DYNAMICS OF THE BERING GLACIER, ALASKA FROM LITHOFACIES, ICE RAFTING, AND FORAMINIFERAL RECORD AT IODP SITE U1421

A major goal of IODP Expedition 341 is to assess the glacial history of the Northwestern Cordilleran ice sheet (CIS) in the Pacific. Site U1421 was drilled 10 km from the Bering Trough on the continental slope in a water depth of 721 m. This near shore location, proximal to the Bering Glacier ice stream, received a large influx of glacimarine sediment and ice rafted debris (IRD). A total of 696 m of sediment was drilled, however, core recovery ranged from 100% within the upper 88 m to less than 10% between 100 and 400 m. We analyzed 314 samples from muddy lithofacies to produce an IRD curve based on the coarse sand fraction (250 microns-2mm). Visual inspection of each sample confirmed the presence of terrigenous sand. In addition, 70 core catcher samples were examined for foraminifera. Core sections with high recovery include the following lithofacies: diatom/radiolarian ooze and diatom-rich mud with abundant foraminifers, including planktics and hypoxic, bathyal benthics, mud with common to abundant clasts and clast-poor diamict containing neritic and upper bathyal foraminifera. The sections with low recovery included washed pebbles, drilled rock, and clast-rich diamict. A chronology developed from planktic foraminifera-based radiocarbon and stable isotopes extends to Marine Isotope Stage (MIS) 5. High recovery intervals correspond with MIS 1 and 2. Cores collected within MIS 5 had moderate recovery and MIS 3 and 4 occur within the interval of low recovery. The lack of muddy lithofacies within MIS 3 and 4 suggests that this period is dominated by debrites and turbidites resulting from downslope transport originating from an advanced position of Bering Glacier, proximal to the drill site. Diamict could also be deposited by ice rafting as mud from turbid meltwater plumes bypassed Site U1421. Retreat of the Bering terminus during interstadials is suggested by intervals of low IRD, hypoxic benthic foraminifera and diatom ooze. Northwestern CIS deglaciation is shown by a gradual drop in IRD from 16.9 to 14.9 ka yrs B.P.