科研之友
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摘要
Multiple sulfur isotope ratios (S-36/S-34/S-33/S-32) and organic carbon isotope ratios (C-13/C-12) were measured from 198 non-conglomeratic sedimentary samples that were collected from five deep diamond drill-cores that cover the majority of the ca. 2.96-2.82 Ga Witwatersrand Supergroup. delta C-13(org). delta S-34(py) and Delta S-33(py) values of the sample set range from -44.3 to -21.9%, -3.7 to +16.5%. and -4.0 to +1.9 parts per thousand, respectively. These geochemical data vary relative to depositional facies (proximal marine, distal marine and fluvial-dominated) and tectonic setting (trailing margin and foreland basin). In the trailing margin setting of the similar to 2.96 Ga Hospital Hill Subgroup, the proximal marine depofacies is characterized by relatively high organic carbon contents (up to 0.9 wt.%) and delta C-13(org) values around -28 parts per thousand, in contrast to the distal marine depofacies that yields low organic carbon contents (0.01 wt.%) and high delta C-13(org) values (up to -22 parts per thousand). Both depofacies yield low sulfur contents (0.02 wt.parts per thousand), a narrow range of delta S-34 values (similar to+3 +/- 2 parts per thousand) and positive Delta S-33 values (up to +1.9 parts per thousand). This data is consistent with photoautotrophic carbon fixation in shallow marine environments and limited organic carbon production/preservation in deeper water settings due to longer transport distances and effective biological degradation (e.g., Fe-reducing microbes). Positive Delta S-33 values imply that sulfur was largely derived from a photochemical elemental sulfur reservoir. In the foreland basin tectonic setting of the similar to 2.94 Ga Government and similar to 2.92 Ga Jeppestown subgroups, shelf deposits associated with fluvial braidplain depofacies are characterized by C-13-depleted organic carbon (similar to-44 to -38 parts per thousand), relatively high sulfur contents (0.2-1.3 wt.%), variable delta S-34 values (-3.7 to +16.5 parts per thousand) and small negative Delta S-33 values (similar to-0.4 parts per thousand). These data suggest that the microbial community in the fluvial-dominated depofacies may have consisted of photoautotrophs, methanogens, anaerobic methanotrophs and sulfate reducers. Sulfate was derived from a variety of sources that include photochemical, crustal and marine sulfur reservoirs. The occurrence of three regionally persistent diamictite deposits in the Government Subgroup coincides with the onset of compressional tectonics and development of pyritic shales with highly C-13-depleted organic carbon, suggesting that an increase in continental sulfur flux and methane oxidation may have triggered the Mesoarchean glaciations (drawdown of H-2 and CH4). However, the link between large Delta S-33 anomalies (-4.0 to +1.2 parts per thousand) and diamictite suggests low levels of atmospheric oxygen and minimal dilution of photochemical signatures.
- 出版日期2012-10
