For nearly three weeks we have been pursuing our research in the Bellingshausen Sea by tracing back the development history of the West Antarctic Ice Sheet. The various geophysical, geological and also biological groups have been active in an alternating sequence by deploying their sampling and surveying equipment. In this issue, we want to describe the work of the marine geologists in more detail.
Ten scientists from eight different nationalities – coming from Great Britain, Chile, South Africa, USA, Belgium, France, Morocco and Germany – are united in the marine geology group. Their motivation and aim of their work on the continental shelf of the Bellingshausen Sea is to reconstruct the ice-sheet retreat after the last phase of maximum glaciation about 20,000 years ago. They work closely together with the two hydroacoustic teams on board, the bathymetry and sub-bottom profiling groups, who generate accurate, detailed maps and profiles of the seafloor and the top sediments beneath (Fig. 1). Both hydroacoustic methods apply the same principle: An acoustic signal is transmitted so that its reflection at the seafloor and from the top sedimentary layers can then be recorded. For bathymetry, multiple beams at different angles downward are transmitted at about 15 kHz to record a three-dimensional image of the seafloor. The sub-bottom profiler, also called sediment echosounder, generates only single acoustics pings downward with a lower frequency of about 4 kHz that can penetrate up to 150 m into the seafloor. They reflect at the various sub-bottom layers to produce a vertical profile of the uppermost sedimentary sequences.
On the basis of these maps and profiles, the geologists find the most promising locations for the collection of sediment cores. They use mostly two different devices for sampling – a so-called box corer (Fig. 2) for grabbing an undeformed sample of the uppermost 60 cm of the seafloor, and a gravity corer that can penetrate several meters into the seafloor, pushed by its 1.5 ton top weight (Figs. 3 and 4). Ideally, this method allows to collect samples from sediment layers that were compacted by the past ice-sheet load thousands of years ago, proving an opportunity to characterize the deposition conditions back then and give indications for the age of the ice-sheet retreat. This sampling method is not only applied at a single location, but at several locations along a transect from the inner to outer continental shelf, so that a temporal sequence of the ice sheet retreat from its maximum extent 20.000 years ago until today can be reconstructed. At the same time, the influence of the morphological formations on the seafloor and the geological structures is assessed with regard to ice-sheet dynamics of the past.
The combination of these surveys and sampling provide a valuable four-dimensional dataset to test and improve ice-sheet models for more accurate estimates of the future global sea-level rise. Being a key region for such studies, the Bellingshausen Sea sector has lost already a large ice mass in the last 20,000 years, and it is one of the regions that react extremely sensitive to any external influences such as the increasing inflow of warm ocean water.
Since the beginning of our research, we have been very grateful for the incredible competent, effective and friendly support of all sections of the ship’s crew, from the kitchen and service team to the deck crew, engine technique, electric and communication technique and to the bridge and ship leadership. We are looking forward to this continued cooperation during our remaining three weeks in the work area.
Best regards and wishes
Johann Klages, Estella Weigelt and Karsten Gohl
Further information on PS134:
Frequent short blogs: https://follow-polarstern.awi.de/
125-year anniversary of Belgica expedition: https://125yearsbelgica.com/