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Detailed marine geological studies of the post-LGM history of the West Antarctic Ice Sheet (WAIS) have been conducted in the Ross Sea and Pine Island Bay. Both areas yield a record of episodic grounding line retreat from the continental shelf. In the Ross Sea the ice sheet advanced to the shelf margin during the LGM and began to retreat from the outer shelf soon after the post-LGM sea-level rise began. Retreat of the grounding line across the western shelf, where ice was debouching from East Antarctica, was fairly continuous. In contrast, grounding line retreat from the eastern shelf was episodic and diachronous between paleo-ice streams. This is attributed to variations in ice stream sliding velocities that were, in turn, controlled by differences in the geology of the bed and physiography of the sea floor encountered by the retreating grounding line. The record in Pine Island Bay is poorly constrained, but does show that the ice sheet had retreated from the shelf by 16 ky and from Pine Island Bay by 10 ky.
During the LGM the Antarctic Peninsula Ice Sheet (APIS) extended across the continental shelf on both sides of the Peninsula. Retreat from the continental shelf began immediately following the LGM and associated rise in sea level between 18 and 15 ky. Two relatively short duration steps, whose timing coincides with MWP-1A and MWP-1B, punctuated retreat from the outer continental shelf. This indicates that sea-level rise was the principle driving force behind the retreat of the ice sheet from the outer shelf. In the South Shetland Islands, the ice sheet retreated from the inner shelf and bays by 14 ky, some 2000 years prior to grounding line retreat from the rugged inner shelf on the mainland peninsula. There, decoupling of the grounding line from the seafloor occurred in diachronous fashion until approximately 9 ky. By 8 ky the bays and fjords of the Peninsula were mostly free of grounded ice. At no time since then has ice been grounded at water depths below approximately 200 meters in any of the bays and fjords of the Peninsula.
Deep troughs with mega-scale glacial lineations mark the positions of paleo-ice streams that drained the WAIS and IPIS during the LGM. This implies a low profile for these ice sheets, at least during the late stages of the LGM, making them especially sensitive to sea-level rise. The timing of grounding line retreat from the continental shelf supports under-penning of the ice sheets by rising sea level. Between 12 ky and 8 ky, grounding line retreat across the rugged bedrock of the inner shelf of the Antarctic Peninsula region was diachronous. There is evidence that subglacial meltwater may have contributed to grounding line instability of both the Marguerite and Pine Island paleo-ice streams.
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