Amino acid racemization dating of some coastal plain sites, southeastern Virginia and northeastern North Carolina

Abstract

Bivalve mollusks (g. Mercenaria and g. Anadara) have been analyzed from 4 southeastern coastal plain sites using the amino acid racemization (AAR) technique for correlation and age estimate purposes. As seen previously (Wehmiller and Belknap, 1982), a conflict exists between age estimates obtained by AAR and V-series methods at the Norris Bridge, VA locality. Here, V-series dating of coral yields a 187 ±20 KA age, while AAR methods suggest a 400 to 500 KA age. The following methods are employed to understand this discrepancy at the Norris Bridge site: 1). Analysis of standard samples to ensure uniform sample preparation and system operation; 2). Quantification of variation in Mercenari'a and Anadara samples for each field site; 3). Qualitative comparison of AAR behavior between Mercenaria and Anadara to assess suitability of Anadara for dating purposes; 4). Aminostratigraphic correlation of field sites using both genera, and 5). Estimation of AAR ages using the the non-linear model of Wehmiller and Belknap (1982). Standard samples analyzed for this study show relatively good preCISIon. Coefficients of variation (CVs) of D/L leucine values are 5.7% (ILC-A) and 9.1% (ILC-B). Overall, both ILC-A and -B show accuracy within 6% when compared to other University of Delaware analyses. Several valves of each genus were analyzed from all sites to determine field site precision. CVs for multiple Mercenaria samples from each site are as follows: Gomez Pit, VA (5.2%); New Light Pit, VA (7.0%); Moyock Pit, N.C. (8.8%) and Norris Bridge, VA (4.4%). CVs for Anadara samples are slightly higher, but comparable for each field site. No Anadara were analyzed from the New Light Pit locality. On the basis of an identical order of racemization rates (i.e. ASP2:ALA> LEU>VAL) and similar ASP/LEU values in both genera, Mercenaria and Anadara qualitatively show similar racemization behavior. 3 clusters of DjL leucine values have been discerned from these field site analyses. These D/L leucine values are used to correlate fossiliferous strata in the southeastern Virginia and northeastern North Carolina Using the non-linear kinetic model, these Coastal Plain. D/L values suggest the following ages: 0.21 to 0.25 (75 KA) and 0.530 (approximately 400 KA). D/L values ranging between 0.33 and 0.38 are tentatively estimated to be 100 to 200 KA in age.