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

根据大量有机质的稳定同位素组成解释化石系统的生物化学，假设分子内成岩反应和与保存环境的相互作用对这些系统的同位素完整性的影响最小。对化石生物成因信号的现实评估需要评估成岩过程中可能发生的同位素分馏的限度，并制定独立的标准来评估成分可能与周围环境交换的情况。该项目将开始量化水解和二次成岩反应(例如腐殖化)对化石中肽和蛋白质的氨基酸组成的稳定碳和氮同位素组成的影响。我们最近开发的用于氨基酸对映体纳米摩尔浓度的直接稳定碳同位素分析的气相色谱/同位素比质谱仪(GC/IRMS)方法使我们首次有机会监测成岩反应对大多数蛋白质氨基酸的影响。我们将继续测试用化石中单个氨基酸成分的D-和L-对映体的稳定同位素组成作为这些成分的本土性的独立检查的可能性。目前，GC/IRMS方法仅限于稳定碳同位素的分析，但我们建议对稳定氮同位素的分析系统进行改进。监测单个氨基酸对映体的13C和15N组成的能力提供了对这些成分的内源性的强大的、独立的检查。最后，还计划开发用于化石贝壳碳水化合物组成的13C分析的GC/IRMS方法。

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.