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?
Résumé
To explore the linkage between mass-independent sulfur isotope fractionation (MIF-S) and d13Corg 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 (d34S, d33S, d36S) isotopic study of the 2.7 Ga Manjeri and 2.65 Ga Cheshire formations of the Ngezi Group (Belingwe Greenstone Belt, Zimbabwe). Multiple sulfur isotope data show non-zero D33S and D36S values for sediments older than 2.4 Ga (i.e. prior to the Great Oxidation Event, GOE), indicating MIF-S thought to be associated with low atmospheric oxygen concentration. However, in several 2.7-2.5 Ga Neoarchean localities, small-scale variations in MIF-S signal (magnitude) seem to correlate with negative excursion in d13Corg, 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 d13Corg between 35.4‰ and 16.2‰ (average of 30.3 ± 6.0‰, 1r), while the Cheshire Formation shales have d13Corg between 47.7&and 35.1‰ (average 41.3 ± 3‰, 1r). The d34S values of sedimentary sulfides from Manjeri Formation vary between 15.15‰ and +2.37‰ (average 1.71 ± 4.76‰, 1r), showing very small and mostly negative D33S values varying from 0.58‰ to 0.87‰ (average 0.02 ± 0.43‰, 1r). Cheshire Formation black shale sulfide samples measured in this study have d34S values ranging from 2.11‰ to 2.39‰ (average 0.25 ± 1.08‰, 1r) and near zero and solely positive D33S anomalies between 0.14‰ and 1.17‰ (average 0.56 ± 0.29‰, 1r). Moreover, D36S/D33S 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 d13Corg 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 d34S 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.