Carbon Isotopic Evidence for a Methane-Supported Benthic Microbial Mat from a Late Archean Silty Shale, Witwatersrand Basin

Stable carbon isotopic analysis and organic carbon/sulfur abundance ratios were determined for thirty-seven samples from a deep-mine core containing a 60 m section of the Krugersdorp Quartzite Formation and Booysens Shale Formation. The studied core was collected at a depth of 1.8 km below the surface in Evander Shaft #8, Harmony Gold Incorporated. Lithologic units in the Evander core are predominantly silty shales in the Booysens Shale Formation but include several meters of underlying gray quartzites in the Krugersdorp Quartzite Formation. Carbonate (Ccarb) and organic carbon (Corg) contents in core samples range from 0.0 to 2.3 wt% and 0.0 to 0.4 wt%, respectively. Notably, ratios of Corg/Acid-Insoluble Sulfur from the Booysens Shale Formation fit the trend for Phanerzoic shale from normal marine settings. Stable carbon isotopic ratios for organic constituents (δ^13Corg) range from -42.2^o/oo to -35.5^o/oo (relative to Pee Dee Belemnite) in the Evander core. Previously published data on the Booysens Formation from other localities in the Witwatersrand Basin document heavier δ^13Corg values, ranging from -34.5^o/oo to -26.2^o/oo (Strauss, 1992; Watanabe, 1997). Distinct stratigraphic shifts occur away from average δ^13Corg (-39.2^o/oo) in the Evander core. Three negative shifts characterize laminated clay-rich intervals with Corg contents above 0.15 wt%, while two positive shifts characterize massive carbonate-rich intervals with Corg contents below 0.12 wt%. The most negative isotopic shifts suggest a restricted and stratified benthic environment, such as a silled sub-basin, in which a benthic microbial mat enhances methanotropic processes. Positive shifts suggest periods of reduced methanotrophy and increased mixing with isotopically heavier waters.

Evander Shaft #8 Harmony Gold Incorporated, Evander, Mpumalanga, RSA Duane Moser, Greg Wanger, and Brett Tipple sampling freshly drilled rock core 1.5 km underground Duane Moser and Brett Tipple filtering fissure water for microbial life at the 104 Level of Mponeng Gold Mine