Geochemical Effects of Early Diagenesis of Organic Matter, Sulfur, and Trace Elements in Devonian Black Shales, Appalachian Basin: ABSTRACT

Abstract

The relation between organic carbon and sulfides in Devonian black shales can be used to identify these units as having been deposited in ancient marine euxinic environments. Based on the modern Black Sea analogy, the euxinic environment is indicated by a positive intercept for sulfur at zero organic carbon on a carbon-sulfur plot. Furthermore, the slope of the plot can be related to position in the basin and to deposition rate. Sulfur-isotope ratios of fine-grained, early diagenetic iron sulfides are typically light, indicating that a majority of the sulfide formed in the water column and near the sediment-water interface. Isolated heavier values are observed, however, which demonstrate that sulfide formation persisted into later diagenesis at least locally. Carbon and o ygen isotopes of carbonate minerals show the effects of both early diagenesis of organic matter by micro-biological processes and later redistribution of carbonate into veins and nodules. A range of ^dgr-values suggests that anaerobic oxidation of organic matter is more important than methane generation for carbonate diagenesis. Trace-element abundances (U, Mo, V, Ni, Hg) are related to organic-carbon and sulfide content. These relations can be explained by invoking an organic-concentration mechanism and aqueous-sulfide protection and redistribution processes. Trace-element/organic and trace-element/sulfide ratios do not change greatly in the basin, although deposition rates vary by more than one order of magnitude. End_of_Article - Last_Page 692------------