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

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4222 U.G. Wortmann et al. / Geochimica et Cosmochimica Acta 71 (2007) 4221–4232Although several details of the fractionation process remaincontroversial, the overall process is well understoodand can be described as the sum of several mass dependentfractionations during the stepwise reduction of sulfate tosulfide (Fig. 1) and the ratio between the forward and backwardreactions (Rees, 1973; Brüchert, 2004; Brunner andBernasconi, 2005). Culture experiments with dissimilatorysulfate reducers and field data show that the evolution ofthe d 18 O SO4 2 value during progressive sulfate reductionEq. (1) is dependent on the oxygen isotope compositionof the water (e.g. Mizutani and Rafter, 1973; Fritz et al.,1989; Böttcher et al., 1998, 1999; Brunner et al., 2005).Thus, if water is strongly depleted in 18 O compared to sulfate,the oxygen isotope ratio of sulfate will decrease whileits sulfur isotope composition increases (e.g. Mizutani andRafter, 1973; Fritz et al., 1989; Brunner et al., 2005). However,the limited number of studies conducted so far do notagree on the cause of this isotope effect and several modelshave been proposed:(A) Microcosm experiments have conclusively demonstratedthat isotope exchange reactions between sulfateand water are the dominant control factor ofthe d 18 O SO4 2 value (e.g., Mizutani and Rafter,1973; Fritz et al., 1989; Böttcher et al., 1998; Brunneret al., 2005; Knöller et al., 2006). However, the possibilityof a kinetic d 18 O SO4 2 fractionation is still discussedin the literature describing marineenvironments. This interpretation is based on theobservation that in certain studies the measuredd 18 O SO4 2 and d 34 S values of dissolved sulfate showa linear correlation (e.g. Aharon and Fu, 2000; Bottrellet al., 2000; Mandernack et al., 2003). Thereported ratios between the fractionation factors ofO and S vary from 1:1.4 to 1:4 (Mizutani and Rafter,1969; Aharon and Fu, 2000). We will thereforeexplore whether this hypothesis is a good explanationfor the ODP Site 1130 data.(B) The d 18 O SO4 2 is a function of an oxygen isotopeexchange between metabolic intermediates and cytoplasmicwater during microbially-mediated sulfatereduction. Fritz et al. (1989) observed that the d 34 Sand d 18 O SO4 2 values of aqueous sulfate in a groundwater environment initially increased together, butthat the d 18 O SO4 2 value asymptotically approacheda constant value whereas the d 34 S value continuedto increase. This observation agrees with results fromanoxic pore waters of marine sediments (Zak et al.,1980; Böttcher et al., 1998, 1999, 2001). It was shownexperimentally that the final steady state d 18 O SO4 2value depends on the d 18 O value of the ambient water(Mizutani and Rafter, 1973; Fritz et al., 1989; Brunneret al., 2005) suggesting an isotope exchange reactionbetween ambient water and sulfate. Theexperimentally determined steady state value for bacterialcultures (29‰ at 5 °C, Fritz et al., 1989) issomewhat lower than the equilibrium value predictedfrom high temperature experiments (36.4‰ and33.6‰, Lloyd, 1968; Mizutani and Rafter, 1973,respectively). As oxygen isotope exchange reactionsbetween water and sulfate are extremely slow atambient temperature and circumneutral pH (Zaket al., 1980; Chiba and Sakai, 1985), it has been suggestedthat this exchange must take place betweenenzymatically activated sulfate (adenosine phosphosulfate,APS) or sulfite and cytoplasmic water (Mizutaniand Rafter, 1973; Fritz et al., 1989).(C) The d 18 O SO4 2 value is a function of (A and B). So far,this possibility has not been studied in great detail,but only mentioned (Fritz et al., 1989; Brunneret al., 2005). The combined effect of a kinetic isotopefractionation and an oxygen isotope exchange withambient water would lead to an offset of the equilibriumisotope value and create an apparent equilibriumisotope factor, which would be greater thanthe actual equilibrium factor.Cytoplasmic Membranef 1 f 2 f 3 f 4 f 5SO 2— 4 SO 2— 4 APSSO 2— 3 H 2 Sb 1 b 2 b 3 b 4 b 5H 2 SCytoplasmAMPH 2 OH 2 OFig. 1. The major fractionation steps and associated fluxes during microbial reduction of sulfate. The overall isotope effect depends on thesum of the individual steps and the ratio between the forward and backward fluxes. Modified after Brunner and Bernasconi (2005).
Oxygen isotope biogeochemistry of pore water sulfate... 4223(D) d 18 O SO4 2 isotope effects are mainly a product of theoxidative part of the sulfur cycle where H 2 S is oxidizedto S 0 2and then disproportionated to SO 4and sulfide. The net reaction describing this processcan be written as4H 2 O þ S 0 2! 3H 2 S þ SO 4þ 2H þð2ÞBöttcher et al. (2001, 2005) observed that the abovereaction results in a net isotope effect for d 18 O SO4 2 ofup to 21‰ at 28–35 °C. They suggested that this isotopeeffect may be facilitated via the formation of sulfite as ametabolic intermediate, which would allow for the oxygenisotope exchange with ambient water. This processhas been used to explain the measurements from interstitialwaters (e.g. Blake et al., 2006; Turchyn et al.,2006) and to explain the d 18 O SO4 2 value of ocean water(Turchyn and Schrag, 2006).(E) Oxidation of sulfide to sulfate with water or dissolvedoxygen. For example, a combination of hypothesis(A and D) was suggested by Ku et al. (1999) andLu et al. (2001).In the following, we present d 18 O SO4 2 data for dissolved2SO 4from interstitial water samples of ODP Leg 182(Feary et al., 2000a), and investigate which of the abovehypothesis best explains the Site 1130 data.1.1. Geologic backgroundThe Great Australian Bight (GAB) forms the centralembayment of Australia’s southern continental margin, locatedbetween 124°E and 134°E and 32°S and 34°S. It representsthe largest cool water carbonate depositional realmon Earth today (Feary and James, 1998) and was the focusof ODP Leg 182 (Feary et al., 2000a). Although the coolwatercarbonate ramp of the GAB represents an unusualsystem today, it is considered a modern analogue of theMesozoic carbonate ramps (Feary and James, 1998) thathost a large share of the world’s petroleum resources.Ocean Drilling Program Leg 182 drilled two transectsthrough this carbonate ramp (see Fig. 2) and recovered244 interstitial water samples down to 500 m below seafloor(mbsf). The interstitial water profiles of these cores indicatethat the margin contains a complex system of differentbrines, with salinity values up to three times that of seawater(see Fig. 3, and Hine et al., 1999; Feary et al., 2000a;Swart et al., 2000; Wortmann, 2006). These brines mayhave been generated during sealevel lowstands on the largeadjacent shelf and emplaced into the sediments of the uppercontinental slope under the influence of a hydraulic head(Swart et al., 2000; Jones et al., 2002). The geochemical signatureof the brine corroborates a seawater origin, andshows that the brine carries up to 84 mM of sulfate. AtODP Site 1130, the advecting brine results in the supplyof SO 42from below, where SO 42concentrations increasewith depth until they stabilize at 300 mbsf at 65 mM (Fearyet al., 2000a). Modeling the conservative chloride ion concentrationdata of Site 1130 shows that Site 1130 is influencedby strong upward advective flow on the order of2 mm/year (Wortmann, 2006). Inverse reaction-transportmodeling of the sulfate concentration data of this site, suggeststhat volumetric sulfate reduction rates (SRR) vary between600 and 65 pmol/cm 3 /year (Wortmann, 2006). Asthese carbonate sediments contain only small amounts ofiron, hypersulfidic conditions prevail throughout most ofthe cores recovered during ODP Leg 182 (Feary et al.,2000a). The sulfur geochemistry of these sites is unusualas the isotopic difference of dissolved sulfide and sulfate approaches70‰ (see Fig. 4, and for a detailed discussion seeWortmann et al. (2001) and Wortmann (2006)).33 S127 E 128 E129 E100200Eyre1134Terrace100011321130Jerboa-11126112911271131113350034 S2000300011284000GAB-13B(alternate)45000 50 kmFig. 2. Locations of the sites drilled during ODP-Leg 182. Contour lines are in m and show water depth. Figure taken from Feary et al.(2000a).
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