TAXONOMIC AND TAPHONOMIC CONTROLS ON AMINO ACID GEOCHRONOLOGY: A TEST IN THE PLIO-PLEISTOCENE SHELL BEDS OF FLORIDA

Amino Acid Racemization (AAR) is a biochemical reaction, the extent of which can be measured in carbonate shells of marine organisms as an indicator of age and temperature history in geologic investigations. Previous AAR studies of Quaternary deposits in Florida have focused on the analysis of robust, ubiquitous fossil shells of the mollusks Chione elevata and Mercenaria mercenaria/campechiensis. However, the utility of other taxa and the effects of preservation remain poorly understood. Because the racemization rates in Chione and Mercenaria are relatively rapid, their chronostratigraphic utility is limited to the middle Pleistocene at best. In this study we examine taxonomic and taphonomic controls on racemization in shells collected from the Fort Thompson “Formation” in southern Florida. Here we investigate fourteen genera to assess suitability for aminostratigraphic applications and determine their relative rates of racemization for several amino acids. Taxa have been classified into three main groups; those that: 1) racemize relatively slowly (e.g., Balanus, Dentalium, Mulinia, Oliva, and Vokesinotus); 2) racemize at intermediate rates (e.g., Cerithium, Donax, Chlamys); and 3) relatively fast racemizing taxa (e.g., Anodontia, Chione, Corbula, Parvilucina). The slower racemizing taxa are useful over a larger temporal range, whereas the relatively fast racemizing taxa provide enhanced temporal resolution in younger samples. Low coefficients of variation for D/L ratios of aspartic acid, glutamic acid, and valine, measured in replicate shell samples, indicate that the taxa most suitable for geochronologic interpretations are Balanus, Chione, Corbula, Donax, and Mulinia. Genera whose coefficients of variation suggest little aminostratigraphic utility include Anodontia, Arca, Cerithium, Dentalium, Donax, Oliva, Parvilucina, Chlamys, and Vokesinotus. To test whether measured amino acid concentrations and D/L values are altered by taphonomic processes, we are currently investigating amino acid racemization data for various taphonomic grades of Chione elevata collected from the Fort Thompson “Formation”.