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Despite the importance of grounding zone sedimentation for ice sheet stability and ice sheet history, evidence for sedimentary deposits beneath West Antarctica's modern grounding zone remains sparse. Seismic surveying shows that Kamb Ice Stream has no detectable grounding zone deposit. As grounding zone deposition relies strongly on ice flow, the absence of a deposit suggests that the transition from the ice stream to the ice shelf has moved after stagnation of Kamb Ice Stream. Further support for a recent grounding zone occupation comes from satellite imagery of sub-ice shelf channel features that likely originated at previous grounding-zone locations. These features begin 25km seaward of the current grounding zone and cut across ice flow streak lines. We estimate that retreat to the modern grounding-zone position was abrupt at rates >0.2km a(-1).

Plain Language Summary The location where an ice sheet goes afloat and becomes an ice shelf is termed the grounding zone. This zone can be thought of as the ice sheet's coastline, and its position determines how much the ice sheets contribute to global sea level. The flow of ice sheets, and the melt of the bottom of ice shelves, result in sediment being deposited at the grounding zone. This sediment can stabilize the ice sheet margin and provides a record of where and how long the ice sheet has occupied a certain location. Here we use acoustic surveying methods to examine the grounding zone of Kamb Ice Stream in West Antarctica. We find that the grounding zone migrated to its current position within the last 150years, indicating that this part of the ice sheet is more dynamic than previously thought.