AMINO ACIDS AND THE COASTAL PLAIN – WHAT HAVE THEY MEANT FOR EACH OTHER OVER THE PAST 40 YEARS?

Mollusks from Quaternary deposits of the US Atlantic Coastal Plain (ACP) have been studied for the extent of amino acid racemization (AAR) for over four decades, ever since the earliest publications by Hare and Abelson in the mid-1960’s. These and all later studies have generally focused on the geochronological applications of AAR, either for relative or numerical dating, but implicit in many of these studies has been the use of field-collected samples to understand more about the geochemistry of AAR. AAR data from superposed sequences (either outcrop or core) demonstrate that D/L values increase with increasing stratigraphic age, a basic criterion for the reliability of AAR as a dating tool. Use of AAR for numerical dating depends on independent calibration, which is currently limited, primarily because there are no reliable mid- or early Pleistocene age-calibrated units from which shells can be sampled. Potential independent calibration of these older units using biostratigraphy, paleomagnetics, and Sr-isotope analysis remains to be pursued in more detail. Reliable calibration exists for several late Pleistocene sites based on U-series coral dating, although chronological “surprises” exist because the elevations of ACP MIS 5 paleoshorelines do not conform to traditional “Barbados sea-level model.” Recent AAR study of the thick Quaternary section preserved in the Albemarle Embayment of the NC Coastal Plain has identified at least 8 aminozones (clusters of D/L values), all statistically significant and all in superposition within the Embayment. These new results also provide insights regarding issues of latitudinal correlation of age-calibrated aminozones between VA and SC. When multiple analyses are conducted, AAR is a particularly useful tool for the recognition of mixed-age assemblages, and results from outcrop, core, and particularly ACP beaches assist in understanding various taphonomic processes. The effect of shell alteration on AAR results can also be evaluated in those cases where field sites contain a rich collection of specimens. Comparison of D/L values from Holocene (14C-calibrated) and MIS 5 (U-Th calibrated) shells assists in the combined effort of evaluating kinetic models of racemization and estimation of the contrast between late Pleistocene and Holocene effective temperatures.