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

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 Corg 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-Corg correlations can arise simply from microbially mediated oxidation of Corg by SO42−. This process produces sulfide and lowers pH, triggering precipitation of FeMoS4, which soon transforms irreversibly to FeMoS2(S2). 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-Corg correlations in sediments due to fluctuations in biological productivity. In some cases, precipitation in this stage begins before MoS42− becomes the predominant dissolved Mo species, in which case FeMoS4 with negative δ98Mo can be produced. In Stage III, the flux of Mo becomes independent of further sulfate reduction progress and Mo-Corg correlations degrade, as reported in extremely Corg-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 MoVI in water to MoIV in sediments. According to the model, spikes in atmospheric PCO2 will markedly increase Mo accumulation and lower δ98Mo 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 MoO42− in seawater but seems more likely to be due to low SO42−.