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

4224 U.G. Wortmann et al. / Geochimica et Cosmochimica Acta 71 (2007) 4221–4232567 mmol/l Cl - 567 mmol/l Cl -1300 mmol/l Cl - 1300 mmol/l Cl -Fig. 3. Isohaline surfaces derived from interpolation of the Cl data of Sites 1130 and 1132. The fact that the Cl concentrations stabilize at asub-horizontal level, suggests that the brine is constantly replenished at depth, possibly by lateral advection. The horizontal distance betweenthe two sites is about 11.6 km, seismic data courtesy of D.A. Feary.Depth [m]-50 0 500SO 4 [mM] DataH 2 S [mM] Data50δ 34 S SO 4 [ 0 / 00 VCDT] Dataδ 34 S H 2 S[ 0 / 00 VCDT] Data1001502002500 / 00[VCDT]SO 4 [mM] ModelH 2 S [mM] Modelδ 34 S SO 4 [ 0 / 00 VCDT] Modelδ 34 S H 2 S [ 0 / 00 VCDT] Model300-50 0 50[mM]Fig. 4. Wortmann et al. (2001) reported d 34 S data from ODP Site 1130 which showed an isotopic difference of more than 70‰ between coexistingdissolved H 2 S and SO 24. They argued that under hypersulfidic conditions and in the absence of any oxidants, this difference must becaused by unusually high S-fractionation factors during microbial sulfate reduction. Reaction transport modeling of this system suggests thatthe fractionation factor was at least 65‰ (Wortmann et al., 2001). A theoretical framework how to explain these large fractionation factorswas given by Brunner and Bernasconi (2005). Note that the modeling results presented in this figure have been obtained by the same nonsteadystate parametrization used throughout in this paper, and that Fig. 7 demonstrates that the observed d 34 S-signatures are not an artifactof the 1-D modeling approach.However, the interstitial water data of the upper 30 mbsfat ODP Site 182 are difficult to explain within the frameworkof a steady state diagenetic model. Previous publications(Wortmann et al., 2001; Wortmann, 2006) suggestedthat the upper 30 mbsf might be affected by an unknownmixing process. While it is difficult to envision a physicalprocess which provides for a steady state mixing depth of30 mbsf, non-steady state mixing by sedimentation eventsis a plausible process. Huuse and Feary (2005) suggestedthat the youngest sediments in the Great Australian Bightsediments might be contourites. Thus we assume that thesediments at Site 1130 are redeposited older sediments,which provides a plausible mechanism for seawater irrigation,and allows us for the first time, to successfully modelthe upper 30 mbsf of OPD Site 1130 (see Fig. 5).2. METHODSThe interstitial water samples were taken on board theJOIDES Resolution following the procedures given in