Abstract
The diffusion of hydrogen sulphide across the sediment–water interface and subsequent liberation to the atmosphere may occur in iron-deficient coastal marine environments with enhanced microbial activity in surface sediments and corresponding accumulation of dissolved H2S in near-surface pore waters. The involvement of analogue processes in periods of global mass extinctions during Earth's history (e.g. at the Permian–Triassic boundary) is currently in discussion [L.R. Kump, A. Pavlov, and M. Arthur, Massive Release of Hydrogen Sulfide to the Surface Ocean and Atmosphere During Intervals of Oceanic Anoxia, Geology 33, 397 (2005)]. The outgassing of H2S is associated with a fractionation of the stable sulphur isotopes, which has so far only been investigated experimentally at selected acidic and neutral pH values, and no experiments with seawater had been carried out. In this communication, we report on sulphur isotope fractionation that takes place during the experimental degassing of H2S from aqueous solution by an inert gas (N2) at 21°C. Experiments were conducted in the pH range between 2.6 and 10.8, corresponding to the dominance fields of dissolved hydrogen sulphide (H2S(aq)), bisulphide (HS), and mixtures of both sulphide species. Overall isotope enrichment factors between−1.6 and +3.0‰ were observed, with the residual dissolved sulphide being enriched or depleted in 34S compared to the liberated H2S at low and high pH values, respectively. The difference in the low and high pH isotope fractionation effects can be explained by isotope exchange between H2S(aq) and HS [B. Fry, H. Gest, and J.M. Hayes, Sulfur Isotope Eeffects Associated with Protonation of HS− and Volatilization of H2S, Chem. Geol. (Isot. Geosci. Sec.) 58, 253 (1986); R. Geßler and K. von Gehlen, Investigation of Sulfur Isotope Fractionation Between H2S Gas and Aqueous Solutions, Fresenius J. Anal. Chem. 324, 130 (1986)] followed by the subsequent transfer of H2S(aq) to the gaseous phase. The assumption of pure physical outgassing of H2S(aq) at low pH values leads to an isotope enrichment factor of−0.9±0.4‰ (n=14) which is caused by the combined differences in dehydration and diffusion coefficients of HS(aq) and S(aq). In the pH range of natural surface and shallow pore waters, 34S will be equal to or enriched in the gaseous phase compared to the aqueous solution, therefore creating no or a slight enrichment of 32S in the aqueous solution. Experiments in seawater solution showed no significant influence of increased ionic strength and changed corresponding aqueous speciation on sulphur isotope effects.
