The Deep Cool Terrestrial Biosphere: Habitability of ancient fracture waters of the Canadian Shield (Invited)

Ancient saline fractures waters in Precambrian rocks of the Canadian Shield contain mM concentrations of dissolved CH4 and higher hydrocarbons, and in particular up to 7 mM H2 derived from radiolysis and/or serpentinization. At 2.8 km depths in the Mponeng gold mine in the Witwatersrand basin South Africa, similar groundwater systems host some of the deepest communities of H2-utilizing sulphate-reducing microbes yet identified, in waters with noble gas derived residence times on the order of tens of Ma [1, 2]. Such H2-rich environments, in fracture waters in gold mines in South Africa, in deep groundwaters from the Canadian and Fennoscandian Shields, in hydrothermal marine vents and terrestrial hot springs, are the focus of research programs designed to expand our understanding of the habitability of Earth. Results on the geochemistry and geobiology of these systems are providing important insights into the habitability of Mars and other planets and moons in our solar system. Despite the fact that Precambrian cratons constitute > 30% of the Earth's exposed continents, the habitability of deep saline fractures waters in these rocks has been significantly under-investigated to date. Unlike high-temperature hydrothermal systems on the seafloor or continental hot springs, where extensive fluid circulation and mixing with ocean or surface waters respectively rapidly deplete the products of water-rock reaction such as H2, the hydrogeologically isolated fracture waters in tectonically quiescent Precambrian Shield rock provide virtual 'time capsules'. Therein, despite the slower rates of water-gas-rock reactions, the products of water-rock reaction, and potential substrates for microbial life can accumulate and build up high concentrations over geologically long time scales. Recent results from a copper-zinc mine near Timmins Ontario Canada revealed free flowing fracture waters at 2.4 km below surface of an unparalleled antiquity. Coupling geochemical evidence from the waters and gases with conservative noble gases (He, Ne, Ar, Kr, Xe) provided bulk residence times on the order of billions of years [3]. These results for the first time suggest a realm of the Earth's hydrosphere that preserves a geochemical (and potentially microbial) environment minimally impacted by hydrogeological mixing with the surface over geologic time scales. Ongoing research is investigating the potential for microbial life in these waters, and the timing of life's penetration of these environments relative to the residence times of the fracture waters. These frontiers of the deep cool biosphere may provide a window into the Earth's biodiversity. The saline fracture waters provide a critical environment in which to investigate habitability and to determine whether the types of chemolithotrophic life recognized at the vents and hot springs are supported in the much larger segments of the Earth's crust where lower temperatures and hence slower rates of water-rock reaction prevail. The deepest fracture water may even provide the opportunity to investigate controls on the biotic-abiotic transition and limits to life in the deep Earth. [1] Lin et al. (2006) Science 314, 479-482. [2] Lippmann-Pipke et al. (2011) Chemical Geology 283, 287-296. [3] Holland et al. (2013) Nature 497, 357-360.