The yield and isotopic composition of radiolytic H 2, a potential energy source for the deep subsurface biosphere

Abstract

The production rate and isotopic composition of H 2 derived from radiolytic reactions in H 2O 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 γ source, and the H 2 abundance in the headspace and its isotopic composition were measured. The H 2 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 ∼2000 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 α and β particles, a radiolytic H 2 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 δD of H 2 ( δ D = ( ( D / H ) sample / ( D / H ) standard − 1 ) × 1000 ) was independent of the dosage, pH (except for pH 4), salinity, and O 2 and yielded an αD H2O-H2 of 2.05 ± 0.07 (αD H2O-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 H 2 and H 2O, the isotopic exchange rate between H 2 and H 2O will erase the heavy δD of radiolytic H 2 if the age of the groundwater is greater than ∼10 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.