Hydrogeochemical and isotopic signatures elucidate deep subsurface hypersaline brine formation through radiolysis driven water-rock interaction

Geochemical and isotopic fluid signatures from a 2.9–3.2 km deep, 45–55 °C temperature, hypersaline brine from Moab Khotsong gold and uranium mine in the Witwatersrand Basin of South Africa were combined with radiolytic and water–rock isotopic exchange models to delineate brine evolution over geologic time, and to explore brine conditions for habitability. The Moab Khotsong brines were hypersaline (Ca-Na-Cl) with 215–246 g/L TDS, and Cl− concentrations up to 4 mol/L suggesting their position as a hypersaline end-member significantly more saline than any previously sampled Witwatersrand Basin fluids. The brines revealed low DIC (∼0.266–∼1.07 mmol/L) with high (∼8.49–∼23.6 mmol/L) DOC pools, and several reduced gaseous species (up to 46 % by volume H2) despite microoxic conditions (Eh = 135–161 mV). Alpha particle radiolysis of water to H2, H2O2, and O2 along with anhydrous-silicate-to-clay alteration reactions predicted 4 mol/L Cl− brine concentration and deuterium enrichment in the fracture waters over a period > 1.00 Ga, consistent with previously reported 40Ar noble gas-derived residence times of 1.20 Ga for this system. In addition, radiolytic production of 7–26 nmol/(L × yr) H2, 3–11 nmol/(L × yr) O2, and 1–8 nmol/(L × yr) H2O2 was predicted for 1–100 g/g 238U dosage scenarios, supporting radiolysis as a significant source of H2 and oxidant species to deep brines over time that are available to a low biomass system (102–103 cells/mL). The host rock lithology was predominately Archaean quartzite, with minerals exposed on fracture surfaces that included calcite, pyrite, and chlorite. Signatures of 18Ocalcite, 13Ccalcite, Δ33Spyrite, 34Spyrite and 87Sr/86Sr obtained from secondary ion mass spectrometry (SIMS) microanalyses suggest several discrete fluid events as the basin cooled from peak greenschist conditions to equilibrium with present-day brine temperatures. The brine physiochemistry, geochemistry, and cellular abundances were significantly different from those of a younger, shallower, low salinity dolomitic fluid in the same mine, and both were different from the mine service water. These results indicate the discovery of one of few long-isolated systems that supports subsurface brine formation via extended water–rock interaction, and an example of a subsurface brine system where abiotic geochemistry may support a low biomass microbial community

Fluctuations in populations of subsurface methane oxidizers in coordination with changes in electron acceptor availability

The concentrations of electron donors and acceptors in the terrestrial subsurface biosphere fluctuate due to migration and mixing of subsurface fluids, but the mechanisms and rates at which microbial communities respond to these changes are largely unknown. Subsurface microbial communities exhibit long cellular turnover times and are often considered relatively static-generating just enough ATP for cellular maintenance. Here, we investigated how subsurface populations of CH4 oxidizers respond to changes in electron acceptor availability by monitoring the biological and geochemical composition in a 1339 m-below-land-surface (mbls) fluid-filled fracture over the course of both longer (2.5 year) and shorter (2-week) time scales. Using a combination of metagenomic, metatranscriptomic, and metaproteomic analyses, we observe that the CH4 oxidizers within the subsurface microbial community change in coordination with electron acceptor availability over time. We then validate these findings through a series of 13C-CH4 laboratory incubation experiments, highlighting a connection between composition of subsurface CH4 oxidizing communities and electron acceptor availability.</p

A New Analysis of Mars ‘‘Special Regions’’: Findings of the Second MEPAG Special Regions Science Analysis Group (SR-SAG2)

A committee of the Mars Exploration Program Analysis Group (MEPAG) has reviewed and updated the description of Special Regions on Mars as places where terrestrial organisms might replicate (per the COSPAR Planetary Protection Policy). This review and update was conducted by an international team (SR-SAG2) drawn from both the biological science and Mars exploration communities, focused on understanding when and where Special Regions could occur. The study applied recently available data about martian environments and about terrestrial organisms, building on a previous analysis of Mars Special Regions (2006) undertaken by a similar team. Since then, a new body of highly relevant information has been generated from the Mars Reconnaissance Orbiter (launched in 2005) and Phoenix (2007) and data from Mars Express and the twin Mars Exploration Rovers (all 2003). Results have also been gleaned from the Mars Science Laboratory (launched in 2011). In addition to Mars data, there is a considerable body of new data regarding the known environmental limits to life on Earth—including the potential for terrestrial microbial life to survive and replicate under martian environmental conditions. The SR-SAG2 analysis has included an examination of new Mars models relevant to natural environmental variation in water activity and temperature; a review and reconsideration of the current parameters used to define Special Regions; and updated maps and descriptions of the martian environments recommended for treatment as ‘‘Uncertain’’ or ‘‘Special’’ as natural features or those potentially formed by the influence of future landed spacecraft. Significant changes in our knowledge of the capabilities of terrestrial organisms and the existence of possibly habitable martian environments have led to a new appreciation ofwhere Mars Special Regions may be identified and protected. The SR-SAG also considered the impact of Special Regions on potential future human missions to Mars, both as locations of potential resources and as places that should not be inadvertently contaminated by human activity

Report of the COSPAR Mars special regions colloquium

International audienceIn this paper we present the findings of a COSPAR Mars Special Regions Colloquium held in Rome in 2007. We review and discuss the definition of Mars Special Regions, the physical parameters used to define Mars Special Regions, and physical features on Mars that can be interpreted as Mars Special Regions. We conclude that any region experiencing temperatures > -25 degrees C for a few hours a year and a water activity > 0.5 can potentially allow the replication of terrestrial microorganisms. Physical features on Mars that can be interpreted as meeting these conditions constitute a Mars Special Region. Based on current knowledge of the martian environment and the conservative nature of planetary protection, the following features constitute Mars Special regions: Gullies and bright streaks associated with them, pasted-on terrain, deep subsurface, dark streaks only on a case-by-case basis, others to be determined. The parameter definition and the associated list of physical features should be re-evaluated on a regular basis