Determination and application of the equilibrium oxygen isotope effect between water and sulfite Scott D. Wankel a,b,⇑ , Alexander S. Bradley c , Daniel L. Eldridge d , David T. Johnston a a Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, United States b Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, United States c Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130, United States d Department of Geology, University of Maryland, College Park, MD 20742, United States Received 30 October 2012; accepted in revised form 15 August 2013; available online xxxx Abstract The information encoded by the two stable isotope systems in sulfate (d 34 S SO 4 and d 18 O SO 4 ) has been widely applied to aid reconstructions of both modern and ancient environments. Interpretation of d 18 O SO 4 records has been complicated by rapid oxygen isotope equilibration between sulfoxyanions and water. Specifically, the apparent relationship that develops between d 18 O SO 4 and d 18 O water during microbial sulfate reduction is thought to result from rapid oxygen isotope equilibrium between intracellular water and aqueous sulfite – a reactive intermediate of the sulfate reduction network that can back-react to pro- duce sulfate. Here, we describe the oxygen equilibrium isotope effect between water and sulfite (referring to all the sum of all S(IV)-oxyanions including sulfite and both isomers and the dimer of bisulfite). Based on experiments conducted over a range of pH (4.5–9.8) and temperature (2–95 °C), where e = 1000 * (a  1), we find e SO 3 –H 2 O ¼ 13:61  0:299  pH  0:081  T  C: Thus, at a pH (7.0) and temperature (25 °C) typifying commonly used experimental conditions for sulfate reducing bac- terial cultures, sulfite is enriched in 18 O by 9.5& (±0.8&) relative to ambient water. We examine the implication of these results in a sulfate reduction network that has been revised to reflect our understand- ing of the reactions involving oxygen. By evaluating previously published data within this new architecture, our results are consistent with previous suggestions of high reversibility of the sulfate reduction biochemical network. We also demonstrate that intracellular exchange rates between SO 3 2 and water must be on average 1–3 orders of magnitude more rapid than intra- cellular fluxes of sulfate reduction intermediates and that kinetic isotope effects upstream of SO 3 2 are required to explain previous laboratory and environmental studies of d 18 O SO 4 resulting as a consequence of sulfate reduction. Ó 2013 Elsevier Ltd. All rights reserved. 1. INTRODUCTION The dual stable isotopic composition of sulfate (d 34 S SO 4 and d 18 O SO 4 ) is widely applied in the interpretation of environmental reconstructions of both modern and an- cient environments (Claypool et al., 1980; Mandernack et al., 2003; Turchyn and Schrag, 2004; Einsiedl and Mayer, 2005; Novak et al., 2006; Turchyn and Schrag, 2006). Central to these studies is a host of biological and abiotic redox and exchange reactions. Most notable is the biogeochemical cycling between sulfate and sulfide. Stable sulfur isotopes are a common vehicle for examina- tion of microbial sulfate reduction, which is almost solely responsible for reductive fluxes (Canfield, 2001). In com- plement, oxygen isotopes fingerprint inorganic equilibra- tion reactions between water and various sulfoxyanions, including sulfite, and are often used to quantify oxidative fluxes within the sulfate reduction process (e.g., Brunner et al., 2005; Turchyn et al., 2010). Thus, it is the balance 0016-7037/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.gca.2013.08.039 ⇑ Corresponding author. Address: Department of Marine Chem- istry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, United States. Tel.: +1 5082893944. E-mail address: sdwankel@whoi.edu (S.D. Wankel). www.elsevier.com/locate/gca Available online at www.sciencedirect.com ScienceDirect Geochimica et Cosmochimica Acta xxx (2013) xxx–xxx Please cite this article in press as: Wankel S. D., et al. Determination and application of the equilibrium oxygen isotope effect between water and sulfite. Geochim. Cosmochim. Acta (2013), http://dx.doi.org/10.1016/j.gca.2013.08.039