ENHANCED AGE RESOLUTION OF THE MARINE QUATERNARY RECORD IN THE ARCTICUSING ASPARTIC-ACID RACEMIZATION DATING OF BIVALVE SHELLS

Aspartic acid (Asp) racemization occurs at a significantly higher rate than isoleucine epimerization and consequently provides better tempor al resolution of Arctic marine deposits (from Alaska, Spitsbergen, and Baffin Island). Heating experiments (at 100 degrees C) on the bivalve s hlya and Hiatella show the Asp racemization rate decreases with incr easing D/L values, as is typical for biogenic carbonates. Based on the se experimental racemization rates and rates determined from racemizat ion of samples radiocarbon dared to ca. 10,000-12,000 yr B.P., activat ion energies for Mya and Hiatella are estimated to be 30.6 and 30.0 kc al/mol, respectively, for Asp racemization, and 29.0 and 29.5 for isol eucine epimerization. Analysis of a time series of Plio-Pleistocene Hi atella from the north coast of Alaska shows that last-interglacial mol lusks can be readily distinguished from modern samples by Asp but not by isoleucine. oh Asp values indicate a younger age for the Fishcreeki an transgression than does isoleucine epimerization. For Spitsbergen, Dir. Asp shows a slight age difference (ca, 12,000 yr) between two uni ts of the ''episode B'' interstadial and suggests that the age of thes e units may be closer to 65,000 than to 80,000 yr B.P., two possible a ges suggested by other evidence. The age of the Loks Land Interstadial on Baffin Island is likely to be greater than that indicated by radio carbon ages. Within deposits from each region, D/L Asp values are less variable among individual shells than isoleucine epimerization values . This may indicate better reliability of Asp for geochronology. (C) 1 996 University of Washington.