Diagenetic isotope effects in an anoxic marine sediment

The carbon isotope effects associated with the remineralization of organic matter in the anoxic sediments of Cape Lookout Bight, NC, have been investigated. Total organic carbon δ13C profiles and an isotopic mass balance of the diagenetic products, σCO2 and methane, indicate that the initial step in the breakdown of the organic matter, i.e. the transformation of biopolymeric material to biomonomers, does not express a significant isotope effect. Intramolecular δ13C measurements of the diagenetic intermediate, acetate indicates that large isotope effects (5-10 per-thousand) occur during the biosynthesis of that compound. Downcore δ13C variations in the acetate appear to be related to fractionations associated with methanogenesis. The fractionation factor for the conversion of the acetate methyl group to methane has been estimated to be 1.02 from the porewater profiles