Geochemical and Mo-S isotopic compositions of black shales overlying the Pc-C boundary (Tal Formation, Lesser Himalaya)

We present here chemical (trace elements and Fe-speciation) and isotopic (δ³⁴S and δ⁹⁸Mo) data for black shales from the Tal Formation, Lesser Himalaya (India). This dataset was used to reconstruct oceanic redox state, their areal extent and oceanic sulfate concentrations during the Precambrian-Cambrian (Pc-C) transition. The Pc-C boundary represents a period of large-scale tectonic, climatic, and biotic changes. Observed enrichment in redox-sensitive elements and variation in relative abundances of Fe-species in these shales point to anoxic and ferruginous (iron-rich) deep water conditions in the shelf regions during Tal shale deposition. Further, the Mo isotopic data reveal higher areal extent of sulfidic conditions during this period (than the modern-day seafloor), whereas sulfur isotopic data confirm a lower sulfate concentration at around 539 Ma. These sulfate concentrations, however, are found to be higher than the Neoproterozoic ocean, confirming increased terrestrial input and oxygen in the ocean-atmospheric system during the Pc-C. These observed environmental conditions would have influenced the subsequent development and diversification of multicellular life during the early Cambrian.