ALBERT

Description: The threat, in terms of sea level rise, posed by the potential rapid deglaciation of West Antarctica means there is an urgent need to know more about the speed and style of marine ice sheet retreat. Quaternary deglacial events recorded in marine sediments provide an opportunity to understand the future of the modern day ice sheet. In this context, we examine the glacial history of a particularly poorly understood sector of the West Antarctic continental shelf the Amundsen Sea Embayment using new data from two recent research cruises. This extended abstract describes how marine geological and geophysical data are being used alongside terrestrial dating methods to understand the full extents, dynamics and retreat pattern of the West Antarctic Ice Sheet in the Amundsen Sea region during the last glacial cycle. These data hold significance for understanding and accurately modelling the stability and climate sensitivity of the West Antarctic Ice Sheet.

Description: Marine geoscience data indicate that during the Last Glacial Maximum (LGM) grounded ice extended to the shelf edge along most, if not all, of the 2500 km-long continental margin from the northern Antarctic Peninsula to the Amundsen Sea. Past extent of grounded ice is indicated by swath bathymetry data from the outer parts of cross-shelf troughs, which reveal relict elongated subglacial bedforms. The bedforms show that the troughs were paths of fast-flowing (streaming) ice. Geomorphological evidence regarding the nature of ice flow over intervening outer shelf banks has been erased through pervasive post-glacial ploughing by icebergs. However, seismic profiles across the banks reveal widespread shelf edge progradation and numerous glacial unconformities that indicate grounded ice has extended across them many times during the Pleistocene, and before. Subglacial tills in the outer parts of shelf troughs are overlain by up to 2 m of postglacial sediments, which are no older than the LGM in any core yet dated. A layer of soft, intermediate shear strength (12¬25 kPa) till, interpreted as deformation till, underlies the postglacial sediments in cores in the troughs. These observations are consistent with the interpretation that streaming ice extended along the troughs during the LGM, but the duration of such flow, and whether or not it spanned the entire period when ice extended to the outer shelf remains undetermined.To determine when, and how rapidly, ice retreated from the continental shelf, ages of core samples from near the base of postglacial sediments in several troughs have been determined by AMS radiocarbon dating. Samples to constrain glacial retreat have been taken from either the base of muds deposited in seasonally open-marine conditions similar to today, or underlying sandy muds interpreted as having been deposited close to the grounding line. Modern sea-floor sediments on some parts of the margin contain sufficient calcareous microfossils for dating to constrain the local marine 14C reservoir correction. However, even where they occur, contents of planktonic foraminifera decrease downcore, and most deglaciation ages have been obtained from acid insoluble organic material (AIOM). In some areas these ages are significantly affected by reworked fossil carbon, as shown by apparent ages from AIOM in modern sea-floor sediments that range up to ~6000 years. Thus radiocarbon results from this margin must be treated with caution and there is a clear need for development of alternative dating methods.Notwithstanding these uncertainties, deglaciation ages obtained thus far suggest variable retreat histories along the margin. Results from the Antarctic Peninsula shelf and Amundsen Sea embayment suggest relatively rapid post-LGM ice retreat from the outer and middle shelf, followed by slower Holocene retreat to the present day ice margin. However, initial results from the Bellingshausen Sea (Belgica Trough) suggest a slower, progressive retreat commencing about 25 ka (corrected radiocarbon years). These results show that local factors are important in controlling the rate of ice retreat, and this needs to be taken into account in numerical models that attempt to predict the dynamic behaviour of large ice sheets.

Description: The Amundsen Sea sector of the West Antarctic Ice Sheet (WAIS) is the most rapidly changing part of the Antarctic ice sheet and could have a significant impact on future sea level rise. However, sea level rise predictions in the recent Intergovernmental Panel on Climate Change Summary for Policymakers excluded the possible effects of future rapid dynamic changes in ice flow because ice sheet model predictions were not considered to be sufficiently reliable.One approach to testing and refining ice sheet models would be to examine their ability to reproduce ice sheet changes since the Last Glacial Maximum (LGM). Records of ice margin retreat and ice surface elevation change since the LGM also provide a context for recent changes. Until recently, however, knowledge of the chronology of change in the Amundsen Sea sector of WAIS since the LGM has been based on just seven radiocarbon dates from continental shelf sediment cores collected in 1999 on RV Nathaniel B. Palmer1. These dates indicated that there was already seasonally open water over the middle part of the shelf by 15,800 +/- 3900 radiocarbon years ago, and open water extended to within 100 km of the modern ice margin in Pine Island Bay before 10,150 +/- 370 radiocarbon years ago. Over the past 18 months we have obtained 34 new AMS radiocarbon dates on samples from 25 Amundsen Sea shelf sediment cores collected during research cruises in early 2006 on RRS James Clark Ross and RV Polarstern. Some dates are on carbonate (foraminifera) but most are on the acid insoluble organic fraction. Several dates are on modern surface sediment samples to evaluate the marine reservoir correction and the effect of reworked fossil carbon. We have also obtained the first surface exposure ages from the region by analysing cosmogenic isotopes in samples collected from sites accessed using helicopters operating from RV Polarstern. These ages provide the first data on long-term changes in surface elevation of ice in the Amundsen Sea sector of the WAIS.Our new radiocarbon dates, together with swath bathymetry data collected on the same cruises and some previous cruises2, confirm that the ice grounding line advanced to the continental shelf edge in the Amundsen Sea at the LGM. The retreat of the ice margin to its present position represents a loss of more than 150,000 km2 of ice sheet, i.e. more than 35% of the area that remains in the Amundsen Sea sector of the WAIS. Our new data are generally consistent with the timing of ice margin retreat suggested previously, and in the western part of the embayment most of the retreat to the present ice margin position was certainly complete by early Holocene time. Average rates of retreat and surface elevation change are more than an order of magnitude slower than those observed over recent decades, but we cannot discount the possibility that there might have been previous short-lived episodes of rapid change since the LGM.

Description: Understanding the past glacial history of regions undergoing potential rapid deglaciation is essential in order to estimate the possible threat of sea level rise. Recently acquired data have given new images of mega-scale glacial lineations on the sea floor of the Amundsen Sea, which provide us a new understanding of the direction of glacial flow on the continental shelf of the Amundsen Sea region. Two adjacent areas of seafloor on the outer shelf of the Amundsen Sea embayment exhibit remarkably different styles of glacial lineations, and allow the interpretation of a divergent glacial trough for the Pine Island Glacier during the last glacial maximum, whereas ice flow from the Abbot Ice Shelf probably converged with that from the Pine Island Glacier (PIG) to the north of a grounding zone wedge.

Description: The presence of a complex bedform arrangement on the sea floor of the continental shelf in the westernAmundsen Sea Embayment, West Antarctica, indicates a multi-temporal record of flow related to theactivity of one or more ice streams in the past. Mapping and division of the bedforms into distinctlandform assemblages reveals their time-transgressive history, which implies that bedforms can neitherbe considered part of a single downflow continuum nor a direct proxy for palaeo-ice velocity, as suggestedpreviously. A main control on the bedform imprint is the geology of the shelf, which is dividedbroadly between rough bedrock on the inner shelf, and smooth, dipping sedimentary strata on themiddle to outer shelf. Inner shelf bedform variability is well preserved, revealing information about local,complex basal ice conditions, meltwater flow, and ice dynamics over time. These details, which are notapparent at the scale of regional morphological studies, indicate that past ice streams flowed across theentire shelf at times, and often had onset zones that lay within the interior of the Antarctic Ice Sheettoday. In contrast, highly elongated subglacial bedforms on sedimentary strata of the middle to outershelf represent a timeslice snapshot of the last activity of ice stream flow, and may be a truer representationof fast palaeo-ice flow in these locations. A revised model for ice streams on the shelf capturescomplicated multi-temporal bedform patterns associated with an Antarctic palaeo-ice stream for the firsttime, and confirms a strong substrate control on a major ice stream system that drained the WestAntarctic Ice Sheet during the Late Quaternary.

Description: Existing landform models of palaeo-ice stream beds in Antarctica often portray a simple snapshot of former ice-basal conditions, but studies have rarely mapped the bedforms in detail. A better understanding of conditions at the ice-bed interface in palaeo-ice sheets is required because: (a) ice streams determine the discharge from large ice sheets, and (b) knowledge of past ice dynamics can be used to constrain predictions of future ice sheetbehaviour. We use an extensive (9950 km2) marine geophysical dataset, comprising multibeam swath bathymetry, sub-bottom and single-channel seismic reflection profiles, to map the geomorphological signature of a large palaeo-ice stream system in the western Amundsen Sea Embayment, West Antarctica. The bedform imprint of past ice streams comprises morethan 4000 elements, which we divide into five landsystem components: (1) a meltwater assemblage, (2) a composite assemblage of bedforms, (3) a sub-ice stream footprint, (4) grounding line retreat morphology, and (5) a pro-marginal deglacial group. Each group demonstrates different levels of overprinting and preservation, indicating a time-transgressive history for the inner shelf morphology, which implies that bedforms can neither be considered part of a single down-flow continuum nor a direct proxy for palaeo-ice velocity, as suggested previously. A main control on the geomorphology is the subglacial geology of the shelf, which is divided between rough bedrock on the inner shelf, and smooth, dipping, layered sediments on the mid-to-outer shelf. Inner shelf bedform variability reveals information about local, complex basal-ice conditions, meltwater flow, and ice dynamics over time, including detail not apparent at the scale of regional morphological studies. This detail leads us to conclude that past icestreams streamed across the entire shelf, and had onset zones that lay within the interior of the Antarctic Ice Sheet today. In contrast, highly-elongated bedforms on sedimentary strata reveal a timeslice snapshot of the last activity of ice streams on the middle to outer shelf, and may be a truerrepresentation of palaeo-ice velocity in these locations. A revised model for ice streams on the shelf captures multi-temporal bedform patterns associated with a West Antarctic palaeoice stream for the first time, and confirms a dominant substrate control on the flow and geomorphic imprint of this particular ice stream pathway.