Multiple sulfur and carbon isotope composition of sediments from the Belingwe Greenstone Belt (Zimbabwe): A biogenic methane regulation on mass independent fractionation of sulfur during the Neoarchean?

Abstract

To explore the linkage between mass-independent sulfur isotope fractionation (MIF-S) and δ13Corg excursions during the Neoarchean, as well as the contemporary redox state and biogeochemical cycling of carbon and sulfur, we report the results of a detailed carbon and multiple sulfur (δ34S, δ33S, δ36S) isotopic study of the ∼2.7Ga Manjeri and ∼2.65Ga Cheshire formations of the Ngezi Group (Belingwe Greenstone Belt, Zimbabwe).Multiple sulfur isotope data show non-zero Δ33S and Δ36S values for sediments older than 2.4Ga (i.e. prior to the Great Oxidation Event, GOE), indicating MIF-S thought to be associated with low atmospheric oxygen concentration. However, in several 2.7–2.5Ga Neoarchean localities, small-scale variations in MIF-S signal (magnitude) seem to correlate with negative excursion in δ13Corg, possibly reflecting a global connection between the relative reaction rate of different MIF-S source reaction and sulfur exit channels and the biogenic flux of methane into the atmosphere during periods of localized, microbiologically mediated, shallow surface-water oxygenation.The Manjeri Formation black shales studied here display a wide range of δ13Corg between −35.4‰ and −16.2‰ (average of −30.3±6.0‰, 1σ), while the Cheshire Formation shales have δ13Corg between −47.7‰ and −35.1‰ (average −41.3±3‰, 1σ). The δ34S values of sedimentary sulfides from Manjeri Formation vary between −15.15‰ and +2.37‰ (average −1.71±4.76‰, 1σ), showing very small and mostly negative Δ33S values varying from −0.58‰ to 0.87‰ (average 0.02±0.43‰, 1σ). Cheshire Formation black shale sulfide samples measured in this study have δ34S values ranging from −2.11‰ to 2.39‰ (average 0.25±1.08‰, 1σ) and near zero and solely positive Δ33S anomalies between 0.14‰ and 1.17‰ (average 0.56±0.29‰, 1σ). Moreover, Δ36S/Δ33S in the two formations are comparable with a slope of −1.38 (Manjeri Formation) and −1.67 (Cheshire Formation), respectively. This differs from the Archean reference line (i.e. −0.9).The sulfur and carbon isotopic signatures recorded here likely reflect different environmental conditions and/or ecosystems within the sampled Cheshire and Manjeri formations paleofacies. The differences in carbon isotopes in different sedimentological facies are interpreted as recording different metabolic pathways, including photosynthesis, methanogenesis and methanotrophy. Low δ13Corg values in the Cheshire Formation black shales suggest a strong influence of methanotrophy (likely preceded by bacterial methanogenesis), while the large range of Manjeri Formation δ34S may record bacterial sulfate reduction. The C and multiple S isotopic variations recorded here may imply different patterns of C and S fractionation, perhaps dependent on variations in the biogenic egress of methane to the atmosphere.