THE STATUS OF CALIBRATION OF MID-ATLANTIC COASTAL PLAIN (ACP) AMINOSTRATIGRAPHY: IMPLICATIONS OF NEW RESULTS FROM NORTHEASTERN NORTH CAROLINA (NC), USA

Amino acid racemization (AAR) is a calibrated dating tool, requiring secure links to independently dated samples. For late Pleistocene ACP mollusks, this calibration has relied on coral U-series dating. Although many U-series results exist, only five sites involve corals and AAR samples collected simultaneously. Assumed associations between dated corals and AAR samples, sample quality, and even lateral variations within exposed collection sites, can all result in inaccurate AAR calibrations.

Results for samples from three commercial excavations in northeastern NC are here related to published regional aminostratigraphy for the Albemarle Embayment (AE). Multiple whole shells (most with evidence of transport) are available, permitting a more thorough understanding of inter- and intra-shell variability than possible in prior AE studies. A superposed section (~7 to ~9 m bmsl) at East Lake Pit (ELP) has both OSL and U-series ages (middle/late MIS 5) and multiple AAR results for two mollusk taxa, Mercenaria and Mulinia. Two clusters of AAR D/L values are apparent in the results for both taxa, the lower D/L values interpreted to represent MIS 5a (associated with MIS 5a corals), the higher D/L values representing a middle Pleistocene unit also observed in a nearby subsurface section.

Mercenaria D/L values from Forbes Pit (FP) fall in two clusters, internally consistent with the outcrop stratigraphy (tidal and overwash units, ~7 to ~11 m bmsl) and within a suite of superposed OSL ages spanning all of MIS 5. The lower FP D/L values equal the lower values from ELP. The higher FP D/L values likely represent early MIS 5 and are also observed in fragmented Mercenaria in cores from earlier AE studies. Nearby Lane Pit (LP) results equal those from the lower unit at Forbes, reinforcing the regional extent of this aminozone. Both FP aminozones are also observed in shells from NC beach collections.

These results affirm the existence of aminozones previously recognized in multiple subsurface AE cores and demonstrate the complexity of ACP Quaternary geochronology. Substantial age ranges, diagenetic, and taphonomic effects can be seen within small vertical stratigraphic sequences. The transient nature of these and similar exposures usually prevents return visits, but both ELP and FP should be re-examined in light of these new results.