THE ASSOCIATION OF SUBGLACIAL MELTWATER ACTIVITY WITH ICE-SHEET RETREAT IN MARGUERITE BAY, ANTARCTICA

The ancestral Marguerite Bay Ice Stream was underlain by extensive networks of meltwater channels and basins whose activity may have played a role in past retreat episodes. Using existing geomorphological data and new sedimentological and radiocarbon data, we demonstrate evidence of meltwater activity during the Holocene that corresponds to large-scale grounding-line retreat. Our refined retreat chronology reveals that postglacial grounding-line retreat from the continental shelf edge first occurred sometime well prior to 14 cal ka BP, with the ice shelf retreating to the middle continental shelf by 13 cal ka BP. The grounding line then continued to form embayments through the deep portions of Marguerite Bay as the grounding line and ice shelf gradually retreated through 11 cal ka BP. Marine radiocarbon ages indicate that by 10 cal ka BP, inner Marguerite Bay was nearly ice-free, corresponding to established records of a 9.6 cal ka BP drawdown of nearby terrestrial ice. We identify fine, relatively well-sorted meltwater silt deposits located within the basins of the rugged bedrock on the inner and middle continental shelf, but not in cores on the shallower outer continental shelf. We attribute this distribution to the process of sediment being expelled from the grounding line at high enough velocities or sediment concentrations to prevent plumes from becoming hypopycnal. Most significantly, we observe these meltwater deposits to have occurred throughout the gradual deglaciation beginning at 13 cal ka BP until just prior to the major 10 cal ka BP ice retreat. We suggest these meltwater deposits may be a sedimentological marker of ice-sheet instability and propose Marguerite Bay provides a model for estimating the future of the unstable Pine Island/Thwaites Glacier system, which is also underlain by an active hydrological system that produces sediment-laden meltwater plumes today.