Mo–total organic carbon covariation in modern anoxic marine environments: Implications for analysis of paleoredox and paleohydrographic conditions

Abstract

Sedimentary molybdenum, [Mo]s, has been widely used as a proxy for benthic redox potential owing to its generally strong enrichment in organic‐rich marine facies deposited under oxygen‐depleted conditions. A detailed analysis of [Mo]s–total organic carbon (TOC) covariation in modern anoxic marine environments and its relationship to ambient water chemistry suggests that (1) [Mo]s, while useful in distinguishing oxic from anoxic facies, is not related in a simple manner to dissolved sulfide concentrations within euxinic environments and (2) patterns of [Mo]s‐TOC covariation can provide information about paleohydrographic conditions, especially the degree of restriction of the subchemoclinal water mass and temporal changes thereof related to deepwater renewal. These inferences are based on data from four anoxic silled basins (the Black Sea, Framvaren Fjord, Cariaco Basin, and Saanich Inlet) and one upwelling zone (the Namibian Shelf), representing a spectrum of aqueous chemical conditions related to water mass restriction. In the silled‐basin environments, increasing restriction is correlated with a systematic decrease in [Mo]s/TOC ratios, from ∼45 ± 5 for Saanich Inlet to ∼4.5 ± 1 for the Black Sea. This variation reflects control of [Mo]s by [Mo]aq, which becomes depleted in stagnant basins through removal to the sediment without adequate resupply by deepwater renewal (the “basin reservoir effect”). The temporal dynamics of this process are revealed by high‐resolution chemostratigraphic data from Framvaren Fjord and Cariaco Basin sediment cores, which exhibit long‐term trends toward lower [Mo]s/TOC ratios following development of water column stratification and deepwater anoxia. Mo burial fluxes peak in weakly sulfidic environments such as Saanich Inlet (owing to a combination of greater [Mo]aq availability and enhanced Mo transport to the sediment‐water interface via Fe‐Mn redox cycling) and are lower in strongly sulfidic environments such as the Black Sea and Framvaren Fjord. These observations demonstrate that, at timescales associated with deepwater renewal in anoxic silled basins, decreased sedimentary Mo concentrations and burial fluxes are associated with lower benthic redox potentials (i.e., more sulfidic conditions). These conclusions apply only to anoxic marine environments exhibiting some degree of water mass restriction (e.g., silled basins) and are not valid for low‐oxygen facies in open marine settings such as continent‐margin upwelling systems.