Stable carbon isotope composition of amino acids in modern and fossil Mercenaria

Amino acids were extracted from the bivalve shells of modern and fossil Mercenaria and analyzed for delta C-13 (PDB). These data were then compared to the expected impacts of diagenesis and contamination on the isotopic composition of fossilized proteins. The delta C-13 Of L-amino acids from modern specimens had enriched glycine, glutamic acid, and isoleucine relative to leucine, and phenylalanine. The range of delta C-13 in amino acids from modern Mercenaria collected from coastal Virginia (-1.6 parts per thousand to -29.6 parts per thousand) and coastal Florida (-11.6 parts per thousand to -35.5 parts per thousand) suggests the preservation of a diet signal. All of the fossil specimens had amino acids with the same 613 C distribution pattern as modern specimens. Alanine in the fossil specimens however, was always relatively enriched and often had delta C-13 values comparable to or more positive than glycine. Fossil specimens commonly contained glycine and glutamic acid with positive delta C-13 values. The variation of delta C-13 among the different enantionters was not predictable. In some specimens, the D- and L-amino acids had the same delta C-13 values. a feature attributed to the presence of original amino acids. In other specimens, one or more enantiomers had different delta C-13 values. suggesting preferential loss of one enantiomer, the formation of secondary amino acids by the degradation of others, a diagenetic affect, or perhaps contamination. (c) 2006 Elsevier Ltd. All rights reserved.