Last Glacial Maximum extent and subsequent retreat of the East Antarctic Ice Sheet from the Mac. Robertson Shelf

Abstract. The future behavior of the Antarctic Ice Sheet is considered to be one of the largest unknowns in global climate projections, with dramatically accelerating ice loss being observed over the past few decades in multiple drainage basins. However, those records only reflect a short moment of limited informative value when considering the length of a full cycle of ice sheet expansion and retreat. East Antarctica’s deglaciation history remains largely understudied compared to the West Antarctic margin. This emphasizes the urgent need for reliable long-term spatiotemporal data on ice sheet change, particularly for sectors that play key roles in supplying the world’s oceans with dense bottom water. In this study, we performed a multi-proxy analysis on a set of sediment cores recovered from two prominent glacial cross-shelf troughs on the Mac. Robertson Shelf. We classified submarine glacial landforms on the shelf along both troughs from combined multibeam swath bathymetry and sub-bottom profiler data to infer ice sheet retreat dynamics. Additionally, combined sedimentological, sediment-physical, and geochemical analyses, including radiocarbon ages of foraminifera, reveal the onset of deglaciation on the Mac. Robertson Shelf and the subsequent retreat of the grounding line (GL). Glacial bedforms indicate an episodic retreat of the ice sheet’s GL, which started with a slow retreat on the outer shelf and accelerated towards the retrograde mid-shelf part. Across the mid-shelf, the GL repeatedly halted, leading to the formation of several small grounding-zone wedges. A mid-shelf bedrock sill likely acted as a pinning point, indicating an additional grounding line stabilization period. Our study reveals that the grounding line advanced to the continental shelf break prior to ~12.5 cal. ka BP, which prevented the formation of dense shelf water on the Mac. Robertson Shelf in its current form and, therefore, suggests either an absent or a different formation mechanism of Antarctic Bottom Water under full glacial conditions. Further, we conclude a retreat at or shortly after the Antarctic Cold Reversal ~12,5 calibrated kiloyears before the present (i.e., 1950 CE, cal. ka BP) from the outer shelf.