RECONSTRUCTING CHANGES IN SEA ICE COVER DURING MARINE ISOTOPE STAGE 11 FROM BERING SEA SEDIMENT CORES

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 the long-term response of sea ice to a warming climate.

Sediments from Integrated Ocean Drilling Program (IODP) sites in the Bering Sea provide a record of paleo-sea ice extent during Marine Isotope Stage 11, an interval of prolonged warming that occurred from 424-374 ka, and is considered a good analogue for future climate change. In this work, laser diffraction particle size analyses of bulk and terrigenous sediments, and scanning electron microscope (SEM) imaging of quartz grains are used to describe variability in sea ice extent at four IODP sites across the Bering Sea (U1339, U1340, U1343 and U1345) during MIS 11. Sediment grain size has been used extensively as a proxy for the presence of glacial and sea ice, as both ice types are effective at carrying terrigenous and near-shore particles far from land. A pronounced decrease in the percentage of clay-sized grains at the onset of MIS 11 suggests that the transition from glacial to interglacial conditions was marked by rapid ice retreat. Despite this, the presence of coarse (>150 µm) grains, interpreted as ice-rafted material, indicates that ice remained present in some parts of the Bering Sea during MIS 11, even at peak interglacial warmth.

Both sea ice and icebergs are capable of transporting a comparable fraction of coarse terrigenous material, so that ice transport mechanisms cannot be reliably determined on the basis of grain size alone. However, St. John et al. (2015, Ann. Glaciol) showed that the surface features of glacially transported quartz grains differ from the surface features of grains transported by sea ice. SEM imaging will be used to quantify the relative importance of sea ice and iceberg rafting during MIS 11 in the Bering Sea, in order to improve our understanding of ice-rafting history in the region, as well as to document changes in sea extent during an interval of extreme warming. Initial data from site U1339 indicate that both glaciers and sea ice were important transport agents during MIS 11.