Precipitation of molybdenum from euxinic waters and the role of organic matter

Abstract

Molybdenum preserved in organic-rich shales is a key archive employed in studying Earth's past oxygenation and Life's evolution. In these shales, positive correlations between Mo and C org are common and have been attributed to Mo scavenging from seawater by organic shuttles. Here, we argue that known organic ligands are too weak or too scarce to capture more than minor amounts of Mo from seawater, especially in competition with sulfide. Alternatively, we demonstrate that Mo-C org correlations can arise simply from microbially mediated oxidation of C org by SO 42− . This process produces sulfide and lowers pH, triggering precipitation of FeMoS 4 , which soon transforms irreversibly to FeMoS 2 (S 2 ). In euxinic basins, the extent of sulfate reduction progress increases with depth and can be modeled as an empirical function of biological productivity and water replacement time. The response of dissolved Mo to sulfate reduction progress occurs in three stages identified by a thermodynamic model. In Stage I, progress is too small to precipitate Mo from waters that are only mildly sulfidic. In Stage II, the flux of Mo to sediments is promoted by sulfate reduction progress, giving rise to positive Mo-C org correlations in sediments due to fluctuations in biological productivity. In some cases, precipitation in this stage begins before MoS 42− becomes the predominant dissolved Mo species, in which case FeMoS 4 with negative δ 98 Mo can be produced. In Stage III, the flux of Mo becomes independent of further sulfate reduction progress and Mo-C org correlations degrade, as reported in extremely C org -rich shales. Even though the model omits a number of potentially important processes, its predicted Mo profiles in waters of several modern euxinic basins are qualitatively reasonable and are consistent with observed reduction of Mo VI in water to Mo IV in sediments. According to the model, spikes in atmospheric P CO2 will markedly increase Mo accumulation and lower δ 98 Mo in euxinic basin sediments. This appears to have occurred repeatedly during the Phanerozoic and may be commencing now as a result of fossil fuel combustion. Minimal deposition of Mo in organic-rich shales during the Archean has been attributed to low MoO 42− in seawater but seems more likely to be due to low SO 42− .