RACEMIZATION-INFERRED AGE DISTRIBUTION OF MOLLUSKS IN THE US ATLANTIC MARGIN COASTAL SYSTEM

The age distribution of shells in surface deposits of the inner shelf or on beaches of the US Atlantic margin represents an integrated response to numerous factors, including sediment sources and supply, shell survival rates, and various anthropogenic activities. Here we summarize results from approximately 900 amino acid racemization (AAR) analyses of mollusks obtained from inner shelf core and grab sites, as well as dozens of beach collection sites, between New Jersey and Florida. North Carolina results dominate the dataset because of the availability of numerous samples along the Outer Banks and the nearby inner shelf. ¹⁴C results on selected samples provide calibration and distinguish Holocene from Pleistocene AAR results, while also identifying samples that were likely exposed to high beach surface temperatures. Mercenaria, Spisula, and Mulinia are the most commonly analyzed taxa. For Mercenaria, in many cases shell characteristics (particularly color and luster) are diagnostic of pre-Holocene ages for samples found on the Outer Banks beaches. Multiple Pleistocene aminozones are recognized in the NC beach aminostratigraphy, aminozones that correlate to units recognized in on-shore deposits with in-situshells. Correlative inner-shelf aminozones represent the sources for beach shell material. Similarly, Pleistocene shells are recognized on both the Maryland shelf and at selected beaches of the Delmarva Peninsula.

Simple histograms of AAR D/L amino acid values for individual ~ 50 km segments of the NC beaches reveal distinct clusters of apparent ages, with Holocene shells dominating the mix from 36.5 to 35.5 N. To the south, increasing relative abundances of Pleistocene beach shells are encountered, with the maximum found between Onslow Beach and Wrightsville Beach, NC. Shelf and beach samples from the Edisto Beach, Greys Reef, and Tybee Island region (SC-GA) also indicate an abundance of Pleistocene shell material. Although results are not uniformly distributed across the entire region of study, and artificial beach nourishment has undoubtedly affected results in selected areas, concentrations of Pleistocene shells (often with multiple ages) in surface deposits (inner shelf or beaches), indicate local sources of pre-Holocene sediment and/or absence of thick Holocene sediment cover.