A review of the geochemical applications of the amino acid racemization reaction

A method of geochronology based on the chemical racemization of amino acids has been developed within the last few years. The various amino acids that make up the proteins of all living organisms consist virtually entirely of thel-enantiomer. After death, thel-enantiomer for each amino acid is slowly racemized over geological time and eventually forms an equilibrium mixture consisting of equal amounts of thed- andl-enantiometer. The increase in D/L ratio can be used to obtain a measure of the time that has elapsed since the organism died. The range of applicability of this method is the Pleistocene and may eventually be useful throughout the Pliocene in some cases. This paper presents a review of the literature on these applications as well as several suggested areas for future research.Unlike radionuclide decay rates, chemical reaction rates are sensitive to changes in such common environmental parameters as temperature, pH, solvent-medium, etc. For this reason, kinetic studies have been conducted at elevated temperatures in various “fossil-types” in order to simulate the changes that occur over long periods of time at the low temperatures found on the surface of the earth. Such studies, while of somewhat limited value for precise extrapolation, do nevertheless provide valuable information on which to base the theoretical concepts necessary for a complete understanding of the geochemical implications of the racemization reaction.Skeletal remains form the most suitable fossils for chronological study. Proteinaceous material is found embedded within the carbonate exoskeletons of invertebrates and the phosphatic endoskeletons of vertebrates. Most of the geochemical racemization studies have been conducted on foraminiferal shells and on bones. However, some work has also been reported on shells of other invertebrates, marine and lacustrine clays, and a few abiogenic concretions.Since the racemization reaction is temperature dependent, it has been used as a paleothermometer to estimate the average temperature to which bones and shells of independently known age have been exposed since their deposition. These average temperatures as a function of time have then been converted into estimates of the magnitude of the Holocene postglacial/Wisconsin glacial temperature change on land. They have also been used to obtain estimates of the “time-averaged” thermal gradient in deep-sea sediments using foraminifera isolated from the sediments.     

Geothermal and other effects on amino acid racemization in selected Deep-Sea Drilling Project cores

Kinetic parameters for the epimerization of isoleucine in multispecific foraminiferal asemblages were used to establish the effects of burial depth and the geothermal gradient on the extent of reaction. It was observed that with a little as thirty meters of burial in a normal thermal regime there were differences between the extent of epimerization measured and that which would have been predicted for thermal equilibrium with bottom water temperatures. As would be expected, these differences are greatest when the heat flow (the geothermal gradient) and/or the sedimentation rates are highest. These effects were observed in most of the DSDP samples studied, and have been used to estimate the average heat flux since the time of sample deposition. Occasional anomalous effects were observed which could not be related to past or present heat flux. These were determined to be due to such geologic occurrences as slumping and reworking or to recent sample contamination. Other problems emerged related to bottom water temperatures including changes over geologic time which are unknown and could not be deduced. Thus, the presence of epimerization anomalies in DSDP cores as noted above limits the effectiveness of amino acid geochronology in such cores, unless these anomalies can be recognized as ab initio.