d13C COMPOSITION OF AMINO ACIDS FROM THE SHELLS OF MODERN AND FOSSIL MERCENARIA

We have measured the composition of d¹³C in acid extractable amino acids from the shells of modern and Holocene to Pleistocene age Mercenaria using compound specific isotope analysis techniques. Methods used for the purification of amino acids from dissolved shell fragments were improved with the help of Glenn Goodfriend. This investigation tested the hypothesis that the presence of original amino acids in fossil shells could be substantiated if the carbon isotopic composition of individual, D- and L- enantiomers were the same. Additionally, amino acids from modern Mercenaria collected from known environments were compared to fossil specimens to test the hypothesis that paleodiet information was also preserved in the carbon isotope signal of proteins in ancient shells. Our results indicate that the d¹³C of amino acids in modern Mercenaria varies with diet. With the exception of alanine, the d¹³C pattern of amino acids in fossil specimens mimics that found in the modern counterparts suggesting the preservation of a paleoecological signal. The d¹³C composition of amino acids however becomes enriched during diagenesis. Values of d¹³C in glycine reached nearly +15 per mil in physically altered fossil specimens. The d¹³C composition of individual stereoisomers in fossils was not consistent. Glutamic acid, leucine, and isoleucine were found to be the most reliable indicators of original amino acids in fossils. The carbon isotopic composition of amino acids in many of the fossil specimens could be explained most simply by diagenetic alteration without envoking contamination. Many of the specimens used for this study were previously analyzed for amino acid racemization investigations and the data collected here supports the conclusion from those studies that were based on an assumed original amino acid pool.