DEGLACIATION TO HOLOCENE SEA-ICE HISTORY AT UMNAK PLATEAU, BERING SEA AS INFERRED FROM DIATOM, ALKENONE, AND STABLE ISOTOPE RECORDS

Modern climate change in the Bering Sea has been characterized by rising sea surface temperatures (SSTs), an earlier spring and wide-spread, anomalous phytoplankton blooms. Sea ice is a defining characteristic of the Bering shelf. It controls the timing and location of the spring phytoplankton bloom and serves as habitat for many trophic levels in the Bering Sea from phytoplankton to large marine mammals. Climate models predict that as Arctic temperatures continue to warm, summer sea ice will disappear from the Arctic Ocean in the next century. Preparation for further change and the effects it will have on the Bering Sea and its fishery (the largest in the US) requires a detailed knowledge of how the system, especially sea ice, responds to rapid warming.

The transition from the Last Glacial Maximum (LGM) into the Holocene was the most recent period in Earth's history that the arctic experienced a rapid warming. During the LGM, the diatom assemblage at the Umnak Plateau was dominated by sea-ice taxa. Annual ice duration began to decrease approximately 18ka and the diatom assemblage shifted to one dominated by a species found in the relatively warm and saline Alaska Stream waters today. The U^'K₃₇ temperature index derived from alkenones shows two distinct, but minor temperature increases (1ºC: 17.8-17.5ka and 16.3-14.9ka) synchronous with 5ºC increases in North Pacific SSTs. Foraminiferal δ18O also become depleted 15.3ka. A fresh water pulse is reflected in the diatom assemblage, while the U^'K₃₇ points to a warming at this time. During the deglaciation, periods of high diatom productivity occurred, while alkenones record large coccolithophorid blooms occurring when both insolation and SSTs are high and diatom production low. Benthic and planktic foraminiferal δ13C verified these periods of high productivity during the deglaciation. A more dramatic (3ºC) warming in the Bering Sea is seen during the Holocene Thermal Maximum (~11.3ka). Throughout the deglaciation, changes in alkenone concentration and U^'K₃₇ are positively correlated with changes in the Sea of Okhotsk indicating that productivity and SST changes were basin wide in the North Pacific. Although SSTs increased in rapid pulses of 1kyrs or less, diatom assemblages indicate that ice retreated slowly over a period of almost 7kyrs and was absent from the basin by 11.3ka.