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

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. 13Corg, 34Spy and 33Spy values of the sample set range from
−44.3 to
−21.9‰,
−3.7 to +16.5‰ and
−4.0 to +1.9‰, respectively.
These geochemical data vary relative to depositional facies (proximal marine, distal marine and fluvialdominated) 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 13Corg values around −28‰, in contrast to the distal marine depofacies that yields low organic carbon contents (0.01 wt.%) and high 13Corg values (up to −22‰). Both depofacies yield low sulfur contents (0.02 wt.‰), a narrow range of 34S values (∼+3 ±2‰) and positive 33S values (up to +1.9‰). 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‰), relatively high sulfur contents (0.2–1.3 wt.%), variable 34S values (−3.7 to +16.5‰) and small negative 33S values (∼−0.4‰). 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 13Cdepleted 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‰) and diamictite suggests low levels of atmospheric oxygen and minimal dilution of photochemical signatures.