UC Santa Cruz

Abstract The extreme warmth of Marine Isotope Stage (MIS) 11 has been extensively studied in the terrestrial realm; however, less is known about its expression in the North Pacific. Here, we present a multiproxy record from Integrated Ocean Drilling Program (IODP) Site U1339 in the southeastern Bering Sea (Umnak Plateau). We use sedimentology, diatoms, and organic geochemistry to reconstruct sea ice and primary productivity from the end of MIS 12 to the beginning of MIS 10 (430–368 ka) at this site. In late MIS 12, the Umnak Plateau experienced extensive seasonal sea ice cover, with spring sea ice concentration near 100%. Based on diatoms and ice-rafted debris, sea ice declined to year-round ice-free conditions during deglaciation but readvanced during the interglacial climatic optimum. In contrast, sea ice has not been present at the Umnak Plateau throughout the Holocene. During MIS 11, sea ice at this site may have persisted due to a more easterly and/or weaker Aleutian Low. Laminated sediments indicate enhanced seasonal productivity during deglaciation. However, productivity does not remain high throughout the interglacial. The decline in productivity during MIS 11 occurs independently of fluctuations in sea ice and upwelling and may be due to increasing nutrient limitation.