JR157: Seabed biology of Marguerite Bay, Antarctica


Cruise Diary

Date: Tuesday 30 January 2007
Position: 68o44.9’ S, 69o34.2’ W, Marguerite Bay

Chris writes:

"Isis completed dive 10 in the early hours of this morning and is currently sat on deck getting its hydraulic oil topped up before the next dive. Isis is depth rated to 6500m, the water pressure at this depth is roughly 650 times that at the surface of the sea. To build a complex remotely operated vehicle, with lots or wiring, joints and connections, which can withstand this pressure is a very challenging task. One of the engineering solutions to this problem is to fill all of the electrical trunking and other systems with hydraulic oil which is the kept above ambient pressure. This ensures that if there are any small leaks in any joint or pipe on the ROV then no sea water will leak into the system and cause rapid electrolysis, corrosion or short circuits in the electrical systems. Every so often the oil reservoir on the ROV needs to be topped up and that is taking place at the moment.

The past few days on ship have been quite a blur of watch keeping, processing samples and catching up on sleep. At times it has been quite difficult to keep track on the current dive number and our location.

Once we had completed the first science dives at the ‘drumlin’ on 23 January, we headed north and further out to sea. Our next series of dives took place at the edge of the continental shelf. A series of video and swath transects were surveyed from approximately 800m down to a depth of about 2000m. This location represented the maximum extent of sea ice during the last glacial maximum and is a site where a large amount of material flowed from the continent, along with the glacier and was then deposited in the sea. The seabed here is a mixture of fine sands and silt with numerous ‘drop stones’. These stones were carried from the Antarctic Continent by glaciers to be dropped at their current position. Well almost, the sea bed is a slope at this point and we found evidence to indicate that some of these stones have slipped down the slope and may not lie in their original position. Nevertheless these stones provide a small oasis for organisms, such as sponges, soft corals, and anemones, which need a hard surface to live on.

After this site was completed we moved further out to sea and on to the abyssal plain. Here Isis conducted her deepest dives (~ 3500m) to investigate the nature of the seabed and some regular sedimentary features which had been seen on earlier swath bathymetry transects. At this site the seabed is quite flat with very little in the way of a megafaunal biological community. Organisms that do occur are at very low abundance and are patchily distributed.

At the end of this series we moved back towards the slope and investigated a series of deep channels cut into the sea bed by turbidity currents. These quite amazing features, some approximately 100m deep, provided a welcome alternative to the flat sea bed of the abyssal plain. At the end of this site the weather conditions worsened with a strong northerly wind developing. The swell that was kicked up as a result made for one quite nerve-wracking recovery of Isis which I don’t think anyone would want to see repeated.

One good aspect of the strong northerly wind was that it would blow sea ice further south. Shortly after the end of dive 9 we received a satellite image of the sea ice coverage in Marguerite Bay which showed that the first planned science site, which had to be aborted on the morning of Sunday 21 January because of the pack ice, was now clear. The decision was taken to steam back into shallow water and visit this more southerly site, a decision which has put us back within sight of the Antarctic Peninsula and in amongst the ice bergs once again.

Through yesterday and last night Isis conducted a video survey of some deep channels. These channels, or sub-glacial melt water gullies, are believed to have been created during the last glacial maximum. At the bottom of every glacier the ice melts and provides a lubricating layer which helps the ice to flow. This water also erodes the land on which the glacier sits. Some of this water is enclosed within channels which can be pressurized and which can allow the water to flow up gradients. Here on the seabed we have evidence of some of these sub-glacial channels which have been preserved ever since as they have remained underwater. Again these channels were visually stunning with vertical walls covered in sponges, anemones, brittlestars and holothurians.

The end of the cruise is rapidly approaching now and discussions are being held about our last dive sites which we’ll attempt before returning to Rothera this Sunday and leaving the ship. I’ll post an update of our plans as we know them...."

Isis recovering samples of the seafloor

The inquisitive rat tails return to supervise sampling

The changing sea bed type in Marguerite Bay

Sea pen Umbellula

Sponges on rock wall

Sponges and anemones at the top of a cliff

Brittlestars on the wall of a channel

Meanwhile… on the surface… the icebergs have returned

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January 2007
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