Search for Life in Deep Time and Space: The role of Precambrian rocks in analogue research
Abstract:Deep saline fracture fluids have been found in billion year old rocks throughout the Precambrian shields of Canada, Fennoscandia and South Africa - geologic terrains that make up more than 70% of the Earth’s continental crust. Dissolved gases in these fluids are dominated by methane and higher hydrocarbons, some of the most radiogenic noble gas signatures ever reported, and up to mM concentrations of H2, making these environments as H2-rich as marine hydrothermal vents and spreading centers. Although the discovery of microbial ecosystems sustained by H2-producing water-rock reactions in the deep ocean vents transformed thinking about where life may have originated on Earth and the search for life on other worlds, such as Europa, the billion year old rocks of Precambrian shields have been under-investigated to date as analogs.
Noble gas studies have provided a range of residence times from millions to tens of millions of years for the subsurface fracture fluids in South Africa. Holland et al. (2013) reported residence times of more than a billion years for fracture waters from 2.4 km depth from mines in Northern Canada – revealing an ancient previously unexplored hydrosphere. Analyses of the DNA, RNA and amino acids from some of these fracture waters reveal an active biosphere supported by chemolithoautotrophs that alters the stable isotopic composition of C, N, and S aqueous and gaseous species. The discovery that ancient fluid environments capable of supporting life can remain isolated for up to billions of years in the Precambrian crust changes our understanding of the extent of the Earth’s crust, and by inference Mars’ crust, that may be habitable, and the role that such potential buried biomes play in preserving, evolving and propagating life on planetary timescales. Mars, like the Precambrian shields on Earth, is dominated by tectonically quiescent geologic terrains which are billions of years old, some with serpentinized ultramafic rocks capable of sustaining production of reduced gases. If such ancient fluids, with mM concentrations of H2 and CH4, are preserved deep in the terrestrial crust on Ga time scales, similar potential buried biomes may be preserved at depth in the subsurface of Mars.  Holland et al. (2013) Nature 497, 357-360.