Oxygen isotope fractionation of dissolved oxygen during abiological reduction by aqueous sulfide

The stable oxygen isotope fractionation factor of dissolved oxygen gas has been measured during abiological reduction by aqueous sulfide at 10–40 °C under circum-neutral conditions in order to understand the stable isotope behavior of oxygen during interaction with sulfide-rich natural waters. The oxygen isotopic composition of O2 becomes isotopically heavier as the reaction proceeds, with the oxygen isotope enrichment factor ε ranging from − 8.2 to − 9.8‰ at 10 °C, from − 8.8 to − 10.9‰ at 23 °C, and from − 6.0 to − 10.7‰ at 40 °C. Values of ε show little dependence on the initial sulfide concentration, reaction temperature, or salinity in the solutions. In addition to the direct reaction of sulfide with O2, aqueous sulfide oxidation can also progress through complex and multiple reaction pathways where O2 can further react with intermediate products of sulfide oxidation such as S0, Sx2−, S2O32−, and SO32−. Hence, the measured values of ε are a result of multiple isotope fractionations during O2 consumption by these sulfur species, where the net measured isotope fractionation factor is dependent upon the relative magnitude of each individual O2-consuming reaction, and the isotope fractionation factor associated with each of these reactions. Nevertheless, the results will have application to the identification and possible quantification of O2 consumption by reaction with sulfide in natural environments.