A multi-proxy reconstruction of sea ice extent and primary productivity during Marine Isotope Stage 11 in the Bering Sea

The ongoing decline in Arctic sea ice extent has prompted concerns about the fate of sea ice in the future, and the stability of sea ice-dependent ecosystems. By studying the natural variability of sea ice cover during past warm intervals, we can better understand how sea ice might respond to future warming. Sediments from Integrated Ocean Drilling Program (IODP) sites in the Bering Sea provide a record of paleo sea ice extent and productivity during Marine Isotope Stage (MIS) 11 (424-374 ka), an interval of prolonged warming that represents a close analog to the Holocene with respect to orbital parameters. The purpose of this dissertation is twofold; firstly, to examine the effectiveness of sediment grain size as a proxy for the sedimentary history of the Bering Sea, and secondly, to reconstruct variations in sea ice extent and productivity in the Bering Sea during the MIS 12-10 cycle. We show that, with some caveats, grain size parameters, including the volume percent of grains in various size fractions, as well as the statistical measures of mean grain size, sorting, and skewness, can be used to infer past changes in sediment transport, ice rafting and productivity at three core sites in the Bering Sea. We present a new, high resolution, multi-proxy record of MIS 11 from IODP Site U1339 (Umnak Plateau, southeastern Bering Sea), based on sediment grain size, diatom assemblages, stable isotopes, and a new diatom-based proxy for sea ice concentration. The record shows that sea ice was more extensive during MIS 11, in comparison to the Holocene, and advanced over the plateau during an interval of high sea level and high global temperatures. As sea ice declined during deglaciation, there was a spike in productivity, but sea ice and productivity trends are not correlated during MIS 11. We then compare the Umnak Plateau record to new and published records from the Bering slope (sites U1345 and U1343). Sediment grain size, diatom assemblages, and the diatom proxy show that sea ice was present at the slope sites during MIS 11, but there is evidence for considerable regional variability in the timing of ice advance and retreat between the Umnak Plateau and the Bering slope. This variability may be explained by east-west differences in sea level pressure and wind direction, which are controlled by shifts in the geographic position of the Aleutian Low