Marine Isotope Stage (MIS) 5 on the Umnak Plateau, Bering Sea (IODP Site U1339): Using diatom taxonomy, grain size and nitrogen 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 look at how sea ice declines during times of climate warming and its impact on marine ecosystems. Marine Isotope Stage (MIS) 5 is the last interglacial prior to the Holocene and has been characterized as having higher summer air temperatures and higher sea level compared to today. However, there is a scarcity of data for sea ice extent during MIS 5. This presents an opportunity to reconstruct sea ice during a previous warming that may be used as an analogue for future change. We aim to provide insight into how sea ice changed throughout MIS 5 and how this change impacted primary productivity at the Umnak Plateau in the southeastern Bering Sea. This region is not currently covered by sea ice at all; however, low-resolution work reveals that sea ice did reach the study area during glacial intervals.

This study uses high-resolution (500 year) bulk geochemistry, diatom taxonomy, and grain size analysis of marine sediments from the Umnak Plateau as proxies for primary production and sea ice conditions during MIS 5. Nitrogen isotopic measurements of marine sediments are a valuable recorder of nitrate utilization by primary producers. Diatom taxonomy gives corresponding ecological affinities, which are relatable to sea ice coverage, productivity, and sea surface temperatures. Grain size analyses from previous studies have shown clay- and silt-sized biogenic content with well-preserved diatom valves to be associated with interglacials and larger silt to sand-size siliclastic particles with fragmented diatom valves to be associated with glacials. In this study, the silt fraction appears to be constant while the clay fraction follows the oxygen isotope record from Lisiecki and Raymo, 2005, increasing during glacial intervals and stadials. This apparent contradiction with earlier work warrants further investigation.