Effects of sea-level, sea-ice extent, and nutrient availability on primary production at the Umnak Plateau, Bering Sea (IODP Site U1339) during Marine Isotope Stage (MIS) 5

Abstract

This study investigates how primary production responded to changes in nutrient availability, sea-level, and sea-ice coverage at the Umnak Plateau during Marine Isotope Stage (MIS) 5 and parts of MIS 4 and MIS 6. The Umnak Plateau during MIS 4 and MIS 6 had low primary production resulting from extensive sea-ice coverage that enhanced stratification and limited nutrient replenishment. This nutrient limitation was relieved during the deglaciation going into MIS 5 when previously exposed continental shelves were flooded and the eastern Aleutian Island passes were opened by an increase in sea-level, resulting in an increase in productivity. Opening and closing of the eastern Aleutian Island passes during the rest of MIS 5 reflected fluctuations in sea-level and likely affected nutrient sources and productivity at the Umnak Plateau. Nutrients during sea-level lowstands were likely sourced by the Aleutian North Slope Current, whereas most of the sea-level high stands throughout MIS 5 had nitrogen sourced by an increase in nitrogen fixation and the assimilation of remineralized nitrogen, which led to highly depleted nitrogen isotope values never before seen for the northern Pacific. Higher sea-level and productivity near the beginning of MIS 5 likely reflected MIS 5e; however, a discrepancy between two diatom productivity proxies suggests that changing environmental conditions stimulated a change in the dominant diatom species. This highlights the complexities associated with productivity in the Bering Sea and emphasizes the need to continue studying how productivity and ecosystems changed during previous warm periods.