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摘要
To explore the linkage between mass-independent sulfur isotope fractionation (MIF-S) and delta C-13(org) 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 (delta S-34, delta S-33, delta S-36) isotopic study of the similar to 2.7 Ga Manjeri and similar to 2.65 Ga Cheshire formations of the Ngezi Group (Belingwe Greenstone Belt, Zimbabwe). Multiple sulfur isotope data show non-zero Delta S-33 and Delta S-36 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 delta C-13(org), 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. %26lt;br%26gt;The Manjeri Formation black shales studied here display a wide range of delta C-13(org) between -35.4 parts per thousand and -16.2 parts per thousand (average of -30.3 perpendicular to 6.0 parts per thousand, 1 sigma), while the Cheshire Formation shales have delta C-13(org) between -47.7 parts per thousand and -35.1 parts per thousand (average -41.3 perpendicular to 3 parts per thousand, 1 sigma). The delta S-34 values of sedimentary sulfides from Manjeri Formation vary between -15.15 parts per thousand and + 2.37 parts per thousand (average -1.71 +/- 4.76 parts per thousand, 1 sigma), showing very small and mostly negative Delta S-33 values varying from -0.58 parts per thousand to 0.87 parts per thousand (average 0.02 +/- 0.43 parts per thousand, 1 sigma). Cheshire Formation black shale sulfide samples measured in this study have delta S-34 values ranging from -2.11 parts per thousand to 2.39 parts per thousand (average 0.25 perpendicular to 1.08 parts per thousand, 1 sigma) and near zero and solely positive Delta S-33 anomalies between 0.14 parts per thousand and 1.17 parts per thousand (average 0.56 +/- 0.29 parts per thousand, 1 sigma). Moreover, Delta S-36/Delta S-33 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). %26lt;br%26gt;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 delta C-13(org) 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 delta S-34 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.
- 出版日期2013-11-15
