The Origin and Age of Biogeochemical Trends in Deep Fracture Water of the Witwatersrand Basin, South Africa

Water residing within crustal fractures encountered during mining at depths greater than 500 meters in the Witwatersrand basin of South Africa represents a mixture of paleo-meteoric water and 2.0–2.3 Ga hydrothermal fluid. The hydrothermal fluid is highly saline, contains abiogenic CH 4 and hydrocarbon, occasionally N 2 , originally formed at ∼ 250–300°C and during cooling isotopically exchanged O and H with minerals and accrued H 2 , 4 He and other radiogenic gases. The paleo-meteoric water ranges in age from ∼ 10 Ka to > 1.5 Ma, is of low salinity, falls along the global meteoric water line (GMWL) and is CO 2 and atmospheric noble gas-rich. The hydrothermal fluid, which should be completely sterile, has probably been mixing with paleo-meteoric water for at least the past ∼100 Myr, a process which inoculates previously sterile environments at depths > 2.0 to 2.5 km. Free energy flux calculations suggest that sulfate reduction is the dominant electron acceptor microbial process for the high salinity fracture water and that it is 10 7 times that normally required for cell maintenance in lab cultures. Flux calculations also indicate that the potential bioavailable chemical energy increases with salinity, but because the fluence of bioavailable C, N and P also increase with salinity, the environment remains energy-limited. The 4 He concentrations and theoretical calculations indicate that the H 2 that is sustaining the subsurface microbial communities (e.g. H 2 -utilizing SRB and methanogens) is produced by water radiolysis at a rate of ∼1 nM yr −1 . Microbial CH 4 mixes with abiogenic CH 4 to produce the observed isotopic signatures and indicates that the rate of methanogenesis diminishes with depth from ∼ 100 at < 1 kmbls, to < 0.01 nM yr −1 at > 3 kmbls. Microbial Fe(III) reduction is limited due to the elevated pH. The δ13C of dissolved inorganic carbon is consistent with heterotrophy rather than autotrophy dominating the deeper, more saline environments. One potential source of the organic carbon may be microfilms present on the mineral surfaces.