The trace element inventory of seawater is likely to have varied in the past owing to changes in the proportion of oxic-suboxic versus anoxic marine facies. Because only ∼0.3% of the modern global seafloor is subject to benthic anoxia, removal of redox- sensitive trace elements from seawater is largely mediated by oxic-suboxic facies. This results in higher concentrations and longer residence times for such elements in seawater compared with strongly anoxic paleoseas such as those of the Late Devonian. Reservoir modeling of Mo in Late Devonian seawater suggests that its concentration could have been reduced to ∼30%–50% of the present-day value. Chemostratigraphic data from Upper Devonian black shales of the Central Appalachian Basin provide empirical evidence of such depletion: sedimentary Mo concentrations decline within the Cleveland Shale despite intensified benthic anoxia, as proxied by degree of pyritization. Mo may have been depleted in penecontemporaneous seawater as a consequence of elevated burial fluxes associated with black shale deposition at time scales of 105 to >106 yr.

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