Determination and application of the equilibrium oxygen isotope effect between water and sulfite

摘要

The information encoded by the two stable isotope systems in sulfate (δ~(34)S_(SO4) and δ~(18)O_(SO4)) has been widely applied to aid reconstructions of both modern and ancient environments. Interpretation of δ~(18)O_(SO4) records has been complicated by rapid oxygen isotope equilibration between sulfoxyanions and water. Specifically, the apparent relationship that develops between δ~(18)O_(SO4) and δ~(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 produce 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 ℃), where ε= 1000 * (a - 1), we find eSO3–H2O = 13:61 - 0:299 * pH - 0:081 * T ℃: Thus, at a pH (7.0) and temperature (25 ℃) typifying commonly used experimental conditions for sulfate reducing bacterial 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 understanding 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 intracellular 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 δ~(18)O_(SO4) resulting as a consequence of sulfate reduction.