SEA ICE AND PRIMARY PRODUCTIVITY IN THE GULF OF ALASKA DURING RETREAT OF BERING ICE STREAM FROM THE LAST GLACIAL MAXIMUM

During the Last Glacial Maximum, the Bering Ice Stream (BIS) extended from the northwestern Cordilleran ice sheet, filling the Bering Trough across the continental shelf in the Gulf of Alaska. The Integrated Ocean Discovery Program’s (IODP) Exp. 341 provided the opportunity to examine the advance and retreat of BIS across several glacial-interglacial transitions. Here we focus on the diatom record at Site U1421 from 27 ka to present as the BIS retreated.

The BIS retreated in several pulses with intermittent advances. Diatoms are nearly absent from the sediments during advances. Initial retreat includes ice-rafted debris, dominated by glacially transported quartz grains, and foraminiferal fauna indicative of glacial meltwater. Rapid retreat occurs prior to 26.6 ka, between 21.5 and 20.5 ka, and between 18.1 and 15.5 ka. These phases are dominated by diatoms indicative of spring sea ice such as Thalassiosira antarctica resting spores (RS) and Fragilariopsis cylindrus. All include an increase in quartz grains transported by sea ice. The youngest of these 3 retreats is also coincident with a peak in dinosterol, a molecular biomarker for phytoplankton, and an increase in diatom accumulation rate by four orders of magnitude.

A quantitative, diatom-based proxy for spring (March, April, May, June) sea ice, developed for the Bering and Chukchi seas, was applied to this diatom data. Surprisingly, it shows low sea ice concentration during the youngest retreat and a peak in spring sea ice immediately after complete retreat of the BIS (15.5 ka). At this point, the diatom assemblage has shifted from sea ice related species to one dominated by Chaetoceros RS. Chaetoceros are typically considered indicators of high productivity; however, certain morphotaxa (Vallodiscus sp. and the hyaline variety) show a strong association with sea ice. Their abundance at 15.5 ka is responsible for the high sea ice concentration returned by our proxy. Additionally, diatoms that would return high concentrations of sea ice in our proxy, Fragilariopsis reginae-jahniae, and Sinerima marigella, are absent from this record. After 15.5 ka, sea ice diatoms and Chaetoceros RS proportionally decrease and are replaced by Neodenticula seminae, a marker of open North Pacific, ice-free waters. After 10 ka, there is no evidence for either ice bergs or sea ice.