Oxygen isotope biogeochemistry of pore water sulfate in the deep ...

4230 U.G. Wortmann et al. / Geochimica et Cosmochimica Acta 71 (2007) 4221–4232Fig. 9. The most likely source of the oxygen used for the oxidation of sulfite to APS is adenosine monophosphate (AMP). AMP is used in thetransformation of sulfite to APS, a reaction that is reversibly catalyzed by the flavoenzyme APS reductase (Fritz et al., 2002).mbsf050100150200250300350400FeP [wt%]0.000 0.025 0.050 0.075 0.1000.0 0.1 0.2 0.3 0.4FeT [wt\%]Fig. 10. Iron and pyrite concentration data for ODP-Site 1130.FeT, total iron content; FeP, pyrite bound iron.From a modeling point of view, we cannot discriminatebetween exchange processes related to sulfate reduction(hypothesis B and C), sulfur disproportionation (D), or directsulfide oxidation to sulfate (E). The latter two processes,however, depend on the availability of an oxidantto oxidize H 2 S to elemental sulfur. While it is currently unclearwhich substance could act as an oxidant at hypersulfidicSite 1130 (Wortmann et al., 2001), we can calculate themolar equivalent needed to sustain the above exchangefluxes, using Eq. (16). Assuming a 2:1 ratio between oxidantand oxidized sulfide would require 3000 mol/m 3 of oxidantat 30 mbsf. If the oxidizing phase were Fe 2 O 3 , thiswould be equivalent to an iron content of 14 wt%. If we allowfor a higher value, the required exchange fluxes wouldbe much smaller, e.g., with an =50‰, the exchange fluxwould be 4-fold, equivalent to 4 wt% Fe 2 O 3 . The iron concentrationdata shows that ODP Site 1130 contains notmore than 0.4 wt% Fe, which is about one order of magnitudeless than the amount discussed above (see Fig. 10).3.1. ConclusionsWe present d 18 O SO4 2 data from interstitial watersamples of ODP Site 1130. The maximum observedd 18 O SO4 2 values coincide within error with the experimentallydetermined steady state equilibrium value of 29‰ at5 °C. Reaction transport modeling shows that it is difficultto explain the d 18 O SO4 2 of ODP Site 1130 invoking kineticfractionation effects. If we consider however isotopic exchangereactions between metabolic intermediates andambient water, the measured data can be explained in aconsistent way with an oxygen isotope equilibrium fractionationfactor =29‰. Our model was built without a prioryassumptions which of the metabolic intermediates facilitatesthe oxygen isotope exchange, and we are therefore unableto differentiate between the individual contributions ofAPS, sulfite, and AMP. However, in a sulfite exchange scenariowith subsequent oxidation to sulfate with oxygen derivedfrom water, the oxygen isotope equilibrium betweensulfite and water would be at least 38‰, much higher thanany reported estimates so far. Therefore, we suggest thatthe oxygen isotope exchange either occurs between APS