A new value for the stable oxygen isotope fractionation between dissolved sulfate ion and water

Abstract

Although the stable oxygen isotope fractionation between dissolved sulfate ion ( SO 4 2 - ) and H 2O (hereafter α ( SO 4 2 - – H 2 O ) ) is of physico-chemical and biogeochemical significance, no experimental value has been established until present. The primary reason being that uncatalyzed oxygen exchange between SO 4 2 - and H 2O is extremely slow, taking ∼ 10 5 years at room temperature. For lack of a better approach, values of 16‰ and 31‰ at 25 °C have been assumed in the past, based on theoretical ‘gas-phase’ calculations and extrapolation of laboratory results obtained at temperatures >75 °C that actually pertain to the bisulfate ( HSO 4 - ) – H 2 O system. Here I use novel quantum-chemistry calculations, which take into account detailed solute–water interactions to establish a new value for α ( SO 4 2 - – H 2 O ) of 23‰ at 25 °C. The results of the corresponding calculations for the bisulfate ion are in agreement with observations. The new theoretical values show that sediment δ 18 O SO 4 2 - -data, which reflect oxygen isotope equilibration between sulfate and ambient water during microbial sulfate reduction, are consistent with the abiotic equilibrium between SO 4 2 - and water.