Amino acids in fossil corals: racemization (epimerization) reactions and their implications for diagenetic models and geochronological studies*

Thirty-eight fossil coral samples of known age from late Pleistocene uplifted reef terraces have been analyzed for amino acid composition and for the degree of racemization of selected amino acids. Particular attention has been given to the epimerization (racemization about the cc-carbon) of isoleucine. The D/L ratios observed in many of these samples do not conform to the concept of increaqing racemization being associated with increasing fossil age. It i:, shown what s~~tc’cn 01 ~hc ~~mplcs demonstrate concordance between their known age and the age estimated from racemization data. Racemization ages are based upon a kinetic model derived from the kinetics observed in foraminifera in marine sediments and published estimates of the temperature dependence of the isoleucine epimerization reaction. Lack of concordance for the remaining samples is explained by either of two separate diagenetic phenomena: extensive leaching of free amino acids from the fossil, or contamination by ‘young’ amino acids. Ambiguities observed in many of the results may be due to the fact that coralline organic matter can have a complex history because of variations in its source and in its relation to the mineral phase. Neither of these factors has been observed to exert such a significant effect on racemization kinetics observed in fossil foraminifera or molluscs. Fossil corals are of only modest value as specimens for amino acid geochronological studies. INTRODUCTION the analyses of these various types of materials is that the mechanisms of diagenetic reactions are a complex NUMEROUS investigators have discussed the kinetics function of fossil type (mineral phase and structure, of amino acid racemization (or epimerization) reacorganic-inorganic interactions, etc.) and diagenetic tions in geological samples: molluscs (HARE and environment. The importance of these various diaABELSON, 1968; HARE and MITTERER, 1969); corals genetic reactions in the evaluation of observed race(WEHMILLER and HARE, 1970; HUSSEINI, 1973); estuarmization kinetics has been pointed out in the case ine sediments (KVENVOLDEN et al., 1970); marine sediof each type of fossil material (HARE and MITTERER, ments (BAL)A et al., 1970; WEHMILLER and HARE, 1969 ; WEHMILLER and HARE, 197 1: WEHMILLER, 197 1; 1971; BADA and SCHROEDER, 1972; KVENVOLDEN et BAIIA and SCHROEDER, 1972; BADA, 1972) though few al., 1973); and Pleistocene bones (BADA, 1972; TUREKquantitative models have been developed. IAN and BAIIA, 1972; BADA et al., 1973a, b; KVENThis paper presents data on the observed kinetics VOLDEN and PETERSON, 1973). The value of these reacof racemization (strictly, epimerization) of isoleucine tions as either chronological (HARE and MITTERER, in fossil corals from several locations. The ages of 1969; HARE, 1969; BADA et al., 1970; WEHMILLER and all samples have been estimated by stratigraphic HARE, 1971; BAIIA, 1972; BADA and SCHROEDER, 1972) and/or radiometric techniques. Data presented here or paleotemperature (MITTERER, 1972; SCHROEIIER includes some previous results (WEHMILLER and and BAUA, 1973; BADA et al., 1973a) tools has freHARE, 1970) but interpretation is based upon models quently been emphasized, though the uncertainties of derived from the kinetics of racemization that have such approaches have been indicated (WEHMILLER been observed in marine sediments (WEHMILLER and and HARE, 1971; BADA et al., 1973b; KVENVOLDEN HARE, 1971; BADA and SCHROEDER, 1972). The major et al., 1973). The conclusion that must be drawn from focus of this work is on a suite of midto late-Pleistocene corals from uplifted reef terraces on the island * Lamont-Doherty Geological Observatory Contribuof Barbados (MESOLELLA, 1968; ME~OLELLA et al., tion No. 2330. 1969). The island consists of a coral cap of uplifted