the mantle beneath, brought up beneath these enormous faults. Or perhaps thin stretched flakes of crust lie tilted like tiles on a roof above the deeper mantle.
Then, as so often happens in science, once the initial discovery was reported, faults like this were recognized from existing surveys in many places. A larger fault was seen close to the Kane Transform Fault, the next major transform to the south, about 15 km across, and other flat faults were seen in the area in between, and in other parts of the oceans. The largest imaged so far is in the ocean south of Australia, where a single fault 31 km long has been seen. The next stage is to find out more about them, to test, for example the Tucholke and Lin model properly. Now new expeditions to both the Atlantis and Kane faults have been agreed, and the deep-ocean drilling ship, Joides Resolution, called after Cook's ship of exploration, may be drilling one of these flat faults in the next three years to test the model further. It appears that such faults may be much more common than we had ever suspected, and may play a crucial role in the stretching of both oceanic and continental crust when no magma is around.
BRIDGE scientists Roger Searle University of Durham; Patience Cowie, Simon Allerton, Javier Escartin University of Edinburgh; Philip Meredith, University College London; Neil Mitchell Oxford University ; Joe Cann, Donna Blackman, Eddie McAllister University of Leeds. International colleagues Debbie Smith, BrianTucholke, Jian Lin Woods Hole Oceanographic Institution.
Life on the ocean wave... with BRIDGE
I was one of the lucky scientists who joined the RRS Charles Darwin Centennial Cruise (nicknamed CD100), one of the BRIDGE cruises, which visited the North Mid-Atlantic Ridge in spring 1996. The scientific schedule is always demanding on these cruises and there is little free time. Data collection and processing goes on around the clock; if you are not on a shift helping with preparation and launching of instruments or supervising data recording, you are probably processing data or planning survey strategies, navigation and logistics. Nevertheless, in the 40 days we spent in the North Atlantic I had the opportunity to discover the personalities of my fellow scientists and see beyond their scientific skills.
For example, the cruise principal investigator Joe Cann insisted on having an almost daily break for "hacky sack", a truly ingenious version of football. The RSS Charles Darwin is anything but large; we were crammed aboard and everybody craved physical exertion. Unfortunately, the ship's stern, filled with cranes and geophysical instruments, was not exactly the ideal sports field. Our excitement and zeal often ended with our precious football disappearing overboard. It was then that we were introduced to one of Joe Cann's particular virtues: he paid special attention to producing new footballs from paper and sticky tape. Somehow he gave me the impression this was his way of concentrating his thoughts!
One day we were told that the ship's rubber dinghy would be launched to be tested and we could take turns to go out for rides with the Chief Engineer. The sea appeared calm enough from the deck that day and I could not figure out why my colleagues were returning so very happy - not to mention soaked to the skin. I decided to try it myself and discovered that the sea was far choppier than it seemed. It took me almost 10 minutes to jump safely into the rocking dinghy from the rope ladder over the side of the ship. But the greatest thrill was when I drove the dinghy into the waves. The little boat accelerated so frantically that we were literally hopping over the waves! Even the Chief Engineer got wet.
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Smoking out metals
Metal deposits on the ocean floor are a key to unlock ore deposits on land: understand one and we can find more of the other. The crux is the plumbing.
We take metals for granted, and most of us wonder only fleetingly - if at all - where the raw materials come from. Maybe we have a vague picture of primitive miners scrabbling in the dust and smelting rocks; perhaps we imagine miners today doing much the same with machinery. But the modern mining industry is highly sophisticated. Lone prospectors and their donkeys have long since given way to sophisticated models, developed from decades of painstaking mapping, drilling, analysis of samples and ultimately trial and error in remote parts of the world. Crucial to all of this are accurate ideas of how mineral deposits form.
Many metal deposits precipitate from hot water. Water can percolate down into the Earth and heats up there. The hot water dissolves traces of metals from the surrounding rocks and rises to colder, shallower levels, where the metals precipitate as sulphides or oxides. Given time, vast volumes of fluids and a suitable setting, a mineral deposit will grow.
Along the mid-ocean ridges, where hot rock lies just below the surface, there are spectacular fields of hot springs. Each one is up to a few hundred metres across and is producing mineral deposits like the ones mined on land. Superheated fluids laden with metals seep, trickle and gush from the seafloor into the dark abyssal depths of the ocean. In places, vents billow black smoke - water thick with specks of metal sulphides - and build spires and chimneys of solid sulphides as the hot water mixes with the cold, deep ocean. In other places, pale grey fluids seep out through porous diffusers looking like old-fashioned beehives, and, over the whole vent field, shimmering plumes of clear, warm water rise into the ocean. The water rises from heaps of clinkery sulphides coloured red from rust, or yellow from sulphur, crawling with strange animals.
But the most spectacular parts of the vent fields, the black smokers, do not hold the key to mineral wealth. They mark places where the metals leak out as smoke and are lost. While the chimneys that they build are spectacular - individual spires can grow as much as a few metres in a year - the mass of metal involved is insignificant on a global scale. Serious mineral growth needs two things: something to trap the fluids, so that the minerals precipitate in one place, and enough time for metals to accumulate. A particular target of BRIDGE was to discover how seafloor metal deposits grow as large and rich as those in the rock record on land.
BRIDGE scientists concentrated on active vent fields along the Mid-Atlantic Ridge, where surveying of the water column suggested that they are spaced tens to hundreds of kilometres apart (see Sniffing out plumes). In particular they targeted two sites, Broken Spur, discovered by BRIDGE scientists close to 29N, and TAG at 26N, discovered in 1986 after many years searching by Peter Rona and his team from the USA. Our work depended on collaboration with many colleagues from several countries, including the joint Russian-British expedition on the research ship RV Keldysh.
Broken Spur is a small deposit at present, probably no more than ten thousand tonnes of sulphides. It consists of a number of sulphide edifices, up to 10 m wide and 40 m high, scattered over an area 100 m across, rising from a surface of young lava flows. Rowena Duckworth and Richard Knott described wonderfully intricate structures of the sulphides seen through microscopes, showing very complex and varied relationships between different iron, copper and zinc sulphides. These arise because the sulphides precipitated very rapidly when hot vent fluids mixed with cold seawater, either as the hot water emerged into cold water, or as the two mixed below the seafloor.
The TAG deposit would certainly be an ore deposit were it on land, because of its size and content of copper and zinc (see TAG facts). During BRIDGE, TAG was not only visited several times by British scientists, but was also the focus of major international cooperative studies, including one expedition of the international Ocean Drilling Program ship JOIDES Resolution, which drilled 17 holes as far as 125 m into the mound, giving the first insights into the interior of an active deposit, and the first good estimates of its size. Here the outside of the deposit showed the same complex relationships between sulphides, but deeper within it, the patterns have gone: the sulphides have recrystallized as hot water trickles through them on its way to the surface.
The big surprise to the scientists on the drill team - apart from the extent of the deposit - was the abundance of the white mineral anhydrite (calcium sulphate). This was prevalent up to 70 m below the sea floor. Geochemists who analysed fluids from the sea floor had predicted that anhydrite would be there, but this was the first time it had been found by drilling. Anhydrite is not itself an ore mineral. But Rachel Mills and Meg Tivey were excited about finding it because it means that seawater must seep deep into the TAG mound. This process turns out to be the key to creating a rich deposit. The continuing percolation of both hot, metal-rich fluids and cold sea water allows the minerals to build up to high grades.
How long does this take? Mike Bickle used the technique of radiometric dating to delve into the TAG mound. "The surprising result," he says, "was that the oldest minerals came from the middle of the deposit." The minerals above and below the middle layer are younger. "This allows us to recreate how the mound of minerals grew on the seafloor."
How long has this been going on?
He thinks chimneys formed early on, then toppled onto the seafloor to form a cap over the hot water. Sea water then found its way into the seafloor under the cap, so that minerals such as anhydrite and metal-rich sulphides could accumulate there. Now and then vent fluids broke through the cap so that the mound grew upwards, at the same time as it continued to grow downwards underground. Claude Lalou and Mike Bickle have shown that the whole mound formed in about 20 000 years - a long time for one vent site but merely a split-second of geological time - and that the increase was not steady: it seems to have grown in short spurts perhaps a century long, separated by a few thousand years of inactivity.
It turns out that anhydrite plays an important role in this story. When part of the mound cools below 150 íC, anhydrite there dissolves, leaving space for more hot fluids to percolate in from below and more minerals to grow. Then that part of the mound becomes hotter. As minerals form, they clog up this pathway and send the hot fluids elsewhere. That part of the mound then cools, anhydrite dissolves and the cycle begins again. The cycle of continued growth, clogging, cooling and dissolution establishes the mound.
Another important mineral in the system is silica (silicon oxide). Silica precipitates from hot fluids as they pass through the mound and cool. The temperature and conditions during formation are imprinted on the silica structure in a way that can be read using a laser probe. Steve Roberts, Laurence Hopkinson and colleagues have studied silica from the TAG mound. Their results corroborate the anhydrite story, hinting at a long and complex history of waxing and waning fluid supply.
While mineral deposits studied during the BRIDGE programme fall a long way outside our definition of an ore, they have proved invaluable in understanding how minerals accumulate on the ocean floor and how metals reach the ocean from the crust. We have moved several strides further towards understanding where mineral deposits come from - invaluable knowledge for finding and husbanding the metal resources of tomorrow.
BRIDGE scientists Rachel A Mills, Steve Roberts, Laurence Hopkinson Southampton Oceanography Centre; Mike Bickle, University of Cambridge; Rowena Duckworth, Richard Knott, University of Cardiff; Richard Herrington, Natural History Museum; Jamie Wilkinson, Imperial College Overseas colleagues: Peter Rona Rutgers University; Meg Tivey Woods Hole; Claude Lalou Gif-sur-Yvette.
I became involved in BRIDGE when I started a PhD at Cambridge. No sooner had I found my office, than BRIDGE sent me off to the USA for a workshop on seafloor hydrothermal systems. Suddenly I was swimming in the same pool as people whose research I had read as an undergraduate. From the word go, I was part of the international research scene and its diverse ideas. I studied hydrothermal plume theory with Russians, manganese geochemistry with Icelanders, and what anemones eat with an American.
On finishing my PhD, I was awarded a BRIDGE-funded Fellowship - somewhat to my surprise, becasue I knoew that one of the reviewers of my proposal was a particular eminent US scientist. My concern was not scientific, more that I had been sick on his feet in the US submersible ALVIN during a stomach-churning recovery from rough seas. (I can say in my defence only that the standard ALVIN packed lunch is peanut butter and jam sandwiches.)
Some of the work for my Fellowship involved laboratory experiments at high temperature and pressure, at the time outside my expertise. But because of my involvement with BRIDGE, I could collaborate with specialists in this field, and spent three months working with them in their laboratory in the USA to achieve what would have taken me at least three years if I'd had to start from scratch.
There are two definitions of the word "career". One is "a swift course through a chosen profession", and the other is "to move or swerve about wildly". I'd like to think that as I move on to take up a Lectureship at the Open University, the former is the most applicable. And this can be at least partly attributed to my participation in BRIDGE.
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Twist and Spout
How does the hot vent fluid, carrying its load of minerals, spread into the oceans? Experiments show that it curls into huge eddies, triggered by the spin of the Earth.
Active hydrothermal vents produce the spectacular plumes of hot, mineral laden water known as black smokers. How they rise into the oceans is fundamentally important in determining how the minerals and the heat they carry spreads through the oceans. In addition, the plumes play a part in the migration of the vent animals. BRIDGE has worked on measuring and documenting the flows in the sea. It has also supported laboratory experiments on the dynamics of the plumes. The models mimic certain puzzling features of the plumes; they also reveal what controls the circulation of the hydrothermal fluid in the oceans.
When researchers began to seek out plumes, measuring the chemistry and cloudiness of seawater (Sniffing for plumes), they found some puzzling data. At Broken Spur, the results suggested that there were two plumes spreading in the oceans. One might be linked to the vent field below. The Russian Mir submersibles dived and dived, seeking the second source, but found none. Similar oddities arose during the discovery of Rainbow: the distribution of murky plume water within the clear ocean was complicated. Although the overall picture was like smoke rising from a chimney, hydrothermal plumes spread in a more complex fashion, developing convoluted shapes in the wider ocean.
Huge eddies are a feature of the atmosphere and oceans; they arise in situations where the rotation of the Earth influences the flow of these fluids. This takes time: the effects are not noticeable in a river, for example, where the flow is too fast. BRIDGE examined whether the unexpected patterns of the plumes in the oceans could result from eddies.
Finding its own level
At a black smoker vent, fluid at 350 or 400C spurts from the seafloor at around 1 m/s, which is 36 km per hour. Because the fluid is hot, it is considerably less dense than the surrounding seawater, so it is buoyant and rises. As the fluid continues to rise, it mixes with the sea water and cools. The cooler water precipitates minerals, which form tiny particles and lead to the characteristic smoky appearance. As the plume continues to rise and mix, the density of the fluid in it gradually decreases as it mixes with the water around it. Sea water is less dense the closer it is to the surface, so the plume mixes with fluid that is less and less dense as it rises. The plume becomes less buoyant the higher it rises, until it is not buoyant at all. For typical plumes, this happens between 100 and 300 m above the sea floor. Then the plume spreads horizontally. This level is known as the neutral level, because the density of the plume fluid is no more and no less than the seawater here.
The fluid takes just a few minutes to rise to this neutral level, where it spreads out in a cloud, looking like a plate spinning on a pole in a magic show. This outward flow is slow enough to be subject to the Earth's rotation. The fluid develops a circular flow as it spreads out from the rising plume. The cloud becomes unstable when it has spread to a particular radius. Eddies develop, then break off from the plume and drift away as lens-shaped vortices, while a new cloud develops above the plume.
Over the past few years, it has become clear that these larger scale dynamics of the neutral cloud above turbulent buoyant plumes have an important impact on the subsequent dispersal of hydrothermal fluid. And they make sense of the observations at Broken Spur and Rainbow. In both cases, there was just one plume sending off spinning vortices of tell-tale smoky fluid.
These plumes also provide invaluable information about the heat flux from the seafloor. It is difficult to estimate the heat flux from an individual vent field directly, because of the harsh environment and the difficulty of taking such measurements from a submersible. But the classical theory for turbulent buoyant plumes provides a simple estimate for the heat issuing from the seafloor in terms of the height to which the plume rises above the rocks, and the density gradient in the sea.
Moving out of the lab...
Experiments by Andy Woods and John Bush demonstrate the evolution of a plume in the laboratory, but in the ocean other factors play a part.
In particular, the shape of the seafloor makes a difference. If the hydrothermal vent lies within a median valley, as is the case at several places on the Mid-Atlantic Ridge, then when fluid spreads out from the top of the plume, it may be confined within the valley walls. If the valley is wider than the neutral cloud when it becomes unstable, then the plume sheds a series of discrete vortices which migrate along the wall of the valley. This corresponds to models that use a relatively slow discharge. But if the valley is narrower than this critical radius, then the plume feeds a continuous flow along the valley walls; this corresponds to laboratory models with a relatively high discharge.
The experiments and models also made sense of some enigmatic observations first made in the Pacific Ocean. On rare occasions, researchers have found huge masses of warm water, 20-40 km across, and, at 700-1000 m above the seabed, much shallower than the usual hydrothermal plume. The water was a few tenths of a degree warmer than the usual plume water - a very small amount but significant when spread through such large bodies of water. These megaplumes are eddies shaped like giant pancakes, a few hundred metres thick.
Such a big, warm plume demands a lot of heat. And the buoyancy needed to drive the plume so high in the ocean is far more than for ordinary plumes, so the heat must be supplied very fast. One idea for the origin of megaplumes was that they happen when a vent field suddenly cracks open and lets all of its hot fluid out in a rush. But American surveys showed that some of the megaplumes had been seen over sites of newly-erupted lava flows, erupted at about the same time as the megaplume was seen. Could the heat supplied from an eruption be enough to make a megaplume? The total amount of heat is about right, some 1015-1016 J, but can the heat be transferred into the water fast enough from the lava?
As part of the modelling effort of the BRIDGE programme, Martin Palmer, Gerald Ernst and Steve Sparks showed that the heat can indeed be extracted from hot lava fast enough to fuel a megaplume, and in doing so appear to have solved a tricky problem.
Two other aspects of plumes that BRIDGE has tackled are the spread of plumes that include lots of gas bubbles, and the deposition of sediment from the plumes. Gerald Ernst and Steve Sparks used simple fish tanks for their experiments, which mimicked some of the observations of oceanographers. Building on experience of the way that ash settles after volcanic eruptions on land, they made the first experiments on how the toxic sediment disperses from hydrothermal plumes alter both in still water and when there is a cross-current. They discovered how plumes hold onto their smoky sediment, rather than dropping it to the seafloor, that they can split into two plumes and even generate the submarine equivalent of tornadoes.
The research has identified how very large, rare eddies may be generated in the water column as a result of the vigourous geological activity below the surface. The size, speed and composition of the hydrothermal fluid influences how it spreads through the oceans, as does the location of the vent. The dynamical evolution of the hydrothermal effluent also has an important impact on the transport and dispersion of particles and vent creatures through the deep ocean.
BRIDGE scientists: Andrew W Woods, John Bush, Gerald Ernst, Martin Palmer, Stephen Sparks University of Bristol; Chris German, Southampton Oceanography Centre.
A fine thing...
William Beebe, the first person to venture into the deep sea in a bathysphere, wrote that there are two kinds of thrill in science. One is the result of long, patient, intellectual study, but the other lies in a completely unexpected discovery. Although BRIDGE has seen its fair share of patient, intellectual study, there have also been plenty of surprises. There are few experiences that compare to waiting on the deck of a ship for a haul to return laden with samples from the ocean depths, as trawling, dredging and diving in submersibles at mid-ocean ridges can be just like playing lucky dip.
On 4 April 1993, BRIDGE scientists patiently cruising the Atlantic aboard the RRS Charles Darwin came across a plume of mineral-rich water spewing from hot springs on the ocean floor. The site, named Broken Spur, was only the third set of black smokers to be discovered on the Mid-Atlantic Ridge. Since then, BRIDGE scientists have sniffed out traces of eight more sites south of the Azores, confounding those who predicted that the vents should be scattered fewer and farther between.
Chance of a lifetime
Lady Luck has also played a hand in helping to fill the gaps in our patchy knowledge of life on the mid-ocean ridges. Whilst collecting rocks from the Reykjanes Ridge, BRIDGE geologists dredged up an unexpected menagerie of creatures that filled two freezers. Expeditions now carry a "biobox" to preserve such chance finds and biologists are still finding new species as they sift through these collections.
But in a realm that is so seldom visited, even the partly familiar still yields surprises. In 1995, some odd-looking shrimp turned up in nets being trawled above Broken Spur. Although the tiny shrimp looked nothing like those teeming around the black smokers, they turned out to be their offspring. These larvae look so different from their parents because they are adapted to dispersing across the large distances from one vent to the next, transforming once they arrive, like caterpillars into butterflies. Remarkably, it appears that at least 50 baby shrimp make the 360 km trip between two particular sets of vents each generation - an epic journey for such tiny crustaceans, and one that poses plenty of questions for the biologists who retrace their steps.
These discoveries contribute to one of the most important aspects of the BRIDGE legacy - they help to fuel our sense of wonder. There are few places on Earth where the unknown is as tangible as at mid-ocean ridges. The ridges themselves were a surprise when they appeared on seafloor maps from ships' echosounder traces in the 1950s. And although geophysicists predicted the existence of deep-sea vents, the riot of life around them came as a complete shock, rewriting some of the rules of biology. Given the surprises that we have found so far - and how much more there is still to explore - it makes you wonder just what else is out there.
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Beyond the black smokers
Black smokers, grey mist and invisible plumes are signs of the hidden plumbing that carries water within the seafloor. New instruments reveal the contribution of these fluids to the oceans, as well as as unravelling the effects of tide and even day and night on the flow.
The Earth is heated externally by about 180 000 terawatts of solar energy and this is the main reason why we live on such a habitable planet. But we also benefit from a little heat from inside the planet, as the Earth cools down. This heat at present is about 43 TW, which does not seem very much. But it turns out to be important for ocean geochemistry, geophysics, geology, marine biology, and, some say, for the origin of life. Much of the heat goes into the ocean crust and the majority is lost to ocean water at mid-ocean ridges by hydrothermal circulation through the seafloor and black smokers, driven by convection.
Hydrothermal vents were first discovered in 1977, at the Galapagos spreading centre using the US submersible ALVIN, by a team led by Jack Corliss and John Edmond. Since then, hydrothermal activity has been found in all oceans. Harry Elderfield, with Peter Rona and Gary Klinkhammer, was part of the team that in 1985 first found vents in the Atlantic at TAG, visited 10 years later as part of the BRIDGE programme. In 1997, BRIDGE scientists led by Chris German discovered the Rainbow site, the largest yet found in the Atlantic (see Sniffing for plumes).
Because the chemistry of the hydrothermal vent fluids differs so much from that of the seawater that enters the oceanic lithosphere, an obvious question for geochemists is: how important is this process? Many chemical elements are transported to and from the oceans by hydrothermal circulation, but how much and at what rate? It is difficult to make precise calculations but many scientists believe that the process of hydrothermal circulation is as important in defining the chemical budget of many elements in sea water as rivers are to the input of chemicals to the oceans. However, this view has been questioned recently.
The basis for this challenge to what had become an established paradigm is an understanding of the convective heat budget of the seafloor. There are two issues here. The first issue has been the increasing realization that the black smokers, whilst spectacular, probably do not transport the majority of the heat that leaves the ridge crests. This was discovered in 1992 by Adam Schultz (then at the University of Washington) using a crude bell jar apparatus on the sea floor of the Juan de Fuca Ridge around vents but some distance away from the vents themselves. This experiment showed that perhaps as much as two thirds of the hydrothermal heat flux at the ridge crest was associated with lower temperature, diffuse venting and not with black smokers. To tackle this problem more precisely, Schultz started designing and building sophisticated diffuse flow monitoring equipment as part of the BRIDGE programme. The equipment, named Medusa, measured the temperature and velocity and hence the heat flux of diffuse fluids discharging from the seabed and collects samples of the fluid (see "Medusa"). Adam Schultz and Harry Elderfield first deployed Medusa at the TAG and Broken Spur hydrothermal sites as part of the British-Russian Bravex expedition using Mir submersibles; this was followed up by an ALVIN dive series the following year.
The results confirmed what had been found at the Juan de Fuca Ridge and came up with a surprise. The high-temperature heat flux at TAG has been estimated at about 200 MW by Peter Rona in 1993 but plume studies by Mark Rudnicki and Harry Elderfield in 1992 suggested that the total heat coming from hydrothermal activity at TAG is around 500-1000 MW. Simultaneous measurements of the temperature and velocity during fluid sampling indicated that the heat flux density associated with diffuse effluent was about 2.5-2.8 MW per square metre. This suggests that 10 times as much heat is spread by diffuse flow than by the high-temperature vents.
A bonus came from chemical analyses made on the Medusa samples by Rachael James, at that time a graduate student. Rachael showed that the chemical composition of elements such as lithium and manganese in the Medusa fluids could be explained by mixing the black smoker fluids with sea water. As shown by modelling work by Penny Dickson, another BRIDGE graduate student, sea water readily seeps into the TAG mound. But the chemistry of other elements such as calcium, silicon and iron could not be explained in this way as the concentrations were lower than predicted from mixing. This observation directly demonstrated that underground mixing of seawater and high-temperature black smoker fluids leads to the precipitation of sulphides, silica and anhydrite and to the zone refining of metals, including copper, zinc and uranium in the TAG mound - exactly what was found by drilling TAG (see Smoking out metals).
Away from the ridge crest
The second reason for questioning the role of black smokers in defining ocean chemistry is that hydrothermal circulation is not restricted to ridges. Rather, the inflow of cool seawater into the cracked permeable ocean floor, and the attendant cooling of the sea floor as the sea water warms is ubiquitous and characteristic of oceanic lithosphere of age 0-65 million years. Globally, this represents fully half of the area of the sea floor and 35% of the total surface area of the planet. This means that roughly 100 to 1000 times more sea water circulates into and through the older part (1-65 Ma) of the oceanic lithosphere than through new oceanic lithosphere (0-1 Ma). We do not yet understand how the circulating sea water interacts, but it moderates the chemistry of the oceans and alters the mineralogy and composition of the crust.
Plumbing the depths
BRIDGE also took the opportunity to use their new instruments to monitor vent sites in the long term. Medusa could be left on the seafloor for months on end; when the Ocean Drilling Program targeted TAG as a site for a borehole, the Medusa team broke all records to design and build a working instrument in time to monitor the speed and temperature of the flow before, during and after drilling. Three Medusas were left on top of the TAG mound by the Mir submersible. Although they had problems (see Machines for the Abyss) one of them worked for most of the time, recording temperature and flow rate, as well as collecting water samples.
The team found that the temperature of the fluid varied with the tides in the ocean, and with day and night. The speed of flow varied daily. These fluctuations come from the effects of tides on the pressure at the sea floor, perhaps opening and closing fine cracks in the mound, altering the rate at which fluids can circulate. The temperature variations could arise from different parts of the mound being differently affected, so that the flow of warmer or cooler fluid would be disrupted.
An ODP borehole, tens of metres deep, might be expected to interact with whatever plumbing system existed within TAG. It certainly did so: the warm water seeping out of the eastern part of the TAG mound got considerably warmer. It seems that the borehole tapped into a pathway for warm fluid within the mound, which looks as if it had flowed through a layer of cracked and broken rock. And because this change in the flow of fluid through the mound made a difference to the heat conducted through the rocks, these results made possible calculations of the thermal diffusivity of the mound materials.
A workable model - for now
While a useful, semi-quantitative understanding of oceanic hydrothermal circulation has emerged, much work is needed to understand better the basic physics of the process. In particular, we need to understand what controls circulation over the full history from young to ancient lithosphere, and ultimately to find integrated fluid fluxes. At present, geochemical fluxes are believed to be large, but no one knows how large; little is known about the depth to which thermal or chemical circulation reminas significant, nor about how efficient water-rock interactions are. And we know very little about just how isolated sedimentary and volcanic rocks become at great age.
Discrete vents are difficult to find away from mid-ocean ridges, but they do occur, and, like their counterparts at mid-ocean ridges, they support life based on chemosynthetic bacteria (see A bug's life). It is also possible that bacteria may be ubiquitous at depth in the crust. We have only just begun to explore these parts of the seafloor and sub-seafloor biosphere.
BRIDGE scientists: Harry Elderfield, Adam Schultz, Penny Dickson, Rachael James, Mark Rudnicki, University of Cambridge; Chris German, Southampton Oceanography Centre. Overseas colleagues: Gary Klinkhammer, Oregon State University; Peter Rona, Rutgers University.
Medusa - measuring hydrothermal fluids
Medusa was built to tackle an urgent scientific problem. Hot water emerging from a black smoker chimney can be sampled, and its temperature and flow rate measured from a submersible. But much of the fluid emerges as warm water diffused over large areas of the seafloor, and the normal measuring and sampling instruments cannot be used there. BRIDGE researchers needed to know how important this diffuse flow is to the overall flow, both chemically and in terms of the energy flux. Are the black smokers carrying most of the flux, or only a small proportion of it?
What was needed was an instrument that could make these measurements, either as a series of spot measurements while carried around by a submersible, or as a long-term deployment on a patch of diffuse flow to discover if the flow changes with time. The instrument had to be able to survive in the hostile vent environment, to measure and record for long periods the flow rate and temperature of the flowing water in order to estimate the energy flux, and to collect samples of diffuse flow for chemical analysis to determine the chemical flux. These demands were complicated by the need to seal off the bottom of the instrument against the rock to stop leaks from the sides, and by the fact that the diffuse flow is immensely variable in flow rate and temperature, so it needed to work over a very wide range of conditions.
Medusa was designed to these specifications and built against a tight deadline. The central column is the tube in which flow rate and temperature measurements are made, closely sealed to the ocean floor. Around that are the six water sample containers, with springs to open and close them. Medusa is exceptionally reliable, only being overwhelmed in the most hostile conditions (see image).
During BRIDGE, Medusa evolved into a powerful and versatile instrument, much in demand around the world. It has been deployed in many different places, for periods up to 14 months, and can collect up to six different samples of water during each deployment. What emerges from the measurements is that in most vent sites the diffuse flow dominates, especially in terms of the energy flux.
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Sniffing for plumes
Hot springs can be tracked through the world's oceans by Bridget, the chemical bloodhound. What the BRIDGE instrument found was a new vent field that breaks the scientist's rules.
Hydrothermal vents are rare. Fewer than 100 are known on the more than 60 000 kilometres of mid-ocean ridge worldwide. On the Mid-Atlantic Ridge, there were just two sites at the start of the BRIDGE programme in 1993. A particular goal of the chemists in BRIDGE was to work out how to find high-temperature hydrothermal activity - black smokers - on a segment of ridge crest. A key element of their success was to be the emergence of new technology to be exploited, or even hijacked!
When vent fluids erupt from a high-temperature hydrothermal field they are so buoyant that they rise hundreds of metres above the sea floor before levelling out and being swept along by deep-ocean currents, like a plume of smoke from a factory chimney on a windy day. The direction of the wind dictates where the smoke goes. Just as you can tell that you are downwind of a smelly chimney by sniffing the air, marine geochemists prospect for hydrothermal activity in the deep ocean by following chemical traces of a plume towards stronger signals. When the signal vanishes, you know that you have passed the source.
To find a new vent field in this way, you need some characteristic measurable tracer of hydrothermal plumes in the deep ocean. The best candidate would seem to be an isotope of helium, helium-3, which comes from the mantle at mid-ocean ridges and spreads with the plume without chemical changes. Furthermore, the helium-3 signal is unambiguous: there is essentially none of it in the atmosphere and hydrosphere, apart from in plumes. This makes helium-3 very sensitive, useful for following plumes over hundreds or even thousands of kilometres. But it is difficult and slow to measure.
Data while you wait
It is easier and quicker to use dissolved manganese and methane. They too are very rare in the deep ocean, but are about a million times more common in hydrothermal vent fluids. Their main advantage is that sea water can be analysed for them on board the ships that take the samples - unlike helium-3. Thus you can launch an expedition to the mid-ocean ridge, taking the necessary expertise with you, and expect to collect updates on your proximity to a potential new vent site as you sail.
Perhaps the single biggest breakthrough in hydrothermal exploration came when deep-ocean research hijacked an instrument more at home in shallow water - a nephelometer. A nephelometer measures the density of particles in water using the light that they scatter. Metal-rich fluids from black smokers react with the oxygen-rich waters they erupt into and form particles of iron and manganese oxides. So hydrothermal plumes tend to be cloudy.
In 1985, Terry Nelsen from the US National Oceanographic and Atmospheric Administration added a nephelometer to the water sampling system used for Gary Klinkhammer's manganese analyses. They wanted to find out if a nephelometer could detect and trace a hydrothermal plume.
The answer was an emphatic yes: the nephelometer readings matched the manganese concentrations, sample for sample. The added advantage of the nephelometer as a proxy chemical sniffer, however, is that it is not restricted to point sampling and shipboard analysis. It can generate continuous records, wherever it is in the ocean, and send that information in real time to a researcher on watch aboard ship.
The first stage was to work out how to use this tool. In the early 1990s, engineers and geologists at the Institute of Oceanographic Sciences in Wormley had launched their new instrument, TOBI, after years of development. TOBI uses sonar to map the seabed. Chris German takes up the story: "Soon after I arrived at IOS I enquired, purely out of curiosity, at exactly what depth the vehicle operated. To my excitement, I discovered that the best imaging height was between 100 m and 500 m above the seabed, ideally at 300 m - exactly where hydrothermal plumes had been discovered in the past." The IOS team, led by Nick Millard, had inadvertently developed an instrument that could fly straight through hydrothermal plumes. And TOBI was designed to collect datacontinuously for days or even weeks, covering hundreds of kilometres of ridge crest at a time. No time was wasted in fitting up the vehicle.
In the next six years TOBI and its optical sensors found hydrothermal activity in four major areas. For example, Bram Murton and Cindy Van Dover discovered the Broken Spur vent field by tracking it down with the augmented TOBI. The results have been most profound for the ridge running southwest from the Azores between 38íN and 36íN. Chris German and Lindsay Parson led a cruise known as HEAT to examine this area. This section of the Mid-Atlantic Ridge has a complicated shape; lengths of spreading ridge are offset at seams in the crust called non-transform discontinuities, NTDs. Chris German endeavoured to keep the nephelometer operational and streaming data up the wire wherever TOBI was - above the volcanoes of the spreading segment, where vents were expected, or over a transform fault at an NTD, where they were not. This continuity of data was to be of key significance.
When all the nephelometer data for the cruise had been processed, the team had a surprise. They had found seven areas of apparent hydrothermal plume activity, but only three were near spreading segments of the ridge, where the volcanoes are. The other four were at the ends of spreading segments - more specifically on the NTDs between segments. This was unexpected, to say the least. As Chris German recalls: "Many researchers did not believe that you could have hydrothermal plumes without volcanism. Just dirty water, they said, with sediment stirred up. I disagreed."
One of the NTD sites had the strongest signal found on the whole expedition. The team returned in the next few weeks to collect water samples. Chemical analysis - including the work of PhD students working with Harry Elderfield and Martin Palmer during subsequent BRIDGE-funded cruises - proved that it was indeed hydrothermal, enriched with iron and all three classic dissolved tracers: helium-3, methane and manganese. The BRIDGE programme later developed a second instrument, called Bridget, to carry out more detailed deep-tow investigations of hydrothermal plumes. In 1997 the team went back to the same area - known as the Rainbow plume - to investigate further.
Vents where no vents should be
Two findings resulted. First, the plume was recognized as the strongest such feature in the Atlantic Ocean with chemical anomalies that could be traced continuously for about 80 km downstream. Kelvin Richards exploited this result to find out how the plume disperses into the ocean. Secondly, we were also able to narrow down the vent source to an area just 200 m across. With such precision, Lindsay Parson and Chris German could go straight back just two weeks later with Yves Fouquet, to guide the French submersible Nautile directly to the vent-site. They found sulphide from extinct vents and at least 10 active chimneys over an area 60 by 250 m.
After three years of disbelief from parts of the scientific community, BRIDGE had found an active hot (363 degrees C) black smoker vent in a faulted part of the Mid-Atlantic Ridge - where no vents should be. Not only was it on a slow-spreading ridge but it was also far from any recent volcanoes Indeed, there were no volcanic rocks at all. Instead, apart from miles of seemingly endless sediment, the smoking chimneys were stacked up against a fault.
Together, the BRIDGE team had found a new geological setting for vents: a result that could mean that venting is just as common on slow-spreading ridges as on fast ones. In the process, they established a new way to explore the oceans - and put Chris German's mind at rest: "When I saw the active vents I was so relieved. My results were definitely not just dirty water!"
BRIDGE scientists: Chris German, Bramley Murton, Martin Palmer, Lindsay Parson, Kelvin Richards, Martin Sinha Southampton Oceanography Centre; Nick Millard IOS, now SOC; Harry Elderfield, University of Cambridge. International colleagues: Cindy Van Dover, William and Mary, Gary Klinkhammer, Oregon State University; Yves Fouquet IFREMER
Bridget, the BRIDGE Towed instrument, is a flexible deep-tow instrument platform for the detection, investigation and sampling of hydrothermal plumes, developed by BRIDGE and the University of Cambridge and the Southampton Oceanography Centre. Bridget is towed up to 6000 m behind a ship, at up to 2 knots, rising and falling like a yo-yo. It carries optical and other tools for detecting and sampling plume water. It led to the first systematic, three-dimensional surveys of the physics and chemistry of a plume.
Bridget's data will help to answer questions about the distribution of vent sites, the chemical and thermal fluxes associated with sea floor spreading and, specifically, the relative importance of hot and cooler fluxes. And once a site has been found, Bridget's sampling and sensing devices devices allow detailed investigations on the seafloor, complementing data that can be collected with a deep-diving submersible or remotely operated vehicle.
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A bug's life
Vents are dark, deep and decidedly unhomely - yet microbes thrive. Without them, the communities of vent creatures would not be there. How do they do it?
The first indication that you are approaching a field of hydrothermal vents in the dark of the deep ocean is a gradual increase in the abundance of life. Away from hot springs, the young lavas of the mid-ocean ridges are nearly barren, with perhaps a sea lily or an anemone perched on an edge of rock, and an occasional squat lobster waiting patiently for food. Then you see more squat lobsters, then perhaps a few more anemones, until eventually, close to the active vents, more and more organisms may carpet the seafloor, all living in uncomfortable proximity to the hot, toxic vents. The following articles describe these larger organisms, but how do they live? And why do they take the risk of living in the vent environment?
The reason is food. There is food at the vents, lots of it, and it is available because chemosynthetic microbes live around the vents. Chemosynthesis is the process by which energy is extracted from reactions between chemicals in the environment, and is used to drive the biological synthesis of organic chemicals, and hence to fuel life.
The vents emit hydrogen sulphide, methane and other reduced molecules. The ocean water contains dissolved oxygen and other oxidized molecules. When the two waters mix, the oxidized and reduced molecules are brought together and start to react, releasing energy. This is the opportunity that the microbes exploit. They are adapted to speed up these reactions, so that the energy can be channelled into biological synthesis. In a vent field, there is about as much energy available per square metre as from the Sun in a tropical rain forest, and nutrients are abundant, so it is not surprising that microbes flourish.
But the environment is a very hostile one to life. Temperatures range from 2 degrees C up to 400 degrees C, sometimes in only a few centimetres. The fluids are rich in metals such as copper, and hydrogen sulphide, which is highly toxic. Yet it is in these tough conditions that energy is most abundant. Most lifeforms cannot survive above 50 degrees C. But hyperthermophilic microbes prefer temperatures over 80 íC, and can live at temperatures at least up to 115 degrees C. Many of these microbes also seem adapted to survive the chemical stresses of this hostile environment.
Microbes in mud
An interesting microbial environment in which much BRIDGE work was concentrated is that of the sediments that surround the larger and longer-lived hot spring deposits, such as TAG. These sediments form when the the sulphides of the mounds weather in the oxidizing ocean water. The results are aprons of fine-grained iron oxide sediments strewn with sulphide rubble, and mixed with carbonate sediment that has sifted down from the sea surface several kilometres above. The mixture of sulphide blocks and oxidized iron in the sediment is a source of energy, and warm fluid seeps into the sediment from below, adding to its potential for life.
One important site, targeted by John Parkes, Barry Cragg, Martin Palmer, Rachel Mills and Silke Severmann, was at the Mir Mound, an inactive hydrothermal deposit close to the TAG hydrothermal field. Here much of the sediment is rich in iron and copper with smaller amounts of other metals such as cadmium, zinc, uranium, chromium and vanadium. Several of these metals can be toxic to microbes, but microbes were found throughout a 2.2 m core of the sediment. Between two sulphide-rich layers within the core was one containing iron oxides. The microbial populations are high in the sulphide zones and noticeably lower in the intermediate oxidised layer.
Yet measurements of various types of microbial processes indicated that the relatively oxidised zone held most microbial activity, and this was also the layer from which most viable microbes were isolated. It seems that this layer is one of high microbial activity but low microbial biomass. The microbes present included forms that can reduce manganese, and both reduce and oxidize iron. Measurements of thymidine incorporation (thymidine is a component of DNA; its rate of incorporation is a measure of population growth) and counting the proportion of cells engaged in cell division both show that this layer is one of relatively rapid population growth. But since the total population is low, it must mean that only a fraction of the population is growing - those that can tolerate, and use, the metals and toxic sulphides in the sediment.
A different approach
Don Cowan and his team approached the problem in a very different way. They set out to isolate and culture individual species of microbes from the vent environment, with the especial aim of investigating whether some of the thermophilic microbes might be aerobic - oxygen-users. (Most previous work had looked specifically at anaerobic microbes.) They discovered aerobic thermophiles in sediments from close to the TAG mound. These appear to be strains of the bacterium Bacillus that have adapted to this environment. Don Cowan showed that these bacteria have important biotechnological potential. Their unique characteristics, that allow them to survive under these conditions, can be put to use. For example, some enzymes from this research have potential in food processing, detergents, paper-making and degradation of toxic waste.
Both pieces of research, converging on the same topic from different directions, indicate that these microbes may be a unique resource for cleaning up places contaminated by humans with toxic metals, in a similar way to their action in the naturally contaminated ocean floor.
BRIDGE scientists: John Parkes, Martin Palmer, Barry Cragg, Jon Telling, Jo Rhodes, University of Bristol; Rachel Mills, Silke Severmann, Southampton Oceanography Centre; Don Cowan, University College London.
A hot spring rather than a warm pool: did life begin at hydrothermal vents?
The most primitive organisms known so far, as judged on the molecular tree of life, are heat-loving microbes, both Eubacteria and Archaea. Many of them live around deep ocean vents. And chemosynthesis must have come before photosynthesis, because the biochemical pathways for the two processes are the same, except for the addition of mechanisms in photosynthesis for converting light energy into chemical energy in the cell.
But conditions when life began must have been very different from now. In particular, there was no molecular oxygen in the atmosphere or ocean, because our oxygen has been produced by billions of years of photosynthesis. So the chemical pathways that are used at vents now would not have worked then. The only simple reactions that can give enough energy to form organic molecules, and can work in a world free of molecular oxygen, involve hydrogen gas. Is there a source of hydrogen in hydrothermal vents?
Most vents emit some hydrogen, but the Rainbow vent field just discovered at the Mid-Atlantic Ridge (see Sniffing for plumes) was found by Jean-Luc Charlou to emit high concentrations of hydrogen. This is apparently because the hot vent water passes through peridotite, the rock that makes up the Earth's mantle, and converts it to serpentine with the formation of hydrogen. This fixes an important link in the chain of evidence. In the early Earth, lavas very like peridotite in composition were much more common than today. More vent fluids would have passed through this rock, so many more vents would have given off significant amounts of hydrogen, and these could have been critical in the origin of life. How it all began? We now know a lot about the conditions for the origin of life.
It must have started in water, in an ocean. The water would have had to contain dissolved carbonate, and there must have been a source of hydrogen, probably at temperatures around 100 degrees C. This in turn implies the presence of peridotite or a similar rock, bathed in, and altered by, hydrothermal fluids. And that peridotite must have been continually renewed, probably by some sort of plate tectonic process. This pretty well describes the early Earth. Why do we need to seek the origin of life in the wilder realms of outer space, when the conditions were right, here, on our own familiar planet?
DNA damage, toxic vents and environmental contaminationtoxic vents and environmental contamination
Deep-sea hydrothermal vents are arguably one of the most toxic environments on the face of the planet. Apart from the high temperature and pressure, vent organisms are exposed to high levels of toxic heavy metals and radionuclides (arsenic, cadmium, mercury and radon), substances that have well-documented mutagenic and carcinogenic properties.
Recent work carried out as part of BRIDGE has revealed surprisingly high levels of DNA damage in the cells of vent mussels (Bathymodiolus), which correlated with the emission toxicity at specific vent fields on the Mid-Atlantic Ridge: Lucky Strike and Rainbow. It would appear that despite their long evolutionary association with environmental contamination (fossil evidence can be traced back to the Mesozoic), vent organisms are not fully resistant to the damaging effects of their toxic environment. Older (i.e. larger) mussels exhibited higher levels of DNA damage than in their cell nuclei compared with younger (smaller) animals, indicating that the latter may be more efficient at repairing DNA lesions.
Paradoxically, the deep-sea hydrothermal environment is characterized by extremely high growth rates fuelled mainly by bacterial chemosynthesis linked with vent fluid chemistry (hydrogen sulphide and methane). Rapid growth and early reproduction, previously thought of as characteristics that enable vent species to cope with the patchy and temporally unstable nature of venting along the ridge axis, can equally well be seen as adaptations that allow them to survive in this highly toxic environment e.g. through an increase in DNA repair activity linked with certain enzymes (telomerases) that are normally active during the formation of embryos. Investigations of the cellular and molecular biology adaptations of vent organisms hold great potential for biotechnological discoveries, particularly in the areas of DNA repair and bioremediation. Given their long geological history, deep-sea vents provide a unique evolutionary insight into the ways that marine organisms are able to adapt to extremely long-term contaminant exposure.
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The creatures that live in the vents
The creatures that populate deep-sea vents live strange yet ordered lives, far from sunlight. Somewhow they thrive and even travel to distant new vents on the deep ocean floor.
It is surprising that the dark inferno of deep-sea vents should be inhabited at all. Water jets from the sea floor, often well above normal boiling point (up to 375 degrees C), loaded with dissolved metals and toxic hydrogen sulphide, and devoid of life-giving oxygen. As the flow meets cold ocean-floor water the metals precipitate to form chimneys and their "smoke". No one on land nowadays would be allowed to discharge anoxic effluents, containing such amounts of heavy metals and sulphide, into coastal waters.
There was great amazement when the first vent animal communities were found in the depths of the Pacific Ocean twenty years ago. The abundance of invertebrate animals was astonishing. Normally, life is difficult on the deep sea floor: little nourishment percolates down from the productive, sunlit surface of the oceans. Many of the bizarre animals around the vents were new to science and the excitement felt by the discoverers is reflected in the picturesque names they chose for their sites: Garden of Eden, Rose Garden and Hanging Gardens.
Life in poisonous water
The secret of the flourishing vent communities lies in the hot water that issues from the sea floor, specifically its sulphide and methane. Sulphide harms most animals but specialized bacteria can use it to grow. Methane is absorbed and oxidized by different bacteria. Wherever vent water mixes with normally-oxygenated sea water, such bacteria thrive. They grow fast enough to make the water cloudy and carpet rocks, sediment and even instruments with filmy growths. Free-living bacteria are food for animals that can filter or graze; additionally, some vent animals have formed a special relationship with bacteria, providing a home for them in or on their bodies, arranging transport of sulphide and/or methane to them and harvesting their bacterial crop in a sustainable way. Such gardening is a form of symbiosis (living together for mutual advantage).
In 1985 and 1986, the first explorers of the Mid-Atlantic Ridge found active hydrothermal vents with black smokers and dense fauna, in more than 3 km of water. The sites are now known as TAG and Snakepit, the latter named from the many snakefish (Synaphobranchidae) seen there. The chief surprise was the abundance of shrimp around vent openings, and the apparent shortage of the kinds of animals expected from exploration in the Pacific.The hydrothermal vent shrimp in the Atlantic turned out to belong to the same family group (Bresiliidae) as one species (Alvinocaris lusca) found at Pacific vents, but they were different in looks and behavior (see The strange tale of the vent shrimp).
When BRIDGE began in 1991,TAG and Snakepit were the only inhabited vent sites known. Since then, BRIDGE scientists have been active in investigating and locating the northern sites: Menez Gwen, Lucky Strike, Rainbow and Broken Spur. The southernmost vent site, at 14 degrees 45 N, was discovered in 1994 by Russian scientists and has since been named Logatchev, after their research vessel.
The biologists need to see and sample these strange new creatures, so manned submersibles (3-person submarines) had to be employed, and this required much international collaboration. In 1993 the US ship Atlantis II and submersible ALVIN took a BRIDGE geologist to the vents at at Broken Spur to begin direct observation and fauna collection. In 1994 a combined UK/Russian expedition, BRAVEX/94, used the Russian ship RV Keldysh with two Mir submersibles to work at Broken Spur and TAG. In 1997 a joint US/UK group of biologists including Alan Southward, Eve Southward, Andrey Gebruk, and Cathy Allen visited all seven vent sites along the Ridge, using the new US ship Atlantis and the refitted submersible ALVIN During the same period,David Dixon and Peter Herring sampled deep-water plankton for larvae from the RRS Charles Darwin. Knowledge of the Mid-Atlantic Ridge vent fauna has grown enormously during this period.
Order amid apparent chaos
The bustling chaos that life around a vent appears to be on first sight, is in fact highly ordered: each type of creature has its place, not in the Sun, but near the vent. Really hot water is uninhabited, though shrimp sometimes dash through, getting scorched on the way. The inner region where Rimicaris exoculata and Chorocaris chacei are densest is washed by warm water at about 10-30 degrees C, shimmering from small jets, cracks or porous "beehives" built up from the sea floor. The shrimp swarm over the surfaces of chimneys like bees in a hive. In warm spots they move around continuously but where it is cooler they are more quiescent. Large, white crabs (Segonzacia mesatlantica) crawl slowly among the shrimps, eating the dead and dying.
A second zone is subject to diluted flow and more gentle seepage of fluid from the seafloor. Sulphide and methane are less concentrated, and the water is fairly cool (5-9 degrees C). Mussels (Bathymodiolus puteoserpentis etc.) are usually the most obvious animals, with yellow-brown shells ranging from a few millimetres to several centimetres long, sometimes coated with white mats of bacteria. Several shrimp species, small limpets, snails and tiny brittle stars may live near them. The mussels attach themselves with fine byssus threads to any hard surface or to one another, just like mussels on the seashore and can build up into large masses. White crabs and small squat lobsters (Munidopsis sp.) may feed among them. On soft sediments there is a large white clam (Family Vesicomyidae) but clam beds have been found only at the southernmost vent site, Logatchev, where there is a lot of muddy sediment. The clam field at Logatchev contains the first live vesicomyids seen on the Mid-Atlantic Ridge. It is at least 12 m across, with up to 120 clams per square metre. Dead shells indicate that there used to be other clam fields at the site. There are also smaller bivalves (Family Thyasiridae).
An outer fringe zone is beyond the reach of vent fluids but within reach of the swimming shrimps. It also receives bacterial and animal remains, raining down. The temperature is a few degrees, normal for the depth. There is a scattering of animals able to exploit these resources, including sea-anemones and filter feeding worms on rocks, and mobile scavengers such as a sea snail (Phymorhynchus), squat-lobsters and shrimps. Normal bottom-feeding fish such as grenadiers and deep-sea cod feed here and wander into the warmer regions.
The seven sites
The seven known sites of hydrothermal activity are strung out over more than 3000 km, in depths ranging from less than 1 km to more than 3.5 km (see Table). Over 100 animal species are known from them and more are being reported all the time. Such a range of sites and their strange creatures raised questions for biologists. Are there different animals at different vent sites? What effect do distance, depth, and water chemistry have? What is the role of animal/bacterial symbioses? How are the dense aggregations of shrimp maintained? How do various species reproduce? How do their young disperse? BRIDGE-sponsored projects have looked at how the communities and some of their members vary. Seven species of bresiliid shrimp are identified: Rimicaris exoculata and Chorocaris chacei form swarms at all sites except Menez Gwen. Mirocaris fortunata (a scavenger) is found from Menez Gwen to Broken Spur while Mirocaris keldyshi overlaps with it at Broken Spur and continues alone to Logatchev. Mussel distribution has a similar pattern: Bathymodiolus azoricus (the Lucky Strike type) forms the mussel zones at the three northern sites. Molecular work by David Dixon has shown some depth-related differences between the Menez Gwen and Lucky Strike populations. The Lucky Strike type is mixed with Bathymodiolus puteoserpentis at Broken Spur, and the latter occurs alone at Snakepit and Logatchev (there are no mussels at TAG). Paradoxically, Ddiier Jollivet showed that the scale worm (Branchipolynoe seepensis), that lives inside the shell of both species of mussel, shows no genetic difference. Logatchev has the most diverse fauna and is notable for the small brittle star, Ophioctenella acies, known from TAG and Broken Spur, but exceptionally abundant on the mussel beds at Logatchev. It has unusual mouthparts that look like chopsticks, which may be adapted to eat filaments of bacteria.
Finding out what creatures actually do eat is more difficult. Studies on food sources of vent animals have concentrated on shrimp and bivalves. Hilary Kennedy and Andrey Gebruk looked at the proportions of stable isotopes of carbon and nitrogen in these creatures. This analysis and analysis of the different elements present shows something of where their food came from.David Pond and Cathy Allen worked on the composition of the fats, to find further clues. Where possible, biologists also observed how and where the creatures fed.
Munching on methane
The two species of mussel host a mixture of sulphur-oxidising and methane-oxidising bacteria, and use both as food, though their relative importance probably varies depending on where the mussels are and how much methane is available. Vesicomyid clams and thyasirids host only bacteria that oxidise sulphide. Shrimp species have been examined from all vent sites and the story, told in the following article, becomes more elaborate the more they are studied. Probably all vent shrimps have a larval/juvenile phase away from the vents, in mid-water. At the vents, adult Rimicaris and Chorocaris convert to a sulphide-oxidizing bacterial diet, some from their own bacterial gardens, while Mirocaris species scavenge on debris etc. and the active and more solitary Alvinocaris species may capture live prey. These shrimp form a graded evolutionary series of adaptations for life at the vents, from Alvinocaris (least changed) through Mirocaris and Chorocaris to Rimicaris (most modified).
BRIDGE scientists: Eve C Southward, Alan J Southward, Marine Biological Association, Plymouth; Andrey Gebruk Shirshov Institute, Moscow; Jon Copley, David Dixon, Peter Herring, Hilary Kennedy, Paul Tyler, Southampton Oceanography Centre; Cathy Allen, Harbor Branch Oceanographic Institution. David Pond University of Stirling.
Unlike astronauts visiting the Moon, those exploring the depths of the ocean cannot leave footprints behind. But rather than feeling disappointed, we should perhaps be glad. Deep-sea vents were discovered just over two decades ago. Since then there has been a flurry of scientific activity around and even under them. To understand how these systems evolve over time, we need to be confident that the changes we see are natural and not the result of our own inadvertent interference.
In the 1980s, Verena Tunnicliffe compared black smokers whose fauna had been sampled with those left untouched and found that their communities changed in different ways. As a result, biologists are eager to set aside reserves free from possible disturbance to study the long-term changes in the communities of animals. BRIDGE researchers are in discussion with their colleagues worldwide on the best way to co-ordinate such efforts.
The decision of the Ocean Drilling Program to drill the TAG vent site in 1994 provided a unique opportunity to study changes in the hydrothermal system that were likely to ensue. News that the site had been perforated with 17 holes initially alarmed some researchers, butmonitoring of drilling with a BRIDGE, funded timelapse video camera showed that the impact on the denizens of the site was both local and short-lived. In fact, the animals responded more vigourously to a change in the pattern of venting that had already started before the drilling ship arrived.
Studying the shrimp that swarm around Atlantic vents poses a further challenge. The shrimp have an unusual "eye" on their backs that may detect the extremely faint glow of the vents. Unfortunately, the bright lights of submersibles dazzle the shrimp, possibly producing lasting changes in the structure of their "eye" according to work by Peter Herring and his colleagues.
Whether this effect impairs the shrimp is uncertain - other deep-sea shrimp blinded by lights can still feed and reproduce. But none of the shrimp behaviour we see at vents can be deemed normal, because the very act of looking may affect the animals. To get round this problem, an international meeting of vent biologists in 1997 mooted the idea of using pencil-beam sonar to study the animals. Submersible pilots, however, are understandably reluctant to wander between black smoker chimneys without any lights.
Mid-ocean ridges are a source of fascination and potential wealth in biotechnology and minerals. By finding out more about this realm, BRIDGE has helped to teach us how to tread lightly in order to preserve it for future generations
Falling to the seafloor
ALVIN's hatch is sealed, we are winched over the back of the RV Atlantis into the ocean and we start our slow fall to the seafloor. The pilot touches a button and The Charge Of The Light Brigade booms out from the tape.
For two hours or so, until we reach the sea floor, I can sit back and relax - or I could sit back if there was enough room. ALVIN is a sphere just six feet across and cramped with three people inside. The pilot sits in the middle, looking out of the largest view-port. My colleague and I have cushions, portholes and video screens on either side.
As we fall, the water gets darker and emptier. Eventually, the depth gauge reads 3000 m. It is cold and I pull on a sweatshirt. We are 200 m above the bottom. The pilot switches on the outside lights and our search for hot springs begins. We have seen video footage and have maps from previous visitors, but it is still like looking for a ball on a football field at midnight with only a torch.
After half an hour or so, we notice yellow, sulphur-stained rocks on the ocean floor below us and we know that we are getting close. Soon we begin to see anemones, shrimp and fish. Suddenly, out of nowhere, comes a black smoker, five metres tall, gushing black, cloudy water. Equally astonishing are the swarms of shrimp jostling for position nearby. There must be thousands of them! The pilot sucks some up with a slurp gun. Then we ask him to grab rock samples with the hydraulic claws and collect water from the smoker - the pilot makes it look easy, but it is anything but.
Finally we shoot video of the hot springs and the animals around them. After three hours on the bottom, the batteries are low and we must return to the ship. We remember to eat and then doze as ALVIN spirals slowly to the surface. When we are winched back on board, I emerge triumphant, into the several bucketsful of water customary for drenching ALVIN novices. With or without the soaking, the dive is something I shall never forget!
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The strange tale of the vent shrimp
The life of an Atlantic vent shrimp is one paradox after another: they live where life seems impossible, they see yet they are blind, they depend on their vents, yet venture hundreds of miles away. BRIDGE took a closer look.
The most striking features of vents in the Atlantic are the swarms of shrimp that crowd the sulphide rocks right next to plumes of black smoke. They live around the vents in vast numbers, depending on the hot, mineral-laden springs for food, but managing to keep just far enough from the hot water. The same shrimp live at different vents hundreds of kilometres apart along the ridge. The discovery of these vast populations of shrimp posed a number of questions for marine biologists. How do they feed? Why do shrimp dominate these vents whereas in other parts of the world other organisms are the most prominent? And how do they colonize existing and, more importantly, new vents?
The first consideration is how the shrimp live. They depend on the vents, and they make considerable efforts to stay near the hot black smoker fluid streaming from the sea floor. The shrimp were first described as having no eyes; certainly they do not have the little shiny black eyes on stalks that normal shrimp have. But Cindy Van Dover, then at Woods Hole Oceanographic Institution, showed that a dorsal organ below the carapace on their backs is a highly modified eye that does not form images, but detects light at very low levels.
The blind shrimp that can see
The organ probably senses the light given off by the hot vent water, a dull glow not visible to human eyes, and ensures both that the shrimp do not lose contact with a vent, and that they do not accidentally become boiled shrimp by swimming into the hot water. In addition, a team of US neurophysiologists showed that the shrimp antennae are sensitive to very small levels of hydrogen sulphide, so that the shrimp have at least two ways of staying close to food, but avoiding trouble.
What are the shrimp feeding on, so close to the hot water? It turns out that they eat microbes, partly by scraping microbe-coated grains of sulphide from the rock surface, and partly by growing their own. Covering the inside of the shell of the shrimp are millions of filament-like bacteria. These bacteria derive chemical energy from the vent waters and live by chemosynthesis. Unlike plants, which use the Sun's energy to turn carbon dioxide into the sugars and fats they need to grow, these bacteria use energy from chemical reactions at the vent (see Chemosynthesis). The shrimp have appendages specially adapted for harvesting the growing bacteria from their carapaces, in order to eat them, and of course they have to stay close to the black smoker water so that the bacteria have energy to live from. It is as though we grew cabbages and apples on our bodies, and could snack on them from time to time. But if we did live like shrimp in this way, we would have to spend much of the day jostling for a place in the Sun.
Home, home at the vent
This raises an important problem. The shrimp have to stay very close to the black smoker fluid if they are not to starve. How do they then manage to colonize new vents, or survive when their home vent closes down, as it must do from time to time? These questions about the life history of the shrimp were central to the work of the BRIDGE scientists.
The first question that the BRIDGE team tackled was how closely the shrimp at the different vents are related to one another. The deep sea vents are classic examples of evolutionary islands, sites of isolation where different species or strains of closely related shrimp might arise. Deft manipulation of the arms of submersibles, holding fishing nets (like those with which children approach rock pools) or a slurp gun, a kind of underwater vacuum cleaner shown in the picture to the right, provided samples of shrimp from different vents. They looked similar, but we could do better with a genetic technique called enzyme electrophoresis. This looks at the gene templates that shape different proteins: a single species has a similar pattern. Alex Rogers and Simon Creasey examined populations of shrimp at TAG and Broken Spur - sites 360 km apart - and found no difference in their genetic makeup. The shrimp can somehow leave their home vent, surviveon the journey and perhaps detect and arrive at a new vent: the vent sites are not as isolated as they first appear.
If the individuals at TAG and Broken Spur are so close genetically, how do they mix? The reproductive biology of the shrimp and the dispersal of shrimp larvae provide the answers. The clear picture of the lifecycle of the shrimp that has emerged from the BRIDGE programme is a result of a variety of different approaches by different scientists.
Sex, eggs and fat
Let's start with sex. The sexes in the shrimp are separate. Sexual reproduction demands that an individual organism produces eggs or spermatozoa. Eggs produced in the ovary can be seen through the shell of the shrimp. Jon Copley and Paul Tyler found that the eggs grow to about 0.4 mm across (around the size of a grain of salt), with a lot of yolk. But within any one shrimp, there are a group of large eggs and a group of small ones. This tells us that the adult reproduces at least twice during its lifetime and produces eggs in batches. Cathy Allen and David Pond looked at the composition of the fats in adult shrimp. They indicate that most of the fats came from chemosynthetic bacteria harboured by the adult - another sign that the shrimp do not depend directly on the Sun's energy at this stage of their lives.
When the shrimp sheds its outer skeleton to grow, the eggs are fertilized by a male. This gives rise to the first enigma of reproduction in vent shrimp. In our extensive samples, we found very few adults carrying eggs. We believe that shrimp that are carrying eggs may move away from the vent, perhaps to avoid any toxic effects from vent waters on the developing eggs (see DNA damage).
Eventually the larvae are released into the ocean to start the next stage in their life history. To collect larvae Peter Herring and Dave Dixon used large plankton nets towed over the hydrothermal vent regions and at control stations away from the vents. The larvae they found, had two odd features: they had eyes like normal shrimp, not the dorsal light-sensing organs of the adults, and their bodies were bright orange, the result of orange oily material within their thorax and abdomen. The smallest shrimp at the vents also share these characteristics. Analysis of the fats in these young shrimp by Cathy Allen and David Pond shows that the fats originate from phytoplankton, the microscopic plants that float near the sea surface. The larvae and juvenile shrimp must have eaten phytoplankton or organisms that feed on phytoplankton.
Free range shrimp
This is astonishing. Not only does hardly any plant debris normally reach such depths in the oceans, but both TAG and Broken Spur are on the edge of the Sargasso Sea, where there is very little life at the surface anyway. The orange oily fat found in the young shrimp is similar to that found in deep-sea shrimp that live in mid-water where food supplies are scarce or intermittent. This kind of fat allows larvae and juveniles to live away from vents for some time and solves a critical problem. If at this stage in their lives the shrimp live on phytoplankton, this will allow them to range widely through the oceans in search of another vent. And this was confirmed when Peter Herring made a mid-water trawl close to Madeira, more than 1500 km from the Mid-Atlantic Ridge, and found one of the shrimp larvae, looking healthy and apparently still travelling hopefully.
We thus have a picture of shrimp larvae ranging widely through the oceans, drifting in the deep-sea currents and feeding on phytoplankton. The next challenge that they must face is finding their original vent again, or some other vent.
How they do that we do not know. Vents are spread out, at different depths and are very small in the vastness of the oceans. The larvae cannot swim fast, certainly not fast enough to swim against a current in response to some sign of a vent from upstream. But somehow they do reach a vent again, as the presence there of the orange juveniles, with their fat derived from phytoplankton, shows. And at the vents the shrimp metamorphose into adults, losing their image-forming eyes, developing the light-sensing dorsal organ instead, and remaining for the rest of their lives, growing and eating bacteria, so that the orange travelling fat is replaced by fats from bacterial chemosynthesis.
It is tempting, but misguided of course, to draw parallels with humans, whose visionary, wide-ranging teenagers travel the world, return looking rather different, and then settle down to a steady job, with a mortgage and a car to wash on Sundays.
The remarkable story of the shrimp, pieced together by BRIDGE scientists, illuminates the lives of these strange vent organisms, showing the complex strategies that need to be followed to survive around hydrothermal vents. There are, of course, many questions yet to be answered, not only about the shrimp, but also about other specialist vent animals, and how they answer the challenges of the vent environment. In turn such answers will be important in deepening our understanding of other challenging ecosystems.
BRIDGE scientists Paul Tyler , David Dixon, Peter Herring, Cathy Allen, Jon Copley Southampton Oceanography Centre; Linda Dixon Plymouth Marine Laboratory; Simon Creasey, Andrey Gebruk, Alex Rogers, Alan Southward, Eve Southward Marine Biological Association, Plymouth; David Pond University of Stirling. International Colleagues Cindy Van Dover William and Mary.
Most living things on Earth are made of organic carbon, which plants make using energy from the Sun, a process called photosynthesis. Plants and the animals that eat them need sunlight to live.
Chemosynthetic bacteria are different. They make organic carbon by chemosynthesis, using the energy from chemical reactions, rather than from sunlight. At hot springs, the reactions of sulphide or methane from the vents with oxygen in sea water fuel chemosynthesis in the unusual bugs that live there.
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Machines for the abyss
Exploring the abyssal ocean - dark, dangerous and a very long way down - takes ingenuity and technical innovation. BRIDGE harnessed both to build new tools that revealed the workings of the deep.
Exploring the science of the deep ocean floor and the rocks below is no easy task. During BRIDGE we developed and built instruments that have changed the way researchers in all our disciplines look at the sea floor - metaphorically as well as literally. We then used these novel instruments to shine fresh light on the strange world of the ocean floor; our new tools are in demand, both from research programmes and for commercial applications around the world.
The problems with investigating the deep oceans fall into two main groups. First, there is the environment, which is decidedly hostile. Most of the deep ocean is a desert of cold, dark water, about 2 degrees C, with the weight of the 3 km or so of water giving a pressure of 300 atmospheres or more. Exploring the oases of life around the hot springs is still more challenging. The plumes of hot water in black smoker jets suck in cold water from round about, so the temperature can rise from 2 to 400 degrees C in only a few centimetres. The smoky fluid is not just hot, it is also corrosive, full of dissolved metals and salts. And seemingly innocuous areas of seafloor can turn into black smokers in just a few months, so instruments can become buried in sulphides and roasted by hot fluids.
Secondly, there is the problem of getting instruments to the seafloor - and getting any data recorded back to the scientists. Some instruments, such as echo sounders and magnetometers can operate at the surface. But the 3 km of seawater between the instrument and the ocean floor blurs the images and clouds the vision. There are great advantages in placing instruments on or very close to the rocky bottom.
To improve our view from the surface, BRIDGE bought the Norwegian-made Simrad multi-beam echosounder that makes maps of the ocean floor by transmitting a fan of 81 beams of sound as the ship steams along, covering a width of three to four times the water depth. This produced wonderfully detailed maps of the seafloor (see Investigating Iceland and the Reykjanes Ridge), with a footprint about 50 metres across, and an accuracy of a few metres in depth, giving maps with about the same resolution as the 1:50 000 Ordnance Survey maps that we use in the UK. It generated maps at a rate of 5000 square kilometres per day (about two 1:50 000 map sheets) which is very rapid, though the oceans are so large that it would take over 150 years of continuous work at this speed to cover them all. The powerful computing system on the ship allows finished maps to be produced only a day or so after a survey is complete. This was very important in the discovery of great faults on the Mid-Atlantic Ridge (see Bending and breaking the ocean floor).
But most of our effort went into getting closer to the ocean floor. One way is to use submersibles, small, specialized submarines strong enough to reach the sea floor with two or three scientists aboard and make measurements or place instruments exactly where they are needed. We have no British submersible yet, and we were glad to be able to use American, Russian and French submersibles. But the main thrust of the technological effort in BRIDGE went into devising new instruments that could be towed near the ocean floor, or left there for months on end.
Both options have their challenges. They require tools that function reliably for long periods at a distance from the engineers who operate them. This demands simplicity and much more redundancy than a similar instrument used elsewhere. Deep-towed instruments get their power and return their results via cable; provided the cable is long enough and strong enough, they can approach the seafloor. But such instruments are vulnerable to the weather, especially getting them in and out of the water. There are also the wider difficulties of using a tool that comes with its own 6 km of cable: getting it to where you want it, keeping it on the right path and, indeed, knowing exactly where it is. Anyone who has ever wrestled with a vacuum cleaner cable on the stairs can imagine some of the problems of manoeuvring these instruments and their considerably longer cables.
TOBI is one of these deep-towed instrument packages. It is towed about 300 m above the seafloor and transmits a narrow fan of high-pitched sound on each side, covering a swath of ocean floor 6 km wide. It makes two kinds of sonar images. One is made by recording the intensity of the sound scattered back. This gives a map of bright and dark areas of the seafloor, those that scatter much of the energy back to the ship, and those that absorb it, and of course of the shadows thrown by hills. It is like searching a dark cellar with a torch: shine it into the dark and a whitewashed wall will send back a bright flash, a rack of wine bottles will show as irregular humps, casting shadows, and a heap of coal will not show up at all until you stumble into it. On the ocean floor, fault planes with bare rock and piles of scree show like a whitewashed wall, rough hummocky lava gives patchy backscatter. And just as in the cellar, there are shadows, some helping to interpret what's there, others looking like monsters. This sort of TOBI image gives us the most direct view of the ocean floor.
Nick Millard, Ian Rouse and Chris Flewellin developed TOBI to give the second type of image, high-resolution swath bathymetry. This uses more of the information in the backscattered sound. As well as recording how much energy comes back from a particular direction, it also measures the phase of the returned sound. From this, it is possible to calculate a much higher resolution map of the ocean floor than Simrad gives. Mike Avgerinos has developed a way to interpret the signals, and has shown that features only dimly visible on the Simrad map have a rich detail of structure on a TOBI map, detail that is vital for scientific interpretation.
An especially valuable modification to TOBI was the incorporation of a three-component magnetometer, to measure the magnetic field of rocks in the seafloor from close up. Roger Searle, Simon Allerton and Gaud Pouliquen masterminded this addition, which they applied to detailed measurements of crustal magnetism, to measure how far blocks of rock had rotated as the crust spreads apart.
Diving deeper with Bridget
A team led by Harry Elderfield applied some of the lessons learnt from TOBI to the design of another deep-towed instrument package, Bridget. Bridget seeks out hydrothermal plumes. It carries a group of instruments to detect the faint traces of hydrothermal plumes as they spread through the oceans. These include a nephelometer, to measure the cloudiness of the water, and devices which sample the water on command. Researchers can monitor the readings from the nephelometer on the ship and trigger the collection of water samples at likely places. Bridget is towed up and down through the water as the ship steams along, making a saw-tooth track, a technique called tow-yoing. Chris German and his team found the Rainbow plume in this way (see Sniffing for plumes), and Bridget has also been used for other applications.
A major effort in BRIDGE went into instruments that work on the ocean floor itself. One great success was the BRIDGE drill, developed by Jack Pheasant and his team at the British Geological Survey, and controlled from a ship at the surface. It is not only the first portable drill that can take short cores in rocks of the deep ocean floor under control from the surface, but also the first to bring back cores marked to show which way they had originally been aligned. This allows researchers to discover the direction in which features such as elongated or magnetized mineral grains lined up within the rock. From this comes the direction of sliding of faults, or the amount of rotation of blocks of the crust. The drill is computer controlled, via a cable, by people on the ship above. Researchers can watch its progress, send commands and ensure that it does not get stuck - a common hazard when drilling at depth and one that could destroy the drill.
Other BRIDGE instruments worked autonomously, some for months on end. Some were relatively simple, such as fish traps, distant relatives of lobster pots, deposited around vents and picked up on a later submersible dive. Also simple are deep-sea video cameras, that can be left to take time-lapse shots, or short sequences of images to watch how animals moved and vents changed over periods of months.
Much more complex was Medusa, an innovative instrument brought by Adam Schultz, Harry Elderfield and their team from design to use in less than two years. Medusa is an exceptionally versatile and robust instrument that continuously measures the properties of the vent fluids, at the same time as taking water samples for later laboratory analysis. It is made almost entirely of titanium, suitable for working up to 6 km underwater, and can be set to work either by scientists in a submersible or by a remotely operated vehicle. So far, its record is 14 months underwater at a site on the Pacific mid-ocean ridge.
A cleverly-designed sensor picks up fluids as they leave the seafloor - before mixing with any seawater - to determine their original composition. Thermocouples measure the temperature of the fluid and the seawater, and the speed of the flow in such a way that researchers can find out how much heat the fluids carry. Chemical sampling uses bottles that can be closed automatically at set times.
The Medusa system could not have been constructed without the unique skills of the technical staff involved, including Martin Walker who provided a key set of mechanical design and machining skills, and Derrick Shulman and Steve Riches who designed and built the electronics. Six Medusa instruments were built under BRIDGE funding, and three more for operation by the Geological Survey of Japan. New successor instruments are now under construction for US agencies and organisations, including NASA.
A wide range of BRIDGE instruments, including Medusa, deep-sea video and fish traps, were used in a major international project at the TAG hydrothermal vent field in the Mid-Atlantic Ridge. In 1992, the international Ocean Drilling Program decided to drill deep rock cores to investigate TAG. BRIDGE decided to monitor the effects of this with the instruments in place before, during and after drilling. With considerable dispatch, BRIDGE rose to this challenge, and a complicated series of negotiations with the Shirshov Institute of Oceanology in Moscow and Woods Hole Oceanographic Institution in the USA culminated in the successful BRAVEX/94 and TAG-95 research cruises. All went well, especially as none of the 17 holes in the TAG mound drilled through any of our instruments! But one surprise was that drilling the holes brought marked changes in the flow of hot water through the top of the mound. One area where we deployed Medusa changed from emitting relatively benign warm water to emissions of hostile black smoker fluid. The instrument was buried in a heap of sulphide and heated until its batteries exploded. Another Medusa and a video camera came close to the same fate (see the image on the previous page), but both carried on working, and the Medusa even returned data of the highest quality, a tribute to its design and construction.
The latest success of BRIDGE technology is aimed at biology. The vent shrimps and mussels, for example, have a larval stage in which they leave the vents and float in the oceans as plankton. To understand how the biological systems function, it is essential to know as much as possible about the larval stages. A team led by David Dixon built an instrument to catch and preserve larvae well enough to carry out DNA analyses on them, the only way to know what species they are, since they look nothing at all like the adult animals. PLASMA contains a pump that takes in sea water through a range of filters in a sequence, trapping different sizes of larvae, then seals each sample, adding preservative. It can be used from the ship, or left at a vent site for a year, collecting samples. It has had great success, and the results from the analyses are just now coming in.
BRIDGE pulled together a wide range of technologies and a lot of and ingenuity to explore the deep oceans. Britain now has new instruments, new techniques and above all, new skills in working in this difficult environment. Many of the instruments and techniques have already found uses in industry and in the research community worldwide. Their potential is huge. In the future we expect to see instruments that spend years on the deep seafloor, monitoring the seafloor until this hidden 70% of the planet is explored as thoroughly as the land. Or maybe BRIDGE's legacy will be the instruments that search for life in the seas of Europa or even on other planetary systems. Wherever they lead, the ideas and expertise fostered by BRIDGE have made their mark.
BRIDGE Scientists David Dixon Chris German, Nick Millard, Ian Rouse and Chris Flewellin Southampton Oceanography Centre; Adam Schultz, Harry Elderfield, Martin Walker, Derek Schulman, Steve Riches University of Cambridge; Jack Pheasant, British Geological Survey; others published in issues of BRIDGE Newsletter.
BRIDGE data stewardship
The BRIDGE Programme has explored many sections of mid-ocean ridge, looking at the sea floor and the overlying oceans. The high-quality datasets collected during this programme are unique, and not only because technical and financial reasons make it difficult to go back. The global BRIDGE dataset is a crucial legacy to future scientific studies of the deep ocean.
Data stewardship is the process of actively protecting and using the data, as opposed to just storing it. The BRIDGE Data Stewardship project has been jointly financed by BRIDGE and the NERC-SEEDCORN programme and is now nearing completion, in a record time of less than one year.
BRIDGE explored and charted a large portion of the Mid-Atlantic Ridge south of the Azores, the whole Reykjanes Ridge south of Iceland and the emergent portions of the mid-oceanic ridge in Iceland itself, as well as the East Scotia Sea in the South Atlantic and the Lau Basin in the southwest Pacific. The collection of data started at regional scales, and progressed to increasingly detailed work focusing on some of the newly discovered hydrothermal systems. The objective of BRIDGE has always been to draw together the findings from disparate disciplines investigating the same locations. All in all, 44 funded projects have provided thousands of megabytes of data from the sea floor and overlying ocean. These include data from multibeam bathymetry, sonar imagery, seismic data, electromagnetic measurements, gravimetry, underway magnetics, rock chemistry and mineralogy, chemical and physical oceanography (including samples and analytical data), macro- and microbiology (including specimens and analytical data), numerical models and audio-visual records.
The amounts of data involved are of course enormous, and maps, reports, detailed sample descriptions and electronic files have been flooding in from all over the UK. They are usually in a variety of formats and sizes, potentially a nightmare for the archiving and description of incoming data. Fortunately, BRIDGE scientists had planned for this and defined common formats and processing techniques in a series of national workshops held in 1993-1994. Each incoming dataset is assessed for readability, completeness, and submitted to many rigorous quality checks before being archived on long-lasting CD-ROMs. Raw and processed data are archived together, to avoid any loss of information. The results are then grouped by geographical areas and disciplines, to be released on CDs. They are complemented by a Web site where researchers can search for and access data on-line. It is our intention to leave a high-quality record of our achievements as a legacy for future researchers. The educational and scientific use of these results will also provide a benchmark for addressing future environmental questions. The BRIDGE legacy will prove a valuable resource for related scientific, commercial and political programmes, addressing resource and hazard management in the deep oceans.
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Bridge to the future
A project such as BRIDGE, if successful, leaves a wide range of legacies. Our most tangible bequest is, of course, the many gigabytes of the BRIDGE database, which will be important for years to come. But BRIDGE has had many results, tangible and otherwise.
BRIDGE has set a style for successful major environmental research programmes, a style of which we are proud. We deliberately focused our efforts in a limited number of areas of the oceans. This decision sometimes gave us doubts, but was also one of our strengths, because one project could build on the groundwork laid by another, as we progressed from a regional scale to the detail, and, in general, from geophysics through geochemistry to biology. The overall shape of BRIDGE was determined firmly by the Steering Committee. In that sense the project was directed, but within that framework, individual research topics could range widely, responding to innovative developments.
In addition, BRIDGE was conceived and carried out as a multidisciplinary programme, drawing scientists from radically different disciplines across the breadth of marine and earth science in NERC. It brought together scientists who had never met before, but who now maintain collaborative research. BRIDGE geophysicists observed shrimp and geochemists hovered anxiously over deep-tow instruments.
This approach was sustained at the international level, too; many of our successes resulted from close collaboration with colleagues from many countries. More trivially, at the BRIDGE Annual Science meetings, we deliberately mixed topics of talks in each session to encourage contacts between scientists, as well as to foster our ability to communicate with scientists from other fields. We hope that these lessons will be applied in future NERC projects; we believe that they will substantially enhance NERC's success.
The instruments constructed during BRIDGE are an important legacy, too. They are in demand both nationally and internationally for deep ocean research, and prove to have applications well beyond the mid-ocean ridges. For example, the Japanese government has bought Medusas for research, and the upgraded TOBI is in demand for surveys in other oceanic environments. Our instruments also have the potential for development beyond their original specification into a new generation of instruments. BRIDGE will then have provided the seedcorn for this line of development, and the benefits of BRIDGE funding will be felt for years to come. And our instruments have given us passports to the wider world of international collaboration. They have given us concrete contributions that we can make to major projects worldwide, such as the recent Japanese cruise to the fascinating ridges of the Southwest Indian Ocean.
As we have shown here, the deep ocean, and especially the hydrothermal vents, are natural extreme environments, hostile to much ordinary life. Our research has revealed a great deal about how organisms cope with, and indeed thrive in, these natural conditions. These results are important for understanding the anthropogenic extreme environments that we otherwise call pollution. Microbes and higher organisms have evolved a number of defences against the toxicity, heat and stress of these environments; our increased understanding has pointed the way to new approaches to pollution - a line of enquiry that has great potential for further development.
Research from the mid-ocean ridges has also illuminated many other areas of science. For example, the continents are old; everywhere they bear the scars of ancient crises, and any new stretching of the crust activates old fractures. But the crust at the mid-ocean ridges is pristine and young, and the patterns of faulting there show how unscarred crust responds to stresses, so that evolution of faults can better be understood there. The results can readily be applied to topics such as oil exploration, where faults are critical in controlling where the oil is to be found. And the new, high-resolution images of seafloor lava flows promise to shednew light on the dynamics of lava flows on land as well as under the sea. Ore deposits in the rocks of the continents are already much better understood from our research at black smokers, and there is more to come from this link. The possibility that ocean floor hot springs may be the site of the origin of life has taken a new step forward, too, from the discovery by BRIDGE scientists of the hydrogen-rich springs of the Rainbow site. This site must have more important messages about this always-fascinating topic.
The big questions
And finally, of course, despite all the advances we and our international colleagues have made during the lifetime of BRIDGE, there are still fundamental questions to answer about the mid-ocean ridges themselves, some of which have been brought into much sharper focus by our work. For example, there are important questions about the evolution of the communities of organisms that surround the hydrothermal vents. How ancient are they? What strategies do they use to cope with the isolated vent environment? How do vent larvae detect and colonize new vents? These questions can partly be answered by discovering and investigating vents in unexplored and remote parts of the ocean, such as the East Scotia Sea close to the Falkland Islands, and partly by deeper study of vents already known, and especially the genetics of vent organisms.
Then there is the question of the chemical fluxes carried by fluids which flow up through the deep ocean floor, of which hydrothermal vents are only a part. How big are such fluxes world wide? How do they compare with other fluxes (including anthropogenic fluxes) to the oceans? For what elements are these fluxes dominant in controlling the chemistry of ocean water? Again further work in the deep oceans is necessary, especially away from the ridges, and around island arcs.
So near yet so far
A final example is the problem of the generation of the lower ocean crust. Four kilometres thick, covering three fifths of the Earth's surface, the lower ocean crust is still an enigma. It is created somehow in the root zones of the mid-ocean ridge volcanoes. There are two conflicting models for the generation of the crust at fast spreading ridges, both probably wrong, and both almost certainly inapplicable to slow-spreading ridges such as the Mid-Atlantic Ridge. That such a major part of the Earth, so close to the surface, is so poorly understood certainly demands attention. It is one of the high priority targets of the international Ocean Drilling Program, of which the UK is a member; a better picture may emerge in the next few years, built on the work we have accomplished.
In short, BRIDGE has built bridges, both between disciplines and between continents. These intellectual bridges will endure and the exploration will continue. The mid-ocean ridges are remote, but they are not detached from the rest of the Earth system. The deep oceans are a critical part of the global ecosystem, and further research here is critical if we are to build on our achievements to date and understand how the Earth works.
Joe Cann, University of Leeds
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