A TALE OF TWO RETREATS – COMPARING AND CONTRASTING PALEOENVIRONMENTAL CONDITIONS DURING THE ULTIMATE AND PENULTIMATE LGM RETREATS OF BERING ICE STREAM, GULF OF ALASKA

A stratigraphically complete, mid-Pleistocene to Recent record of advance and retreat of Bering Ice Stream (BIS) was recovered from the Gulf of Alaska slope at IODP Site U1421. Foraminifera, diatom, and geochemical analyses and a radiocarbon-dated, ice-rafted debris mass accumulation (IRDMAR) curve are used to test our hypothesis that differences in the two youngest advance/retreat sequences of BIS are in response to changing temperatures, sea level, and sea ice.

Each sequence lasts ~ 4,000 yr and starts with a shelf edge advance where clast-rich diamicts, with moderately preserved, low-diversity, foraminifera, formed by debris flows and turbidity currents. Subsequent retreat is marked by mud-rich diamicts with increased foraminiferal preservation, diversity, and abundance.

Penultimate BIS advance (26.6-25.2 Kyr) has consistently high, yet variable, IRD flux and rapid retreat by 21.1 Kyr. This sequence, formed when eustatic sea level was stable and 120 m lower than today, is dominated by Elphidium excavatum indicating nutrient-rich meltwater flow directly onto the slope. Numbers of Neogloboquadrina pachyderma (sin.) and Globigerina bulloides increase in response to nutrient input and reduction in IRD as BIS grounded on a moraine in Bering Trough.

Ultimate advance occurs from 20-19.2 Kyr with complete retreat from the sea by 15.5 Kyr. Sea ice related diatoms including Thalassiosira antarctica resting spores and Fragilariopsis spp. are present, as are open water diatoms such as Neodenticula seminae, Actinocyclus spp. and Thalassiosira spp. Foraminiferal biofacies are variable, reflecting eustatic sea level rise and warming after initial advance. Earliest retreat, with high, yet declining IRD, is dominated by E. excavatum. Between 17.2-17.6 Kyr reduced IRD, low total organic carbon, elevated dinosterol, and sea ice diatoms suggest phytoplankton communities responding to changing sea ice conditions. After retreat, sea ice diatoms are rare to absent with Chaetoceros resting spores or N. seminae dominating the diatom assemblages, indicating highly productive and/or North Pacific-influenced waters. From 13.5-12.5 Kyr an increase and then reduction of bolivinids/buliminds indicate expansion then constriction of the oxygen minimum zone (OMZ), coincident with expansion of OMZs across the North Pacific.