Marine Isotope Stage (MIS) 5 on the Umnak Plateau, Bering Sea (IODP Site U1339): Diatom taxonomy, grain size and isotopic composition of marine sediments as proxies for primary productivity and sea ice extent

The current rapid reduction of sea ice in the arctic has motivated numerous studies to observe how sea ice declines during times of climate warming and its impact on marine ecosystems. Marine Isotope Stage (MIS) 5, the last interglacial prior to the Holocene, is characterized as having higher summer air temperatures and sea level compared to today; however, there is a scarcity of data for how sea ice extent and ecosystems changed during MIS 5. The Umnak Plateau is not currently covered by sea ice due to the influence of the warm Alaskan Coastal Current entering through the eastern Aleutian Island passes; however, low-resolution studies from the Last Glacial Maximum (LGM) demonstrate that sea ice extended to the Umnak Plateau when sea level dropped and restricted flow of the Alaskan Coastal Current over the Umnak Plateau. This study uses a multi-proxy approach consisting of grain size, diatom assemblages, and isotopic analyses to determine how environmental conditions changed at the Umnak Plateau (IODP Site U1339) during MIS 5 as well as the end of MIS 6 and the beginning of MIS 4 (146ka -- 65ka), both of which are glacial periods.;The research presented in this thesis reveals that the glacials MIS 6 and MIS 4 are both characterized as having decreased primary productivity combined with increased nutrient utilization and increased terrestrial organic matter deposition, suggesting there may have been an extensive sea ice cover at the Umnak Plateau and a limited influence of the Alaskan Coastal Current. In contrast, MIS 5 is characterized as having higher primary productivity combined with decreased nutrient utilization and decreased sea ice extent. MIS 5e, the warm substage of MIS 5 that has been correlated with the Eemian Interglacial from terrestrial records, shows that decreased productivity at the Umnak Plateau may be related to an intensified stratification associated with increased warming of the surface waters that resulted from increased insolation and a prolonged summer season. Comparing the stable nitrogen isotope (delta15N) record with other sites in the North Pacific reveals noteworthy similarities between delta15N patterns during the warm substages of MIS 5 from the Umnak Plateau and from the Gulf of Alaska, the origin of the Alaskan Coastal Current. Thus, the delta15N record from the Umnak Plateau may be displaying changes relating to the source of the nitrates over the Umnak Plateau from a more western Bering Sea source during the cold substages of MIS 5 to being sourced from the Alaskan Coastal Current during parts of the warm substages of MIS 5.