Linking sedimentary sulfur and iron biogeochemistry to growth patterns of a cold-water coral mound in the Porcupine Basin, S.W. Ireland (IODP Expedition 307).

Challenger Mound, a 150-m-high cold-water coral mound on the eastern flank of the Porcupine Seabight off SW Ireland, was drilled during Expedition 307 of the Integrated Ocean Drilling Program (IODP). Retrieved cores offer unique insight into an archive of Quaternary paleo-environmental change, long-term coral mound development, and the diagenetic alteration of these carbonate fabrics over time. To characterize biogeochemical carbon-iron-sulfur transformations in the mound sediments, the contents of dithionite- and HCl-extractable iron phases, iron monosulfide and pyrite, and acid-extractable calcium, magnesium, manganese, and strontium were determined. Additionally, the stable isotopic compositions of pore-water sulfate and solid-phase reduced sulfur compounds were analyzed. Sulfate penetrated through the mound sequence and into the underlying Miocene sediments, where a sulfate-methane transition zone was identified. Small sulfate concentration decreases (<7 mM) within the top 40 m of the mound suggested slow net rates of present-day organoclastic sulfate reduction. Increasing δ(34)S-sulfate values due to microbial sulfate reduction mirrored the decrease in sulfate concentrations. This process was accompanied by oxygen isotope exchange with water that was indicated by increasing δ(18)O-sulfate values, reaching equilibrium with pore-water at depth. Below 50 mbsf, sediment intervals with strong (34)S-enriched imprints on chromium-reducible sulfur (pyrite S), high degree-of-pyritization values, and semi-lithified diagenetic carbonate-rich layers characterized by poor coral preservation, were observed. These layers provided evidence for the occurrence of enhanced microbial sulfate-reducing activity in the mound in the past during periods of rapid mound aggradation and subsequent intervals of non-deposition or erosion when geochemical fronts remained stationary. During these periods, especially during the Early Pleistocene, elevated sulfate reduction rates facilitated the consumption of reducible iron oxide phases, coral dissolution, and the subsequent formation of carbonate cements.

Stable sulfur isotope fractionation during the reduction of thiosulfate by Dethiosulfovibrio russensis.

Stable sulfur isotope fractionation was investigated during reduction of thiosulfate by growing batch cultures of Dethiosulfovibrio russensis at a cell-specific reduction rate of 2.4 +/- 0.72 fmol cell(-1) d(-1) (28 degrees C). Citrate was used as carbon and energy source. The hydrogen sulfide produced by this sulfur- and thiosulfate-reducing bacterium was depleted in 34S by 11% compared to total thiosulfate sulfur, in agreement with previous results observed for sulfate-reducing bacteria. This indicates the operation of a similar pathway for thiosulfate reduction in these phylogenetically different bacteria.

The Stable Isotopic Geochemistry of the Sulfur and Carbon Cycles in a Modern Karst Environment.

The stable isotopic geochemistry of the sulfur and carbon cycles in a modern karst environment at the southwest border of the Harz Mountains (Germany) has been studied during a 7-year period. Beside major, minor and trace elements, river and sulfate-carbonate ground water were analyzed for the 13C/12C ratios of the dissolved carbonate species, the 18O/16O ratio of water, and the 34S/32S and 18O/16O ratios of dissolved sulfate. Additionally, the carbon, sulfur and oxygen isotopic composition of the major karst aquifers (carbonate and gypsum anhydrite rocks) of the Permian Zechstein formation was measured. Nowadays, the stable isotope biogeochemistry of the river and karst water in the studied area results from the complex interactions between dissolution of biogenic CO2 in the water-unsaturated zone, (minor) subterrestrial microbial decomposition reactions of organic matter (recent DOC or fossil organic matter), interactions with carbonate and sulfate aquifer minerals, and input of acids from atmospheric pollution (e.g., sulfuric acid). Subsurface precipitation of secondary calcite due to the incongruent dissolution of gypsum and/or dolomite is deduced from the hydrogeochemical composition of selected ground waters and the isotopical composition of calcite found in gypsum aquifer material. Spring and river waters were additionally influenced by the liberation of carbon dioxide into the Earth's atmosphere, a process which is accompanied by the preferrential desorbtion of 12CO2. The sulfur isotopic composition of dissolved sulfate from ground and spring waters indicates a mixture of sulfate derived from surface waters (mainly originating from atmospheric pollution) and geogenic sulfate from the subsurface dissolution of gypsum/anhydrite of the Zechstein formation. The oxygen isotopic composition of dissolved sulfate is generally far away from the exchange equilibrium with water, but consistent with the two-component mixing model derived from the sulfur isotope ratios.

Methane-derived carbonates in a native sulfur deposit: stable isotope and trace element discriminations related to the transformation of aragonite to calcite.

Abstract Stable isotope ((13)C, (18)O, (34)S) and trace element (Sr(2+), Mg(2+), Mn(2+), Ba(2+), Na(+)) investigations of elemental sulfur, primary calcites and mixtures of aragonite with secondary, post-aragonitic calcite from sulfur-bearing limestones have provided new insights into the geochemistry of the mineral forming environment of the native sulfur deposit at Machów (SE-Poland). The carbon isotopic composition of carbonates (δ(13)C = -41 to -47‰ vs. PDB) associated with native sulfur (δ(34)S = + 10 to + 15‰ vs. V-CDT) relates their formation to the microbiological anaerobic oxidation of methane and the reduction of sulfate derived from Miocene gypsum. From a comparison with experimentally derived fractionation factors the element ratios of the aqueous fluids responsible for carbonate formation are estimated. In agreement with field and laboratory observations, ratios near seawater composition are obtained for primary aragonite, whereas the fluids were relatively enriched in dissolved calcium during the formation of primary and secondary calcites. Based on the oxygen isotope composition of the carbonates (δ(18)O = -3.9 to -5.9‰ vs. PDB) and a secondary SrSO(4) (δ(18)O = + 20‰ vs. SMOW; δ(34)S = + 59‰ vs. V-CDT), maximum formation temperatures of 35°C (carbonates) and 47°C (celestite) are obtained, in agreement with estimates for West Ukraine sulfur ores. The sulfur isotopic composition of elemental sulfur associated with carbonates points to intense microbial reduction of sulfate derived from Miocene gypsum (δ(34)S ≈ + 23‰) prior to the re-oxidation of dissolved reduced sulfur species.

(18)O/(16)O and (13)c/(12)c fractionation during the reaction of carbonates with phosphoric Acid: effects of cationic substitution and reaction temperature.

Abstract The factors for (18)O/(16)O fractionation between carbonates and CO(2) gas produced by the dissolution of the carbonates in phosphoric acid (sealed vessel method) have been investigated as a function of reaction temperature (20-90°C) and cationic substitution in the solid. Synthetic CaCO(3), Ca(0.75) Mn(0.25) CO(3), MnCO(3), BaCO(3) and SrCO(3) powders, and a natural kutnahorite sample were used as solids. The δ(18)O values of the gaseous CO(2) liberated by the reaction with phosphoric acid decrease with increasing temperature and seem to be a linear function of T(°K)(-2). The slopes are specific for different carbonates. No temperature-depended (13)C/(12)C fractionation seems to exist.