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

Although the stable oxygen isotope fractionation between dissolved sulfate ion (SO(4)(2-)) and H(2)O (hereafter alpha((SO42--H2O))) 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(2)O is extremely slow, taking similar to 10(5) years at room temperature. For lack of a better approach, values of 16 parts per thousand and 31 parts per thousand at 25 degrees C have been assumed in the past, based on theoretical 'gas-phase' calculations and extrapolation of laboratory results obtained at temperatures >75 degrees 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-interactions to establish a new value for alpha((SO42--H2O)) of 23 parts per thousand at 25 degrees C. The results of the corresponding calculations for the bisulfate ion are in agreement with observations. The new theoretical values show that sediment delta(18)O(SO42-)-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. (C) 2009 Elsevier Ltd. All rights reserved.