Stable Isotope Geochemistry at the Molecular Level: An Assessment of the Diagenesis of Organic Matter in Fossil Shells

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Interpretation of the biochemistry of fossil systems based on the stable isotope composition of bulk organic matter assumes that intramolecular diagenetic reactions and interactions with the preserving environment have a minimal effect on the isotopic integrity of these systems. A realistic assessment of biogenic signals from fossils requires an appraisal of the limits of isotopic fractionation that can occur during diagenesis and the development of independent criteria for evaluating the possible exchange of components with the surrounding environment. This project will begin to quantify the effects of hydrolysis and secondary diagenetic reactions (e.g. humification) on the stable carbon and nitrogen isotope compositions of amino acid constituents of peptides and proteins in fossils. Our recent development of a combined gas chromatography/isotope ratio mass spectrometry (GC/IRMS) methods for the direct stable carbon isotope analysis of nanomole concentrations of amino acid enantiomers permits, for the first time, an opportunity to monitor the effects of diagenetic reactions on most of the protein amino acids. We will continue to test the possibility of using the stable isotope compositions of the D- and L-enantiomers of individual amino acid constituents of fossils as an independent check on the indigeneity of these components. At present, the GC/IRMS method is limited to the analysis of stable carbon isotopes; however we propose to modify the system for the analysis of stable nitrogen isotopes. The ability to monitor both 13C and 15N compositions of individual amino acid enantiomers affords a powerful, independent check on the indigeneity of these components. Finally, experiments are also planned to develop GC/IRMS methods for the 13C analysis of carbohydrate constituents of fossil shells.