The grounding-zone wedge inventory on the Amundsen Sea Embayment shelf, West Antarctica: formation processes and significance for establishing reliable post-LGM retreat chronologies

Abstract:

Grounding-zone wedges (GZW) have been mapped on the sea floor in various sectors of the formerly glaciated continental shelf around Antarctica. In most cases, these wedges record periods of grounding-line stillstands during ice-sheet retreat following the Last Glacial Maximum (~26-19 ka BP; kiloyears before present). The presence of GZWs along the axis of a palaeo-ice stream trough therefore indicates a style of episodic retreat of the grounding line from its LGM to modern position. However, precise chronological constraints for both the onset and duration of these stillstands are still lacking. Consequently, the role of grounding-zone wedge formation in modulating post-LGM ice-sheet retreat cannot be reliably quantified. This information is vital, however, for calculating reliable retreat rates during the past, which are essential for evaluating and understanding the significance of modern retreat rates. Here we present the currently known inventory of GZWs on the continental shelf of the Amundsen Sea Embayment, West Antarctica. The glaciers draining into this embayment are retreating at exceptionally high rates compared to the ice sheet as a whole. We will discuss geological preconditions at the ice sheet bed that led to GZW formation, and draw possible relations between GZWs in two neighbouring outer shelf palaeo-ice stream troughs that are separated by an inter-ice stream ridge. Furthermore, we will present our approach how to i) reliably date the onset of stabilization periods, and ii) constrain the actual duration of stillstands. This knowledge will help refine available post-LGM retreat chronologies, which, in turn, serve as a basis for validating and improving ice-sheet models in an area where these are urgently needed.