Oceanic Redox State During the Early Cambrian: Insights From Mo-S Isotopes and Geochemistry of Himalayan Shales

Abstract

The Precambrian-Cambrian (Pc-C) boundary marks significant biological, atmospheric, and oceanic changes. These changes include extinction of the Ediacaran fauna, initiation of complex lifeforms, and oxygenation of the atmosphere and oceans. In this contribution, elemental and Mo-S isotopic compositions of organic-rich shales overlying the Pc-C boundary from the Tal Formation, Lesser Himalaya, have been investigated. These datasets are used to reconstruct past oceanic redox state and sulfate concentrations. The principal component analysis of the elemental dataset identifies six major factors, with factors associated with organic matter and sulfide phases accounting for about half of the total variance. Iron speciation and Mo/U data suggest that the shales were deposited in anoxic and ferruginous deep water conditions. The δ98Mo data (1.5 ± 0.2‰) and their mass balance calculations indicate that the areal extent of sulfidic waters and pyrite burial rates were about 2–4 times higher during the Pc-C transition than in the modern ocean. The pyrite-δ34S values (3.6–8.3‰) for the Tal shales are isotopically heavier compared to modern-day sedimentary pyrites (∼−21‰). Calculations involving earlier-reported δ34S values for early Cambrian seawater and our measured pyrite-δ34S data estimate the seawater sulfate concentration (8 ± 3 mM) during their deposition. This sulfate value for the Tal basin is higher than that reported for the late Neoproterozoic ocean (<5 mM), attributable to increasing oxygen availability and continental supply during this time. The observed basinal conditions and high terrestrial input may have influenced metazoan diversification.