Circumpolar Deep Water upwelling and West Antarctic Ice Sheet stability

The project ?Circumpolar Deep Water Upwelling and West Antarctic Ice Sheet Stability? (CIRCUMWAIS) was hosted by Department of Geology, University of Tromsø ? The Arctic University of Norway (UiT). It involved prof. Tine Lander Rasmussen (project leader), Dr. Patrycja Ewa Jernas and Dr. Matthias Forwick (all UiT), as well as scientists from the Alfred Wegener Institute (Germany), the British Antarctic Survey (UK) and the Adam Mickiewicz University (Poland). The aim of the research was to establish proxies (indicators) for modern environmental conditions along a transect from close to the Antarctic ice sheet, off Pine Island Glacier, to the middle-outer shelf in the eastern Amundsen Sea Embayment (ASE), western Antarctica. The West Antarctic Ice Sheet (WAIS) is considered the most unstable part of the Antarctic Ice Sheet. As the WAIS is mostly grounded below sea level, its stability is of great concern. A collapse of large parts of the WAIS would result in a significant global sea-level rise. At present, the WAIS shows dramatic ice loss in its Amundsen Sea sector. The upwelling of warm and saline Circumpolar Deep Water (CDW) onto the Amundsen Sea shelf is one of the key climatic elements considered to cause the rapid melting of the ice sheet. Seafloor-surface sediments from 10 different locations across Amundsen Sea Embayment were studied within this project to look at the distribution of marine single-cell microorganisms (foraminifera) and sediment properties, and their correlation to the inflow of CDW. As different species of foraminifera demand specific oceanographic conditions i.e. low or high salinities, specific temperatures and certain levels of pelagic productivity (food supply), the species occurrences and assemblages can indicate e.g. reduced or strengthened inflow of CDW, as well as the influence from glaciers draining into the sea. A total of 75 foraminifera species were identified within the analysed samples. The most important assemblage of living foraminifera for the ASE, the Angulogerina earlandi/Bulimina aculeata assemblage, is associated with the flow of warm and saline CDW onto the shelf, but also overall higher bottom-water current velocities (as indicated by sediment properties) and higher pelagic productivity at the middle-outer shelf. On the contrary, Nonionella bradyi can be applied as indicator for fresher and colder water masses originating from ice-sheet runoff. Moreover, the fauna flourishes (high abundance and diversity) in the presence of the polynyas, i.e. areas of open water within areas generally covered by sea ice. The group of foraminifera characteristic for such polynya-like, high productive conditions are associated with the dominant species Globocassidulina sp. On the other hand, the low fauna abundance and diversity, as well as the presence of Astrononion echolsi, is related to harsh environments in the closest proximity to the glaciers. As the foraminifera tests can be preserved in marine sediments as fossils for thousands of years after the organism?s death, the knowledge obtained from this study provides a valuable tool for the interpretation of sedimentary sequences that are analysed to reconstruct past environmental changes in the Amundsen Sea. Based on the fossil foraminiferal occurrences in 10 longer (c. 50 cm) marine sediment cores along the same transect from Pine Island Bay to open shelf of ASE it is possible to reconstruct the spatial and temporal variations of CDW upwelling and ice-ocean interactions several thousands of years back in time. The research performed in this project was the first systematic study of living benthic foraminiferal assemblages from the Amundsen Sea. Due to the logistic difficulties in connection with extremely harsh climate conditions (e.g. fast sea-ice cover), this region is rarely studied despite of its great scientific importance. Thus, the results provide new knowledge that shall find application in many research fields as e.g. Antarctic ecology, paleoceanography or reconstruction of the glacial activity. Results from this study were presented to the international research community during scientific conferences in Norway, Italy, Austria, Poland and New Zealand. They will also be published in peer-reviewed publications, e.g. focusing on the live foraminiferal fauna distribution in the Amundsen Sea, as well as focusing on the development of past environmental conditions in the Amundsen Sea during the past c. 8000 years.

The project aims to increase the understanding of the flow of warm Circumpolar Deep Water (CDW) onto the shelf of the Amundsen Sea Embayment (ASE), and its impact on sub-glacial melting and the stability of ice streams draining the West Antarctic Ice Shee t (WAIS). As the WAIS is mostly grounded below sea level, its stability is of great concern because its break-up can result in a collapse of large parts of the WAIS and consequently contributes to a significant global sea-level rise. In our project we wan t to focus on the most dynamic sector of the WAIS, the Pine Island Glacier, located in the Amundsen Sea Embayment. The Pine Island Glacier is characterised by high-velocity flow, fast thinning and grounding line retreat. Those processes are particularly i ntensified by the inflow of warm CDW. The future evolution of PIG and its potential instability may be classified as a sea level tipping point, when considering the theoretical possibility of widespread ice-sheet collapse triggered by the ongoing climate change. Marine proxies (based on foraminifera, sediment properties and oceanographic data) reflecting the present conditions will be established and applied on sediment records in order to reconstruct spatial and temporal variations of CDW upwelling and P ine Island Glacier - ocean interactions in the ASE since the Last Glacial maximum, i.e. during the past c. 23,000 years. The project emerged from the ongoing collaboration between scientists from the University of Tromsø (Norway), the Alfred Wegener Insti tute for Polar and Marine Research (Germany) and British Antarctic Survey (United Kingdom). It will be based on the analyses of marine sediment samples collected during earlier international marine research expeditions that several of the project partners participated in.