SEA ICE EXTENT AND PRIMARY PRODUCTIVITY ON THE UMNAK PLATEAU, BERING SEA, DURING MARINE ISOTOPE STAGE 11

The current rapid decline in Arctic sea ice extent has prompted questions about how sea ice will respond to a warming climate. Sea ice exerts a fundamental control on primary productivity in the oceans, so as Arctic sea ice continues to decline, another major concern is how productivity will respond. Sediments from IODP Site U1339 in the Bering Sea afford the chance to examine changes in sea ice and productivity during a past interval where climate warmed dramatically, the transition to the ‘super-interglacial’ Marine Isotope Stage (MIS) 11 at ~424 ka, which is considered a good analogue for future change. Because of its location as a gateway between the Pacific and Arctic Oceans, the Bering Sea records large seasonal variability in sea ice extent and is a highly productive region of the world’s oceans. Accordingly, Site U1339, located on the Umnak Plateau in the eastern Bering Sea, is well situated for the study of past variability in sea ice and associated ecological changes.

This work adopts a multi-proxy approach (sediment grain size, diatom assemblages and stable isotopic analyses) to examine the relationship between sea ice and productivity at the Umnak Plateau during MIS 11. The presence of coarse (>150μm) grains and clasts interpreted as ice-rafted material indicates that sea ice was consistently present in some parts of the Bering Sea during MIS 11. Furthermore, an increase in the relative percent abundance of sea ice-associated diatoms following deglaciation suggests that sea ice persisted in the Umnak Plateau region even during peak interglacial warmth. Laminated sediments at the boundary between MIS 12 and 11 point toward intervals of enhanced seasonal productivity during deglaciation. High productivity during deglaciation is characterized by an increase in organic and inorganic carbon, and also by a significant increase in Chaetoceros resting spores, a diatom species associated with high productivity. An increase in sedimentary δ¹⁵N values during laminated intervals suggests that deglaciation may also have been characterized by more complete surface nitrate utilization. In addition, the onset of MIS 11 is marked by an increase in the relative abundance of the diatom Neodenticula seminae, which suggests a greater influence of warm Alaskan Stream water at the site.