The West Antarctic Ice Sheet retreat in Amundsen Sea, West Antarctica
Prof. Dr. Werner Ehrmann
Dr. Claus-Dieter Hillenbrand
British Antarctic Survey, Cambridge, United Kingdom
Dr. Gerhard Kuhn et al.
Alfred-Wegener-Institut für Polar- und Meeresforschung, Bremerhaven
Dr. Robert D. Larter, Dr. James A. Smith, Dr. Alastair G.C. Graham et al.
British Antarctic Survey, Cambridge, United Kingdom
Deutsche Forschungsgemeinschaft, Bonn
British Antarctic Survey, Cambridge
The history of glacial-interglacial fluctuations in the extent of the some two million km2 West Antarctic Ice Sheet can provide clues about its future stability, with clear implications for global sea-level rise and possible effects on thermohaline circulation. However, understanding of the causes and effects of glacial-interglacial fluctuations in West Antarctic Ice Sheet extent is hampered by the fact that relatively little is known about the nature and stability of ice, which advanced onto continental shelf areas during glacial intervals. Furthermore there are few areas where a reliable chronology has been established for retreat of the grounding line since the last glacial maximum.
Figure 1: Map of the Amundsen Sea, with seafloor surface samples recovered during cruises of the British research vessel James Clark Ross and cruises of the German research vessel Polarstern. At many sites additional long sediment cores have been taken (vibro corer, gravity corer). From Ehrmann et al. (2011).
The study aims in improving our understanding of the deglaciation history of the West Antarctic Ice Sheet after the last glacial maximum. It builds on investigations in Bellingshausen Sea and concentrates on the continental shelf of the Amundsen Sea. This region is extremely important, because it is located offshore from Pine Island Glacier and Thwaites Glacier, which exhibit the most rapid ice thinning and grounding line retreat in present-day Antarctica.
Mainly sediments from the continental shelf, but also from a few sites at the continental slope are studied for their clay mineral assemblages. Surface samples are investigated to map the distribution of the individual clay minerals in order to identify different clay mineral provinces, source areas and present transport pathways and processes. Core samples are analysed to decipher temporal and spatial changes in the clay mineralogical signature in order to reconstruct the maximum glacial extent of the ice sheet and glaciers, the ice flow directions, the subglacial and gravitational depositional processes and the retreat history of the ice sheet.
The study will help to differentiate between consequences of anthropogenic warming or recent sea-level rise, and consequences of a natural continuation of recession that was triggered in early Holocene time.
The investigation is linked to comprehensive research activities at British Antarctic Survey in Cambridge and at Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, which comprise the full range of geophysical, sedimentological, geochemical, stratigraphical and palaeontological studies.
Figure 2: Distribution pattern of kaolinite in seafloor surface samples of the Amundsen Sea (after Ehrmann et al. 2011). Concentrations of up to 29% indicate the occurrence of sedimentary source rocks in the hinterland.
Figure 3: Clay mineral composition of late Quaternary sediments in cores recovered close to the present-day ice edge. At each site, the post-glacial assemblage is distinctly different from the glacial one indicating a change of the catchment area. LGM = Last Glacial Maximum.
The most striking feature in the present-day clay mineral distribution is the high concentration of kaolinite, which is mainly supplied by the Thwaites glacier system and indicates the presence of hitherto unknown kaolinite-bearing sedimentary strata in the hinterland, probably in the Byrd Subglacial Basin. Because kaolinite cannot form under polar conditions, these sediments have to be at least Eocene in age. In contrast, the Bentley Subglacial Trench, which is drained by Pine Island Glacier, hosts the main modern source for illite. Smectite originates from the erosion of volcanic rocks in Ellsworth Land and western Marie Byrd Land.
The clay mineral assemblages in diamictons deposited during the last glacial period are distinctly different from those in corresponding surface sediments. The changes in sediment composition indicate that glacial sediment sources were different from modern ones, which could reflect changes in the catchment areas of the glaciers and ice streams. This probably indicates a migration of the ice divides. Our interpretation has major implications for the long-term stability of the West Antarctic Ice Shelf, because it points towards a dynamically evolving drainage system in the past, similar to that observed today. Furthermore it highlights that one has to proceed with caution when reconstructing palaeo-drainage patterns based on modern sediment sources in this area.
Ehrmann, W., Hillenbrand, C.-D., Smith, J.A., Graham, A.G.C., Kuhn, G. & Larter, R.D. (2011): Provenance changes between recent and glacial-time sediments in the Amundsen Sea Embayment, West Antarctica: clay mineral assemblage evidence. - Antarctic Science, 23(5): 471-486. DOI:10.1017/S0954102011000320.
Smith, J.A., Hillenbrand, C.-D., Kuhn, G., Larter, R.D., Graham, A.G.C., Ehrmann, W., Moreton, S.G. & Forwick, G. (2011): Deglacial history of the West Antarctic Ice Sheet in the western Amundsen Sea Embayment. - Quaternary Science Reviews, 30: 488-505.