John Wehmiller

New perspectives on US Atlantic coastal plain aminostratigraphy gleaned from extensive analyses of shell specimens from inner continental shelf vibracores

John Wehmiller §; Kelvin Ramsey £ ; Scott Howard #

§ University of Delaware, Newark DE 19716 USA; £ Delaware Geological Survey Newark DE 19716; # South Carolina Geological Survey Columbia SC 29212

Amino acid racemization (AAR) results have been incorporated into studies of stratitgraphy and geochronology of US Atlantic coastal plain sites for over 50 years (Wehmiller, 2013a: Quaternary Geochronology). Several analytical methods have been employed (IE, GC, and RP) depending on currently available technology, sample size, or other criteria. Over the past four years, an extensive campaign of coring to assess offshore sand resources has occurred on the inner continental shelf from New Jersey (~ 40N) to Georgia (~ 30N), conducted by the Bureau of Ocean Energy Management (BOEM) and several state agencies. AAR results for ~900 mollusk specimens from >50 cores are now available. Analyses have been by RP at Northern Arizona University after initial sample preparation at the Delaware Geological Survey. To compare these results with prior IE or GC results for both onshore and offshore samples in the region, previously analyzed samples (or sites) have been included in the study. Taxa employed in the offshore studies include Chione, Spisula, Ensis, Mulinia, and Mercenaria, the latter two (and especially Mercenaria) being the taxa most commonly analyzed by GC (Wehmiller, 2013a).

In those instances where two or more methods have been compared for specific shells (or collections from the same site), agreement is usually excellent, although direct equivalence of only a few amino acid D/L values by multiple methods should be expected (Wehmiller 2013b; Whitacre et al., 2016: Quaternary Geochronology). In spite of these limitations, multiple methods always lead to the same relative age interpretations. For larger shells, intrashell variability is more of an issue in the RP results, simply because this method is so sensitive that small samples can be analyzed.

Analytical results, including collection sites, stratigraphic information, and related independent chronologic data are maintained in an online map database. Over 50 of the AAR results are for shells with paired 14C analyses, providing intergeneric calibration for Holocene samples and confirmation of Pleistocene AAR age estimates. At least three mid- to late Pleistocene units are identified at one or more shelf sites, and examples are seen of age-mixing on a variety of time scales, including Pleistocene shells within Holocene deposits. The age distribution of mollusks of different ages, both on the inner shelf and on nearby beaches, is heavily influenced by local and regional sediment supply as well as the history of sea-level transgression/regression cycles during the late Pleistocene.