PS104 - Weekly Report No. 2 | 13 - 20 February 2017
The second week
A broad belt of thick pack ice normally blocks the northern entrance to the shelf of the Amundsen Sea Embayment. But we encountered an unusually small sea-ice cover as also observed by satellite data from around entire Antarctica this southern summer. With only an average 3/10 to 5/10 ice cover, Polarstern easily managed to pass this belt so that we soon arrived at our first station west of Thurston Island to sample marine sediments.
We did not remain for long at this first sampling site, because our main destination for first drilling with the MeBo70 seafloor drill device is the southern Pine Island Bay close to the mighty Pine Island Glacier. This glacier and the neighbouring Thwaites, Smith, Pope and Kohler Glaciers consist of huge ice streams which transport about 100 Gigatons per year of ice from the central West Antarctic Ice Sheet to the Amundsen Sea. This is about two thirds of the total ice mass that Antarctica is presently losing into the oceans every year. In front of Pine Island Glacier, a 300 m thick sedimentary basin is embedded in surrounding hard rock basement that outcrops on the seafloor of the inner continental shelf. The sediments of this basin were likely transported by the glacier, but they could also contain deposits with micro fossils from algae of the water column that can be used for age dating. With this and other sedimentological and geochemical analysis methods it is possible to reconstruct the retreat of the glacier from this area.
The team of technicians from MARUM, University of Bremen, where the MeBo drill device was developed, prepared the MeBo for its first deployment in Antarctica. As the German name abbreviation says, the device is deployed on the sea floor – in this case at over 1000 m water depth – and is connected to the ship by a strong cable. All power and communication line go through this cable (or umbilical). Two “pilots” of the MeBo team control the device using control consoles and screens that are built into a specially designed control container placed on the working deck of Polarstern. Although the drill device can drill up to 80 m into the subsurface, we first equipped it with only 10 drill pipes making 23 m in total for a test, because the composition of the sediments is not yet known and the opinion on their properties varies. This first drill attempt reached the planned drill depth and brought back sediment-filled drill pipes. However, the enthusiasm of the geologist were dampened, because the recovery of these extremely soft, fine to coarse grained glacially transported sediments was only 33%. In addition, the drilled material hardly contained any microfossils that are important for age dating.
A comprehensive maintenance and preparation must be done with MeBo after every drill operation, so that there is time to apply other sampling and surveying techniques in between drill sites. As the extent and internal structure of the Pine Island Glacier basin is not well known, we used the time for seismic surveying and for sampling of the uppermost sediments using conventional coring devices such as gravity corer, multi-corer and box corer. Some nautical miles away from the first drill site, we planned drilling up to 50 m at the second MeBo site of this basin. Drilling started well until just after midnight an iceberg alert came from the bridge. First, there was hope it would pass by the ship, but then its drift path turned and it approached the ship. MeBo drilling had to be abandoned and the MeBo lifted from the sea floor. A short distance away, MeBo was lowered back to the sea floor for a second attempt to drill. But then a malfunction of the flush-water pump forced us to cancel this drilling attempt. Therefore, only 17 m were drilled at this site. As the sediments are also very soft and of similar origin as at the first site, a recovery of only 23% was not a surprise anymore.
The high-resolution satellite images we receive every day from the German Aerospace Agency (DLR) show that Pine Island Glacier retreated again recently to uncover another large area of sea floor. We did not miss the opportunity to map this opened-up sea floor with the multi-beam echosounder for the first time. One of the survey tracks close to the ice shelf and glacier front, rising 50 to 80 m above the sea, made a memorable impression on all expedition participants. But despite the beauty of the ice wonder world outside, we discussed during the MeBo repair and maintenance time intensively which of the initially selected drill sites we will next aim for ….
With best regards and wishes from all expedition participants