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Article Volume 6, Number 7 12 July 2005 Q07003, doi:10.1029/2004GC000907
AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society
ISSN: 1525-2027
Radiolytic H2 in continental crust: Nuclear power for deep subsurface microbial communities Li-Hung Lin and James Hall Department of Geosciences, Princeton University, Princeton, New Jersey 08540, USA (
[email protected]) Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, N.W, Washington, DC 20015, USA
Johanna Lippmann-Pipke Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, New York 10964, USA GeoForschungsZentrum Potsdam, Telegraphenberg, Haus B, 320, D-14473 Potsdam, Germany
Julie A. Ward and Barbara Sherwood Lollar Stable Isotope Laboratory, Department of Geology and Geophysics, University of Toronto, Toronto, 22 Russel Street, Toronto, Ontario, Canada M5S 3D1
Mary DeFlaun GeoSyntec Consultants, 1 Airport Place, Suite 3, Princeton, New Jersey 08540, USA
Randi Rothmel Shaw Group, 4100 Quakerbridge Road, Lawrenceville, New Jersey 08648, USA
Duane Moser and Thomas M. Gihring Pacific Northwest National Laboratory, 902 Battelle Boulevard, Mailstop P7-50, P.O. Box 999, Richland, Washington 99352, USA
Bianca Mislowack and T. C. Onstott Department of Geosciences, Princeton, New Jersey 08540, USA
[1] H2 is probably the most important substrate for terrestrial subsurface lithoautotrophic microbial
communities. Abiotic H2 generation is an essential component of subsurface ecosystems truly independent of surface photosynthesis. Here we report that H2 concentrations in fracture water collected from deep siliclastic and volcanic rock units in the Witwatersrand Basin, South Africa, ranged up to two molar, a value far greater than observed in shallow aquifers or marine sediments. The high H2 concentrations are consistent with that predicted by radiolytic dissociation of H2O during radioactive decay of U, Th, and K in the host rock and the observed He concentrations. None of the other known H2-generating mechanisms can account for such high H2 abundance either because of the positive free energy imposed by the high H2 concentration or pH or because of the absence of required mineral phases. The radiolytic H2 is consumed by methanogens and abiotic hydrocarbon synthesis. Our calculations indicate that radiolytic H2 production is a ubiquitous and virtually limitless source of energy for deep crustal chemolithoautotrophic ecosystems. Components: 8658 words, 2 figures, 4 tables. Keywords: continental crust; hydrogen; radiolysis; subsurface ecosystems. Index Terms: 0448 Biogeosciences: Geomicrobiology; 0456 Biogeosciences: Life in extreme environments. Received 26 December 2004; Revised 6 February 2005; Accepted 29 April 2005; Published 12 July 2005.
Copyright 2005 by the American Geophysical Union
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Lin, L.-H., et al. (2005), Radiolytic H2 in continental crust: Nuclear power for deep subsurface microbial communities, Geochem. Geophys. Geosyst., 6, Q07003, doi:10.1029/2004GC000907.
1. Introduction
them for one subsurface environment has not been undertaken until this study.
[2] One constraint on the extent of subsurface microbial life is whether sufficient energy exists to sustain a minimal metabolism [McKay, 2001]. Pedersen [2001] and Stevens and McKinley [1995] have proposed that some subsurface microbial communities rely on substrates derived from geochemical processes rather than from photosynthetically derived organic carbon. These lithoautotrophic ecosystems necessitate abiogenically produced H2. H2 generates substantial energy for various electron acceptors [Amend and Shock, 2001] and its high diffusivity makes it readily available to microorganisms in confined pore spaces. H2 is also crucial for Fischer-Tropsch (F-T) synthesis of organic compounds [Cody et al., 2000; Rushdi and Simoneit, 2001; Sherwood Lollar et al., 2002] where H2, catalyzed by metallic phases, reacts with CO/ CO2. The C and H isotopic signatures of these hydrocarbons are distinct from those of thermogenic hydrocarbons and microbial CH4 [Sherwood Lollar et al., 2002]. Microorganisms utilizing F-T synthesized organics as substrates would be independent of photosynthetically derived organic matter.
[4] To investigate the origins of high H2 concentrations in the deep subsurface and to determine whether water radiolysis plays a role in H2 generation, we analyzed the radiolytic and radiogenic products, dissolved H2 and He, in 24 groundwater samples collected from 6 Au and coal mines of the Witwatersrand Basin, South Africa. We evaluated the thermodynamic potential of each candidate H2 generation reaction under the in situ conditions to account for the observed H2 concentrations.
[3] The few dissolved H2 analyses for deep subsurface environments vary dramatically from 1% H2 the RGA and Varian 3400 GC analyses agreed to within a factor of 2 despite the 1000 times dilution for the RGA analyses. All analyses were run in triplicate and mean values are reported. Reproducibility for triplicate analyses was ±5% relative error. Noble gas analyses, including He, were performed on Cu tube samples using the procedures of Lippmann et al. [2003]. [8] The measured H2 and He concentrations were corrected for loss through diffusion to the fracture
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headspace that is created behind the tunnel face as the fracture is dewatered [Lippmann et al., 2003]. The correction was based on the observation that the concentration of the dissolved atmospheric noble gases, 20Ne, 36Ar, 84Kr, 132Xe, progressively increased with increasing atomic mass in a manner consistent with diffusive loss at in situ temperatures from initial concentrations that were in equilibrium with the atmosphere during recharge [Lippmann et al., 2003]. The deficiency of other dissolved gases can be adjusted according to the differences between their diffusion coefficients and that of 20Ne and the following relationship [Lippmann et al., 2003]: Ci ¼ Ci0
CNe 0 CNe
qffiffiffiffiffiffiffiffiffiffi DDi ðTðTs ÞÞ Ne
s
;
ð1Þ
where Ci is the measured concentration of specific gas i, Coi is the concentration of specific gas i in equilibrium with atmosphere composition, and Di is the diffusion coefficient [Jaehne et al., 1987] of specific gas i at the in situ temperature Ts. [9] The corrected values were converted to the dissolved concentrations following the procedure of Andrews and Wilson [1987]. Flow rate measured during sampling varied by less than 10%. The uncertainty for the correction for diffusion loss is ±10%. Error propagation yielded an uncertainty less than ±20% for the reported values (after the correction for diffusive loss). [10] The cations were measured by an Optima 4300 DV inductively coupled plasma-atomic emission spectrometer (Perkin-Elmer, Wellesley, MA). Anions were measured with a DX-320 ion chromatograph (Dionex, Sunnyvale, CA). Dissolved inorganic carbon was determined by an infrared spectrometer (Licor 6352, Lincoln, NE). The total organic carbon (TOC) was measured from acidified samples as CO2 generated by catalytic combustion using an infrared detector (Tekmar Dohrman DC190, Perkin-Elmer, Wellesley, MA) and represented nonpurgeable organic fraction. The pH and Eh of the fracture water were measured in the field. The uncertainty for aqueous chemistry is ±5%. [11] The microcosm experiment for H2-utilzing metabolisms was carried out by supplying the headspace with 80% of H2 and 20% of CO2, and 0.05% (final concentration) of Na2S as a reducing agent. The control sample was prepared in the same way but autoclaved right after the purging 3 of 13
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of headspace gases. The samples were incubated at the in situ temperature and the abundances of headspace gases were measured by a RGA 5 and compared to those of the control.
data overlap published dissolved H2 and He concentrations for groundwater from Precambrian Russian [Vovk, 1987] and Fennoscandian Shields [Haveman and Pedersen, 1999] and the Triassic Dunbarton Basin [Marine, 1979] (Figure 1).
3. Dissolved H2 and He Abundances
[15] Although anomalously high H2 concentrations that have been reported for shallow sedimentary boreholes are artifacts of the drilling process [Bjerg et al., 1997; Bjornstad et al., 1994], the H2 concentrations reported herein are not due to drilling for the following reasons:
[12] H2 abundances ranged up to 27% of the combustible gases, and the measured concentrations spanned over five orders of magnitude with a maximum of 7.41 mM (Table 1). The sample with the second highest measured H2 concentration (4.98 mM for sample 12) was collected from a 3day-old, tens of meters long, diamond-drilled borehole into metavolcanic rocks at 3.0 kilometers below land surface (kmbls). The 10 L min1 flow rates for the high-salinity (0.23 M Cl), nonmeteoric fracture water and gas (mostly CH4, H2 and He) indicate that the borehole had been flushed by >2000 borehole volumes of water (Table 1) prior to sampling. By contrast the lowest measured H2 concentration (16 nM for sample 4) was collected from a one-week-old borehole intersecting the contact between quartzite and a diabase dyke at 1.29 kmbls where the flow rates of lowsalinity (0.04 M of Cl) meteoric fracture water and gas (mostly CH4, N2 and He) [Ward et al., 2004] were 37.5 and 0.04 L min1, respectively. The H2 concentrations were not correlated with depth, salinity, pH, rock type, borehole age, fracture water age or any other measurement (correlation coefficients were all < 0.5), but the highest concentrations were typically found in the deeper, highly saline, nonmeteoric, older fracture water [Lippmann et al., 2003]. [13] The measured He concentrations (mostly 4He) (Table 1) ranged over three orders of magnitude and were not correlated with the H2 data, but were correlated with Cl (correlation coefficient = 0.98). Depending on the assumptions regarding crustal fluxes of 4He, 40Ar and 134,136Xe, the He concentrations correspond to subsurface residence times of 1.5 to 20 Myr [Lippmann et al., 2003] during which time the Witwatersrand Basin was at present pressure and temperature conditions [Omar et al., 2003]. [14] Nine samples were corrected for degassing into the fracture headspace as the fracture was dewatered using noble gas analyses [Lippmann et al., 2003]. The corrections increased H2 and He concentrations by 10 (Table 1) but did not impact the H2 to He ratios due to their comparable diffusion coefficients [Jaehne et al., 1987]. Our
[16] 1. Bjornstad et al. [1994] and Bjerg et al. [1997] used percussion drilling into shallow unconsolidated sands with no drilling fluid circulation, whereas mine boreholes are drilled with high-rpm diamond bits cooled by mine water circulating at a rate of L min1. [17] 2. Bjornstad et al. [1994] reported that tiny fragments of metal casing generated by pulverization of the rock during percussion drilling created a slurry that reacted with static groundwater to produce H2 at a rate of 1–10 nM s1, but that flushing the borehole with groundwater removed this effect within 12 hours. In mine drilling the circulating water removes cuttings and bit fragments from the borehole before they have time to react. Furthermore, mine boreholes are drilled under pressure so that upon intersection of a fluid-filled fracture (at least several MPa) residual mine water and cuttings are removed by the flushing of thousands of borehole volumes of fracture water (Table 1). [18] 3. Bjerg et al. [1997] reported H2 concentrations of 50 mM during drilling and a slow decline over days, whereas H2 concentrations in mine boreholes are observed to slowly increase over days after drilling (Table 1). Four samples of sample 14 over 54 days (2700 to 5700 borehole volumes) documented increasing H2 concentrations (0.3 to 3.7 mM) with increasing gas/water flow rate ratios and constant gas composition (H2 ranged from 10–12 mole%). The H2 concentration of sample 8c immediately after fracture intersection (10 borehole volumes) was 9 times less than that 2 weeks later (sample 8d; 5000 borehole volumes). The increasing gas/water flow rates probably represent degassing of water within the fracture as the fracture is drained as inferred from atmospheric noble gas analyses [Lippmann et al., 2003]. [19] 4. Bjerg et al. [1997] observed
