AUTOGENIC CONTROLS PROLONGED LAST GLACIAL MAXIMUM GROUNDING LINE RETREAT OF THE BERING ICE STREAM

The Cordilleran Ice Sheet discharged to the NE Pacific as ice streams cut troughs across the Alaskan shelf at the Last Glacial Maximum (LGM). We have a radiocarbon dated, high-resolution sedimentary record produced by fluctuations of the Bering Ice Stream (BIS) on the continental slope, 10 km from the mouth of the Bering Trough. IODP Site U1421 in 721 m water depth recovered proglacial deposits recording advance and retreat of the BIS grounding line without erosional hiatuses typical of shelf sites. The grounding line is inferred to have advanced to the shelf break between 26.6-25.2 Kyr and 20-19.2 Kyr, marked by low core recovery with inferred debris flows and turbidites initiated by the grounded terminus at the top of the slope. These lithofacies correlate with chaotic, unstratified, low-amplitude seismic facies. Core recovery increased dramatically within lithofacies recording terminus retreat from the shelf edge: mud with varying amounts of ice rafted debris (IRD) was deposited producing seismically stratified units. Iceberg flux recorded by IRD mass accumulation rates (MAR) coupled with foraminiferal and diatom abundance indicates the timing and rate of BIS retreat. In this temperate and active tectonic setting, two retreats, each lasting over 4000 years, were modulated by autogenic controls regulated by high sediment flux produced by glacial erosion. Several large, short duration IRD MAR peaks followed by an abrupt drop signal sudden grounding line collapse and restabilization over a 200-year period. Based on modern tidewater glacier analogs, BIS grounding line collapsed in deep water with later prolonged retreat caused by stabilization on pinning points where morainal banks were deposited on tectonic highs within the trough. In addition, IRD MAR was suppressed from 17.2-17.6 Kyr by the formation of perennial coastal sea ice indicated by low, presumably terrigenous-derived organic carbon, presence of sea ice diatoms, and a reduction in abundance of Elphidium excavatum, a meltwater indicator species. Elevated concentrations of the phytoplankton biomarker dinosterol, however, may indicate enhanced primary productivity along the ice-edge or formation of polynya at the drill site. BIS retreated completely on land by 15.5 Kyr coinciding with rapidly warming sea surface temperatures and sea level rise.