Isotopic and geochemical tracers of fracture fluids in Precambrian cratons: Time capsules for the deep biosphere

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Some of the most isolated microbial ecosystems on Earth have recently been discovered by our international team at 2.8 km below the Earth's surface in saline fracture waters discharging from exploration boreholes in the Precambrian rocks of South Africa (Lin et al. 2006, Science). Like the hydrothermal vents on the ocean floor, these deep fracture waters are the most H2-rich environments on the planet, and are repositories of other products of extensive water-rock reaction including abiogenic hydrocarbons and radiogenic noble gases (Sherwood Lollar et al., 2002 Nature). Recent results by our team in South Africa, and preliminary results at our sites on the Precambrian Shield in Canada, indicate components of the noble gases in these free flowing fracture waters are more than 2 Billion years old - attesting to the isolation of these groundwaters from the surface photosphere on a previously unrecognised timescale (Lippmann-Pipke, Sherwood Lollar et al., 2011 Chemical Geology; Holland, Sherwood Lollar et al., 2012 in revision at Nature). While much has been learned, these exciting results have opened up yet more intriguing scientific questions of fundamental importance for understanding the distribution of life on Earth and the habitability of the deep subsurface; the origin and age of deep crustal fluids and reduced gases (methane, hydrogen); and the degree to which geochemistry (water-rock alteration reactions in the subsurface) can support subsurface life. Focused on as yet under investigated sites in Precambrian rocks on the Canadian Shield, this Discovery research program is focused on addressing the above questions - with their profound implications for our understanding the habitability of this planet, the evolution of the life on Earth; and the potential for life in the subsurface on other planets or moons.