KINETIC MODELS OF AMINO ACID RACEMIZATION IN FOSSIL SHELLS: APPLICATIONS TO GEOCHRONOLOGY AND PALEOTHERMOMETRY OF FLORIDA'S PLIO-PLEISTOCENE RECORD

Amino Acid Racemization (AAR) is the interconversion of an amino acid between its L-and D-form stereoisomers. Measuring the extent of racemization, expressed as the D/L ratio, in the carbonate shells of marine fossils can be used to assess either the relative ages of specimens from different strata, or the effective diagenetic temperature of a fossil's post-depositional history. In order to quantify either time (numerical age) or (paleo)temperature, the kinetics of the racemization reaction must be known for the amino acids and taxa of interest. In this study we construct and test the applicability of different kinetic models of racemization for aspartic acid, glutamic acid, valine, phenylalanine, and isoleucine in Chione elevata, a commonly occurring bivalve from the fossiliferous Plio-Pleistocene interglacial high sea-level deposits in Florida.

Fragments of live-collected Chione elevata shells were heated in the laboratory at 120° and 140°C, for times ranging from 2 hours to 2 months. These high-temperature studies accelerate natural racemization in order to model the progression of D/L ratios and calculate the Arrhenius parameters of racemization for the amino acids of interest. Kinetics are modeled using a linear, first-order approximation for the initial part of the racemization reaction (D/L < ~ 0.4), while parabolic and power-law models are applied to heated samples over the full range of D/L values. Amino acid racemization rates vary in different taxa, making it necessary to model the kinetics for each genus independently. Chione has been shown to be widely occurring and well suited to AAR studies. For comparison, we are also examining racemization kinetics in the bivalves Mercenaria mercenaria and Macrocallista nimbosa, the gastropod Oliva sp., and the cerripide Balanus sp. While these heating experiments are still underway, the results will ultimately be used to evaluate the age and paleotemperature estimates determined using Chione data.