Authigenic molybdenum formation in marine sediments: A link to pore water sulfide in the Santa Barbara Basin

Pore water and sediment Mo concentrations were measured in a suite of multicores collected at four sites along the northeastern flank of the Santa Barbara Basin to examine the connection between authigenic Mo formation and pore water sulfide concentration. Only at the deepest site (580 m), where pore water sulfide concentrations rise to >0.1 μM right below the sediment water interface, was there active authigenic Mo formation. At shallower sites (550, 430, and 340 m), where pore water sulfide concentrations were consistently <0.05 μM, Mo precipitation was not occurring at the time of sampling. A sulfide concentration of ∼0.1 μM appears to be a threshold for the onset of Mo-Fe-S co-precipitation. A second threshold sulfide concentration of ∼100 μM is required for Mo precipitation without Fe, possibly as Mo-S or as particle-bound Mo.