SEA ICE ACROSS THE BERING SEA SINCE THE LAST GLACIAL MAXIMUM

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Several factors influence sea ice extent in the Bering Sea. Sea ice is spread by prevailing winds across the shelf. Years with decreased sea ice coincide with winter cyclonic storms entering the Bering Sea from the south and effectively pushing sea ice to the north. Consequently, when the Aleutian Low intensifies, sea ice extent is reduced, and when it is diminished, cold, continental and/or Arctic air masses over the Bering Sea result in increased sea ice. Today sea ice is largely constrained to the Bering Sea shelf because heat flux from the ocean melts the ice when it is pushed over deeper waters. This process is magnified by the Bering Slope Current which runs along the shelf-slope break and carries relatively warmer water to the western Bering Sea. Here we evaluate past sea ice during the marine transgression from the last glacial maximum (LGM) through Holocene to determine the pattern of sea ice decline and evaluate the influence of oceanic and atmospheric forcings. We examine 18 new and previously published records of sea ice diatoms, a diatom-based quantitative proxy, the molecular biomarker, IP25, and ice rafted debris. During the LGM, when low sea level cut off the southeastern Bering Sea from the Pacific, much of the deep Bering Sea was covered with extensive sea ice. Two sites at Bowers Ridge had brief excursions of sea ice during the glacial period but remain ice free otherwise. As sea level rose, sea ice decreased in the Bering Sea, starting at about 15 ka. At Shirshov Ridge in the western Bering Sea, sea ice disappeared during the Bølling-Allerød (14.7-12.9 ka) warm period and re-advanced during the Younger Dryas. In contrast, sites along the Bering Sea shelf/slope break and Umnak Plateau in the eastern Bering Sea all have evidence of continued sea ice during the Bølling-Allerød and during the Younger Dryas; most do not show an advance of sea ice during the Younger Dryas. This pattern of diminished sea ice in the west may be attributed to a combination of a weakened Aleutian Low allowing sea ice to expand southward and an intensified Bering Slope Current bringing warmer waters into the western Bering Sea after the Alaskan Stream breached the eastern passes in the Aleutian Island chain. Sea ice disappears from the deep Bering Sea between 10 and 11.7 ka.