THREE RADIOCARBON CALIBRATION TECHNIQUES FOR AMINO ACID RACEMIZATION IN HOLOCENE CHESAPEAKE BAY MOLLUSKS

A radiocarbon-calibrated aminochronology is presented for the bivalve Mulinia lateralis from Chesapeake Bay core MD03-2661, a 25 m piston core extracted near Kent Island (38º 53.21’ N, 76º 23.89’ W) during the 2003 USGS Marion-Dufresne cruise. Three separate approaches were used to calibrate radiocarbon data with the amino acid racemization (AAR) of aspartic acid. Variability in AAR ratios from a myriad of sources was recognized, including: analytical error, intrashell variability, inherent variability, time averaging, and contamination. Most of these variability sources were controlled for through experimental design or by the application of three mutually independent dating techniques. In the first of these techniques, a direct or paired analysis calibration incorporated eight articulated specimens, thereby allowing for the application of AAR and radiocarbon analysis of the same specimen and controlling for both intrashell variability and time averaging. A second paired analysis approach to calibration relied on shells that were bilaterally split to facilitate both AAR and radiocarbon dating, thus controlling for time averaging. For the third approach, nine independent radiocarbon dates were combined with 129 Asp D/L ratios from the same core depths to produce an indirect calibration model, from which intershell variability and time averaging can be estimated.

Although influenced by several sources of variability, both the direct calibration of articulated shells and the indirect calibration yielded virtually identical age models, well within their respective 95% confidence intervals. This study establishes an aminostratigraphic reference section for the Holocene record of the Chesapeake Bay and demonstrates the usefulness of multiple calibration approaches and the potential utility of AAR for future studies of sedimentary processes and chronologies in the Bay.