Subglacial sedimentary basins: a last resort or oasis for microbial life?
Ice sheets cover ~10% of the Earth’s surface at present, rising to 30% during Quaternary glacial periods with potentially global coverage during the Neoproterozoic “Snowball Earth” Era (600 Myr ago).
Beneath thick ice cover, substantial sedimentary sequences remain thawed and may provide favourable habitats for micro-organisms, forming one of the most remote and deepest parts of the Earth’s biosphere. This is exemplified by the Antarctic Ice Sheet which is underlain by extensive sedimentary sequences up to 14km in thickness.
While these perennially dark, cold and isolated sedimentary basins might seem inclement to life, a range of unique physical and geochemical conditions suggest habitability. At the ice-bed interface, accelerated rates of physical erosion by moving ice may liberate nutrients and microbial energy sources from bedrock. Iron sulphide minerals, commonly released during rock crushing, are oxidised using Fe(III) by anaerobic microbes, while Fe(III) minerals such as ferrihydrite may stimulate dissimilatory iron reduction.
Preliminary experiments suggest the potential for substantial release of hydrogen from rock and carbon dioxide/methane from fluid inclusions during partial crushing. They indicate that glacial erosion could be one of the largest generators of abiotic H2 in zones of continental crust on the planet. These gases may serve as energetically favourable substrates for hydrogen/methane oxidising bacteria and methanogenic archaea beneath ice sheets. This contrasts with many remote, deep environments on Earth where organic carbon and energetically favourable electron acceptors become exhausted. Accumulation and burial of crushed by-products beneath ice sheets over time may sustain microbial activity in deep sediments over geological time scales. Additionally, glacial advance and retreat cycles promote burial of organic carbon and associated nutrients beneath ice sheets. This, together with substrates from crushed rock, may sustain microbial respiration and the production of carbon-dioxide and methane under ice. Methane may be stored as hydrate beneath the ice, with the potential to stimulate greenhouse warming upon release during deglaciation. Increased temperature/pressure with depth in sedimentary basins may further influence microbial metabolism which may be accentuated in geothermally active zones such as West Antarctica.
Counter-intuitively, therefore, subglacial sedimentary basins could be considered one of the more habitable parts of the deep biosphere, providing relevant analogues for extreme extraterrestrial environments where liquid water is or was once present (eg early Mars and Europa). Generation of greenhouse gases and water-borne nutrients also creates the potential for impact on neighbouring parts of the Earth system.
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