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 δ13C (PDB). These data were then compared to the expected impacts of diagenesis and contamination on the isotopic composition of fossilized proteins. The δ13C of L-amino acids from modern specimens had enriched glycine, glutamic acid, and isoleucine relative to leucine, and phenylalanine. The range of δ13C in amino acids from modern Mercenaria collected from coastal Virginia (−1.6‰ to −29.6‰) and coastal Florida (−11.6‰ to −35.5‰) suggests the preservation of a diet signal. All of the fossil specimens had amino acids with the same δ13C distribution pattern as modern specimens. Alanine in the fossil specimens however, was always relatively enriched and often had δ13C values comparable to or more positive than glycine. Fossil specimens commonly contained glycine and glutamic acid with positive δ13C values. The variation of δ13C among the different enantiomers was not predictable. In some specimens, the D- and L-amino acids had the same δ13C values, a feature attributed to the presence of original amino acids. In other specimens, one or more enantiomers had different δ13C values, suggesting preferential loss of one enantiomer, the formation of secondary amino acids by the degradation of others, a diagenetic affect, or perhaps contamination.