Organic Matter Alteration at Active and Relict Sedimentary Oxidation Fronts

Gregory L. Cowie, Stephen E. Calvert, John I. Hedges, Richard G. Keil, Fredrick G. Prahl, Gert J. de Lange, and John Thomson

Sedimentary oxidation fronts, generated by downward diffusion of O₂ into previously reducing sediments, provide an opportunity to study the effects of O₂ exposure on organic matter preservation, where other factors are invariant. Oxidation fronts have now been identified in a number of environments, including turbidites found on the Madeira abyssal plain (NE Atlantic) and in sapropels from the Mediterranean. A multifaceted study is being carried out on a series of relict fronts in turbidites of varied origin and composition, recovered from the Madeira abyssal plain on ODP Leg 157. A similar study is being made on an active front in the most recent, S1 sapropel, from the central Mediterranean. In both cases, the relatively organic-rich turbidites and apropels are intercalated with typical, organic-poor pelagic marls. Analyses include major and minor inorganic elements, stable C and N isotopes, palynomorphs, surface areas and a comprehensive suite of biochemicals.

Although the extent of alteration varies, striking changes in organic content and composition occur across many of the oxidation fronts, and the oxidized horizons resemble the interclated marls. A common trend is that organic C contents drop from "monolayer equivalent" sorptive loadings in the unoxidized horizons (typical of continental margin sediments) to submonolayer loadings, typical of the pelagic marls. Shifts are also observed in redox-sensitive trace metals and in stable C and N isotopic compositions, although the patterns and extent are not uniform. The results clearly indicate that in these pelagic settings, where O₂ exposure is typically long, organic material deposited and then preserved for extended periods under anomalous reducing conditions, can be extensively altered on relatively short-term exposure to O₂. Implications of these findings will be discussed.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California