HIGH RESOLUTION AMINOSTRATIGRAPHIC INTERPRETATION OF QUATERNARY COASTAL UNITS, ALBEMARLE EMBAYMENT, NORTH CAROLINA

The geochronology of Quaternary units within the Albemarle Embayment, North Carolina coastal plain, is based on limited isotopic data (radiocarbon and U-series), plus a growing database of amino acid racemization (AAR) results. Our continuing study of molluscan AAR results from 23 rotosonic drill holes from mainland and barrier island sites (primarily in Dare and Hyde counties) seeks to improve the understanding of the age-resolution capabilities of the method by analysis of multiple Mercenaria samples from closely-spaced superposed sections. Eight aminozones, or clusters of amino acid D/L values, are statistically resolved in the regional dataset, spanning from the Plio-Pleistocene Chowan River Fm to the late Pleistocene (MIS5). These clusters are based primarily on two slowly racemizing amino acids, leucine and valine, both having excellent chromatographic resolution and analytical precision; other amino acids conform to this zonation. The parameter VL is the average of the valine and leucine values in the analyzed samples. Four older aminozones have VL values of 0.85, 0.75, 0.63, and 0.53, all well resolved from each other, with some seen in superposition within a single hole. Four younger aminozones have VL values of 0.394, 0.335, 0.305, and 0.255, the youngest two of these being in superposition (1m apart) at one site, MLD01. Enough samples have been analyzed from each of these younger aminozones to provide high confidence (p<0.001 in most cases) in their resolution, even though many other aminostratigraphic studies would indicate that such high resolution would not be possible because of “typical” coefficients of variation of at least 10%. “Pooling” of all these D/L values into a single zone with a mean of ~0.34 is rejected using ANOVA tests, so we conclude that these zones are valid aminostratigraphic units. The lowest two VL values are interpreted to represent intervals within MIS5; middle Pleistocene (~200-700 kyr) ages are represented VL values ranging from ~0.34 to ~0.53, with early Pleistocene ages being represented by the VL values up to ~0.75. In a few cases, a regional aminozone range may be affected by small thermal differences between mainland and island holes, but our results imply a high degree of aminostratigraphic resolution when superposed sequences are studied in detail.