The pivotal role of sulfite species in shaping the oxygen isotope composition of sulfate: new insights from a stable isotope perspective

Sulfate (SO4) is highly abundant on our planet and is one of the most important terminal electron acceptors during the degradation of organic matter by microorganisms in anoxic environments. During the dissimilatory sulfate reduction (DSR) SO4 is reduced to other sulfur compounds at lower oxidation state such as hydrogen sulfide (H2S) which is excreted from the microorganisms and in most cases is oxidized back to SO4. The H2S or other reduced sulfur compounds can be oxidized via different pathways depending on the absence/presence of sulfur oxidizing microorganisms, as well as on the chemical conditions of the environment. For example, the chemical conditions in the environment can vary with respect to the presence of air O2 or ferric iron (Fe) as oxidizing agents. Changes in the abundance of these oxidants lead to slight differences in the oxidation pathways, but both pathways will produce in the end SO4. Adding to the complexity of the sulfur cycle is the fact that sulfoxy intermediates (e.g. S2O3, SO3) or elemental sulfur (S) can be disproportionated into sulfur compounds with higher and lower oxidation state. The sulfur compound with higher oxidation state is typically SO4 and the compound with lower oxidation state can be subsequently oxidized to SO4. All these different processes will leave a specific oxygen isotope imprints on the produced SO4. Sulfate is a very inert molecule that does not exchange its oxygen with the water (only under extreme conditions), thus the oxygen isotope composition of SO4 can be considered an archive for the various production processes. In my dissertation I was especially interested in the role of sulfite (SO3) during the oxidative part of sulfur cycling and if the presence of SO3 as an intermediate in sulfur cycling has the potential to affect the oxygen isotope composition of SO4. Sulfite is a highly interesting sulfur compound because it is often consider the final intermediate during the oxidation of reduced sulfur compounds. In contrast to SO4, SO3 rapidly exchanges its oxygen with water quickly approaching equilibrium isotope exchange conditions. Due to these characteristics the presence of SO3 as final sulfoxy intermediate in oxidative sulfur cycling must be visible in the oxygen isotope composition of the produced SO4. Although SO3 is often considered an important intermediate and despite its potential in affecting the oxygen isotope composition of sulfate during the oxidative part of sulfur cycling, direct studies on the isotope effects caused by the presence of SO3 are scarce. To unravel the role of sulfite on the oxygen isotope patterns of SO4 we first had to determine the oxygen isotope equilibrium fractionation between SO3 and water by performing anoxic experiments where sodium sulfite was dissolved in de-ionized water (Chapter 2: “The oxygen isotope equilibrium fractionation between sulfite species and water”). After pinpointing the oxygen isotope equilibrium fractionation between SO3 and water we could use this value to determine the oxygen isotope effects during the abiotic oxidation of SO3 to SO4 (Chapter 3: “Isotopic evidence of the pivotal role of sulfite oxidation in shaping the oxygen isotope signature of sulfate”). As a the final project of this dissertation we analyzed the isotope effects that take place during the disproportionation of sulfur dioxide (SO2) to HSO4 and S at hydrothermal temperatures. This process appears to occur more often than expected in hydrothermal systems at the seafloor at subduction zones, contributing high amounts of sulfate and sulfur to the marine environment. We hypothesize that the disproportionation of SO2 in hydrothermal