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

Abstract

[1] 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.Citation: Algeo, T. J., and T. W. Lyons (2006), Mo -total organic carbon covariation in modern anoxic marine environments: Implications for analysis of paleoredox and paleohydrographic conditions, Paleoceanography, 21, PA1016,

A model for the oceanic mass balance of rhenium and implications for the extent of Proterozoic ocean anoxia

“…Indeed, the Re concentration and burial flux of Unit I sediments in the Black Sea are generally lower than those of the Cariaco Basin and other relatively open-ocean anoxic settings (Table 2; keeping in mind that sedimentation rate and organic carbon flux also locally influence Re concentrations in organic-rich sediments). A similar relationship between metal concentrations in bottom waters and organic-rich sediments in anoxic basins has been noted for other redox-sensitive elements (e.g., Mo, U, and Zn; Algeo and Lyons, 2006;Scott et al, 2008Scott et al, , 2013Partin et al, 2013).…”

“…This approach enables an internally consistent compilation that includes samples deposited from weakly to mildly oxygenated waters (with sediment anoxia) and from fully anoxic waters. We assume that levels of authigenic Re enrichment in anoxic sediments generally scale to firstorder with water column Re concentrations (i.e., following Algeo and Lyons, 2006;Scott et al, 2008, for Mo), thus allowing us to track broad first-order changes in the Re concentration of ORM via comparison of time-bin averages. Although Re concentrations of less than 5.0 ppb may occur in some Archean ORM deposited beneath a fully anoxic atmosphere and oceans, only a minority of Archean samples had sufficiently low Re concentrations to be excluded from the compilation.…”

“…For a firstorder mass-balance approach, a is set to unity to reflect the principle that, within a particular sink setting, the burial rate of a metal scales linearly with the size of its seawater reservoir (cf., Algeo and Lyons, 2006;Scott et al, 2008;Partin et al, 2013). Substitution and rearrangement of equations 3-5 yield a generalized expression for seawater Re concentration under new steady-state conditions following a perturbation to the global Re oceanic mass balance (cf., Reinhard et al, 2013a):…”

Basinal restriction, black shales, Re‐Os dating, and the Early Toarcian (Jurassic) oceanic anoxic event

“…Because of this removal, the renewal time i.e., degree of restriction, of the subpycnoclinal water mass in modern, euxinic, marine basins, can be estimated from the slope of the regression line between Mo and TOC (rsMo/TOC hereinafter, to distinguish it from raw Mo/TOC ratios) in a crossplot of TOC against Mo in sediments [ Algeo and Lyons, 2006]. The method works only where euxinic conditions exist(ed) both in the sediment and the overlying water column.…”

“…[8] The rsMo/TOC value of a sediment reflects the balance of fluxes into and out of a restricted parcel of overlying seawater from which the nonclastic component of the sedimentary Mo is derived; i.e., the frequency of deepwater renewal [Algeo and Lyons, 2006]. Molybdenum concentrations in euxinic bodies of seawater decline if the removal rate of Mo into underlying sediments exceeds the rate of replenishment of Mo to the water column; under such conditions, the Mo/TOC of depositing sediment declines as the Mo reservoir in solution is drawn down.…”