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Supplementary material to “Improving Constraints on Paleo Ice Sheets in the Amundsen Sea Embayment”
26 January 2010
Robert D. Larter, British Antarctic Survey, Cambridge, UK
Karsten Gohl, Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany
Michael J. Bentley, University of Durham, Durham, UK; and British Antarctic Survey, Cambridge, UK
Citation:
Larter, R. D., K. Gohl, and M. J. Bentley (2010), Improving constraints on paleo ice sheets in the Amundsen Sea Embayment, Eos Trans. AGU, 91(4), 33. [Full Article (pdf)]
Workshop on tectonic, climatic and glacial evolution of the Amundsen Sea Embayment, West Antarctica; Granada, Spain, 9 September 2009
Geoscientists working on the Amundsen Sea Embayment (ASE) of West Antarctica met at a workshop during the First Antarctic Climate Evolution Symposium to discuss recent advances from, and future priorities for, work in this region. The ASE is the most rapidly changing sector of the West Antarctic Ice Sheet (WAIS) and contains enough ice to raise global sea level by 1.2 meters. Changes observed over the past two decades, mainly from satellite studies, include rapid ice shelf thinning, decreasing surface elevation, glacier flow acceleration and grounding line retreat. Ice sheet modelling studies suggest that this sector of the WAIS is potentially unstable because most of its base is grounded far below sea level and has a reverse gradient (i.e. dipping down towards the centre of the ice sheet).
In recent years considerable efforts have been made through several national Antarctic programs to acquire new data to improve knowledge of the geological structure, subglacial topography, continental shelf bathymetry, and glacial history of this remote region. These data are important for establishing boundary conditions for ice sheet modelling (i.e., bathymetry, subglacial topography, substrate type, heat flow), for providing constraints on past ice sheet changes that can be used to test and refine models, and for putting recent changes into a longer term context. Hence geoscientific data from the ASE have a key role to play in improving the next generation of ice sheet models that will be used to predict the contribution from Antarctica to global sea level rise.
During the workshop current knowledge on the tectonic, climatic and glacial evolution of the ASE was reviewed and priorities for future work were identified. Plate tectonic reconstructions suggest that the region has been subject to three distinct phases of extension since New Zealand started to rift away from the continental margin at ~90 million years ago. Seismic measurements of crustal thickness have been made in only a few locations, but reveal relatively thin continental crust that is consistent with this extension history. On the basis of these results, and earthquake tomography studies, elevated heat flow is predicted, but as yet no actual measurements of heat flow have been made in the region. In the western part of the region, volcanic activity in Marie Byrd Land has persisted since about 30 Ma. Recent aerogeophysical surveys have revealed evidence of a volcanic eruption further east, in the Hudson Mountains, <3000 years ago, and aeromagnetic data reveal other anomalies that could represent subglacial volcanic centers. Airborne radio-echo sounding surveys have transformed knowledge of the subglacial topography over most of the ASE, and aerogravity data are providing insight in the locations of subglacial sedimentary basins. However, aerogeophysical data coverage remains sparse in neighboring areas to both east and west, and aeromagnetic data coverage over the continental shelf is also incomplete.
Recent seismic reflection surveys on the continental shelf revealed alternating acoustically stratified and reflection-poor sedimentary sequences similar to those that studies on other polar margins have shown to be characteristic of deposition from waxing and waning ice sheets. However, no stratigraphic drilling has been carried in the region and so the age range and periodicity of these glacial cycles remains unknown. Seismic reflection surveys on the continental rise reveal sediment drifts that record a history of changes in deep water circulation extending back to early Tertiary or Late Cretaceous time.
Compilation of data from several multibeam swath bathymetry surveys carried out over the past decade has provided a much improved definition of topographic features on the continental shelf, slope and rise. Broad cross-shelf troughs show the paths followed by ice streams in the more extensive ice sheet that existed during glacial periods. The same troughs now channel inflowing, relatively warm Circumpolar Deep Water (CDW) across the shelf and beneath the floating termini of the modern ice streams, where it enhances basal melting. Detailed examination of multibeam data reveals bedforms generated beneath the formerly more extensive ice sheet and provide insight into its past extent, dynamic behavior and pattern of retreat since the Last Glacial Maximum (LGM, about 20,000 years ago). Sediment cores collected from the continental shelf contain sediments that record changes during the last deglaciation. Radiocarbon dates on core samples exhibit some scatter, but in general suggest that the grounding line retreated fairly rapidly from the outer and middle shelf before 12 thousand years ago, then subsequent retreat has been more gradual, prior to a dramatic acceleration over recent decades or centuries.
Changes in the ice sheet surface elevation since the LGM have recently been investigated using cosmogenic isotopes to determine surface exposure ages on nunataks. Results available so far contrast with the offshore record in that they indicate relatively slow change initially following the LGM, then more rapid change over the last few thousand years.
Some sediment cores collected on the continental rise contain continuous, albeit low-resolution, records spanning the entire Quaternary (about 2.5 million years ago to the present). These cores do not record any clear indication of a late Quaternary collapse of the WAIS, which has been proposed on the basis of recovery of marine microfossils from beneath Whillans Ice Stream (Ice stream B) on the Siple Coast. However, it is not clear that collapse of a marine ice sheet would necessarily yield a prominent layer of ice-rafted debris in areas close to the Antarctic margin. The cores do record a depositional anomaly spanning Marine Isotope Stages 13 to 15, which has the characteristics of one long interglacial period and may therefore be the most likely time for a late Quaternary collapse to have occurred.
Workshop participants agreed that priorities for future research in the ASE are to: (1) establish the deglacial history of each palaeo-ice stream trough through complete multibeam swath bathymetry coverage of and targeted sediment coring, (2) develop proxies to recognize ice sheet collapse events and CDW incursions onto the shelf, and to estimate past surface water temperatures, (3) intensify surface exposure age studies and extend them to the western part of the region, (4) extend aerogeophysical surveys to areas adjacent to the ASE and to complete coverage over the continental shelf, (5) obtain direct measurements of heat flow, (6) extend seismic reflection profile coverage on the continental shelf, slope and rise and use these profiles to identify targets for (7) stratigraphic drilling that will reveal the long-term glacial history of the ASE and its sensitivity to changing climate, sea level, and oceanography.
Workshop participants: John B. Anderson (Rice University, Texas); Michael. J. Bentley (Durham University/British Antarctic Survey, UK); Donald. D. Blankenship, Jamin Greenbaum, and Dustin Schroeder (University of Texas Institute for Geophysics, Austin, Texas); Graeme Eagles (Royal Holloway University of London, UK); Werner Ehrmann (University of Leipzig, Germany); Karsten Gohl, Ansa Lindeque, Gabriele Uenzelmann-Neben, Estella Weigelt, and Florian Wobbe (Alfred Wegener Institute for Polar and Marine Research, Germany); Alastair G.C. Graham, Claus-Dieter Hillenbrand, Robert D. Larter, and James A. Smith (British Antarctic Survey, UK); Frank O. Nitsche (Lamont Doherty Earth Observatory, Columbia University, New York); Martin J. Siegert (University of Edinburgh, UK); Charlotte Sjunneskog (Florida State University, USA); and Julia Wellner (University of Houston, Texas, USA).
The workshop Programme and Abstracts are available to be downloaded from http://nora.nerc.ac.uk/8740/
ROBERT D. LARTER, British Antarctic Survey, Cambridge, UK E-Mail: rdla@bas.ac.uk; KARSTEN GOHL, Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany E-Mail: karsten.gohl@awi.de; MICHAEL J. BENTLEY, University of Durham/British Antarctic Survey, UK E-mail: m.j.bentley@durham.ac.uk.