ENGEL, MICHAEL H., and Y. QIAN, Geology Dept., University of Oklahoma, Norman, OK; and S. A. MACKO, University of Virginia, Charlottesville, VA

ABSTRACT: Stable Isotopes as Probes for the Early Diagenesis of Organic Matter in Fossils and Sediments

The recent development of combined gas chromatography/isotope ratio mass spectrometry (GC/IRMS) permits a direct assessment of early diagenetic reactions based on the stable carbon isotope composition of individual organic compounds in geologic materials. Two applications of this method are presented.

Partial hydrolysis of synthetic dipeptides resulted in carbon and nitrogen isotope fractionations of several per mil between free amino acids (depleted) and the residual dipeptides (enriched). In this study, modern shell was heated (80 degrees C) for up to 266 days. Proteinaceous material isolated from the shells was separated into molecular weight fractions

(<500 to >10,000 daltons). Preliminary GC/IRMS analyses of individual amino acids in the various fractions suggests that isotope fractionation during partial hydrolysis of the proteinaceous material in the shell matrix is minimal. It is suggested that the fractionation previously observed for the partial hydrolysis of dipeptides is masked in these higher molecular weight fractions.

A second set of experiments were performed to assess changes in the isotopic composition of amino acids and sugars during early diagenesis of organic matter in recent sediments. Seagrass was allowed to decompose in the presence and absence of marine sediments for up to 4 weeks. Preliminary results reveal complex pathways for proteins and carbohydrates and indicate that; a portion of the organic matter eventually preserved in organic-rich deposits can be attributed to new production in the deposit, rather than simple losses of biomonomers via hydrolysis. Stable isotopes were also used as probes to follow diagenetic pathways during the condensation of amino acids and sugars to form melanoidins. Preliminary results indicate that stable isotope fractionation that occurs during the transfo mation of organic matter to humic materials might be at least partially explained by kinetic effects during condensation reactions rather than decarboxylation of the primary amino acids.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.