Amino acid racemization geochronology of reworked Quaternary mollusks on U.S. Atlantic coast beaches: implications for chronostratigraphy, taphonomy, and coastal sediment transport

Abstract

Approximately 200 shells (primarily Mercenaria) from 21 beach sites between New Jersey and Florida have been analyzed for the extent of racemization (epimerization) of their fossilized amino acids. The greatest concentration of sites is in North Carolina, in regions least affected by human modification of natural beach sediment-transport processes. These results can be used to estimate the frequency of age mixing of Pleistocene shell material in Holocene depositional environments. Selected shells have also been dated by 14C (conventional and/or AMS) to provide direct calibration of the amino acid epimerization ratios. Shell taphonomic characteristics (particularly color) can be qualitatively related to apparent ages. Data for shell fragmentation, combined with amino acid age estimates, provide insights into probable transport distances of reworked shells. Shells with apparent Pleistocene ages have epimerization values equivalent to those seen in onshore exposures of Pleistocene coastal units. Amino acid epimerization measurements are a comparatively rapid and inexpensive chronologic tool for use in taphonomic and sediment(shell-) transport studies. Paired amino acid- 14C analyses on single shells provide insights into geochemical alteration of shells and permit modelling of the residence time of shells on beaches. Greater numbers of analyses at each site are needed before statistically valid estimates of age mixing can be obtained, but the apparent distribution of Pleistocene shells on Atlantic coast beaches appears to be related to the distribution of Pleistocene units in the shoreface and inner shelf, and the thickness of Holocene sedimentary cover in specific areas. These observations provide useful criteria for the evaluation of possible age mixing of shells collected at outcrops of Pleistocene units in the Coastal Plain.