Unravelling the extent of ocean euxinia during the late Paleoproterozoic: Constraints from Re–Os and Mo isotopes

Oceanic euxinia was one of the prime constraints for the Proterozoic nutrient availability and hence, eukaryotic expansion. Although available studies mostly point to the occurrence of sulfidic to ferruginous conditions in the Paleoproterozoic ocean, data on the areal extent of sulfidic settings are scarce. In this study, geochemical (major and trace elements, and iron-speciation), and Re–Os and Mo isotopic data for black shales from the Cumbum Formation, Cuddapah Basin (India) are presented. These datasets are used to constrain the depositional age and environment, and euxinic extent for the late Paleoproterozoic ocean. The Re–Os isochron for these shales provides a depositional (Model 3) age of 1658 ± 50 Ma (2σ, n = 10), with an initial ¹⁸⁷Os/¹⁸⁸Os (Os_i) ratio of 0.03 ± 0.53. Distribution of enrichment factors (EF) for redox-sensitive elements and their (Mo_EF/U_EF) ratio for the Cumbum shales indicates shale deposition in an anoxic basin. Further, high Fe_HR/Fe_T (0.61 ± 0.18 (1σ)) and Fe_Py/Fe_HR (0.72 ± 0.14 (1σ)) ratios confirm euxinic to ferruginous bottom-water conditions. The δ⁹⁸Mo values of these shales (+0.68 ± 0.13 ‰ (1σ)) are comparable to Paleoproterozoic shales (+0.51 ± 0.57 ‰ (1σ)) and systematically lower in comparison to the present-day average seawater (+2.34 ± 0.10 ‰ (1σ)) and euxinic sediments (+1.8 ± 0.4 ‰ (1σ)) values. A mass balance modelling using Mo isotopic values of euxinic shales, their major sources and sinks, and related fractionation factors suggests an order of magnitude higher extent of ocean euxinia (∼5% of seafloor area) at 1.66 Ga than that observed for modern oceans (0.1 to 0.3 %). Such extensive euxinia in shallow-water environments under a weakly oxygenated atmosphere (≥0.1 % PAL to ≤40 % PAL) could have played a major role in delaying the emergence of eukaryotes and multicellular life.