MOLYBDENUM ACCUMULATION IN ORGANIC-RICH SEDIMENTS AND SEDIMENTARY ROCKS

The co-accumulation of Mo and organic carbon (OC) is well documented in the sediments of many oxygen-deficient settings. Significant covariation, however, is not universally observed in these settings, and the slope of the line and the strength of the correlation can vary temporally within and among anoxic basins. Primary and secondary causes for these differences will be discussed in the context of water-column, diagenetic and deeper burial processes.

Mechanisms of Mo enrichment have been linked to, among other things, high concentrations of dissolved sulfide; reactions at mineral surfaces, including pyrite; and the amount and type of organic matter present. Ultimately, these relationships can yield a strong positive correlation between Mo enrichment (as expressed in Mo/Al ratios) and OC concentration. However, our mechanistic understanding of this relationship has lagged behind the growing empirical evidence for coupled OC-Mo accumulation. The principal role of OC in Mo sequestration may be in the generation of hydrogen sulfide through bacterial sulfate reduction. This relationship is much like the C-S covariation observed in oxic marine sediments, where greater local OC burial yields higher production of pore-water sulfide and therefore diagenetic pyrite. In euxinic settings, however, local OC accumulation is typically decoupled from sulfide availability and pyrite formation in the overlying water. Thus, the relative amounts of water-column versus diagenetic Mo sequestration should influence the strength of the correlation between Mo enrichment and local OC burial.

Alternatively, the organic matter may play a more direct role in sequestering Mo, in which case the type of organic matter and the extent to which it is sulfurized appear to be critical. Mo accumulation is also ascribed to co-mineralization with pyrite, and Fe would play competing roles by providing pyrite substrates while reducing dissolved sulfide concentrations. In the Cariaco Basin, OC and Mo/Al covary within the sediments but appear decoupled from pyrite S. We will further explore mechanistic models through use of published sediment trap results from anoxic sites, as well as through analysis of OC-rich carbonate muds, where sulfide concentrations are in excess of several mM but pyrite formation is precluded by the lack of Fe.