Stable isotope compositions and amino acid preservation in Mercenaria shells

This dissertation focused on the use of stable isotope techniques to distinguish between diagenesis and contamination of organic matter in fossil shells. The use of compound specific isotope analysis as an independent method to verify the presence of original amino acids in fossils was tested on a suite of Holocene and Pleistocene, Mercenaria spp. from the Atlantic Coastal Plain, U.S. Bulk, stable isotope analysis of organic matter from modern and fossil shells was also used to determine if information about past food webs was preserved in the fossils.; The δ13C compositions of the amino acids stereoisomers from the shells of well preserved fossil Mercenaria spp. were not always the same. D- and L-alanine were often different. Secondary alanine is generated by the diagenetic alteration of relatively unstable hydroxy amino acids. Alloisoleucine and isoleucine were often different but these amino acids are typically present in low concentrations and are difficult to measure. D- and L-glutamic acid are difficult to separate from phenylalanine; and hence, are difficult to analyze isotopically. D- and L-leucine are the most reliable amino acids to test for the presence of contamination. Most of the fossils studied for this research have isotopically equivalent leucine stereoisomers.; Amino acids in fossils were often enriched relative to modern counterparts. This results from the preferential alteration and subsequent leaching of isotopically light amino acids. The pattern of isotopic distributions of amino acids however remains largely unchanged in well-preserved fossils, except for enriched alanine.; The isotopic composition of bulk organic matter in amino acids from the biominerals of modern and fossil Mercenaria spp. can be evaluated with the same methods used for soft tissue in estuarine food web studies. This research demonstrated that biominerals of Mercenaria record annual variations in diet. The preservation of diet signals was also found in Holocene and Pleistocene fossils.; This dissertation has improved the ability to identify original amino acids in fossils. The data from Mercenaria spp. have been used to further the understanding of past food web dynamics in estuarine systems. Knowledge of past changes to estuarine systems should help understand and predict impacts to these systems in the future.