On the influence of post-LGM ice shelf loss and grounding zone sedimentation on West Antarctic ice sheet stability

The Whales Deep Basin in eastern Ross Sea was occupied by the paleo-Bindschadler Ice Stream during the Last Glacial Maximum (LGM). Previous studies showed that megascale glacial lineations (MSGLs) extend to the continental shelf edge. In a landward direction, these lineations are buried by an overlapping stack of backstepped grounding zone wedges (GZWs). The constraints require that the West Antarctic Ice Sheet (WAIS) grounded at least seven times within 60 km of the shelf edge but the morphological relationships in the Whales Deep Basin cannot be used to uniquely reveal the magnitudes of either grounding line and/or calving front retreat and re-advance between the successive groundings. Here we show a new regional transect of cores from the basin which demonstrates that there were no major back and forth oscillations of either the grounding line or calving front during the overall retreat of grounded and floating ice. Instead, the new sedimentologic data show that a small ice shelf formed as the grounding line retreated from the shelf edge and backstepped over the outer continental shelf. An event bed records breakup of the ice shelf that covered the outer shelf. The stratal relationships indicate that the ice-shelf collapse occurred during the fourth grounding and that the grounding line remained on the outer continental shelf for three additional groundings. We infer that accelerated ice stream flow (due to loss of buttressing) increased sediment flux to the grounding line and thinned the WAIS at the grounding line. Following the seventh grounding, thinning contributed to an abrupt 200 km retreat of the grounding line to the middle continental shelf and a large ice shelf re-formed over the middle continental shelf. A series of small-scale moraine ridges formed as the grounding line shifted south towards Roosevelt Island. The facies relationships on the middle continental shelf indicate that the calving front then shifted abruptly to its modern position. The ground-truth of the seafloor geomorphology presented here is important because it is a necessary framework for constraining retreat chronology for a central part of the WAIS that was not influenced by East Antarctic Ice Sheet overflow.