A multiple sulfur and organic carbon isotope record from non-conglomeratic sedimentary rocks of the Mesoarchean Witwatersrand Supergroup, South Africa

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• 作者：B.M. Guy ; S. Ono ; J. Gutzmer ; A.J. Kaufman ; Y. Lin ; M.L. Fogel ; N.J. Beukes
• 关键词：Diagenetic pyrite ; Mesoarchean ; Sulfur mass-independent fractionation ; Witwatersrand Supergroup ; Anaerobic methanotrophy ; Diamictite
• 刊名：Precambrian Research
• 出版年：2012
• 出版时间：October, 2012
• 年：2012
• 卷：216-219
• 期：Complete
• 页码：208-231
• 全文大小：4090 K

文摘

Multiple sulfur isotope ratios (³⁶S/³⁴S/³³S/³²S) and organic carbon isotope ratios (¹³C/¹²C) 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. ¦Ä¹³C_org, ¦Ä³⁴S_py and ¦¤³³S_py values of the sample set range from ?44.3 to ?21.9¡ë, ?3.7 to +16.5¡ë and ?4.0 to +1.9¡ë, 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 ¡«2.96 Ga Hospital Hill Subgroup, the proximal marine depofacies is characterized by relatively high organic carbon contents (up to 0.9 wt. % ) and ¦Ä¹³C_org values around ?28¡ë, in contrast to the distal marine depofacies that yields low organic carbon contents (0.01 wt. % ) and high ¦Ä¹³C_org values (up to ?22¡ë). Both depofacies yield low sulfur contents (0.02 wt.¡ë), a narrow range of ¦Ä³⁴S values (¡«+3 ¡À 2¡ë) and positive ¦¤³³S values (up to +1.9¡ë). 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 ¦¤³³S values imply that sulfur was largely derived from a photochemical elemental sulfur reservoir. In the foreland basin tectonic setting of the ¡«2.94 Ga Government and ¡«2.92 Ga Jeppestown subgroups, shelf deposits associated with fluvial braidplain depofacies are characterized by ¹³C-depleted organic carbon (¡«?44 to ?38¡ë), relatively high sulfur contents (0.2-1.3 wt. % ), variable ¦Ä³⁴S values (?3.7 to +16.5¡ë) and small negative ¦¤³³S values (¡«?0.4¡ë). 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 ¹³C-depleted organic carbon, suggesting that an increase in continental sulfur flux and methane oxidation may have triggered the Mesoarchean glaciations (drawdown of H₂ and CH₄). However, the link between large ¦¤³³S anomalies (?4.0 to +1.2¡ë) and diamictite suggests low levels of atmospheric oxygen and minimal dilution of photochemical signatures.