Paper No. 7
Presentation Time: 8:00 AM-12:00 PM
ISOTOPIC EVIDENCE FOR MICROBIAL SULFATE REDUCTION AND METHANOTROPHY DURING THE LATE ARCHEAN, WITWATERSRAND BASIN, SOUTH AFRICA
Stable isotope ratios were determined for carbon (d 13C) in organic matter and carbonate and for sulfur (d 34S) in pyrite for a stratigraphic profile from a core containing 60 meters of the late Archean Booysens/Kimberly (B/K) Shale Formation and a portion of the underlying Krugersdorp Quarzite Formation. The studied core was collected at a subsurface depth of 1.8 km in Evander shaft #8 operated by Harmony Gold Inc. The Evander sub-basin is located on the northeast rim of the Witwatersrand Basin. Silty shales are the dominant lithofacies in the core but interbedded siltstones and several meters of underlying gray quartzites are present. Carbonate (Ccarb) and Organic carbon (Corg) contents in core samples range from <0.05 to 2.26 wt% and <0.05 to 0.42 wt%, respectively. Values of d 13C for organic matter (d 13Corg) and carbonate minerals (d 13Ccarb) range from -42 to -35 and 14.6 to -10.1, respectively. Distinct negative shifts in d 13Corg values occur in laminated clay-rich intervals while positive shifts occur in carbonaterich intervals. Values of d 34S for Cr-reducible disulfide minerals (predominantly pyrite) range from -3.2 to +2.3. The most negative d 34S values occur in the laminated intervals that contain the most negative d 13Corg values. d 13Ccarb values lower than -10 are inferred to reflect bicarbonate derived from methane. Microprobe imaging shows partial to complete chlorite replacement of original rhombohedral carbonate grains. The combined carbon and sulfur isotopic evidence suggest a thriving community of methanotrophic and sulfate-reducing bacteria in laminated, muddy sediment deposited during maximum marine flooding of the Witwatersrand Basin.