10,000 Feet Below in the Indian Ocean

Professor Alex Rogers  writes a fascinating account of a recent dive to the Kairei Vent Field on the Central Indian Ridge – more than 10,000 feet (3382m) underwater – in the submersible Shinkai 6500 to research the habitats surrounding deep-sea hydrothermal vents.

The Ocean Research and Conservation (ORC) group carries out research focused on biodiversity hotspots in the ocean. These are areas which are species rich or which have a high proportion of endemic species, those which occur nowhere else. Deep-sea hydrothermal vents are one such habitat where up to 70% or more of the biota are not found elsewhere depending on the group of organisms. Vents are formed when hot rock lies beneath the seabed, mostly along mid-ocean ridges where they are often associated with magma chambers beneath. They also occur on island arcs or at non-ridge associated submarine volcanos both areas associated with high levels of tectonic activity and submarine eruptions. At mid-ocean ridges the vents are formed when seawater penetrates freshly-formed ocean crust and comes into contact with hot rock. The seawater becomes superheated, stripped of its oxygen and enriched with a variety of elements and compounds include hydrogen sulphide, methane and various heavy metals.

Superheated water of course is buoyant and so it rushes up to the surface and exits at great speed. When the vent fluids are not mixed with cold seawater under the surface they leave the seabed at temperatures up to more than 400oC and on contact with cold deep-sea water metal sulphides precipitate as fine particles forming the appearance of black smoke underwater. The sulphide forms chimneys on the seabed which can be tens of meters tall. Some of the chemicals in the vent fluid can be oxidised by bacteria in a reaction that releases energy which can then be used to fix carbon. This is known as chemosynthesis as opposed to photosynthesis where light provides the energy for carbon fixation. This was one of the reasons the discovery of hydrothermal vents back in the late 1970s caused such excitement, life could exist in the absence of sunlight.

Chemosynthetic bacteria form the base of the food chain at hydrothermal vents and the high in-situ primary production means that there is a high abundance and biomass of animals at vents. These animals either graze bacteria growing on rocks or have formed symbiotic relationships with them, growing the bacteria on external surfaces or in specialised internal organs. Deep-sea environments outside vents usually rely on a rain of organic material from the sunlit ocean surface. Most of this food is consumed as it sinks and so life in the deep sea is thin on the ground.

Despite the abundance of energy at hydrothermal vents few organisms have adapted to the extreme conditions including high temperatures, low oxygen, the high toxicity of chemicals such as hydrogen sulphide and heavy metals and acidic fluids. This is why the vent fauna is relatively species poor but the few species that have adapted to vent conditions do not generally occur outside these ecosystems. Vents are therefore island like oases in the deep ocean for their inhabitants, surrounded by huge areas of unsuitable habitat. We are interested in vents as one model for understanding how species disperse in the deep ocean and how communities of animals are structured over different spatial scales. This includes evolution of the vent biota over time, how both the physical environment and biological interactions such as predation and competition determine what species are present at any one locality.

In recent years such understanding has become more urgent. This is because the seabed massive sulphides formed at vents contain high concentrations of metals such as copper, gold and silver and are now of interest as potential sites of deep-sea mining. An understanding of the environmental impacts of such activities on hydrothermal vents is therefore critical if this activity is to be undertaken in an environmentally sustainable manner. This is one reason why the Indian Ocean has become of interest to us in our vent studies. One of the vents we have already worked on located on the South West Indian Ridge south of Madagascar is now subject to a license to explore for deep-sea mineral deposits by China. Last year (2015) we were therefore very grateful when Dr Ken Takai of JAMSTEC invited us to participate in a cruise on the Research Vessel Yokosuka to sample hydrothermal vents with the deep-sea submersible Shinkai 6500.

Christopher Nicolai Roterman and I from the ORC met with the Japanese scientists in the Port of St Louise in Mauritius. Tied up in the harbour were no less than five research vessels including the enormous International Ocean Drilling Programme ship Resolution, two Chinese vessels, the Da Yang Yi Hao and the Xi Angyanghong, the Indian research vessel the Sagar Nidhi and our own vessel the Yokosuka. Both the Chinese vessels were participating in research related to deep-sea mineral deposits in the licensed area on the South West Indian Ridge and despite the region being in the high seas there had been some delicate negotiations between Japanese and Chinese scientists which had gone to State level prior to use leaving.

On the evening before our departure we enjoyed an Indian meal overlooking the port and the next day we sailed in the morning past many anchored fishing vessels, parts of the distant water fleets of various nations as well as the Cunard Line’s Queen Elizabeth. There followed several days sailing down to a part of the Central Indian Ridge on which a chemical signature likely indicating a new vent area had been detected. This meant we had to undergo training to act as the scientific observer on Shinkai 6500 dives.

The submersible comprises a titanium sphere which measures about 2 m across inside and houses three people, the pilot, co-pilot and scientist. It is crammed with equipment including the controls for the submersible, life support equipment, navigation gear, camera operation equipment and controls for the manipulators, large articulated steel limbs terminating in claws. There are steel boxes painted dark grey with banks of switches and indicator lights robust and resembling something out of a cold-war bomber from the 1950s or 1960s. Red digital read outs for depth and other information. Overhead are canisters for scrubbing the air clean of CO₂ and large black cylinders for oxygen can be seen behind equipment racks. The bottom of the sphere is padded so three crew can crouch, sit or lie down in whichever way is most comfortable for them. This was important as dives tended to last for 7 hours.

By the time I had finished my lecture and also a demonstration inside the submersible by veteran pilot Tomaki San I knew how to operate the cameras, what to do if there was a fire and how to drop ballast from the submersible so it can surface in an emergency. The history of submersible diving is not without accidents, some fatal. The safety systems on the Shinkai 6500, down to the thermally insulated clothing you are provided with for a dive to protect from the extreme cold, are the result of years of experience in the operation of research submersibles by the global marine research community.

After several days of sailing and quite rough seas we arrived at our site. The first task was to search for vent sites using a towed camera system, the YKDT. This comprises a rigid steel frame on which are cameras and environmental sensors all attached to the ship via a steel wire. The Deep Tow as it is known was deployed to a depth of more than 3000m and we headed up to the ships gym where a bank of four large video screens and control gear had been set up.

The seabed was made up of broken areas of black volcanic rock, basalt and pale grey sediment covered in the trails and faecal deposits of deep sea creatures such as sea cucumbers. The towed camera revealed little sign of hydrothermal vents until towards the end when we encountered areas of broken basalt and blackened and red sediment This was sulphide and indicated that even if there was not venting in the precise location now there may have been in the recent past.

The following day the aim was to try and survey around the edge of the submarine hill on which the sulphides had been seen. As before YKDT was launched and we proceeded up to view the video screens as before. Again the vehicle revealed areas of sulphide and blackened basalts. However, shortly afterwards YKDT encountered several cliffs made up of broken basalt rock. Everyone clenched their teeth as the camera bounced off solid granite blocks and spun around to face the opposite direction kicking up clouds of sediment. YKDT was moved away from the extreme topography and we recommenced the survey on sediment.

About 20 minutes later the vehicle went dead and it became clear after the engineers spent several minutes trying to revive the cameras that it was not a problem ship side. YKTD was hauled to the surface and found to be hanging on by just a few strands of wire. The spinning of the vehicle had twisted the thick cable and sheered through most of it and the conducting cable beneath. This meant the search for the vents would switch to using Shinkai 6500. We met that evening and it was decided that the following day a JAMSTEC scientist, Masa, would try and find the vents as well as trial some new chemical sensor equipment. If he failed I would then go on a search the following day.

Watching the Shinkai 6500 launch the following day was quite amazing. There was a short delay in the morning whilst the new equipment was fitted to the front of the submersible. It included a rosette of metal pipes, each with a metal bulb part way along. This was attached to several pumps via a snaking array of plastic tubes, several large handles which could be twisted by the Shinkai’s manipulators and a metal sampling probe, like a giant needle which could also be picked up by the manipulators. On the starboard basket of the submersible was a large plastic cylinder with smaller canisters inside attached to a large corrugated pipe. This was an underwater vacuum cleaner for collecting animals which were deposited in the rotating canisters.

Technicians and ship’s crew, all in blue uniform scurried around loading the submersible with metal ballast, filling up tanks with water, checking everywhere around the submersible with torches. All was coordinated by a senior technician in constant communication with the bridge with voices coming over the tannoy systems and echoing in the vast metal hanger for Shinkai, confirming checks and status prior to launch.

Out on the deck the crew formed a circle to discuss the launch and ensure everyone understood their jobs and give the opportunity for questions, this was a ritual prior to launching of any equipment. Finally the equipment was in place and Masa appeared in his dive overalls above us to walk across the gangway to the top of the Shinkai 6500, giving a wave before disappearing into the sail of the submersible, the equivalent of a conning tower on a submarine. The parallels to astronauts climbing into a rocket to space were not lost on us onlookers. Shinkai 6500 was moved on a giant blue platform on rails out onto the aft deck. The huge A-frame was lowered and two giant ropes attached by metal couplings to the top of the submersible.

Commands were given by the ships First Mate and another master sailor, blasts on a whistle being used to coordinate teams on either side of the submersible. Once stays were removed the submersible was hauled into the air and drawn up to a gimbled coupling at the A-Frame and the whole assembly hauled upwards, the submersible reaching a height of twenty feet or more above the deck. It then rotated over behind the ship where an inflatable with divers waited. When the submersible was lowered into the water the divers lept into the sea and swam over to the submersible, climbing onto it to release the couplings and ropes used by the crew to steady the submersible attached to the front. They jumped back into the sea and were recovered by the inflatable and within a minute Shinkai 6500 disappeared from site. The operation was truly impressive.

The day was spent working in the scientific meeting room with TV screen showing a grainy photograph every 6 seconds or so from Shinkai. The submersible surfaced at 17.00 and the recovery was just as impressive as the launch. Masa reappeared after his 7 hour dive and received a ritual dousing in cold seawater as it was his first dive in Shinkai. No vents had been located and the following day it was to be my turn. I was not quite sure how I felt. I was certainly excited and a little tense, the weather could blow out operations anytime. I think the truth was that despite familiarisation with the inside of the submersible I did not quite know what to expect but showered and went to bed. Showers were not allowed on the mornings when Shinkai dives took place as freshwater attracted sharks, not a comfortable thought for the skin divers that released the submersible.

Following a good night’s sleep I awoke and went for Japanese breakfast which included fish and rice. I fidgeted about getting cameras ready in my cabin and then headed down to the hanger. Again there was Takai San and several other scientists and technicians assembling the equipment. I waited with the odd butterfly appearing in my stomach eager to go. Finally the pilot, Akihisa Ishikawa, and co-pilot, Hirofumi Ueki, appeared and we went into the submersible Commander’s room to get our overalls and to discuss the dive. Aki looked in his twenties and had about 50 dives under his belt, his Shinkai team photo showed him climbing a rock wall. Ueki was probably in his late forties or early fifties and had made over 200 dives. Then, very suddenly it was time to go, and I walked out into the hanger and upstairs to the gangway laden with a bag, a fold up chair to sit on if required and a Go Pro camera. Across the gangway to the roof of Shinkai with a wave to the other scientists down below, I removed my shoes and climbed down to the sphere followed by my possessions.

It was cramped and I managed to lay on my side with my head close to the scientists viewing port on the port side of the capsule. Aki and Ueki sat down and began to go through a list of switches to turn on checks to do, each of which was ticked off. I got the inside camera for photographing through the window and got my notebook ready. A helmeted face appeared above and the ladder was withdrawn giving us some more room, then a pipe providing cool air was withdrawn. Checks were made from outside that the hatchway was completely free of grit or other obstructions and then the hatch was lowered. Ueki locked the hatch with a wheel on the inside of the door. Suddenly we were moving along the rails. There were various muffled thumps and noises as the stays were removed from the submersible but no sound from above as the ropes were attached. Aki beckoned me and I peered out of the pilot’s porthole to see people waving below.

The deck was crowded with crew and technicians involved in the launch. I sat back down and we were then hoisted into the air. I was amazed at how smooth it felt from inside the submersible and guessed that this was why rope was used and not steel cable. I peered at Nicolai through the porthole who waved and took a photo and then we were over the back of the ship and finally lowered into the water. Bright turquoise blue water came splashing up over the porthole and we floated in clouds of bubbles both from the submersible and from the twin propellers of the Yokosuka which I could see on the pilot’s video screen straight ahead of us.

Communication from the ship via radio informed us that the divers were on board and when we were detached but no sound from above reached us. Aki reported that we were ready to dive, the divers were clear and then we began to drop. Sound from outside dropped away to nothing whilst inside there was the sound of a small fan behind me and the faint bubbling as gas passed through a clear container of water just next to the fan, something to do with the air supply. There was a feeling of floating, like the entire diving bell was suspended on a thread swaying slightly as it descended.

I watched the depth gauge slowly increase, the red digits seeming to hesitate now and again. Aki asked me if I had dived before and I replied that I had – to 500m depth in a submersible belonging to a foundation. He laughed, “Very shallow”. I guess it is all relative. To a SCUBA diver a 30m dive is a deep one and caution must be taken to avoid the “bends” a potentially fatal condition caused by too rapid an ascent and nitrogen gas forming bubbles in the body.

Last summer Oxford’s Thinking Deep expedition pushed these depths to 90m using technical diving methods, especially the use of closed-circuit rebreathers and helium, oxygen, nitrogen gas mixes. Such diving requires meticulous planning and rigid discipline to prevent problems such as the “bends”, nitrogen narcosis and oxygen toxicity, a particularly nasty problem that can result in a diver having a fit, passing out and drowning. When diving in a submersible these problems are avoided because the atmosphere inside is maintained at the same pressure as on land at sea level. However, the pressure outside increases for 1 atmosphere every 10m depth.

At 500m there is 50 atmospheres of pressure outside and at our planned diving depth for this day, 340 atmospheres pressure. If the submersible somehow sprung a leak the water would enter the sphere which such force that it would cut through flesh and bone. If such vessels are compromised at such a pressure there is the prospect of an implosion. I have heard of neither of these problems occurring although occasionally penetrators, places where the sphere is bored to take wires for control purposes and power etc. will leak causing a dive to be aborted.

This is one of the major differences with space travel where a capsule is designed to keep something in, air. For a submersible sphere strength is critical to keep water at extraordinary pressures, out. Submersibles have been trapped on the seabed leading to the eventual death of the occupants as happened with the Johnson Sea Link when it became fowled in a ship wreck at 350 feet depth in 1973. Fires have also broken out on submersibles, as happened to the French Archimede, also in 1973, not a good year for submersibles.

Outside the water changed from bright blue to dusk to the deep blue of a brightly lit night and finally black. Every 500m Ueki reported by radio to the Yokosuka that all was well and the ship acknowledged. The sphere was still warm from the surface and Ueki got out a pair of Japanese-style paper fans and both the pilot and co-pilot sat facing each other vigorously fanning themselves. Static on the radio was a constant background originating from the acoustics from the submersible or ship.

I was peering out of the porthole and Ueki offered to turn the outside lights off “yes please” was my reply. Every year I teach Oxford second year Biological Sciences students about adaptations to life in low light environments. More than 80% of the organisms that produce bioluminescence live in the ocean and it occurs in everything from microscopic plankton to jellyfish, shrimps and fish. The twilight or mesopelagic zone from 200m to 1000m depth is by far the part of the ocean where bioluminescence is most prevalent. Immediately I began to see bright green bioluminescent flashes. Many of them I guessed from the structures outlined were gelatinous animals, ctenophores (comb jellies), siphonophores (colonial jellyfish like the Portuguese man of war), and scyphozoans (jellyfish).

There were smaller whirling or spiralling points of light, possibly copepods (water fleas), shrimps or even salps (jellyfish like relatives of sea squirts). Now and again there was a curtain like display of light from a siphonophore and sometimes animals disintegrated when they hit the submersible in a shower of green glowing dust. At 894m depth there were several very large and bright displays, one possibly as fish and the other a large salp with its muscle bands glowing green. We hit 1000m depth and Ueki reported to Yokosuka. The bioluminescence began to fade with increasing depth.

At 1683m down I saw one last glowing jellyfish in the distance and that was pretty much it. Shortly afterwards the lights went on again and we were passing a high density of particles raining down towards the seabed. Some of these were quite large, several millimetres or even a centimetre across. This was marine snow, the sinking food supply of the deep sea which was constantly being consumed by animals and microorganisms all the way to the seabed. Food was perhaps the major constraint to life in the deep sea as so little reached it from the surface where it was produced by photosynthesis.

There was still life. At 2093m a small crustacean flicked its tail and vanished into the darkness beyond Skinkai’s lights. At 2181m depth a large jellyfish passed and then what looked like an arrow made of jelly, an arrow worm or chaetognath, with chitinous jaws armed with venom. At 2260m depth a small jellyfish looking like the top half of a lemon squeezer. At 2417m a small swimming pearl-white paddleworm passed the porthole. At 3030m depth we passed a large jellyfish with pink organs, 20cm or more across. As we dropped below 3200m there was a noticeable increase in the numbers of ctenophores, animals which looked a bit like gelatinous lightbulbs.

Finally, after nearly an hour and a half we were nearing the seabed. At 3298m depth Aki and Ueki dropped the first load of ballast from the Shinkai 6500 and trimmed the submersible to acquire neutral buoyancy at depth. The main thrusters were then used to descend the last 100m or so to the seabed. A buzzing alarm went off and Aki flicked a switch to silence it. A few seconds later it went off again, and then a third time. Now it got the attention of Ueki and after a few anxious moments of flicking switches and looking back and forth from various control panels the problem seemed to be resolved. Apparently it was an issue with the main thruster. The sonar display next to Aki showing a blue bullseye now showed orange flecks coalescing into solid returns around us.

At 3353m depth I looked out of the porthole and suddenly a large decapod shrimp, 30cm or more long, with a reddish carapace, bright green shining eyes and a pink body rose over the front of the submersible sampling basket. Its legs gently undulating in a coordinated slow motion swimming motion, a fantastic sight. At 12.06 the seabed came into view, fine grey sediment, scattered boulders of black basalt and pillow lavas. The latter looked like giant beads of striated black toothpaste extruded out onto the seabed, the very bones of the Earth, this is what the crust that forms the bottom of the ocean is made of. Motionless a meter or two from the seabed just beyond Shinkai was a pale blue grey fish with a robust head and body tapering towards the tail almost eel-like and around half a meter in length. This was a cusk eel (ophidion), probably a Bassogigas.

We landed on the seabed at 3382m depth. Clouds of fine silt billowed up around the submersible and Ueki and Aki orientated themselves to navigate to the first and then second waypoints as well as reporting to the Yokosuka that we were at the seabed and in good shape. We prepared to lift off and go on our search for hydrothermal vents at a depth of more than 10,000 feet on the Central Indian Ridge, in the southern Indian Ocean, a very, very long way from home.

The first thing that struck me was the evidence of life down at such depths. The fine pale grey sediment of the seabed was covered in tiny burrows and everywhere there were what looked like inch-long deflated balloons. I had no idea what they were but then saw a worm extending along one so they were some kind of mucus structure used in feeding or as protection from predation. Faecal trails of sea cucumbers were scattered across the silt, appearing like spiral sausages of mud, piles of pillow-like sediment or tube-like extrusions. There were meandering trails where the animals had bulldozed through the surface layers of the fine mud. Less commonly a larger burrow with spoke-shaped depressions around it, the tell-tail signature of echiuran worms, an animal that looks like a bag with a large strap-like proboscis attached to one end. Small dark anemones dotted the sediment and I also saw a large anemone atop a sediment tube, a cerianthid, something familiar to me from diving off the Isle of Man in the Irish Sea.

Now and again we past lurid purple sea cucumbers, the size of a shoe, Benthodytes, and sometimes smaller translucent pink specimens, through the body wall of which I could see the coiled pale gut, Peniagone. More cusk eels hovering stealthily above the seabed. Small black fish lying on the mud with a bright green reflective band on top of the head where the eyes should be. This was Ipnops, and whether the eye band was to detect bioluminescent prey above the seabed or to act as a lure to attract prey is unknown. There was the odd reddish shrimp gently swimming along on the seabed, most smaller than the monster I had seen approaching the bottom. Life obviously very much moved in the slow lane at such depths but it was not as scarce as I would have expected from much of the literature on the deep-sea.

We crossed hummocks of fine deep-sea mud divided by gulleys formed of pillow basalts and shattered black rock. Shinkai 6500 was gently propelled up the northern slope of the knoll that was thought a possible location for venting. At 3260m depth we crossed a particularly large crevasse again comprising tumbled basalt blocks and again at 3244m with abundant large pillow lavas. I strained at the porthole to get a view down these mini valleys as it was in such fractures that we had seen hydrothermal venting in the East Scotia Ridge in Antarctica. I could not be sure whether or not I could see a blue haze in the distance in these crevasses or whether it was just the gloom at the edge of Shinkai’s lights or silt suspended by our passage across the fine muds of the seabed.

This was one of the critical differences between a remotely operated vehicle (ROV), a robot attached to the ship by a cable, and a submersible. In an ROV vision is restricted to the scope of the camera whereas in a submersible observers can look around getting a much greater panoramic view of the deep-seabed. On a mission like this where we were looking for a site perhaps a few tens of meters across such a difference was important. A small pool of water had formed at the bottom of my porthole. I dipped my finger in and tasted it; freshwater, that was just fine. I mopped it with the cloths lying below the porthole for the purpose.

We reached and in fact passed Waypoint 2 on our map as Shinkai 6500 continued up the slope following a ridge of broken basalts, pillow lavas and other extraordinary forms of rock, some looking like dribbled icing, in other areas like the pages of a book. A call down from the ship drew our attention to our deviation from the planned route so Aki and Ueki changed the heading to due east along a spur of the knoll trending in the same direction. As we crossed a band of mud I spotted an extraordinary animal. It comprised a stalk at least a meter tall ending in a translucent pinkish white bulb from which at least 6 pinnate sturdy arms extended out into the water. I was not sure whether this was a form of sea pen or a stalked crinoid, living fossils found only in the deep sea. We moved downslope passing alternating bands of silt and broken basalt finishing with a broken basalt pavement at 3280m on Waypoint 3. At this point Ueki and Aki had to jump through water to the opposite knoll to Waypoint 4 where Masa had observed basalts the previous day.

It was time to quickly eat a few of the delicious sandwiches provided by the galley on Yokosuka and swallow some black coffee. However, I also kept an eye out of the porthole in case we passed through any columns of turbidity that might indicate a black smoker below. This also gave us a chance to shift positions a bit and relieve a few aches and pains, we had been closed in the sphere for 4 hours. Despite all the equipment, displays, lights and digital readouts surrounding us the bare brushed steel-like surfaces of the titanium sphere we were sitting in was visible behind. Drops of condensation coated the “ceiling” and hatch and occasionally dripped cold water onto us. The metal was now noticeably cold. Outside it was about 1.6^oC. The impression of being in a re-entry space capsule struck me again.

We arrived on the opposite bank and were greeted again with black boulders and pale grey sediment. Trails where small rocks had tumbled down the slope were also visible and the submersible began to move up a steep slope toward the next waypoint. Within 10 minutes we found an area of basalt with some orange sulphides and a small dead sulphide chimney. This was promising, however, as we continued there was no further sign of hydrothermalism. At Waypoint 4 we turned and tracked along the side of the knoll towards the west crossing a slopes of fine mud with outcropping basalt and rocky rubble.

We reached the final waypoint of the dive and so there was a discussion between Aki and I as to where to go. A felt we had seen most basalts and fractures in the deeper areas of the slopes and so a good target might be a narrow saddle between the two knolls where there was some steep topography and where the fractures I had seen on the early part of the dive possibly led. We aimed for this and Aki and Ueki determined a new heading for the submersible and we left the slope to jump to this point. However, we realised that we were running out of time and so after another discussion we decided to head back to the slope of the northern and move up towards the sulphides we had observed.

When the submersible dived I actually had the sensation of falling forwards which was somewhat disconcerting. We reached the seabed again at 3287m and tracked up the slope stopping to take a water sample for standardisation of the methane sensors. Again we encountered some aged sulphides as orange mineral encrusting some low-lying basalt but again no sign of active venting. After this it was time to leave the Central Indian Ridge and return to the surface. The weights were released from Shinkai 6500 and we began to float upwards. It was a peculiar feeling, like free fall in reverse, the 27 tonne submersible rising with no assistance from power. Bioluminescence began at 2103m depth and by 200m or so it was getting noticeably lighter outside.

It took just over an hour to reach the surface and we were quickly recovered onto the ship to emerge into daylight. I removed my overalls and received the ritual dowsing in several buckets of ice-cold seawater that all Shinkai 6500 rookies are rewarded with on return to the ship. Unfortunately we had not found any hydrothermal vents this time but this was always a difficult task. The experience had simply been breath taking and left me hungry for more visits to the most remote environment on Earth.