Feb 16 - And so to leaving the Amundsen Sea
And so to leaving the Amundsen Sea
When you left us last week, the JCR had managed to break through the pack ice and back out into open water. As said then, the pack had moved quite a long way further south, thus leaving the edge of the shelf free of ice. On the passage in, heading west, the whole of the area of the shelf had been covered with ice, making it impractical to do any survey work there, other than at the shelf edge itself, or in deeper water. The ship had still got a week or so of survey time left, before being required to be back at Rothera on 15th Feb. The southwards movement of the pack ice , had uncovered a large area of the northern portion of the shelf, making it possible to undertake a wide variety of survey work on the ships passage back east.
Only a few hours after clearing the pack ice, scientific work was resumed, with a CTD having been completed before the day was out. We remained in the Amundsen Sea for about another week, filling in missing areas of swath survey data as we moved, as well as undertaking a number of box-cores, vibro-cores and more CTDs. Two sessions of seismic survey were undertaken within this period. (see last week's webpage for descriptions) One of these was as far south-east into the Amundsen Sea as the ice would allow, being the closest we would actually get to the planned survey area of Pine Island Bay Glacier. After finishing swath surveys in this area it was at last time to leave the Amundsen Sea, though the science work was still not yet quite over. The JCR was heading in the direction of Rothera to the east-northeast, and time had been left to allow 12 hours of work in the Bellingshausen Sea on the way back. JCR arrived at the planned study area in the early hours of 12th Jan, and a CTD cast commenced at 01.30. Station work, including a box-core and a vibro-core, continued until about 05.30, after which time a period of swath survey work was undertaken.
And so, the science survey work for this one month Geophysics, Geology, and Oceanography cruise was now actually over, although there was still more work to be done. The final important tasks included dismantling the vibrocorer, to make it ready to be packed into the hold on arrival at Rothera, and the backing-up on tape or DVD of all of the data collected during the cruise, for analysis back in the northern hemisphere. The passagetowards Rothera was also marked just after lunch on the 13th by the the recapture of the lost satellite links, restoring the ship's phone and internet links to the outside world.
Many of the present ship's scientific company would be disembarking at Rothera, to take the Dash-7 flight north to the Falklands. Before this could happen, though, the End of Cruise Dinner was still to come. This was to be a less formal affair than the previous two, with a buffet being prepared by the catering staff and laid out in the conference room next to the Officers' and Scientists' Lounge. Everyone assembled at 6pm for pre dinner drinks, and this was then followed by the traditional end of cruise speeches by the PSO, Rob Larter and Captain Jerry Burgan. These were followed by buffet curry and the first evening that all members of science staff could enjoy together since the start of the cruise, since everyone had been working shifts since the first morning on board.
And so to the last science spot for this cruise, with Claus-Dieter Hillenbrand
Science Bit In The Middle.
By Claus-Dieter Hillenbrand.
During the last three weeks we investigated the seafloor in the Amundsen Sea, which is one of the least studied and least accessible areas around the Antarctic continent because its remoteness and its sea-ice coverage for most of the year. Even with an ice-strengthened research vessel like “James Clark Ross”, it is only possible to work here for about two months in most years, between mid-January and mid-March. As a consequence only JCR and the ice breakers “Glacier”, “Nathaniel B. Palmer” (both U.S.A.) and “Polarstern” (Germany) had managed to advance onto the shelf of the Amundsen Sea during the past 25 years.
Collecting mud samples from the seafloor
On cruise JR141 we have been trying to find out how far the West Antarctic Ice Sheet advanced onto the shelf in the Amundsen Sea during the last ice age, which processes formed the large-scale bedforms characterizing the present seafloor, in which direction the ice masses flowed, and how fast the ice retreated back to the coast at the end of the last ice age. As explained in last week’s diary, we were using the ship's advanced sonar systems for mapping bedforms on the seafloor and for reconstructing ice flow patterns. However, for deciphering the origin and age of these seabottom features, and for reconstructing the timing of ice retreat from the shelf we collected sediment samples from the seafloor using sophisticated coring equipment.
On this cruise we used the vibrocorer from the British Geological Survey for recovering sediment cores up 6 metres long, and a box corer for retrieving undisturbed surface sediment samples from the seafloor. The vibrocorer is mounted in a metal frame and lowered to the seafloor via JCR’s stern. As soon as the frame reaches the sea bottom an electric mechanism starts the vibration of the corer, which pushes the coring device into the seabed. This vibration mechanism is particularly suitable for the coring of relatively stiff deposits. Such deposits are very common in polar marine environments, where thick ice masses may have advanced onto the shelf during the past, thereby compacting and consolidating the underlying seabed sediments.
The vibrocorer in its frame (painted orange) is deployed over the stern of the ship. The corer is lowered into the sea floor to collect a cylinder of sediment that may be up to 6 m long. (Picture by R. Larter).
A sediment core recovered with the vibrocorer in the wet lab of JCR. The sediment core sticks in a 6 m long plastic liner, which is cut into 1 m long sections before transferred into the lab or the scientific fridge. (Picture by J. Smith)
The topmost layer of the modern seafloor normally consists of sediments deposited in an open water setting. As a consequence the sediments in this layer are quite “soupy” and therefore difficult to recover with the vibrocorer. Therefore, we are using the box corer, which allows us to recover undisturbed surface sediments from the seafloor. The box corer is deployed via JCR’s midships gantry and consists of two spade-like arms arranged around an open metal box (30x30x98 cm). When the corer reaches the seafloor the box easily penetrates into the soft seabed surface due to the weight of the box corer of about ˝ ton. As soon as the box corer is lifted the hinged arms are triggered and the spade like arms close the bottom of the box with the sediment sample inside.
The box corer is deployed using the midships gantry, and is used to recover an undisturbed sediment surface, which may be missing in vibro-cores. It samples an area 30 cm square. (Picture by J. Smith).
What we found so far
We collected vibrocores and box cores at numerous sites on the continental shelf and slope in the Amundsen Sea. We found two different categories of surface sediments in the study area. On the slope and the outer shelf the sediments consist of foraminiferal muds. Foraminifera are microscopic, single-celled animals resembling amoebae. In contrast to the latter, however, foraminifera build a shell made of calcite. The calcareous shells in the outer shelf and slope sediments of the Amundsen Sea are from so-called “plankonic” foraminifera, which means that these animals were floating in the surface waters, thereby feeding on algae and other tiny organisms living there. When the foraminifera died, their shells settled through the water column down to the seafloor and accumulated in the sediments together with other fine-grained particles, supplied from the Antarctic coastal areas by wind-driven and tidal currents.
The foraminiferal muds in the Amundsen Sea contain large amounts of gravel grains and pebbles. These coarse-grained stones were obviously supplied from different sources and show indications for transport by ice. We interpret them as “dropstones” or iceberg-rafted debris (IRD), i.e. rocks, which were eroded by glaciers in the West Antarctic hinterland and incorporated into the basal ice of these glaciers. When a glacier reaches the coast, icebergs containing the rock debris break off the glacier (they are “calved” by the glacier). The icebergs melt during their drift through the ocean waters, thereby releasing the IRD to the seabed. Many of the dropstones in the Amundsen Sea are brown or black coloured due to the growth of manganese coatings. Such manganese crusts develop very slowly while the IRD is lying on the seafloor, and their presence documents that recent rates of sediment deposition on the shelf and slope have been very low - not more than a few centimetres per thousand years.
An undisturbed seafloor sample, recovered in a box core from the outer continental shelf. The pebbles on the surface were transported and dropped by icebergs (“dropstones”). (Picture by C.-D. Hillenbrand)
In contrast, surface sediments which we recovered from the inner shelf of the Amundsen Sea are much more fine-grained. They are barren of foraminifera shells, but contain large amounts of diatoms. Diatoms are planktonic algae, which belong to the green plants, thus depend on photosynthesis, and form tests made of opal. Diatoms can only live in the surface waters if sufficient sun-light is available, and therefore predominatly thrive during the Antarctic spring and summer, when the sea-ice coverage is at its minimum and the sun shines up to 24 hours per day. The diatomaceous muds in the Amundsen Sea contain only a few dropstones lacking any manganese coating. This finding indicates that the accumulation rates of sediments on the inner shelf were significantly higher compared to the outer shelf and slope.
A split-core taken from a box core recovered from the inner continental shelf (core top is to the left). The sediments contain large amounts of microscopic algae with opaline shells called diatoms. (Picture by C.-D. Hillenbrand)
With the vibrocorer we recovered more than 100 metres of sediment cores. Most of these cores are between 3 m and 6 m long and penetrated the seafloor deep enough for the recovery of sediments deposited during the last ice age and during the transition from the last ice age to the present warm period. The deposits of the last ice age indicate that ice had advanced across the shelf to at least the outer shelf of the Amundsen Sea, because they consist of completely unsorted rock debris comprising pebbles, cobbles, and sandy, silty, and clayey particles. Such sediments called “diamictons” can result from different depositional processes. The diamictons may have been deposited at the base of a glacier (“sub-glalcially”), under floating ice directly in front of a glacier (“sub-ice shelf”), or by a debris flow, which re-deposited this glacial debris further seaward. Back at BAS in Cambridge we will analyze the physical properties, the sedimentary structures, the grain-size composition and the mineralogy of these diamictons. These investigations will help us to reconstruct not only the process responsible for the deposition of a particular diamicton unit, but also where the ice depositing this diamicton unit came from.
Accompanying the retreat of the ice masses from the shelf at the end of the last ice age, conditions for planktonic organisms became more favourable. Consequently, the sediments deposited on the Amundsen Sea shelf during this period of global warming contain foraminifera shells, diatom tests and traces of organic matter of these micro-organisms. Radiocarbon dating of the calcareous tests and the organic matter will give us a clue, when and how fast the glaciers retreated back to the modern coast and the ocean reconquered the shelf.