PS104 - Weekly Report No. 4 | 27 February - 5 March 2017
A week passed by that was full of alternating expectations and events. Sea ice got in the way, but such is life when working in polar environments. But there is always a Plan B, and this time we continued in the eastern part of the Amundsen Sea Embayment, where sea ice and weather conditions were more favourable.
During the coldest part of the last ice age, the last glacial maximum some 20,000 years ago, this area was the location where two glaciers merged. The Amundsen Sea Embayment hosts two very large glaciers, the Pine Island Glacier and the Thwaites Glacier. It was, however, two smaller local glaciers, which created an interesting structure on the seafloor, which was already noted during previous expeditions. We were confident that we had a good target in mind. It was, however, only when we surveyed the area with our hydroacoustic tools on board that we realised the full potential. The combination of swath bathymetry and Parasound sub-bottom profiling allowed us to map the exact seafloor topography as well as the sedimentary structures under the seafloor. Both data sets together revealed characteristic sedimentary deposits that were created when the front of a glacier became stationary at a location over an extended amount of time. During this stabilising period, sediments derived from the continental source area of the glacier, were deposited below its front to form a wedge. The swath bathymetry and sub-bottom profiling taught us that we were not just looking at a localised structure, but that we were facing a system of grounding zone wedges over an extent of 50 kilometres – a significant finding for understanding of glacial dynamics. Full of expectations, we lowered the MeBo seafloor drill device to drill sediment cores from this structure through 40-50 m below the seafloor. But unfortunately, a technical problem occurred first at a screwed connection between drill barrels, and then the umbilical cable of MeBo was severely damaged while the device was already on deck. This caused a longer downtime for repair.
Motivated by this discovery of the grounding zone wedges, we continued our hydroacoustic mapping efforts and partnered it with a seismic survey of the area, which allows visualising sedimentary structures hundreds of meters below the seafloor. As a result of all this work we could decide on ten locations where we collected material from the seafloor in order to unravel the type of sedimentary deposits in the wedges, and - hopefully - their age and hence the time the glacier rested in this location. The instrument we used for this purpose was a gravity corer - a metal pipe, equipped with 1.5 tonnes of weight at the top. Once deployed over the side of the ship on a very thick and sturdy cable, the gravity corer is sent 500 to 600 metres down through the water column to subsequently penetrate ~5 metres into the seafloor. Inspection of the cores revealed that the topmost part of the seafloor is made up of sediment, which was deposited under open ocean conditions after the glaciers retreated from the Last Glacial Maximum position to their present position further south. But we also retrieved the topmost part of the wedges deposited under the ice we were so interested in. Future work, at home in our laboratories, is now eagerly awaited. In conjunction with the hydro acoustic data we collected during the last week, the detailed analysis of the sediment will enable us to paint a more detailed picture of the glacial history in the Amundsen Sea Embayment. Such insights are vital for our scientific understanding of glacial dynamics. Understanding these dynamics is particularly crucial in the Amundsen Sea, an area of rapid ice retreat today. Modern observations tell us that glacial ice in this area is retreating faster than anywhere else in Antarctica, contributing in a growing amount to observed sea level rise.
In the meantime, the well-skilled MeBo team managed to complete the complicated repair job on the thick umbilical cable with a new termination in a single day, so that we were ready to deploy MeBo again to the seafloor. But the direction of ice drift had changed and wind speed increased enormously. We had to give up in order not to waste any more time. Thus, we decided to return to the southern Pine Island Bay where we thought the winds are less intense, and where we aimed for a small basin with sediments supposedly deposited in a former subglacial lake environment. But there we encountered the same strong winds coming from the coastal ice sheet, and we had to give up this drill site as well. Our meteorologist Max then identified an area in the central Pine Island Bay in which the wind should be abated. And yes, when we arrived in this area a few hours later, it was almost no-wind conditions. Immediately, we deployed the MeBo to water and lowered it to the 1400 m deep seafloor of a isolated sedimentary basin. After a while of drilling, iceberg alert came up. An iceberg approached the vessel on starboard side. Do we have to abandon drilling and hoist the MeBo? The moving radius of the vessel is only a few tenths of metres large when the MeBo is deployed. It was indeed a masterly job of the captain and his nautical officers when they manoeuvred the vessel slowly around the iceberg and gave it a "push" with the bow so that it could continue drifting bypassing the vessel. After 36 m drill depth, we ended the drilling at this site. Almost 50% of the core barrels were filled with sediments, which was surprisingly much, because of the difficulties to recover glacial-marine sediments by drilling in general. So far, there has not been the time to analyse these sediments thoroughly, but we hope that we will have material from a former subglacial lake in them. Such subglacial lakes are well known to exist beneath the present-day, several kilometre thick Antarctic ice sheet, but their sediments have not been sampled due to the ice thickness. Therefore, sediments from former subglacial lakes that are now under the present-day seafloor can provide valuable information on former and present ice sheet dynamics. We also mapped the deeper structure of the basin with a seismic survey to image the spatial distribution of the sedimentary deposits.
This expedition shows nicely how geologists, geophysicists and geodesists work together to better understand the development and the mechanisms of the West Antarctic Ice Sheet. In the area where sediments were sampled by conventional coring or by drilling, Ricarda and Katharina measured the temperature gradient in the sediments by using a temperature probe. Highly sensitive temperature sensors are connected to a 6 m long steel rod that has a heavy weight on top. This device is then lowered to the seafloor and pushed into the sediments. Through measuring the difference in temperature with depth, the geothermal heat flux can be determined. High heat flux may have an influence on the ice sheet flow dynamics. The Pine Island Bay is a particular candidate for high geothermal heat flux, because it is one of the few Antarctic regions with active volcanism only a few thousand years ago.
We now left Pine Island Bay and are sailing to the western shelf of the Amundsen Sea Embayment where we will complete our last part of the MeBo drilling program and the other research activities.
All are well and send their best regards and wishes
Johann Klages & Karsten Gohl (with translating help by Tina van de Flierdt)