Follow the Water: Insights into deep carbon and deep life derived from noble gases

The deep gold mines of the Witwatersrand Basin, South Africa have gained recent attention not only because of investigations of the deep fracture water and associated CH4- and H2-rich gases found there, but because of recent reports of deep microbial communities persisting to depths of almost 3 kilometres - an exotic outpost of the Earth's deep biosphere. While shallower fluids in the basin (to approximately 1 km) were found to contain abundant populations of methanogens and sulphate-reducing bacteria, the deepest, oldest, most saline fracture waters in the basin hosted hitherto unrecognized low biomass and low biodiversity chemoautotrophic ecosystems independent from the photosphere. Shallow and deep fluids also show distinct differences in gas and fluid geochemistry. Paleometeoric waters are dominated by hydrocarbon gases with compositional and isotopic characteristics consistent with production by methanogens utilizing the CO2 reduction pathway. In contrast the deepest, most saline fracture waters contain gases that are dominated by high concentrations of H2 gas, and CH4 and higher hydrocarbon gases with isotopic signatures attributed to abiogenic processes of water-rock reaction. The high salinities (up to hundreds of g/L), highly altered ?18O and ?2H signatures, and both 36Cl and measurements of co-occurring nucleogenic noble gases for these fracture waters are consistent with extensive water-rock interaction over geologically long time scales in these high rock/water ratio environments. In a recent paper by Lippman-Pipke, Sherwood Lollar and others, Neon isotope results show distinct differences in neon composition that correspond to the different fluid geochemical end-members previously identified. Typical crustal neon signatures (type A) are identified in the paleometeoric waters populated with abundant methanogens. In contrast, the deep more saline fracture waters contain an enriched nucleogenic neon signature unlike any previously reported in crustal fluids. These samples show the highest 21Ne/22Ne ratios (0.160 ± 0.003) ever reported in groundwater. Fluid inclusions in these rocks yield even higher 21Ne/22Ne ratios between 0.219 to 0.515, consistent with an extrapolated 21Ne/22Ne value of 3.3 ± 0.2 at 20Ne/22Ne = 0. We show that this enriched nucleogenic neon end-member represents a fluid component that was produced in the fluorine-depleted Archaean formations and trapped in fluid inclusions ? 2 Ga ago. The observation of enriched nucleogenic neon signatures in deep fracture water implies the release of this billion year old neon component from the fluid inclusions and its accumulation in exceptionally isolated fracture water systems.