HOLOCENE AMINOCHRONOLOGY AND TIME-AVERAGING FOR CHESAPEAKE BAY MULINIA

The geochrononology of Holocene sediment cores is essential for determining the timing of both natural and anthropogenic environmental changes. The aminochronology for 81 Mulinia lateralis shells has been generated for a long sediment core from central Chesapeake Bay, MD03-2661. Our core preserves a 24.5 meter, nearly complete Holocene sequence that permits a reliable calibration curve of 14C dated shells with amino acid racemization (AAR). This 14C-AAR calibration can then be used to assess the extent of sediment reworking and time-averaging in this core, and allow the application of this method to other Chesapeake Bay sediment cores with old carbon complications. Aspartic acid (Asp), the most rapid racemizer on this time scale, is the principal amino acid of interest. Similar applications of 14C-calibrated AAR kinetics have also been used to assess taphonomic bias and time-averaging (e.g. Goodfriend & Stanley, 1996; Kowalewski et al., 1998).

Two approaches to calibrating 14C and AAR are used. Interval calibration involves multiple AAR analyses (>10) of Mulinia from previously 14C dated core intervals. Both linear and non-linear regressions of D/L Asp against 14C age yield comparable R-squared values (0.91), but the intercept value has not yet been determined by analysis of modern samples. Direct calibration, currently in progress for 8 samples, involves both AAR and 14C analysis of separate valves from articulated Mulinia individuals. Because direct calibration results are not influenced by time-averaging, they should provide insights into the reliability of the interval calibration.

Use of AAR data to assess time-averaging not only requires a calibration curve but also an understanding of all factors that cause a spread in D/L values for a given core interval. For intervals with >10 analyses, the coefficient of variation (CV) for D/L Asp is between 3 and 10%, with only two intervals having CV's >7%. These ranges may be interpreted as "normal scatter" around an analytical mid-point, with all samples being essentially the same age, however, time-averaging (as represented by larger CV's) seems most pronounced within a region of slow or interrupted deposition at ~1150 cm core depth , between 2800 and 5600 cal yrs BP.