Sulfur isotopic evidence of microbial activity during deposition of a Neoarchean shale and in modern deep groundwater, Witwatersrand Basin, South Africa

An international team of geochemists studying deep-subsurface microbes sampled water in fracture zones and rock in exploration cores using mine excavations in South Africa accessible from 1999 to 2002. Three projects were pursued at Indiana University utilizing these samples. The first project examined geochemical and sulfur isotopic characteristics of deep groundwater in gold mines extending to depths of 4 km below the surface in the Witwatersrand Basin. Sulfur isotopic differences (Delta34S) for sulfate compared to sulfide in Witwatersrand deep groundwater range from 7.7‰ to 32.2‰. Sulfate-reducing bacteria (SRB) are inferred to be active at depths of 1.5 to 3.5 km on a regional scale in the Witwatersrand Basin. The second project studied experimentally a strain of sulfate-reducing bacteria (Desulfovibrio vulgaris) isolated from a water intersection at 2 km in the Witwatersrand Basin. Optimal growth temperatures for D. vulgaris range from 32°C to 43°C and cell-specific sulfate reduction rates (10 -12 to 10-14 moles/cell/day) are high compared to other strains of SRB. Values of Delta34S for D. vulgaris are about 5‰ and are consistent over a wide range of temperatures. The third project profiled stratigraphic shifts in stable carbon (delta13C) and sulfur isotopic values (delta 34S) through 60 meters of Neoarchean strata including the Booysens Shale and a portion of the underlying Leandra Quartzite. Isotopic values for organic matter (delta13Corg) and carbonate minerals (delta13Ccarb) in the Booysens Shale range from -42‰ to -35‰ and -14.6‰ to -10.1‰, respectively. Pyrite in the Booysens Shale has delta34S values ranging from -3.8‰ to +3.0‰. A coeval negative isotopic shift in carbon and sulfur values suggests interdependence of methanotrophic and sulfate-reducing microbes in fine-grained sediment deposited during marine flooding of the Witwatersrand Basin in Neoarchean time.