Although widely used as a paleoredox proxy, sedimentary Mo concentrations are in fact not closely correlated to benthic redox conditions in anoxic marine environments with restricted circulation, i.e., silled basins (Algeo and Lyons, in review, Paleoceanography).  In such settings, the rate of Mo removal to the sediment can exceed the rate of Mo resupply through deepwater renewal, resulting in drawdown of aqueous Mo in the subpycnoclinal watermass.  Depletion of deepwater [Mo]_aq reduces the quantity of Mo scavenged per unit organic carbon burial, i.e., (Mo/TOC)_s.  Further, aqueous and sedimentary Mo concentrations are closely related to the residence time of the deepwater mass (τ_res).  For four modern silled basins, Saanich Inlet, Cariaco Basin, Framvaren Fjord, and the Black Sea (in order of increasing restriction): (1) [Mo]_aq values are 80-100%, 70-85%, 20-30%, and 3-5% of the seawater value; (2) (Mo/TOC)_s equals 45±5, 25±5, 9±2, and 4.5±1; and (3) τ_res is <10 y, ~100 y, ~1600 y, and ~2000 y, respectively.  These relationships suggest that (Mo/TOC)_s may have considerable potential as a paleohydrologic proxy, allowing estimation of degrees of watermass restriction in ancient anoxic marine environments.

We investigated the potential of (Mo/TOC)_s as a paleohydrologic proxy through analysis of >40 Middle Devonian-Lower Mississippian marine black shales of the North American craton.  Most formations exhibit statistically significant, positive Mo-TOC covariation with regression-line slopes ranging from ~7 to 50 (similar to slopes of 4.5-45 for modern silled basins).  The highest Mo/TOC values are found in the Sunbury Shale and its stratigraphic equivalents, which record a major eustatic transgression in the earliest Mississippian that resulted in relatively unrestricted watermass conditions across the craton.  The lowest Mo/TOC values are associated with geographically restricted black shales, e.g., the Portwood and Blocher members of the New Albany Shale.  However, a majority of North American Devono-Mississippian black shales yield Mo/TOC values between 12 and 25.  Based on comparison with modern silled basins, these values indicate a moderate degree of watermass restriction (i.e., intermediate between the Cariaco Basin and Framvaren Fjord) and suggest deepwater residence times of ~70-400 y.