The application of Monte Carlo modelling to quantify in situ hydrogen and associated element production in the deep subsurface

The subsurface production, accumulation, and cycling of hydrogen (H2), and cogenetic elements such as sulfate (SO42-) and the noble gases (e.g., 4He, 40Ar) remains a critical area of research in the 21st century. Understanding how these elements generate, migrate, and accumulate is essential in terms of developing hydrogen as an alternative low-carbon energy source and as a basis for helium exploration which is urgently needed to meet global demand of this gas used in medical, industrial, and research fields. Beyond this, understanding the subsurface cycles of these compounds is key for investigating chemosynthetically-driven habitability models with relevance to the subsurface biosphere and the search for life beyond Earth. The challenge is that to evaluate each of these critical element cycles requires quantification and accurate estimates of production rates. The natural variability and intersectional nature of the critical parameters controlling production for different settings (local estimates), and for the planet as a whole (global estimates) are complex. To address this, we propose for the first time a Monte Carlo based approach which is capable of simultaneously incorporating both random and normally distributed ranges for all input parameters. This approach is capable of combining these through deterministic calculations to determine both the most probable production rates for these elements for any given system as well as defining upper and lowermost production rates as a function of probability and the most critical variables. This approach, which is applied to the Kidd Creek Observatory to demonstrate its efficacy, represents the next-generation of models which are needed to effectively incorporate the variability inherent to natural systems and to accurately model H2, 4He, 40Ar, SO42- production on Earth and beyond

Hydrogen isotope exchange between n-alkanes and water under hydrothermal conditions

Author Posting. © The Author(s), 2011. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 77 (2012): 582–599, doi:10.1016/j.gca.2011.10.008.To investigate the extent of hydrogen isotope (2H and 1H) exchange between hydrocarbons and water under hydrothermal conditions, we performed experiments heating C1–C5 n-alkanes in aqueous solutions of varying initial 2H/1H ratios in the presence of a pyrite-pyrrhotite-magnetite redox buffer at 323ºC and 35–36MPa. Extensive and reversible incorporation of water-derived hydrogen into C2–C5 n-alkanes was observed on timescales of months. In contrast, comparatively minor exchange was observed for CH4. Isotopic exchange is facilitated by reversible equilibration of n-alkanes and their corresponding n-alkenes with H2 derived from the disproportionation of water. Rates of δ2H variation in C3+ n-alkanes decreased with time, a trend that is consistent with an asymptotic approach to steady-state isotopic compositions regulated by alkane-water isotopic equilibrium. Substantially slower δ2H variation was observed for ethane relative to C3–C5 n-alkanes, suggesting that the greater stability of C3+ alkenes and isomerization reactions may dramatically enhance rates of 2H/1H exchange in C3+ n-alkanes. Thus, in reducing aqueous environments, reversible reaction of alkenes and their corresponding alkanes facilitates rapid 2H/1H exchange between alkyl- and water-bound hydrogen on relatively short geological timescales at elevated temperatures and pressures. The proximity of some thermogenic and purported abiogenic alkane δ2H values to those predicted for equilibrium 2H/1H fractionation with ambient water suggests that this process may regulate the δ2H signatures of some naturally occurring hydrocarbons.This research received financial support from the Department of Energy (grant DE-FG02-97ER14746), the National Science Foundation (grant OCE-0549829) and the WHOI Deep Ocean Exploration Institute Graduate Fellowship (to E.P. Reeves)

The yield and isotopic composition of radiolytic H2, a potential energy source for the deep subsurface biosphere

Author Posting. © The Authors, 2004. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 69 (2005): 893-903, doi:10.1016/j.gca.2004.07.032.The production rate and isotopic composition of H2 derived from radiolytic reactions in H2O were measured to assess the importance of radiolytic H2 in subsurface environments and to determine whether its isotopic signature can be used as a diagnostic tool. Saline and pure, aerobic and anaerobic water samples with pH values of 4, 7 and 10 were irradiated in sealed vials at room temperature with an artificial γ source, and the H2 abundance in the headspace and its isotopic composition were measured. The H2 concentrations were observed to increase linearly with dosage at a rate of 0.40 ± 0.04 molecules (100 eV)-1 within the dosage range of 900 to 3500 Gray (Gy; Gy =1 J Kg-1) with no indication of a maximum limit on H2 concentration. At ~2000 Gy, the H2 concentration varied only by 16% across the experimental range of pH, salinity and O2. Based upon this measured yield and H2 yields for α and β particles a radiolytic H2 production rate of 10-9 to 10-4 nM sec-1 was estimated for the range of radioactive element concentrations and porosities typical of crustal rocks. The δD of H2 (δD = ((D/H)sample/(D/H)standard –1) × 1000) was independent of the dosage, pH (except for pH 4), salinity, and O2 and yielded an αDH2O-H2 of 2.05 ± 0.07 (αDH2O-H2 = (D/H)H2O to (D/H)H2), slightly less than predicted radiolytic models. Although this radiolytic fractionation value is significantly heavier than that of equilibrium isotopic exchange between H2 and H2O, the isotopic exchange rate between H2 and H2O will erase the heavy δD of radiolytic H2 if the age of the groundwater is greater than ~103 to 104 years. The millimolar concentrations of H2 observed in the groundwater of several Precambrian Shields are consistent with radiolysis of water that has resided in the subsurface for a few million years. These concentrations are well above those required to support H2-utilizing microorganisms and to inhibit H2-producing, fermentative microorganisms.This work is supported by grant from NSF LExEn program (EAR-9978267) to T.C. Onstott

A primordial origin for the atmospheric methane of Saturn's moon Titan

The origin of Titan's atmospheric methane is a key issue for understanding the origin of the Saturnian satellite system. It has been proposed that serpentinization reactions in Titan's interior could lead to the formation of the observed methane. Meanwhile, alternative scenarios suggest that methane was incorporated in Titan's planetesimals before its formation. Here, we point out that serpentinization reactions in Titan's interior are not able to reproduce the deuterium over hydrogen (D/H) ratio observed at present in methane in its atmosphere, and would require a maximum D/H ratio in Titan's water ice 30% lower than the value likely acquired by the satellite during its formation, based on Cassini observations at Enceladus. Alternatively, production of methane in Titan's interior via radiolytic reactions with water can be envisaged but the associated production rates remain uncertain. On the other hand, a mechanism that easily explains the presence of large amounts of methane trapped in Titan in a way consistent with its measured atmospheric D/H ratio is its direct capture in the satellite's planetesimals at the time of their formation in the solar nebula. In this case, the mass of methane trapped in Titan's interior can be up to 1,300 times the current mass of atmospheric methane.Comment: Accepted for publication in Icaru

Thermoanaerosceptrum fracticalcis gen. nov. sp. nov., a novel fumarate-fermenting microorganism from a deep fractured carbonate aquifer of the US Great Basin

Deep fractured rock ecosystems across most of North America have not been studied extensively. However, the US Great Basin, in particular the Nevada National Security Site (NNSS, formerly the Nevada Test Site), has hosted a number of influential subsurface investigations over the years. This investigation focuses on resident microbiota recovered from a hydrogeologically confined aquifer in fractured Paleozoic carbonate rocks at 863 – 923 m meters below land surface. Analysis of the microorganisms living in this oligotrophic environment provides a perspective into microbial metabolic strategies required to endure prolonged hydrogeological isolation deep underground. Here we present a microbiological and physicochemical characterization of a deep continental carbonate ecosystem and describe a bacterial genus isolated from the ecosystem. Strain DRI-13T is a strictly anaerobic, moderately thermophilic, fumarate-respiring member of the phylum Firmicutes. This bacterium grows optimally at 55°C and pH 8.0, can tolerate a concentration of 100 mM NaCl, and appears to obligately metabolize fumarate to acetate and succinate. Culture-independent 16S rRNA gene sequencing indicates a global subsurface distribution, while the closest cultured relatives of DRI-13T are Pelotomaculum thermopropionicum (90.0% similarity) and Desulfotomaculum gibsoniae (88.0% similarity). The predominant fatty acid profile is iso-C15:0, C15:0, C16:0 and C14:0. The percentage of the straight-chain fatty acid C15:0 is a defining characteristic not present in the other closely related species. The genome is estimated to be 3,649,665 bp, composed of 87.3% coding regions with an overall average of 45.1% G+C content. Strain DRI-13T represents a novel genus of subsurface bacterium isolated from a previously uncharacterized rock-hosted geothermal habitat. The characterization of the bacterium combined with the sequenced genome provides insights into metabolism strategies of the deep subsurface biosphere. Based on our characterization analysis we propose the name Thermoanaerosceptrum fracticalcis (DRI-13T = DSM 100382T = ATCC TSD-12T)

Analytical modelling of stable isotope fractionation of volatile organic compounds in the unsaturated zone

Analytical models were developed that simulate stable isotope ratios of volatile organic compounds (VOCs) near a point source contamination in the unsaturated zone. The models describe diffusive transport of VOCs, biodegradation and source ageing. The mass transport is governed by Fick's law for diffusion, and the equation for reactive transport of VOCs in the soil gas phase was solved for different source geometries and for different boundary conditions. Model results were compared to experimental data from a one-dimensional laboratory column and a radial-symmetric field experiment, and the comparison yielded a satisfying agreement. The model results clearly illustrate the significant isotope fractionation by gas-phase diffusion under transient state conditions. This leads to an initial depletion of heavy isotopes with increasing distance from the source. The isotope evolution of the source is governed by the combined effects of isotope fractionation due to vaporization, diffusion and biodegradation. The net effect can lead to an enrichment or depletion of the heavy isotope in the remaining organic phase depending on the compound and element considered. Finally, the isotope evolution of molecules migrating away from the source and undergoing degradation is governed by a combined degradation and diffusion isotope effect. This suggests that in the unsaturated zone, the interpretation of biodegradation based on isotope data must always be based on a model combining gas-phase diffusion and degradation.Comment: 11 pages, 6 figure

The role of porosity in H2/He production ratios in fracture fluids from the Witwatersrand Basin, South Africa

Abiotic H2 produced in the Precambrian lithospheric crust is a key substrate at the base of the metabolic chain of chemosynthetic and photosynthesis-independent microbial communities, significant to our understanding of life on early Earth and other planets. H2 cycling processes are also relevant to recent hydrogen exploration efforts and engineered subsurface environments such as radioactive waste disposal sites. In the lithospheric crust, H2 is produced through water-rock reactions (serpentinisation) and radiolysis; the latter directly linked to He through radioelement decay (U, Th). The Witwatersrand Basin in South Africa is an ideal place to study the radiolytic production pathway in particular, because of the low abundance of ultramafic and mafic minerals and therefore low potential for serpentinisation reactions. Gas samples and gas flow rate data (n = 12) were collected from the surface of exploration boreholes tapping the Witwatersrand and Ventersdorp Supergroups. The samples were predominantly composed of CH4 (65–99%), N2 (3–27%), He (0.1–15%), and trace amounts of C2+ hydrocarbons. Notably, H2 in these samples was below detection limit, despite the presence of He - providing a critical indicator of processes removing H2 from the system. Using a Bayesian modelling approach, we test the hypothesis that the observed fluids are generated in-situ, driven by radioelement decay and subsequent microbial methanogenesis, and controlled by porosity of the host rock. The observed data is consistent with this hypothesis, and can be accounted for by a variation in porosity between 0.3 and 2.2% (typical values to Precambrian basement) across the different sampling sites. These He-rich hydrocarbon gases observed at the surface originate from a hydrogeological system that is porosity-constrained and isolated from externally-sourced fluids. Radioelement decay is the primary process driving the generation of H2 and therefore energy production in this subsurface system, utilised by hydrogenotrophic methanogens at the base of the deep carbon cycle. Microbial utilisation is the key mechanism for H2 consumptions and, conversely, preservation, suggesting that conditions favourable to commercial H2 discoveries are likely constrained to hypersaline environments where microbial activity is inhibited. The model results under the proposed hypothesis (consistent N2/H2 ratio between different boreholes) raises the possibility that N2, which often co-occurs with He-rich deep fluids, is also produced through radiolysis, and future work is needed to fully evaluate this hypothesis

Challenges for Coring Deep Permafrost on Earth and Mars

This is the published version. Final publication is available from Mary Ann Liebert, Inc., publishers http://www.dx.doi.org/10.1089/ast.2007.0159.A scientific drilling expedition to the High Lake region of Nunavut, Canada, was recently completed with the goals of collecting samples and delineating gradients in salinity, gas composition, pH, pe, and microbial abundance in a 400 m thick permafrost zone and accessing the underlying pristine subpermafrost brine. With a triple-barrel wireline tool and the use of stringent quality assurance and quality control (QA/QC) protocols, 200 m of frozen, Archean, mafic volcanic rock was collected from the lower boundary that separates the permafrost layer and subpermafrost saline water. Hot water was used to remove cuttings and prevent the drill rods from freezing in place. No cryopegs were detected during penetration through the permafrost. Coring stopped at the 535 m depth, and the drill water was bailed from the hole while saline water replaced it. Within 24 hours, the borehole iced closed at 125 m depth due to vapor condensation from atmospheric moisture and, initially, warm water leaking through the casing, which blocked further access. Preliminary data suggest that the recovered cores contain viable anaerobic microorganisms that are not contaminants even though isotopic analyses of the saline borehole water suggests that it is a residue of the drilling brine used to remove the ice from the upper, older portion of the borehole. Any proposed coring mission to Mars that seeks to access subpermafrost brine will not only require borehole stability but also a means by which to generate substantial heating along the borehole string to prevent closure of the borehole from condensation of water vapor generated by drilling. Astrobiology 8, 623–638

Geochemistry of low-molecular weight hydrocarbons in hydrothermal fluids from Middle Valley, northern Juan de Fuca Ridge

Author Posting. © Elsevier B.V., 2006. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 70 (2006): 2073-2092, doi:10.1016/j.gca.2006.01.015.Hydrothermal vent fluids from Middle Valley, a sediment-covered mid-ocean ridge on the northern Juan de Fuca Ridge, were sampled in July, 2000. Eight different vents with exit temperatures of 186 to 281°C were sampled from two areas of venting: the Dead Dog and ODP Mound fields. Fluids from the Dead Dog field are characterized by higher concentrations of ΣNH3 and organic compounds (C1-C4 alkanes, ethene, propene, benzene and toluene) compared with fluids from the ODP Mound field. The ODP Mound fluids, however, are characterized by higher C1/(C2+C3) and benzene:toluene ratios than those from the Dead Dog field. The aqueous organic compounds in these fluids have been derived from both bacterial processes (methanogenesis in low-temperature regions during recharge) as well as from thermogenic processes in higher-temperature portions of the subsurface reaction zone. As the sediments undergo hydrothermal alteration, carbon dioxide and hydrocarbons are released to solution as organic matter degrades via a stepwise oxidation process. Compositional and isotopic differences in the aqueous hydrocarbons indicate that maximum subsurface temperatures at the ODP Mound are greater than those at the Dead Dog field. Maximum subsurface temperatures were calculated assuming that thermodynamic equilibrium is attained between alkenes and alkanes, benzene and toluene, and carbon dioxide and methane. The calculated temperatures for alkene-alkane equilibrium are consistent with differences in the dissolved Cl concentrations in fluids from the two fields, and indicate that subsurface temperatures at the ODP Mound are hotter than those at the Dead Dog field. Temperatures calculated assuming benzene-toluene equilibrium and carbon dioxide-methane equilibrium are similar to observed exit temperatures, and do not record the hottest subsurface conditions. The difference in subsurface temperatures estimated using organic geochemical thermometers reflects subsurface cooling processes via mixing of a hot, low-salinity vapor with a cooler, seawater salinity fluid. Because of the disparate temperature dependence of alkene-alkane and benzene-toluene equilibria, the mixed fluid records both the high and low temperature equilibrium conditions. These calculations indicate that vapor-rich fluids are presently being formed in the crust beneath the ODP Mound, yet do not reach the surface due to mixing with the lower-temperature fluids.This work was funded by NSF OCE-9906752