Multiple sulfur isotope geochemistry of Dharwar Supergroup, Southern India: Late Archean record of changing atmospheric chemistry

Abstract

Earth's tectonic and climatic systems may have changed fundamentally before the Great Oxidation Event (GOE) at about 2.3 Ga. Sulfur Mass Independent Fractionation (S-MIF) has demonstrated that Earth's atmosphere was virtually oxygen-free before the GOE. During 3.0 to 2.4 Ga, the change in Δ33S and Δ36S signals may reflect the perturbation of atmospheric chemistry, though the mechanisms of the change are uncertain. Here, we reported multiple sulfur isotopic studies of Archean volcano-sedimentary sequences of the Dharwar Supergroup, distributed in the Chitradurga Schist Belt (CSB), Southern India. New field mapping and zircon U–Pb dating allows us to reconstruct detailed lithostratigraphy of the Dharwar Supergroup. The lower unit consists of post-3.0 Ga conglomerate, stromatolitic carbonate, siliciclastics with diamictite, chert/BIF and pillowed basalt in ascending order, all of which are older than the 2676 Ma dacite dyke that had intruded into the lower unit. The upper unit unconformably overlies the pillow basalts at the top of the lower unit, and consists of conglomerate/sandstone with ∼2600 Ma detrital zircons, komatiitic basalt, BIF and siliciclastic sequence with mafic volcanics. Sulfur isotope analysis of extracted sulfides shows MIF signals (ΔS33>+1‰) with clear ΔS33–ΔS36 correlations. The lower group of the Dharwar Supergroup shows a ΔS36/ΔS33 slope of −1.48, the middle group shows −1.16 and −1.07, and the upper group shows −0.94. Reassessment of all the Archean S-MIF records from sedimentary rocks indicates that the ΔS36/ΔS33 slope systematically changed during the Archean period. The observed trend in the Indian section is similar to those of its Pilbara–Kaapvaal equivalents, thus it could reflect a global atmospheric signature. Moreover, the isotopic trend seems to correlate with mid-Archean glaciation. Thus, the ΔS36/ΔS33 slope could be a useful tracer for atmospheric chemistry and its link with climate change before the GOE.