A Euxinic-Wedge Model for Mo Cycling in the Early Cambrian Oceans

Abstract:

Ocean redox conditions during the early Cambrian, a critical period in Earth history marked by rapid evolution of animals, are uncertain. Previous studies have suggested well oxic conditions based on Mo isotopes and stratified, anoxic deep waters based on Fe-S-C and trace metal data. The Niutitang Formation (529-514 Ma) at Yangjiaping (South China) consists of a lower black shale member (LM, 0-18 m) and an upper gray silty shale member (UM, 24-80 m). The LM is moderately to strongly enriched in Mo, U and V, and heterogeneous in Mo-isotopic compositions (δ⁹⁸Mo = ‒0.65‰ to +2.14‰), while the UM shows lesser enrichment of Mo, U and V, higher Mo/U ratios, and intermediate and more uniform δ⁹⁸Mo (+1.16‰ to +1.71‰). The metal data and previously reported Fe-speciation data suggest a redox change from dominantly euxinic conditions for the LM to suboxic or weakly anoxic-euxinic conditions for the UM, consistent with a shift of the chemocline from the water column to the sediment during a eustatic fall.

We propose an “euxinic wedge” model for the δ⁹⁸Mo patterns observed here. In this model, the early Cambrian ocean exhibited a lateral transition from ferruginous conditions in nearshore areas to euxinic conditions at mid-depths and back to ferruginous conditions in offshore areas. This pattern was the result of terrigenous Fe-Mn_ox reduction, sulfate resupply via rivers, and hydrothermal inputs of reactive Fe into the deep basin. Within this framework, sedimentary δ⁹⁸Mo variation in the study units can be explained through the combined effects of Mo isotopic fractionation associated with Fe-Mn_ox adsorption and depth-dependent H₂S concentrations. Importantly, the heaviest δ⁹⁸Mo values, which were previously interpreted to reflect full oxygenation, can be explained by the operation of a globally widespread Fe-Mn particulate shuttle that removed large quantities of isotopically light Mo to the sediment, leaving seawater enriched in ⁹⁸Mo.