Abiotic (Prebiotic?) Organic Chemistry in a Potentially Ancient Hypersaline Brine: New Insights on the Limits of Microbial Life Inhabiting 3.1 km Deep Fracture Fluid in South Africa
Abstract
Deep subsurface fracture fluid environments are often saline and are associated with elevated temperatures relative to the surface. Consequently, inhabiting microbial life is expected to have adapted to the significant physical and energetic constraints of these environments. As these conditions may be similar to those imposed by subsurface Martian environments, characterization of hypersaline subsurface habitats may aid in the search for life under analogous conditions. The recent discovery of near saturated brines 3 km below land surface in the South African gold mine, Moab Khotsong (26.98 S, 26.78 E), presents an opportunity to characterize microbial life in potentially ancient brines hosted within the 3.1-2.9 Ga Witswatersrand Supergroup. To enhance our understanding of how subsurface habitability changes with increasing abiotic constraints, we collected fracture fluid samples between 1.2-3.1 km depth from Moab Khotsong. Fluid temperature and salinity increased with depth, ranging from 26.7-55°C and 3-240 ppt, respectively, with pressures >100 bars at deeper levels. Isotopic signatures for δ2H/δ18O of the deeper brines plot far from the global meteoric water line consistent with removal of water by low temperature clay formation reactions. Gas composition by GC-MS revealed a predominance of methane and ethane within C1-5 hydrocarbon compounds, but a lack of C6+ hydrocarbons, also suggestive of fluid derived from a more abiotic end member. The gases contained a suite of odorous C2-5 alkyl sulfides that may be sustained by thermochemical sulfate reduction; however, this is inconsistent with the paucity of H2S. The δ2H and δ13C values for C2-3 hydrocarbons relative to that of methane suggest their production dominantly occurs via abiogenic polymerization from water-rock interactions. Initial metagenomic analysis revealed similar lithoautotrophic communities between shallower fluid and the deep brines, with the latter containing a higher relative abundance of halophiles and thermophiles. These preliminary results suggest fluids, whose chemical composition has been long affected by water-rock interaction, do impose constraints strong enough to alter microbial composition and functionality within the subsurface, and may lend insight to habitability of the Martian subsurface.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.B11K2202N
- Keywords:
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- 0448 Geomicrobiology;
- BIOGEOSCIENCES;
- 0456 Life in extreme environments;
- BIOGEOSCIENCES;
- 0463 Microbe/mineral interactions;
- BIOGEOSCIENCES;
- 0465 Microbiology: ecology;
- physiology and genomics;
- BIOGEOSCIENCES