Trace elemental and multi-isotopic (δ⁹⁸Mo, δ³⁴S, δ¹⁵N) study of Proterozoic shales: Implications to ocean euxinia and productivity patterns

Abstract

Oxygenation of the ocean-atmosphere system had a profound effect on the origin and evolution of multicellular life on the Earth. Available geological and geochemical studies document significant fluctuations in Proterozoic seawater redox state and biogeochemical cycling, capable of imparting primary control on eukaryotic expansion. However, the exact linkage between environmental changes and the emergence of complex life is not well-established due to temporal gaps. This thesis work presents a detailed geochemical and isotopic study of organic-rich shales from three major Proterozoic sedimentary successions (Cuddapah Supergroup, Vindhyan Supergroup and the Lesser Himalaya) to (a) reconstruct the depositional environment, (b) quantify the areal extent of ocean euxinia, and (c) evaluate the modes of nitrogen cycling in these basins. A multi-proxy approach using redox-sensitive trace elemental abundances (e.g., Mo, U, Mn and Cd), stable isotopic compositions (δ¹⁵N, δ³⁴S and δ⁹⁸Mo), and Fe-speciation data has been adopted to achieve these objectives.

The ¹⁸⁷Re−¹⁸⁷Os radiometric dating of organic-rich shales from the Cumbum Formation (Nallamalai Group, Cuddapah Supergroup) have yielded their direct depositional age (1658 ± 50 Ma (2σ, n = 10)). Distributions of multiple redox indices (Mo/TOC, MoEF/UEF and Cd/Mo) for these shales and their comparison with datasets from modern-day settings indicate a limited productivity regime with restricted basinal settings during the shale deposition. Further, high FeHR/FeT (0.61 ± 0.18) and FePy/FeHR (0.72 ± 0.14) ratios support a fluctuating (euxinic to ferruginous) bottom-water condition in this basin. The δ⁹⁸Mo values of these shales (+0.68 ± 0.13‰) and their mass balance modelling (for euxinic shales) suggest a wide extent of ocean euxinia (~5% of seafloor area) at 1.66 Ga, an order of magnitude higher than that observed for modern oceans (0.1% to 0.3%). A similar modelling effort using previously reported δ⁹⁸Mo data of euxinic shales from other global sections supports a relatively stable but high (average 5 ± 4%) extent of Proterozoic oceanic euxinia. These evaluations also reveal episodes of intensified oceanic euxinia during the Proterozoic, which often follow the fluctuations in atmospheric oxygen levels.

Shale δ⁹⁸Mo data and their mass balance calculations have also been used to quantify the extent of euxinia around the Precambrian-Cambrian (Pc-C) transition. These shales belong to the Tal Formation (Lesser Himalaya), and their Iron speciation and Mo/U data point to anoxic and ferruginous deep water conditions. Calculations involving the δ⁹⁸Mo (1.5 ± 0.2‰) data for these shales estimate ~4 times higher pyrite burial rates and ~2 times extensive extent of sulfidic conditions during the Pc-C transition when compared to that for present-day oceanic conditions. Further, these shales were also characterized by isotopically heavier pyrite-δ³⁴S values (3.6‰ to 8.3‰) compared to modern-day sedimentary pyrites (~ −21‰). Earlier reported δ³⁴S values for early Cambrian seawater, measured δ³⁴S pyrite data and their empirical relationship 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), potentially related to the increasing oxygen availability and continental supply during the Pc-C transition.

Further, the effect of the temporal and spatial redox heterogeneities in the primary productivity was investigated by comparing the nitrogen isotopic compositions of organic-rich shale sequences from these different basins. The (molar) TOC/TN ratios of these shales vary between 12 and 52 and are systematically higher than the Redfield ratio. The δ¹⁵N values for these shales are consistently positive and range from +0.8‰ to +8.6‰. The lack of correlations between δ¹⁵N-TN, δ¹⁵N-TOC/TN and δ¹⁵N-δ¹³C suggests a limited role of thermal effects, post-depositional alterations and mobilization processes in affecting the isotopic signals. The bulk nitrogen isotopic composition (δ¹⁵Nbulk) of the Bijaigarh and Lower Tal shales (~1−2‰) overlap with sedimentary δ¹⁵N values characteristic of environments where N-fixing processes using Mo-nitrogenase are dominant. The highly enriched δ¹⁵Nbulk values (>3‰) for the Tadpatri, Kajrahat and Cumbum shales suggest the occurrence of aerobic modes of nitrogen cycling in these basins, where processes such as nitrification, denitrification/anammox and ammonium assimilation were active. Further, a steady-state box model using the measured shale δ¹⁵N values establishes the prevalence of nitrate rich-waters and the dominance of nitrate assimilators in most of these sections.