Sulfur mass-independent fractionation in subsurface fracture waters indicates a long-standing sulfur cycle in Precambrian rocks.

The discovery of hydrogen-rich waters preserved below the Earth's surface in Precambrian rocks worldwide expands our understanding of the habitability of the terrestrial subsurface. Many deep microbial ecosystems in these waters survive by coupling hydrogen oxidation to sulfate reduction. Hydrogen originates from water-rock reactions including serpentinization and radiolytic decomposition of water induced by decay of radioactive elements in the host rocks. The origin of dissolved sulfate, however, remains unknown. Here we report, from anoxic saline fracture waters ∼2.4 km below surface in the Canadian Shield, a sulfur mass-independent fractionation signal in dissolved sulfate. We demonstrate that this sulfate most likely originates from oxidation of sulfide minerals in the Archaean host rocks through the action of dissolved oxidants (for example, HO· and H2O2) themselves derived from radiolysis of water, thereby providing a coherent long-term mechanism capable of supplying both an essential electron donor (H2) and a complementary acceptor (sulfate) for the deep biosphere.

Hydrogen Isotopic Composition of Arctic and Atmospheric CH4 Determined by a Portable Near-Infrared Cavity Ring-Down Spectrometer with a Cryogenic Pre-Concentrator.

In this study, near-infrared continuous wave cavity ring-down spectroscopy was applied to the measurement of the δ2H of methane (CH4). The cavity ring-down spectrometer (CRDS) system consisted of multiple DFB laser diodes to optimize selection of spectral line pairs. By rapidly switching measurements between spectral line peaks and the baseline regions, the long-term instrumental drift was minimized, substantially increasing measurement precision. The CRDS system coupled with a cryogenic pre-concentrator measured the δ2H of terrestrial atmospheric CH4 from 3 standard liters of air with a precision of ±1.7‰. The rapidity with which both C and H isotopic measurements of CH4 can be made with the CRDS will enable hourly monitoring of diurnal variations in terrestrial atmospheric CH4 signatures that can be used to increase the resolution of global climate models for the CH4 cycle. Although the current instrument is not capable of measuring the δ2H of 10 ppbv of martian CH4, current technology does exist that could make this feasible for future spaceflight missions. As biological and abiotic CH4 sources have overlapping carbon isotope signatures, dual-element (C and H) analysis is key to reliable differentiation of these sources. Such an instrument package would therefore offer improved ability to determine whether or not the CH4 recently detected in the martian atmosphere is biogenic in origin. Key Words: Arctic-Hydrogen isotopes-Atmospheric CH4-CRDS-Laser. Astrobiology 16, 787-797.

The contribution of the Precambrian continental lithosphere to global H2 production.

Microbial ecosystems can be sustained by hydrogen gas (H2)-producing water-rock interactions in the Earth's subsurface and at deep ocean vents. Current estimates of global H2 production from the marine lithosphere by water-rock reactions (hydration) are in the range of 10(11) moles per year. Recent explorations of saline fracture waters in the Precambrian continental subsurface have identified environments as rich in H2 as hydrothermal vents and seafloor-spreading centres and have suggested a link between dissolved H2 and the radiolytic dissociation of water. However, extrapolation of a regional H2 flux based on the deep gold mines of the Witwatersrand basin in South Africa yields a contribution of the Precambrian lithosphere to global H2 production that was thought to be negligible (0.009 × 10(11) moles per year). Here we present a global compilation of published and new H2 concentration data obtained from Precambrian rocks and find that the H2 production potential of the Precambrian continental lithosphere has been underestimated. We suggest that this can be explained by a lack of consideration of additional H2-producing reactions, such as serpentinization, and the absence of appropriate scaling of H2 measurements from these environments to account for the fact that Precambrian crust represents over 70 per cent of global continental crust surface area. If H2 production via both radiolysis and hydration reactions is taken into account, our estimate of H2 production rates from the Precambrian continental lithosphere of 0.36-2.27 × 10(11) moles per year is comparable to estimates from marine systems.

Measurement of the 13C/12C of atmospheric CH4 using near-infrared (NIR) cavity ring-down spectroscopy.

A near-infrared (NIR) continuous-wave-cavity ring-down spectrometry (CW-CRDS) device was developed with the goal of measuring seasonal changes in the isotopic composition of atmospheric CH4 on Earth and eventually on Mars. The system consisted of three distributed feedback laser diodes (DFB-LDs), two of which were tuned to the absorption line peaks of (12)CH4 and (13)CH4 at 6046.954 cm(-1) and 6049.121 cm(-1), respectively, and a third that measured the baseline at 6050.766 cm(-1). The multiple laser design improved the long-term stability of the system and increased the data acquisition rate. The acquisition frequency was further increased by utilizing a semiconductor optical amplifier (SOA) to initiate cavity ring-down events. The high repetition rate combined with the superhigh reflectivity mirrors yielded precise isotopic measurements in this NIR region, even though the line strengths of CH4 in this region are 200 times weaker than those of the strongest mid-IR absorption bands. The current system has a detection limit of 1.9 × 10(-12) cm(-1), corresponding to 10 pptv of CH4 at 100 Torr. For ambient air samples that contained 1.9 ppmv CH4, the δ(13)C of the CH4 was determined to be -48.7 ± 1.7‰ (1σ).

Deep fracture fluids isolated in the crust since the Precambrian era.

Fluids trapped as inclusions within minerals can be billions of years old and preserve a record of the fluid chemistry and environment at the time of mineralization. Aqueous fluids that have had a similar residence time at mineral interfaces and in fractures (fracture fluids) have not been previously identified. Expulsion of fracture fluids from basement systems with low connectivity occurs through deformation and fracturing of the brittle crust. The fractal nature of this process must, at some scale, preserve pockets of interconnected fluid from the earliest crustal history. In one such system, 2.8 kilometres below the surface in a South African gold mine, extant chemoautotrophic microbes have been identified in fluids isolated from the photosphere on timescales of tens of millions of years. Deep fracture fluids with similar chemistry have been found in a mine in the Timmins, Ontario, area of the Canadian Precambrian Shield. Here we show that excesses of (124)Xe, (126)Xe and (128)Xe in the Timmins mine fluids can be linked to xenon isotope changes in the ancient atmosphere and used to calculate a minimum mean residence time for this fluid of about 1.5 billion years. Further evidence of an ancient fluid system is found in (129)Xe excesses that, owing to the absence of any identifiable mantle input, are probably sourced in sediments and extracted by fluid migration processes operating during or shortly after mineralization at around 2.64 billion years ago. We also provide closed-system radiogenic noble-gas ((4)He, (21)Ne, (40)Ar, (136)Xe) residence times. Together, the different noble gases show that ancient pockets of water can survive the crustal fracturing process and remain in the crust for billions of years.

Hydrogeologic controls on episodic H2 release from precambrian fractured rocks--energy for deep subsurface life on earth and mars.

Dissolved H(2) concentrations up to the mM range and H(2) levels up to 9-58% by volume in the free gas phase are reported for groundwaters at sites in the Precambrian shields of Canada and Finland. Along with previously reported dissolved H(2) concentrations up to 7.4 mM for groundwaters from the Witwatersrand Basin, South Africa, these findings indicate that deep Precambrian Shield fracture waters contain some of the highest levels of dissolved H(2) ever reported and represent a potentially important energy-rich environment for subsurface microbial life. The delta (2)H isotope signatures of H(2) gas from Canada, Finland, and South Africa are consistent with a range of H(2)-producing water-rock reactions, depending on the geologic setting, which include both serpentinization and radiolysis. In Canada and Finland, several of the sites are in Archean greenstone belts characterized by ultramafic rocks that have under-gone serpentinization and may be ancient analogues for serpentinite-hosted gases recently reported at the Lost City Hydrothermal Field and other hydrothermal seafloor deposits. The hydrogeologically isolated nature of these fracture-controlled groundwater systems provides a mechanism whereby the products of water-rock interaction accumulate over geologic timescales, which produces correlations between high H(2) levels, abiogenic hydrocarbon signatures, and the high salinities and highly altered delta (18)O and delta (2)H values of these groundwaters. A conceptual model is presented that demonstrates how periodic opening of fractures and resultant mixing control the distribution and supply of H(2) and support a microbial community of H(2)-utilizing sulfate reducers and methanogens.

Abiogenic formation of alkanes in the Earth's crust as a minor source for global hydrocarbon reservoirs.

Natural hydrocarbons are largely formed by the thermal decomposition of organic matter (thermogenesis) or by microbial processes (bacteriogenesis). But the discovery of methane at an East Pacific Rise hydrothermal vent and in other crustal fluids supports the occurrence of an abiogenic source of hydrocarbons. These abiogenic hydrocarbons are generally formed by the reduction of carbon dioxide, a process which is thought to occur during magma cooling and-more commonly-in hydrothermal systems during water-rock interactions, for example involving Fischer-Tropsch reactions and the serpentinization of ultramafic rocks. Suggestions that abiogenic hydrocarbons make a significant contribution to economic hydrocarbon reservoirs have been difficult to resolve, in part owing to uncertainty in the carbon isotopic signatures for abiogenic versus thermogenic hydrocarbons. Here, using carbon and hydrogen isotope analyses of abiogenic methane and higher hydrocarbons in crystalline rocks of the Canadian shield, we show a clear distinction between abiogenic and thermogenic hydrocarbons. The progressive isotopic trends for the series of C1-C4 alkanes indicate that hydrocarbon formation occurs by way of polymerization of methane precursors. Given that these trends are not observed in the isotopic signatures of economic gas reservoirs, we can now rule out the presence of a globally significant abiogenic source of hydrocarbons.