Raymond M. Coveney, Jr., W. Lynn Watney, Christopher G. Maples; Contrasting depositional models for Pennsylvanian black shale discerned from molybdenum abundances. Geology ; 19 (2): 147–150. doi: https://doi.org/10.1130/0091-7613(1991)019<0147:CDMFPB>2.3.CO;2
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Some black shales contain >1000 ppm Mo, comparable to conventional ores. These include Mo-rich Pennsylvanian black shales that were deposited rapidly near shore as epeiric seas inundated peat swamps, while coeval beds, less enriched in Mo, accumulated slowly offshore. Concentration of Mo probably resulted from retention of the metal by organic matter in acidic pore fluids characteristic of nearshore deposits. Thus, in Pennsylvanian shales where organic productivity is significant, Mo variations may be used to distinguish nearshore from offshore conditions. Enrichments in Mo occur in black shales of other ages; therefore a high Mo content may be useful as a general criterion for nearshore environments and other conditions with strong pH gradients.