Flora Hochscheid

The co-authors and their affiliations are the following:

Andrew C. Turner, affiliation: Central Energy Resources Science Center, U.S. Geological Survey, Denver, CO 80225 USA

Markus Bill, affiliation: Energy Geosciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720 USA 

Daniel A. Stolper, affiliation: Department of Earth and Planetary Science, University of California, Berkeley, CA 94720 USA and Energy Geosciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720 USA 

Molecular hydrogen (H2) is found in a variety of settings on and in the Earth from low-temperature sediments to hydrothermal vents and is actively being considered as an energy resource for the transition to a green energy future. When examining H2 in the environment, it is important to constrain its origins. One way to do this is measure its stable isotopic composition, given as the deuterium/hydrogen [D/H] ratio or 𝛿D. In nature, the 𝛿D of molecular hydrogen (𝛿DH2) varies by hundreds of per mil from ~-800‰ in hydrothermal and sedimentary systems to ~+440‰ in the stratosphere. This range reflects a variety of processes, including kinetic isotope effects associated with formation and destruction and equilibration with water, the latter proceeding at fast (order year) timescales even at low temperatures (<100°C; e.g., Pester et al., 2018). In hydrothermal and sedimentary systems, H2 is commonly assumed to be at equilibrium with local fluids. At isotopic equilibrium, the D/H fractionation factor between liquid water and hydrogen (D𝛼H2O(l)-H2(g)) is a function of temperature and can thus be used as a geothermometer for H2 formation and re-equilibration temperatures. In order to use values of 𝛿DH2 for geothermometry, it is necessary that D𝛼H2O(l)-H2(g) as function of temperature be known accurately over the range of environmentally relevant temperatures where liquid water is present. Multiple studies have produced theoretical calculations for hydrogen isotopic equilibrium between hydrogen gas and liquid or vapor water. However, only three published experimental calibrations exist in the H2O-H2 system: two between 51 and 742°C for H2O(g)-H2(g) (Suess, 1949; Cerrai et al., 1954), and one in the H2O(l)-H2(g) system for temperatures <100°C (Rolston et al., 1976). Evaluation of the accuracy of these calibrations is difficult as both the measurement techniques and standardization procedures do not follow modern practices. Indeed, many studies use theoretical calculations over experimental calibrations to estimate D𝛼H2O(l)-H2(g).
Here we present a new experimental calibration of the equilibrium hydrogen isotopic fractionation factor for liquid water and hydrogen gas (D𝛼H2O(l)-H2(g)) from 3 to 90°C, measured and standardized using modern techniques. Equilibration was achieved using platinum catalysts and verified via experimental bracketing by approaching final D𝛼H2O(l)-H2(g) values at a given temperature from both higher (top-bracket) and lower (bottom-bracket) initial D𝛼 values. Final D𝛼H2O(l)-H2(g) values representing the equilibrium, given by averaged top- and bottom-brackets are in general agreement with previous experimental data and theoretical calculations. We will discuss the implications of this work on the estimation of formation and re-equilibration temperatures of H2 in low-temperature settings on Earth.