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

The production rate and isotopic composition of H-2 derived from radiolytic reactions in H2O were measured to assess the importance of radiolytic H-2 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 gamma source, and the H, 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 H-2 concentration. At similar to2000 Gy, the H-2 concentration varied only by 16% across the experimental range of pH, salinity, and O-2 Based upon this measured yield and H-2 yields for a and O-2 particles, a radiolytic H2 production rate of 10(-9) to 10(-4) nM s(-1) was estimated for the range of radioactive element concentrations and porosities typical of crustal rocks. The deltaD of H-2 (deltaD = ((D/H)(sample)/(D/H)(standard) - 1) x 1000) was independent of the dosage, pH (except for pH 4), salinity, and O-2 and yielded an alphaD(H2O-H2) of 2.05 +/- 0.07 (alphaD(H2O-H2) = (D/H)(H2O) to (D/H)H-2), slightly less than predicted radiolytic models. Although this radiolytic fractionation value is significantly heavier than that of equilibrium isotopic exchange between H-2 and H2O, the isotopic exchange rate between H-2 and H2O will erase the heavy deltaD of radiolytic H-2 if the age of the groundwater is greater than similar to10(3) to 10(4) yr. The millimolar concentrations of H-2 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 H-2-utilizing microorganisms and to inhibit H-2- producing, fermentative microorganisms. Copyright (C) 2005 Elsevier Ltd.