INTRA-STAGE 5 HIGH SEA-LEVEL STANDS IN BERINGIA: 5E/D OR 5A/4?

The most laterally continuous marine deposits in the Beringian region are those of the last interglacial, oxygen-isotope stage 5. Deposits of sub-stage 5e are represented on the Alaskan coast by marine deposits of the Pelukian transgression and a later high sea level event within stage 5 represented by the Flaxman formation. Extensive stage 5 deposits can also be found on Chukotka Peninsula, northeastern Siberia, as well as on St. Lawrence Island within glaciotectonically deformed marine and glacigenic sequences. An analysis of the stratigraphy and alloisoleucince/Isoleucine ratios obtained from fossil mollusk shells by Brigham-Grette et al. (2001) collected in Russia have indicated post sub-stage 5e high sea level stands within these sequences were deposited simultaneously with a rapid and intense glaciation in northeast Russia and St. Lawrence Island. However the timing within stage 5 of this glaciation is still unknown. Shells were reanalysed at Umass using the faster racemizing amino acid, aspartic acid, to increase temporal resolution and separate out intra-stage 5 deposits.

Previous work by Kaufman et al. (1995) suggest similar deposits occur in the North Slope of Alaska in the Flaxman Formation, a glaciomarine fossiliferous unit standing approximately 2-5 meters asl. Thermoluminescence ages and amino acid ratios (using alloisoleucine/isoleucine) give indeterminate dates to the sediment and shell samples but an intra-stage 5 age is likely based on the previous data. Shells were collected from the Flaxman Foramtion this summer and analyzed to determine if they also were deposited at the same time as their Russian counterparts. Presently our hypothesis calls for a single rapid and intense glaciation occurring in Beringia while eustatic sea level remained high either during the 5e/d or 5a/4 transitions concurrent with two of the largest insolation drops in the past 125 ky. Using the faster racemizing aspartic acid along with Electron Spin Resonance and Optically Stimulated Luminescence measurements, our findings indicate the latter transition, stage 5a/4, to be the age of the deposits. These finding have significant implications to our understanding of the timing and relationship between Beringian, Canadian, and Russian ice-sheets that exisited during the last interglacial.