Quaternary West Antarctic Deglaciations (QWAD) is a component project of the Glacial Retreat in Antarctica and Deglaciation of the Earth System (GRADES) research programme, part of the British Antarctic Survey research strategy Global Science in an Antarctic Context (GSAC) 2005–2009
Current predictions of sea level rise do not correctly quantify contributions from Antarctica. Some sectors of the Antarctic ice sheet are showing considerable rates of change; are these changes a forerunner of imminent ice-sheet collapse? The onshore and offshore records of deglaciations in the geologically recent past are key to understanding the stability and climate sensitivity of ice sheets. We have already shown the value and precision of the glacial record in marine sediments. In the next five years the GRADES-QWAD project will integrate further high-resolution marine studies with onshore records of deglaciation provided by studies of past sea level changes and dating of the times at which mountain peaks became exposed due to thinning of the ice sheet. Our studies will employ advanced radiocarbon and cosmogenic dating techniques. As constraints for computer models of ice sheet behaviour, well-dated deglaciation histories for West Antarctica will greatly improve predictions of future sea level change.
We propose to investigate five aspects of deglaciation:
1) the baseline - how large was the glacial maximum ice sheet?
2) thinning of the ice sheet, recorded by formerly buried and/or eroded rock outcrop.
3) retreat of the ice sheet, recorded by marine sediments.
4) relative sea level change at the periphery of the ice sheet, recording the response to glacial unloading and global sea level change.
5) evidence for previous collapse of the ice sheet during the past few hundred thousand years.
To set possible future sea-level change in the context of past environmental change, we will produce a unified history of glacial retreat in the West Antarctic ice sheet (WAIS) and the Antarctic Peninsula ice sheet (APIS). A key strength of the GRADES programme is the complementary use of data and modelling. Synthesis of the various datasets will be achieved in the GRADES-IMAGE project by running inverse models of the ice sheet constrained by published data already collated in the BAS-led Antarctic glacio-geological database, and by new data collected by GRADES-QWAD.
At glacial maximum, ice sheets buried almost the entire land surface of Antarctica and extended across the continental shelf, depositing sediment on the continental shelf, slope and rise. Sea level was some 120-135 m lower than today, with 12-26 m of this locked up in the Antarctic ice sheet. Since glacial maximum, the ice sheet has thinned by hundreds of metres in some areas and retreated inland as much as 1000 km, leaving its imprint on mountain ranges and on the seabed, and a detailed history in marine sediments. Deglaciation of the Antarctic ice sheet may have been master or slave to the deglaciation of Northern Hemisphere ice sheets, but it probably contributed to a rapid rise in sea level and to substantial changes in the forcing of ocean circulation. The QWAD project will address deglaciations and sea level, while ocean circulation forms part of two other BAS programmes.
While computer models of ice sheets include increasingly more realistic physics, all such models of complex Earth Systems can only be shown to be robust predictors if they accurately simulate present and past behaviour. In particular, models of the future evolution and possible collapse of the WAIS are severely limited by the lack of a coherent history of the initiation and course of past deglaciations. There are few parts of Antarctica where a reliable chronology of retreat of the grounding line since the Last Glacial Maximum (LGM) has been established, though evidence is accumulating that maximum global ice volume (the LGM at 18-21 ka) did not coincide with maximum lateral ice extent everywhere. There is a similar lack of well-dated observations of changes in the extent, thickness and flow pattern of the ice sheet during previous glacial periods, which have occurred approximately every 100,000 years since 800,000 years ago. The strongest evidence that the present-day WAIS may be prone to collapse would come from a record of similar events in the past. Some scientists have interpreted data from sub-glacial and marine sediments as suggesting that part of the WAIS may have collapsed at least once during the last 600,000 years, though others dispute this.
Delivering the Science
We will focus on two key areas, the WAIS and the APIS, which have long been considered particularly susceptible to change. They have sectors that are currently changing significantly in extent (APIS ice shelves) and thickness (Pine Island Glacier), and are considered to be a potential source of dramatic sea level rise. We will concentrate on the most recent deglaciation, as this has left the most complete onshore and offshore records, and on a particularly warm interglacial period about 400,000 years ago ('Marine Isotope Stage 11') that is the most likely time for previous ice sheet collapse.
How large was the glacial maximum ice sheet?
Glacial geological techniques will provide direct evidence of past maximum ice thickness. These will involve examination of boulders transported by ice ('erratics'), glacially-striated bedrock, glacial sediment deposits ('moraines'), and erosional limits on isolated peaks within the icesheet ('nunataks'). We will focus on nunataks in the Pine Island and Thwaites glacier basins of the WAIS. The length of time that rocks have been exposed on the surface to cosmic rays will be determined by precise analysis of the concentration of cosmogenic isotopes.
Offshore, surveys using advanced sonar systems (multibeam echo sounder and parametric sub-bottom profiler) on RRS James Clark Ross, together with high resolution seismic profiles and sediment cores, will be used to define the former lateral extent of the glacial maximum ice sheet. These data will also be used to determine the former distribution of fast-flowing glaciers ('ice streams') on the continental shelf, and to understand controls on their location and flow mechanisms. Most ice discharge from the Antarctic ice sheet occurs through ice streams, so to predict future behaviour of the ice sheet it is important to understand how they operate. We will focus surveys on the area offshore from the Pine Island and Thwaites glacier basins, but will also compile existing and opportunistically collected data from other parts of the Antarctic Peninsula and West Antarctic continental margins.
Thinning and retreat of the ice sheet
We will determine the history of deglaciation using both the marine record (lateral retreat) and the onshore record (vertical thinning, i.e. elevation history of the inland ice surface). Geomorphological studies can provide direct evidence of post-glacial thinning; we aim to identify altitude-related sequences of features such as striated bedrock and moraines, and use cosmogenic dating to determine the time of ice thinning down the sequence.
The marine record of deglaciation on the shelf includes subglacial sediments, transitional sediments reflecting movements of the ice grounding line, and postglacial open marine sediments deposited after grounding line retreat. We will use radiocarbon dating to determine the time of onset of marine conditions, dating either organic carbon or the calcium carbonate tests of marine microfossils (foraminifera) where they are present.
Relative sea level change
Sea level at any locality around Antarctica reflects the interplay between changes in global ocean volume (eustasy) and local changes in elevation in response to changes in the load of ice on the Earth's crust in the surrounding region (isostasy). Since past eustatic sea level change is now reasonably well known, a record of past variation of sea level at any location in Antarctica can be used to infer past changes in ice volume close to that location. We will examine raised shorelines and isolation basins (lakes that were formerly inlets of the sea) to determine relative sea level changes on the west side of the Antarctica Peninsula since the last glacial period, and will use the resulting data to constrain changes in the volume of the APIS.
Evidence for previous collapse of the ice sheet
During the last glacial period, sudden episodes of deglaciation affecting the Laurentide ice sheet (on North America) released large numbers of debris-laden icebergs to the North Atlantic, producing widespread ice-rafted debris (IRD) layers ("Heinrich layers"). However, such widespread layers of IRD are virtually unknown in age-equivalent sediments in the Southern Ocean. The Pine Island and Thwaites glacier basins are presently the most dynamic part of the WAIS. We will collect sediment cores along a transect running northwards from the continental margin offshore from these glacier basins to the Polar Front (the most important oceanographic boundary in the Southern Ocean that surrounds Antarctica). We will identify all IRD layers in these cores and determine their lateral extent, age and provenance.
Sedimentation rates generally decrease with distance from the Antarctic continental margin, so several hundred kilometers from the margin it is possible to obtain sediment cores that contain a record extending back more than 400,000 years. Therefore, we will collect additional sediment cores along a transect parallel to, and several hundred kilometers north of the Pacific margin of Antarctica, to compare the IRD record of the five most recent interglacial periods. These IRD records will allow us to assess the hypothesis that there was a collapse of the WAIS during Marine Isotope Stage 11 or one of the subsequent interglacial periods.