TAXONOMIC COMPARISON OF AMINO ACID GEOCHEMISTRY AMONG LATE PLEISTOCENE MARINE INVERTEBRATES FROM FLORIDA

Amino acid racemization geochemistry and geochronology can be used to estimate the relative ages of marine fossils through analyses of particular amino acid concentrations and/or ratio between the D- and L-enantiomers for certain species. It is expected that both overall amino acid concentrations and D/L values would be comparable among individuals of the same species from deposits of similar age. The purpose of this taxonomic research is to test the degree of similarity of amino acid concentration and racemization values within different phylogenetic groups.

Eighty-eight specimens from the families Balanidae (n=10), Chamidae (n=4), Lucinidae (n=35), and Veneridae (n=39) were selected from a bulk sample of the upper Ft. Thompson Formation at Caloosa Shell Co. pit, from Ruskin, Florida. The samples were prepared for reverse phase HPLC analysis, and their concentrations and D/L ratios were calculated by normalizing to the internal standard L-homoarginine. Statistical analyses, particularly the coefficient of variation (CV), show that intrageneric amino acid concentrations are highly variable, with CVs at 2-62%, whereas intrageneric amino acid D/L ratios show greater consistency, with CVs ranging from 0-15%. Aspartic acid and phenylalanine consistently display higher D/L ratios, indicating faster racemization, than glutamic acid, valine, and alloisoleucie/isoleucine. In contrast, examination of single and paired analyses of amino acid D/L ratios indicates that the greatest difference among these taxa is reflected at the order level. Sub-samples from Balanidae (Order Thoracica) are clearly distinguished from the remaining families (all of Order Veneroida).

Ongoing research includes verification that the most significant difference is reflected by different orders. This entails comparing amino acid concentrations and D/L ratios of modern samples of the aforementioned families that have undergone laboratory heating, interlaboratory comparison of D/L ratios derived from reverse phase HPLC and gas chromatography (GC), and analysis of samples from additional orders that are abundant in the upper Pleistocene Fort Thompson Formation.