MEASURING INTRA-CRYSTALLINE AMINO ACID D/L RATIOS IN FOSSIL CHIONE IMPROVES AMINOSTRATIGRAPHIC RESOLUTION IN THE PLIO-PLEISTOCENE OF FLORIDA

Amino acid geochronology is a useful tool for establishing relative ages and stratigraphic correlations of fossilferous Pleistocene sediments representing past high sea-stands in Florida. The extent of racemization is a function of many variables, most important of which are time, temperature, and taxonomy. Typically amino acid D/L ratios are measured in the total preserved organic material in the carbonate shells of marine fossils. While these data may be reliably used to establish relative ages of Pleistocene and younger fossils in Florida, the concentration of total amino acids is increasingly affected by diagenesis, and racemization is near equilibrium in shells from older deposits. Therefore, the application of aminostratigraphy to older fossils is problematic. In this study we propose a sample preparation protocol for fossil Chione bivalves that isolates intra-crystalline amino acids from total shell organics. Our goal is to minimize the influence of diagenetic effects on intra-shell variability in amino acid concentration and D/L ratios by selectively removing intercrystalline amino acids, which could potentially reduce intra-sample variability and enhance the dating capability of aminostratigraphy.

In order to devise an optimum preparation protocol, we carried out several experiments using modern and fossil Chione shells. Results of the first test showed that there is minimal variability in D/L ratios measured in fragments from different parts of a single fossil shell. In the second experiment, we pulverized whole shells and subjected different grain-size fractions to 10% NaOCl for times ranging from 2 to 36 hours. Results showed that bleaching the 125μm – 250μm size fraction for 24 hours maximized the removal of inter-crystalline, while minimizing loss of intra-crystalline amino acids. This protocol was then applied to Chione shells from different geologic formations across Florida, ranging from Early Miocene to Late Pleistocene in age. Results of this study suggest a lower and relatively consistent rate of racemization of intra-crystalline amino acids compared to total shell organics, offering improved resolution for Pleistocene deposits. However, in fossils from Miocene to Pliocene aged deposits, racemization equilibrium still limits the temporal range of applicability.