Detection of ancient biosignatures along deep groundwater flow paths

The globally vast deep biosphere in rock-hosted aquifers is, particularly in igneous-rock hosted systems, spatially confined to open space provided by fractures that offer pathways for water, gases, nutrients and organic carbon essential to sustain microbial life. Processes in this anaerobic and oligotrophic realm related to microbial metabolisms, such as methanogenesis, anaerobic methane oxidation and bacterial sulphate reduction, are particularly confined in space and time to fluid mixing events and to boundary zones between fluids of different origins, such as meteoric influx into old saline brines. These processes causes kinetic δ13C and δ34S fractionation that can be recorded by authigenic calcite and pyrite precipitating on the fracture walls.

Tiny growth zones of crystals in single fracture coatings can be utilized to constrain the timing of the biological activity through high spatial resolution U-Pb and Rb-Sr geochronology combined with molecular biosignature and stable isotope analyses (of C, O and S isotopes via SIMS). Fluid inclusion microthermometry, clumped isotope geochemistry, O and Sr isotope geochemistry and trace element geochemistry give additional input about the environmental context of periods with enhanced microbial activity, particularly of fluid temperature and origin. This multi-method approach has been shown abililty to disclose discrete events of microbial activity within a wide time frame (potentially billions of years) and over large depths (kilometres). The potential of finding records of deep life in deep time is highly site-specific and dependent on the habitability in deep time, essentially being controlled by the exhumation and burial history of the studied setting. A very important, but still neglected aspect in reconstructing habitatability records in deep time, is thus to involve the thermochronology perspecitve. Such thermochronological records may guide us to periods of habitable conditions as well as high-temperature periods of "sterilization" that inhibited microbial life.

The knowledge of the deep biosphere development through deep time is still extremely scarce, despite that it is one of the largest microbial habitats on our planet but recent applications of biosignature detection, coupled to geochronology and thermochronology have started to give us new insights into the history of deep life in deep time and to the environmental and geological factors involved.