HOLOCENE MOLLUSCAN AMINOSTRATIGRAPHY OF CHESAPEAKE BAY SEDIMENTS

The geochronology of mid- to late Holocene sediment cores from the Chesapeake Bay provides a temporal framework for understanding natural and anthropogenic environmental changes. Detailed chronologies are biased by a variety of factors, including variable carbon sources for radiocarbon dating, age mixing and diagenetic sample alteration. Amino acid racemization (AAR), suitably calibrated, provides an additional method for assessing sedimentation rates, discontinuities in sediment accumulation, and possible age mixing. Here we report preliminary AAR results for samples spanning a calibrated ¹⁴C age range of 800 to 8900 years before present from core MD03-2661 (Latitude 38 53.21, Longitude 76 23.89) dated using AMS¹⁴C ages on Mulinia for calibration of the AAR. Our initial results from the Northern Arizona University's reverse-phase liquid chromatograph indicate an increasing trend of D/L values with sample age for Aspartic, Alanine and Glutamic amino acids. Aspartic acid is the most extensively racemized and hence most useful for high resolution Holocene chronology. Individual Mulinia shells from four depths (371cm, 1087cm, 1613cm, and 2026cm) yielded Aspartic D/L ratios of 0.192, 0.239, 0.307, and 0.347 respectively. Because AAR methods are economical than carbon-14, numerous samples can be analyzed from single depths in order to estimate the magnitude of age mixing or reworking. Our calibration curve will be refined using paired AAR-¹⁴C analyses, and multiple AAR analyses at several more depths, may thereby quantify the levels of age mixing within single cores. Samples from cores with both high- and low-sedimentation rates, and those with known breaks in sedimentation, may be studied in the future to evaluate the utility of AAR for Chesapeake Bay geochronology.