Maximum extent and subsequent retreat of the grounding line from the Mac. Robertson Shelf (East Antarctica) during and since the Last Glacial Maximum and its implications for Antarctic Bottom Water formation

The future behaviour of the Antarctic Ice Sheet is considered as one of the largest unknowns in global climate predictions and dramatically accelerated ice loss has been observed over the past few decades for numerous of its 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 build-up and retreat. The deglaciation history of the East Antarctic sector is largely understudied compared to the West Antarctic margin. This emphasizes the urgent need for reliable long-term spatiotemporal data of mass balance change, particularly for sectors along the East Antarctic margin that play key roles in supplying the world’s oceans with dense bottom water. Marine ice sheet dynamics are strongly influenced by interactions between ocean, ice, and bedrock, which so far remain poorly understood along the East Antarctic margin. Here, we performed a multi-proxy analysis on numerous sediment cores recovered from two prominent glacial cross-shelf throughs on the Mac. Robertson Shelf. Combined sedimentological, sediment-physical, and geochemical analysis as well as radiocarbon dating of calcareous foraminifers reveal the onset of deglaciation on the Mac. Robertson Shelf and the subsequent retreat of the grounding line (GL). Additionally, we analyzed submarine glacial landforms on the shelf along both troughs from combined multibeam swath bathymetry and sub-bottom profiler data, providing new evidence on initial GL retreat and the pattern of its subsequent retreat. Our study reveals a retreat at or shortly after the Antarctic Cold Reversal ~12,5 cal. kiloyears before the present (cal. ka BP), it did not contribute to meltwater pulse (MWP) 1A but may have contributed to MWP-1B. Glacial bedforms indicate an episodic retreat of the ice sheet’s GL starting with a slow retreat on the outer shelf, accelerating towards the retrograde mid shelf part. At the mid shelf, the retreat underwent a further stagnation leading to the formation of two small grounding zone wedges. A mid-shelf bedrock sill likely acted as a pinning point representing an additional ice sheet stabilization event. We conclude GL advance to the continental shelf break until ~12.5 cal. ka BP. This maximum position implies the prevention of dense shelf water formation 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.