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.
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The shale samples were powdered to >125 mesh using an agate mortar and pestle. Concentrations of major oxides were measured using a wavelength dispersive- X-ray Fluorescence Spectrometer by preparing fusion beads at 1100°C. Trace elemental analyses were carried out following the analytical methodology of Tripathy et al. (2014). Samples were dissolved using HF-HNO₃-HCl acids using a microwave digestion system, and these solutions were measured in Q-ICPMS for trace elemental concentrations. Inorganic carbon content of these shales was measured using a UIC CO₂-Coulometer, whereas total carbon, nitrogen and sulfur were measured by combusting the samples in a CHNS elemental analyzer. The difference between total and inorganic carbon values yielded the organic carbon content. Principal component analysis (PCA) of the geochemical dataset was done using the PAST (Paleontological Statistics; v. 4.08; Hammer et al., 2001) software. Abundances of iron species in the samples were quantified following the analytical methodology of Poulton and Canfield (2005). The Fe content of extracted iron pools was measured using an Atomic Absorption Spectrometer. For pyrites, the iron content was quantified gravimetrically by separating Ag₂S precipitates using a Cr-reduction approach (Canfield et al., 1986). The sulfur isotopic compositions of these Ag₂S precipitates were also measured using an Isotope ratio mass spectrometer to determine the pyrite-δ³⁴S values. Mo-isotopic measurements were carried out using MC-ICPMS following a double spike technique (Goswami et al., 2022). Towards this, the digested solution of shales was appropriately equilibrated with a known amount of ⁹⁷Mo-¹⁰⁰Mo tracers, and the pure Mo fraction of the equilibrated solution was extracted following a conventional anion-chromatographic approach. Isotopes of these pure Mo cuts were measured in an MC-ICPMS instrument. These isotopic signals were corrected for instrumental fractionation and isotope dilution to yield δ⁹⁸Mo for the samples.
Department of Science and Technology, Ministry of Science and Technology, India
Science and Engineering Research Board