Hydrographic conditions of the Devono–Carboniferous North American Seaway inferred from sedimentary Mo–TOC relationships

Abstract

Deepwater anoxia developed widely within a series of silled intracratonic basins in North America during the Middle Devonian to Early Carboniferous, but the hydrographic factors contributing to this development are virtually unknown. The present study demonstrates that the degree of enrichment of strongly hydrogenous elements such as molybdenum (Mo) in the organic-rich shales that accumulated in these basins has the potential to provide information about paleohydrographic conditions. Most of the study units (49 of 55) exhibit significant positive covariation between elemental Mo and TOC concentrations, yet there is substantial variation in the statistical parameters that define these relationships. Specifically, the regression lines characterizing Mo–TOC covariation have slopes ( m) ranging from ∼2 to 65 (× 10 − 4 ), although a majority of the study units yield m values between 10 and 25 (× 10 − 4 ). The parameter m exhibits systematic stratigraphic variation within individual basins and geographic variation among basins, suggesting that Mo–TOC relationships were controlled by slowly evolving watermass characteristics of the Devono–Carboniferous North American Seaway. A recent study of modern anoxic silled basins [Algeo, T.J., Lyons, T.W., 2006. Mo-total organic carbon covariation in modern anoxic marine environments: Implications for analysis of paleoredox and paleohydrographic conditions. Paleoceanography 21, PA1016. doi:10.1029/2004PA001112] demonstrated that m is closely linked to the degree of restriction of the subpycnoclinal watermass, as reflected in aqueous Mo concentrations ([Mo] aq) and deepwater renewal times ( τ dw). These relationships reflect control of sedimentary Mo enrichment by both the amount of sedimentary organic matter (i.e., host-phase availability) and the concentration of aqueous Mo (i.e., source-ion availability). When applied to black shales of the Devono–Carboniferous North American Seaway, these relationships permit both qualitative and quantitative inferences regarding the degree of restriction of deepwaters in cratonic basins and the evolution of hydrographic conditions through time. Black shales associated with early stages of sea-level rise commonly exhibit low m values (< 10) owing to limited transgression of basin-margin sills, implying [Mo] aq < 20% of the normal seawater concentration and τ dw > 1000 yr. Higher sea-level stands resulted in more open-marine conditions and enhanced deepwater renewal, leading to deposition of black shales with m values up to ∼ 65, equivalent to [Mo] aq > 70% of the normal seawater concentration and τ dw < 100 yr. The regression lines characterizing Mo-TOC covariation also exhibit considerable variation in their X-intercepts ( b x ). The source of this variation is less well understood but probably reflects facies-specific relationships between organic carbon sinking fluxes and benthic redox potential, which are influenced by sedimentation rates, organic matter lability, the oxygen status of renewing deepwaters, and other factors. This study demonstrates that sedimentary Mo–TOC data have considerable potential for analysis of paleohydrographic conditions in anoxic marine facies.