Equilibrium isotope fractionation factors of H exchange between steam and soil clay fractions

Abstract

RationaleSteam equilibration overcomes the problem of the traditional measurements of H isotope compositions, which leave an arbitrary amount of adsorbed water in the sample, by controlling for the entire exchangeable H pool, including adsorbed water and hydroxyl‐H. However, the use of steam equilibration to determine nonexchangeable stable H isotope compositions in environmental media (expressed as δ2Hn values) by mathematically eliminating the influence of exchangeable H after sample equilibration with waters of known H‐isotopic composition requires the knowledge of the equilibrium isotope fractionation factor between steam‐H and exchangeable H of the sample (αex‐w), which is frequently unknown.MethodsWe developed a new method to determine the αex‐w values for clay minerals, topsoil clay fractions, and mica by manipulating the contributions of exchangeable H to the total H pool via different degrees of post‐equilibration sample drying. We measured the δ2H values of steam‐equilibrated mineral and soil samples using elemental analyzer‐pyrolysis‐isotope ratio mass spectrometry.ResultsThe αex‐w values of seven clay minerals ranged from 1.071 to 1.140, and those of 19 topsoil clay fractions ranged from 0.885 to 1.216. The αex‐w value of USGS57 biotite, USGS58 muscovite, and of cellulose was 0.965, 0.871, and 1.175, respectively. The method did not work for kaolinite, because its small exchangeable H pool did not respond to the selected drying conditions. Structurally different mineral groups such as two‐ and three‐layer clay minerals or mica showed systematically different αex‐w values. The αex‐w value of the topsoil clay fractions correlated with the soil clay content (r = 0.63, P = 0.004), the local mean annual temperature (r = 0.68, P = 0.001), and the δ2H values of local precipitation (r = 0.72, P < 0.001), likely to reflect the different clay mineralogy under different weathering regimes.ConclusionsOur new αex‐w determination method yielded realistic results in line with the few previously published values for cellulose. The determined αex‐w values were similar to the widely assumed values of 1.00–1.08 in the literature, suggesting that the adoption of one of these values in steam equilibration approaches is appropriate.