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 (Brigham-Grette and Hopkins, 1995) 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. (in press) have indicated post sub-stage 5e high sea level stands within these sequences was preceded by a rapid and intense glaciation in northeast Russia and St. Lawrence Island. However, the epimerization reaction of L-isoleucine to D-alloisoleucine occurs at an insufficient rate to separate out intra-stage 5 events. An alternative geochronologic method must be used to separate these events and determine whether post sub-stage 5e high sea level events in Beringia occurred during the 5e/5d or 5a/4 transition. Gas chromatographic (GC) analysis has the ability to separate D/L ratios of all common amino acids found in mollusk shells. Goodfriend et al. (1996) analyzed amino acid ratios in bivalves of Arctic marine deposits and determined that the higher racemization rate of aspartic acid provided significantly higher temporal resolution. This method, not widely applied to the arctic, has been used to reanalyze shells from high sea level stands of sub-stage 5e and post 5e collected from the Flaxman formation, Alaska, the Val'katlen and Nunyamo sections, northeast Russia, and marine deposits of St. Lawrence Island. To support our results, electron spin resonance (ESR) geochronology was also utilized to serve as an independent proxy to test the reliability of the GC ratios.