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

Volatilization, dissolution and equilibrium isotope effects (2H, 13C, 37Cl) of trichloromethane, trichloroethene and methanol dissolved in water

This draft of a paper presents isotopic enrichment factors obtained from volatilization, dissolution and equilibration experiments of three organics dissolved in water. <br /

Estimation of nuclear volume dependent fractionation of mercury isotopes in equilibrium liquid-vapor evaporation experiments

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/155720/1/Ghosh_et_al_2013_Estimation_of_nuclear.pd

Bioenergetic Constraints on Microbial Hydrogen Utilization in Precambrian Deep Crustal Fracture Fluids

Precambrian Shield rocks host the oldest fracture fluids on Earth, with residence times up to a billion years or more. Water–rock reactions in these fracture systems over geological time have produced highly saline fluids, which can contain millimolar concentrations of H2. Mixing of these ancient Precambrian fluids with meteoric or palaeo-meteoric water can occur through tectonic fracturing, providing microbial inocula and redox couples to fuel blooms of subsurface growth. Here, we present geochemical and microbiological data from a series of borehole fluids of varying ionic strength (0.6–6.4 M) from the Thompson Mine (Manitoba) within the Canadian Precambrian Shield. Thermodynamic calculations demonstrate sufficient energy for H2-based catabolic reactions across the entire range of ionic strengths during mixing of high ionic strength fracture fluids with meteoric water, although microbial H2 consumption and cultivable H2-utilizing microbes were only detected in fluids of ≤1.9 M ionic strength. This pattern of microbial H2 utilization can be explained by the higher potential bioenergetic cost of organic osmolyte synthesis at increasing ionic strengths. We propose that further research into the bioenergetics of osmolyte regulation in halophiles is warranted to better constrain the habitability zones of hydrogenotrophic ecosystems in both terrestrial subsurface, including potential future radioactive waste disposal sites, and other planetary body crustal environments, including Mars

Carbon and Hydrogen Isotopic Fractionation during Anaerobic Biodegradation of Benzene

Compound-specific isotope analysis has the potential to distinguish physical from biological attenuation processes in the subsurface. In this study, carbon and hydrogen isotopic fractionation effects during biodegradation of benzene under anaerobic conditions with different terminal-electron-accepting processes are reported for the first time. Different enrichment factors (ɛ) for carbon (range of −1.9 to −3.6‰) and hydrogen (range of −29 to −79‰) fractionation were observed during biodegradation of benzene under nitrate-reducing, sulfate-reducing, and methanogenic conditions. These differences are not related to differences in initial biomass or in rates of biodegradation. Carbon isotopic enrichment factors for anaerobic benzene biodegradation in this study are comparable to those previously published for aerobic benzene biodegradation. In contrast, hydrogen enrichment factors determined for anaerobic benzene biodegradation are significantly larger than those previously published for benzene biodegradation under aerobic conditions. A fundamental difference in the previously proposed initial step of aerobic versus proposed anaerobic biodegradation pathways may account for these differences in hydrogen isotopic fractionation. Potentially, C-H bond breakage in the initial step of the anaerobic benzene biodegradation pathway may account for the large fractionation observed compared to that in aerobic benzene biodegradation. Despite some differences in reported enrichment factors between cultures with different terminal-electron-accepting processes, carbon and hydrogen isotope analysis has the potential to provide direct evidence of anaerobic biodegradation of benzene in the field

Carbon isotope systematics of Turrialba volcano, Costa Rica, using a portable cavity ring-down spectrometer

Texto completo del documentoOver the past two decades, activity at Turrialba volcano, Costa Rica, has shifted from hydrothermal to increasingly magmatic in character, with enhanced degassing and eruption potential. We have conducted a survey of the d13C signatures of gases at Turrialba using a portable field-based CRDS with comparison to standard IRMS techniques. Our d13C results of the volcanic plume, high-temperature vents, and soil gases reveal isotopic heterogeneity in the CO2 gas composition at Turrialba prior to its recent phase of eruptive activity. The isotopic value of the regional fault system, Falla Ariete (–3.460.1&), is in distinct contrast with the Central crater gases (–3.960.1&) and the 2012 high-temperature vent (–4.460.2&), an indication that spatial variability in d13C may be linked to hydrothermal transport of volcanic gases, heterogeneities in the source composition, or magmatic degassing. Isotopic values of CO2 samples collected in the plume vary from d13C of 25.2 to 210.0&, indicative of mixing between atmospheric CO2 (–9.260.1&), and a volcanic source. We compare the Keeling method to a traditional mixing model (hyperbolic mixing curve) to estimate the volcanic source composition at Turrialba from the plume measurements. The predicted source compositions from the Keeling and hyperbolic methods (–3.060.5&and 23.960.4&, respectively) illustrate two potential interpretations of the volcanic source at Turrialba. As of the 29 October 2014, Turrialba has entered a new eruptive period, and continued monitoring of the summit gases for d13C should be conducted to better understand the dominant processes controlling d13C fractionation at Turrialba.En los dos últimos decenios, la actividad del volcán Turrialba (Costa Rica) ha pasado del carácter hidrotérmico al carácter cada vez más magmático, con un mayor potencial de desgasificación y erupción. Hemos llevado a cabo un estudio de las firmas de d13C de los gases en Turrialba usando un CRDS portátil basado en el campo con comparación con las técnicas estándar del IRMS. Nuestros resultados del d13C de la pluma volcánica, respiraderos de alta temperatura, y los gases del suelo revelan la heterogeneidad isotópica en la composición del gas CO2 en Turrialba antes de su reciente fase de actividad eruptiva. El valor isotópico del sistema de fallas regionales, Falla Ariete (-3.4 6 0.1&), está en claro contraste con los gases del cráter Central (-3,9 6 0,1&) y el respiradero de alta temperatura del 2012 (-4,4 6 0,2&), un indicación de que la variabilidad espacial del d13C puede estar vinculada al transporte hidrotérmico de los gases volcánicos, a las heterogeneidades en la composición de la fuente o a la desgasificación magmática. Los valores isotópicos de las muestras de CO2 recogidas en la pluma varía de d13C de 25.2 a 210.0&, indicativo de mezcla entre el CO2 atmosférico (-9.2 6 0.1&), y una fuente volcánica. Comparamos el método de Keeling con un modelo de mezcla tradicional (mezcla hiperbólica ) para estimar la composición de la fuente volcánica en Turrialba a partir de las mediciones de la pluma. Las composiciones de la fuente pronosticadas a partir de los métodos Keeling e hiperbólico (-3,0 6 0,5& y 23,9 6 0,4&, respectivamente) ilustran dos posibles interpretaciones de la fuente volcánica de Turrialba. A partir del 29 de octubre 2014, Turrialba ha entrado en un nuevo período eruptivo, y la vigilancia continua de los gases de la cumbre para el d13C para comprender mejor los procesos dominantes que controlan el fraccionamiento del d13C en Turrialba.McGill University, Montreal, Quebec, CanadaUniversidad Nacional, Heredia, Costa RicaUniversity of Toronto, Toronto, Ontario, CanadaObservatorio Vulcanológico y Sismológico de Costa Ric