THE ROLE OF SULFATE REDUCTION IN ORGANIC MATTER DEGRADATION AND MOLYBDENUM ACCUMULATION: THEORETICAL FRAMEWORK AND APPLICATION TO A CRETACEOUS ORGANIC MATTER BURIAL EVENT

In this study a new methodology for assessment of the controls on organic matter (OM) burial in ancient organic-rich strata is developed. A critical aspect of this methodology is the use of authigenic molybdenum accumulation as a geochemical tracer of OM remineralization rates during anaerobic metabolism. Sensitivity experiments with a geochemical box model for organic matter degradation highlight the dependence of authigenic molybdenum accumulation on organic matter production and export, bulk sedimentation rate, OM dilution, and the location (depth) of the upper interface of the sulfate reduction zone (SRZ) within sediments. These box model results form a conceptual framework for the interpretation of new primary production estimates and geochemical burial fluxes (calculated based on a high-resolution orbital time scale) for the Cenomanian-Turonian Oceanic Anoxic Event II (OAE II) interval in the Western Interior basin. The analysis results in a re-evaluation of the controls on OM burial during OAE II: (1) OM accumulation was controlled by the rate of export of organic matter to the SRZ; (2) the depth of the upper boundary of the SRZ was the first-order control on OM export into the SRZ; and (3) changes in the location of the SRZ are attributable to the balance between metabolic demands within pore waters (i.e., consumption of oxygen and sulfate due to OM degradation), rates of hydrogen sulfide production (the presence of which excludes aerobic bioturbators, the principal agents of oxygen penetration in sediments), and rates of sulfide depletion (dominantly through formation of iron sulfides, limited mainly by reactive Fe supply). These results suggest that the strong correlation between source rock development and intervals of transgression in the geologic record could dominantly reflect a confluence of biogeochemical processes within sediments without invoking fundamental changes in primary production levels or oceanographic conditions (such as prolonged periods of stable water column stratification).