Relative and Absolute Dating of Quaternary Mollusks With Amino Acid Racemization: Evaluation, Applications and Questions

Abstract

Amino acids are entrapped in living skeletal carbonates as com-ponents of the structural protein upon which the carbonate phase forms. After death of the organism, the protein undergoes a complex array of reactions, involving hydrolysis into short-chain polypeptides and free amino acids, decomposition and/or leaching, and racemization of the amino acids in both free and bound forms. The racemization reaction involves conversion of each amino acid from its original 100% L form (“left-handed”) into an equilibrium mixture (usually 50-50) of D (“right-handed”) and L amino acids. The chronological utility of amino acid racemization has been studied in several genera of mollusks from more than 200 Quaternary marine localities along both the Atlantic and Pacific coasts of the United States. In these studies, both relative and absolute age estimates have been proposed for a large number of previously undated localities. In addition, a number of criteria for the evaluation of the method have been developed and tested, using samples with some form of absolute or relative age control. These criteria include: 1) acceptable precision (5 to 10% depending upon the amino acid) for multiple analyses of shells of the same genus from the same outcrop; 2) mineralogical and structural preservation; 3) ability of each genus of interest to consistently demonstrate increasing D/L values in samples of increasing relative age (Such as vertical terrace sequences or in superposed strata); 4) abillty of each genus of interest to achieve racemic equil-ibrium in “old” samples (early Pleistocene to Miocene, de-pending on temperature); 5) consistent relative rates of racemization of different amino acids in the same sample; 6) increasing D/L values with decreasing latitude (increasing temperature) in samples of known age equivalents. Genera that do not meet one or more of these criteria must only be used with caution in chronological applications. Multiple genera should be employed in chronological studies because systematic generic effects on apparent racemization kinetics do exist and because only rarely would one genus be found in abundance at all localities of in-terest. The use of multiple genera provides cross-checks on age esti-mates, and occasionally reveals ambiguities that would be unrecognized if only single genera were used. The simplest application of amino acid racemization is as a relative stratigraphic tool for closely spaced (but discontinuous) locaiities that can be assumed to have had similar or identical temperature histories. Results from southern California and the mid-Atlantic coastal plain provide good examples of this approach. If amino acid data are available for at least one absolutely dated locality from within local “aminostratigraphic” sequences, then absolute age estimates can be derived from the D/L values using one of several kinetic models of diagenetic racemization that are under consideration. Correlation of separate aminostratigraphic sequences between two widely-spaced regions (hence with different temperature histories) requires estimates of the differences in temperature histories as well as a quantitative kinetic model of the temperature dependence of racemization in samples of equal age. These kinetic models, and their inherent assumptions, have been calibrated and successfully tested in a few cases, mostly with samples of known age from a broad latitude range (35° N to 25° N) along the Pacific coast of North America. On the Atlantic coast, some conflicts between U-series coral ages and both relative and kinetic model age estimates have been encountered.