JR157: Seabed biology of Marguerite Bay, Antarctica


Cruise Diary

Date: Tuesday 23 January 2007
Position: 67o13.4’S, 70o27.5’ W, in transit between sites in Marguerite Bay

Chris writes:

"Life on board the RRS James Clark Ross has settled into a quiet routine of watch keeping while the scientific part of this cruise is carried out. The geologists and biologists have been paired up and we have been assigned four-hour watches. This means that we can work 24-hours a day and make the most of this valuable time in Marguerite Bay. With this arrangement at any one time a number of us are asleep, whilst others catch up on missed meals or plan the next dive sites and survey transects. The cruise principal scientists, Julian Dowdeswell (a geologist from SPRI) and Paul Tyler (a deep-sea ecologist from NOCS) have agreed that we should attempt to target about 10 sites during the cruise, weather and time permitting. These sites have been determined from swath bathymetry data (sea floor maps) which Julian and his colleagues have collected on a previous cruise and which show areas of potentially interesting topography and geology. At each site we plan to run Isis at an altitude of 20m above the sea bed to carry out a high-resolution swath bathymetry transect providing fine detail of the sea bed topography. Isis will also drop down to just a few metres above bottom and re-run the same transect recording digital video and still images of both geology and biology. Isis will then be recovered to the ship and fitted out for sampling. During recovery, the science team will select point sites from the initial transect which Isis will then sample for biology and geology in a second, much shorter, dive. At the same time additional samples will be collected as necessary using the gravity and box corers.

On Saturday 19th, after finishing a number of safety drills in Ryder Bay near Rothera, Isis conducted a dive to search for the missing deep sea mooring. After almost six hours of, at times, quite taxing search the Isis team located two sets of mooring sinker weights (railway locomotive wheels) and wire, which marked the original location of the mooring. We can only conclude that either the mooring has been ‘taken out’ by a collision with an iceberg or that the acoustic release (a device which is supposed to separate the mooring from the weights when it receives an electronic command from the ship) operated prematurely and the mooring is now adrift in the Southern Ocean. In spite of this outcome, being able to find the relatively small target of the sinker weights in the wide expanse of Marguerite Bay represents an amazing effort on behalf of the Isis team.

On Sunday (20th) Isis carried out her first science dive in the Antarctic. We arrived at our planned initial dive site in the early hours of Sunday morning only to find that it was blocked with pack ice. Because it is unsafe for both the ship and, particularly, Isis to work through pack ice it was decided to move to a second station further north that would be in open water. We hope to return to the first site later in the trip and carry out dives then.

We arrived at the second site shortly after lunch and Isis entered the water at approximately 2pm. She spent a total of almost 24 hours in the water on this first dive. By working in shifts and making use of ROV technology, supplied with power from the surface, scientist can now maximize the time available for high resolution deep-sea science.

On this first dive Isis conducted a video survey of a ‘drumlin’ on the sea floor – a tear-drop shaped mound which rises about 100m above the surroundings. This gave us a valuable insight into the nature of the surface sediments and the resident biological community. The sediment consists of fine silts and mud which suggests that current speeds at this first site are quite low. The sediment is covered with a layer of ‘phytodetritus,’ which is comprised of phytoplankton that sink out of the productive surface waters at the end of the spring/summer bloom and which Tom Bibby and colleagues were investigating as part of their earlier cruise (JR161, http://www.classroomatsea.net/). Phytodetritus forms a valuable food source for the biological community which lives on the sea bed. As evidence of this we have seen a number of species of sea cucumbers (holothurians) which ingest the sediment and digest the phytodetritus as the sediment passes through their gut. Another common group of animals at 850m are the sea pens; these stalked animals look very much like plants and are again indicative of a low current environment.

After the video survey was completed Isis was flown at an altitude of 20m above the bottom and was used to conduct a fine scale swath bathymetry survey of a transect across the drumlin and the surrounding ‘moat’. At the end of this the ROV was recovered and fitted out with a suction sampler, bio-box and push cores. Late last night we carried out a second, sample collecting, dive at the drumlin. Biological specimens of sea pens, sponges and anemones were gathered for species identification and molecular comparison to their temperate-water relatives. Sediment push cores were also recovered for the geologists. These cores will be used to radiocarbon date the age of the surface sediments and to help calibrate the age of the sediments collected in the longer gravity cores. The dexterity with which Peter Mason and his team operated both Isis and its manipulator arms to pluck sea pens from the sediment, as though picking flowers in a field, was simply fantastic.

Isis was recovered at about 2am local time and we all gathered to process the biological material. As we finished and headed to our beds at 4am the James Clark Ross was already making her way through the fog to our next sample site at the edge of the continental shelf..."

Recovering the gravity corer

Swath bathymetry data on the chart table
– used to plan future dives

Arranging the sampling devices on the Isis’ tool tray.

Inside the ROV control van, preparing for the dive.

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January 2007
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