Low Sulfate Reduction Rates Research Articles

Hydrothermal activity fuels microbial sulfate reduction in deep and distal marine settings along the Arctic Mid Ocean Ridges

Microbial sulfate reduction is generally limited in the deep sea compared to shallower marine environments, but cold seeps and hydrothermal systems are considered an exception. Here, we report sulfate reduction rates and geochemical data from marine sediments and hydrothermal vent fields along the Arctic Mid Ocean Ridges (AMOR), to assess the significance of basalt-hosted hydrothermal activity on sulfate reduction in a distal deep marine setting. We find that cored marine sediments do not display evidence for sulfate reduction, apart from low rates in sediments from the Knipovich Ridge. This likely reflects the overall limited availability of reactive organic matter and low sedimentation rates along the AMOR, except for areas in the vicinity of Svalbard and Bear Island. In contrast, hydrothermal samples from the Seven Sisters, Jan Mayen and Loki’s Castle vent fields all demonstrate active microbial sulfate reduction. Rates increase from a few 10s to 100s of pmol SO42- cm-3 d-1 in active high-temperature hydrothermal chimneys, to 10s of nmol SO42- cm-3 d-1 in low-temperature barite chimneys and up to 110 nmol cm-3 d-1 in diffuse venting hydrothermal sediments in the Barite field at Loki’s Castle. Pore fluid and sediment geochemical data suggest that these high rates are sustained by organic compounds from microbial mats and vent fauna as well as methane supplied by high-temperature hydrothermal fluids. However, significant variation was observed between replicate hydrothermal samples and observation of high rates in seemingly inactive barite chimneys suggests that other electron donors may be important as well. Sediment sulfur isotope signatures concur with measured rates in the Barite field and indicate that microbial sulfate reduction has occurred in the hydrothermal sediments since the recent geological past. Our findings indicate that basalt-hosted vent fields provide sufficient electron donors to support microbial sulfate reduction in high- and low-temperature hydrothermal areas in settings that otherwise show very low sulfate reduction rates.

Revisiting the applicability and constraints of molybdenum- and uranium-based paleo redox proxies: comparing two contrasting sill fjords

Abstract. Sedimentary molybdenum (Mo) and uranium (U) enrichments are often used as redox proxies to reconstruct bottom water redox changes. However, these redox proxies may not be equally reliable across a range of coastal settings due to varying depositional environments. Fjords vary greatly in their depositional conditions, due to their unique bathymetry and hydrography, and are highly vulnerable to anthropogenic and climatic pressures. Currently, it is unknown to what extent Mo and U sequestration is affected by variable depositional conditions in fjords. Here, we use pore water and sequential extraction data to investigate Mo and U enrichment pathways in sediments of two sill fjords on the Swedish west coast with contrasting depositional environments and bottom water redox conditions. Our data suggest that sedimentary authigenic Mo and U pools differ between the two fjords. At the (ir)regularly dysoxic (oxygen = 0.2–2 mL L−1) Gullmar Fjord, authigenic Mo largely binds to manganese (Mn) oxides and to a lesser extent to iron (Fe) oxides; Mo sulfides do not play a major role due to low sulfate reduction rates, which limits the rate of Mo burial. Authigenic U largely resides in carbonates. At the (ir)regularly euxinic (oxygen = 0 mL L−1; total hydrogen sulfide ≥ 0 mL L−1) Koljö Fjord, authigenic Mo is significantly higher due to binding with more refractory organic matter complexes and Mo-Fe-sulfide phases. Uranium is moderately enriched and largely bound to organic matter. We found no direct evidence for temporal changes in bottom water redox conditions reflected in Mo and U enrichments at either Gullmar Fjord or Koljö Fjord. While sulfidic bottom waters favor Mo sequestration at Koljö Fjord, enrichment maxima reflect a combination of depositional conditions rather than short-term low-oxygen events. Our data demonstrate that secondary pre- and post-depositional factors control Mo and U sequestration in fjords to such an extent that bottom water redox conditions are either not being systematically recorded or overprinted. This explains the large variability in trace metal enrichments observed in fjords and has implications for applying Mo and U as proxies for environmental redox reconstructions in such systems.

Sulfur isotope fractionation derived from reaction-transport modelling in the Eastern Equatorial Pacific

Microbial sulfate reduction in subseafloor sediments regulates a significant portion of the marine organic matter burial flux. Over secular timescales, sulfate reduction is the fundamental process connecting the biogeochemical cycles of sulfur, carbon, oxygen and phosphorus. Similar to carbon reduction, sulfate reduction is associated with a strong isotope fractionation process that allows us to track this process through time. It depends on a variety of factors and it has been argued previously that systematic differences between shallow and deep-sea environments might explain secular changes in the marine S-isotope ratio. However, observational data of in situ fractionation from deep-sea areas are scarce. Here we use a reaction-transport model to analyse the S-isotope fractionation during microbial sulfate reduction in the interstitial water of Ocean Drilling Program (ODP) Site 1226 (Leg 201). We find that the upper 100 m below seafloor are best explained with fractionation around −76‰, whereas below this depth the degree of fractionation drops to around −42‰. We propose that this shift is caused by changes in the ratio of the rate of microbial sulfate reduction relative to the rate of abiotic sulfide oxidation. Because large parts of deep oceans are characterized by exceedingly low sulfate reduction rates, this process may be widespread and possibly explain why pyrite S-isotope data suggest that the average S-isotope fractionation is around −50‰, rather than the theoretically predicted value below −70‰. Supplementary material: Supplementary figures and model results are available at https://doi.org/10.6084/m9.figshare.c.5976772 Thematic collection: This article is part of the Sulfur in the Earth system collection available at: https://www.lyellcollection.org/cc/sulfur-in-the-earth-system

Modeling of biogeochemical processes in a barrier island freshwater lens (Spiekeroog, Germany)

Freshwater lenses present valuable water resources on barrier islands. Yet, the biogeochemical processes that control the groundwater quality of these freshwater lenses and how they are affected by the prevailing groundwater dynamics is largely unexplored. In this study we investigated the biogeochemistry of a barrier island freshwater lens with a focus on understanding and quantifying organic matter mineralization, sulfur cycling, and chemical fluxes to the land-ocean interface. We analyzed a comprehensive set of hydrogeochemical field data from Spiekeroog Island (Germany), including stable sulfur isotope signatures of dissolved sulfur species, with a reactive transport modeling approach. Tritium-Helium groundwater ages were used to constrain the hydrogeochemistry as a function of residence time. Our results revealed that freshwater lenses can act as archives for anthropogenic pollution, conserving the high sulfur loads associated with historic atmospheric deposition. We observed two distinct (hydro)biogeochemical patterns, which we attribute to a heterogeneous distribution of reactive organic matter. Those patterns were well replicated by two separate reactive transport models that considered the variations in organic matter reactivity. Simulation and field results demonstrated that net sulfur cycling is dominated by microbial sulfate reduction and subsequent iron sulfide precipitation. In the absence of dissolved oxidants, we attribute the observed high stable sulfur isotope fractionation between dissolved sulfate and sulfide of up to 67‰ to low (<20 pmol mL−1 d−1) microbial sulfate reduction rates. We show that older groundwater becomes progressively enriched in ammonium and phosphate due to the mineralization of organic matter, and we speculate that lens-derived nutrient fluxes may be important for the benthic zone at local groundwater discharge sites, at least seasonally in spring and summer.

Environments favoring dolomite formation at cold seeps: A case study from the Gulf of Mexico

Dolomite rarely occurs in modern marine environments, while it is ubiquitous in the geological record. This apparent contradiction made the formation mechanism of dolomite a long-standing problem haunting sedimentologists. However, dolomite is a constituent of many modern seep carbonates, providing an opportunity to investigate the conditions favoring dolomite formation at low temperatures in the shallow subseafloor. This study reports the petrology, mineralogy, carbon and oxygen stable isotopic compositions, as well as sulfur isotopic compositions of both carbonate-associated sulfate (CAS) and chromium reducible sulfur (CRS) of a set of dolomite-bearing carbonates from two seep sites (GC140 and GB382) on the northern slope of the Gulf of Mexico. The δ13C values of carbonates vary from −50.0‰ to 3.2‰ (V-PDB), confirming that carbonate minerals mostly resulted from methane oxidation. The corresponding δ18O values range from 2.6‰ to 5.9‰ (V-PDB), reflecting formation close to equilibrium with seawater composition. All carbonate samples with dolomite show similar petrologies and lack skeletal remains of seep-dwelling metazoans. The homogenous texture of microcrystalline dolomite agrees with its formation in the shallow subseafloor. Dolomite-bearing samples reveal δ18OCAS/δ34SCAS slopes of 0.425 and 0.435 for the two study sites, respectively. Such high slopes are significantly different from the smaller slopes of seep carbonates consisting of calcite from Gulf of Mexico seeps, suggesting low overall sulfate reduction rates during dolomite formation. This explanation is also supported by relatively high δ34SCRS values ranging from −29.4‰ to 19.0‰ (V-CDT) and the positive correlation of these values with dolomite contents. Interestingly, an approximately stoichiometric dolomite sample yielded the most positive δ34SCRS value, indicating a formation environment typified by low replenishment of seawater sulfate at a supposedly relatively deep sulfate methane transition zone (SMTZ). Overall, these results confirm that dolomite formation is facilitated where the SMTZ is situated at relatively great depth, where sulfate reduction rates are slowed down due to the buildup of high contents of dissolved sulfide and low seawater sulfate replenishment.

Sulfur Cycling in an Iron Oxide-Dominated, Dynamic Marine Depositional System: The Argentine Continental Margin

The interplay between sediment deposition patterns, organic matter type and the quantity and quality of reactive mineral phases determines the accumulation, speciation and isotope composition of pore water and solid phase sulfur constituents in marine sediments. Here, we present the sulfur geochemistry of siliciclastic sediments from two sites along the Argentine continental slope—a system characterized by dynamic deposition and reworking, which result in non-steady state conditions. The two investigated sites have different depositional histories but have in common that reactive iron phases are abundant and that organic matter is refractory—conditions that result in low organoclastic sulfate reduction rates. Deposition of reworked, isotopically light pyrite and sulfurized organic matter appear to be important contributors to the sulfur inventory, with only minor addition of pyrite from organoclastic sulfate reduction above the sulfate-methane transition (SMT). Pore-water sulfide is limited to a narrow zone at the SMT. The core of that zone is dominated by pyrite accumulation. Iron monosulfide and elemental sulfur accumulate above and below this zone. Iron monosulfide precipitation is driven by the reaction of low amounts of hydrogen sulfide with ferrous iron and is in competition with the oxidation of sulfide by iron (oxyhydr)oxides to form elemental sulfur. The intervals marked by precipitation of intermediate sulfur phases at the margin of the zone with free sulfide are bordered by two distinct peaks in total organic sulfur. Organic matter sulfurization appears to precede pyrite formation in the iron-dominated margins of the sulfide zone, potentially linked to the presence of polysulfides formed by reaction between dissolved sulfide and elemental sulfur. Thus, SMTs can be hotspots for organic matter sulfurization in sulfide-limited, reactive iron-rich marine sedimentary systems. Furthermore, existence of elemental sulfur and iron monosulfide phases meters below the SMT demonstrates that in sulfide-limited systems metastable sulfur constituents are not readily converted to pyrite but can be buried to deeper sediment depths. Our data show that in non-steady state systems, redox zones do not occur in sequence but can reappear or proceed in inverse sequence throughout the sediment column, causing similar mineral alteration processes to occur at the same time at different sediment depths.

Sulfate reduction and formation of iron sulfide minerals in nearshore sediments from Qi'ao Island, Pearl River Estuary, Southern China

Sulfate reduction usually leads to the formation of iron sulfide minerals in marine sediments and strongly influences the global coupled carbon-iron-sulfur biogeochemical cycles in marine sediments. Here, we report sulfate reduction and pyrite formation from the coastal sediments near Qi'ao Island, Pearl River Estuary using a combination of sedimentary pore water and solid sediment geochemical analyses. Strong enrichments in reactive iron (276.8–358.2 μmol/g) were observed, especially at the sites located near the river outlets. High contents of reactive iron and low values of the degree of iron pyritization (DOP) indicate that the limiting effect of reactive iron on pyrite formation can be excluded at these sites. In the upper sedimentary layer, conservative mixing of river water and marine water reinforces the fact that sulfate reduction is not appreciable. The limited sulfate supply and low sulfate reduction rates lead to not enough dissolved sulfide available for formation of iron sulfide minerals. However, below this dilution-mixing layer, pore water geochemical profiles indicate that a fraction of sulfate consumption is mediated by upward-diffusing methane. An additional sulfide is produced from anaerobic oxidation of methane (AOM) and later reacts with reactive iron, resulting in enhancement of pyrite formation in the sediments. By contrast, it appears that sulfate concentration is a primary factor controlling for pyrite formation in these estuarine environments. Our study highlights the need in future work for an integrated analysis of the hydrologically driven change in the bottom seawater sulfate concentrations to better understand the regulation factors of global sulfur reservoirs in coastal sediments of estuarine environments, as sulfur burial is an important factor in determining the past atmospheric oxygen level.

Sedimentary pyrite δ34S differs from porewater sulfide in Santa Barbara Basin: Proposed role of organic sulfur

Santa Barbara Basin sediments host a complex network of abiotic and metabolic chemical reactions that knit together the carbon, sulfur, and iron cycles. From a 2.1-m sediment core collected in the center of the basin, we present high-resolution profiles of the concentrations and isotopic compositions of all the major species in this system: sulfate, sulfide (∑H2S), elemental sulfur (S0), pyrite, extractable organic sulfur (OS), proto-kerogen S, total organic and dissolved inorganic carbon, and total and reducible iron. Below 10cm depth, the core is characterized by low apparent sulfate reduction rates (<0.01mM/yr) except near the sulfate-methane transition zone. Surprisingly, pyrite forming in shallow sediments is ∼30‰ more 34S-depleted than coexisting ∑H2S in porewater. S0 has the same strongly 34S-depleted composition as pyrite where it forms near the sediment–water interface, though not at depth. This pattern is not easily explained by conventional hypotheses in which sedimentary pyrite derives from abiotic reactions with porewater ∑H2S or from the products of S0 disproportionation. Instead, we propose that pyrite formation in this environment occurs within sulfate reducing microbial aggregates or biofilms, where it reflects the isotopic composition of the immediate products of bacterial sulfate reduction. Porewater ∑H2S in Santa Barbara Basin may be more 34S-enriched than pyrite due to equilibration with relatively 34S-enriched OS. The difference between OS and pyrite δ34S values would then reflect the balance between microbial sulfide formation and the abundance of exchangeable OS. Both OS and pyrite δ34S records thus have the potential to provide valuable information about biogeochemical cycles and redox structure in sedimentary paleoenvironments.

Determination of dissimilatory sulfate reduction rates in marine sediment via radioactive 35S tracer

Rates of dissimilatory sulfate reduction in aquatic sediments have been measured over many years with 35S‐radiotracer, and the method has been continuously modified and optimized. This article discusses the sequence of procedures that constitutes the method from sediment handling before incubation, via incubation and distillation, to statistical analysis of the results. We test modifications that have been added since previous method descriptions, and we recommend sound experimental procedures. We discuss the measurement of extremely low sulfate reduction rates whereby only one count per minute labeled sulfide may be produced. We show by numerical modeling that the measured rates are mostly representative for a small volume around the point where 35SO42− is injected and that this can be used as an advantage to avoid edge effects. Finally, we show that oxidation will spoil samples during storage unless the samples are stored frozen. The main focus is on marine sediment, but the discussions are equally relevant for freshwater.

Anaerobic methanotrophic community of a 5346‐m‐deep vesicomyid clam colony in the Japan Trench

Vesicomyidae clams harbor sulfide-oxidizing endosymbionts and are typical members of cold seep communities where active venting of fluids and gases takes place. We investigated the central biogeochemical processes that supported a vesicomyid clam colony as part of a locally restricted seep community in the Japan Trench at 5346 m water depth, one of the deepest seep settings studied to date. An integrated approach of biogeochemical and molecular ecological techniques was used combining in situ and ex situ measurements. In sediment of the clam colony, low sulfate reduction rates (maximum 128 nmol mL−1 day−1) were coupled to the anaerobic oxidation of methane. They were observed over a depth range of 15 cm, caused by active transport of sulfate due to bioturbation of the vesicomyid clams. A distinct separation between the seep and the surrounding seafloor was shown by steep horizontal geochemical gradients and pronounced microbial community shifts. The sediment below the clam colony was dominated by anaerobic methanotrophic archaea (ANME-2c) and sulfate-reducing Desulfobulbaceae (SEEP-SRB-3, SEEP-SRB-4). Aerobic methanotrophic bacteria were not detected in the sediment, and the oxidation of sulfide seemed to be carried out chemolithoautotrophically by Sulfurovum species. Thus, major redox processes were mediated by distinct subgroups of seep-related microorganisms that might have been selected by this specific abyssal seep environment. Fluid flow and microbial activity were low but sufficient to support the clam community over decades and to build up high biomasses. Hence, the clams and their microbial communities adapted successfully to a low-energy regime and may represent widespread chemosynthetic communities in the Japan Trench. In this regard, they contributed to the restricted deep-sea trench biodiversity as well as to the organic carbon availability, also for non-seep organisms, in such oligotrophic benthic environment of the dark deep ocean.

High Sulfide Intrusion in Five Temperate Seagrasses Growing Under Contrasting Sediment Conditions

Five temperate seagrasses (Amphibolis antartica, Halophila ovalis, Posidonia australis, Posidonia sinuosa and Zostera nigricaulis) were surveyed along the south-west coast of Western Australia. These morphological different seagrasses grow in contrasting sediments with large variations in sedimentary organic matter, carbonate and iron contents. We tested if sulfur composition in the plants responded to sulfur dynamics in the sediments and if plant morphology affected the sulfur composition of the plants. The sediments were characterized by low sulfate reduction rates (<9 mmol m−2day−1), low concentrations of dissolved sulfides in the pore waters (<74 μM) and low burial of sulfides (total reducible sulfur <0.8 mol m−2) in the sediments. However, all seagrasses showed high intrusion in the below-ground parts with up to 84 % of the sulfur derived from sedimentary sulfides. There were no direct links between sulfur in the plants and sulfur dynamics in the sediments, probably due to low iron contents in the sediments limiting the buffering capacity of the sediments and exposing the plants to sulfides despite low rates of production and low pools of sulfides. The intrusion was linked between plant compartments (roots, rhizomes and leaves) for the two small species (H. ovalis and Z. nigricaulis), whereas the intrusion into the leaves was limited for the larger species (P. australis and P. sinuosa) and for A. antarctica, where extensive rhizomes and roots and the long stem for A. antarctica separate the leaves from the sediment compartment. Elevated intrusion was observed at two study locations, where natural deposition of organic matter or nutrient enrichment may be contributing factors to enhanced sulfide pressure.

The imprint of methane seepage on the geochemical record and early diagenetic processes in cold-water coral mounds on Pen Duick Escarpment, Gulf of Cadiz

The diagenetic history and biogeochemical processes in three cold-water coral mounds located in close proximity to each other on Pen Duick Escarpment in the Gulf of Cadiz were examined. The influence of ascending methane-rich fluids from underlying sediment strata delineated two mound groups: Alpha and Beta Mound showed evidence for the presence of a sulfate-methane transition zone (SMTZ) at shallow depth, whereas Gamma Mound appeared to lack a shallow SMTZ. In the methane influenced Alpha and Beta Mound, upward diffusion of hydrogen sulfide from the shallow SMTZ caused extensive pyritization of reactive iron phases as indicated by values for the degree-of-pyritization >0.7. This secondary pyritization overprinted the sulfur isotope composition of sulfides formed during organoclastic sulfate reduction. The almost complete consumption of reactive iron phases by upward diffusing sulfide limited dissimilatory iron reduction to the top layer in these mounds while organic matter in the pyritized zones below was primarily degraded by organoclastic sulfate reduction. Hydrogen sulfide produced during sulfate reduction coupled to the anaerobic oxidation of methane (AOM) diffused upward and induced aragonite dissolution as evidenced in strongly corroded corals in Alpha Mound. This mound has been affected by strong fluctuations in the depth of the SMTZ, as observed by distinct layers with abundant diagenetic high-Mg calcite with a 13C-depleted carbon isotope composition. In the non-methane influenced Gamma Mound low sulfate reduction rates, elevated concentrations of dissolved iron, and solid-phase iron speciation indicated that organic matter mineralization was driven by dissimilatory iron reduction and organoclastic sulfate reduction coupled to oxidative sulfur cycling. The latter process led to 34S-depletion in pyrite of up to 70‰ relative to pore-water sulfate.

Sulfur Isotope Enrichment during Maintenance Metabolism in the Thermophilic Sulfate-Reducing Bacterium Desulfotomaculum putei

Values of Delta(34)S (=delta(34)S(HS)-delta(34)S(SO(4)), where delta(34)S(HS) and delta(34)S(SO(4)) indicate the differences in the isotopic compositions of the HS(-) and SO(4)(2-) in the eluent, respectively) for many modern marine sediments are in the range of -55 to -75 per thousand, much greater than the -2 to -46 per thousand epsilon(34)S (kinetic isotope enrichment) values commonly observed for microbial sulfate reduction in laboratory batch culture and chemostat experiments. It has been proposed that at extremely low sulfate reduction rates under hypersulfidic conditions with a nonlimited supply of sulfate, isotopic enrichment in laboratory culture experiments should increase to the levels recorded in nature. We examined the effect of extremely low sulfate reduction rates and electron donor limitation on S isotope fractionation by culturing a thermophilic, sulfate-reducing bacterium, Desulfotomaculum putei, in a biomass-recycling culture vessel, or "retentostat." The cell-specific rate of sulfate reduction and the specific growth rate decreased progressively from the exponential phase to the maintenance phase, yielding average maintenance coefficients of 10(-16) to 10(-18) mol of SO(4) cell(-1) h(-1) toward the end of the experiments. Overall S mass and isotopic balance were conserved during the experiment. The differences in the delta(34)S values of the sulfate and sulfide eluting from the retentostat were significantly larger, attaining a maximum Delta(34)S of -20.9 per thousand, than the -9.7 per thousand observed during the batch culture experiment, but differences did not attain the values observed in marine sediments.

Effect of hydrogen limitation and temperature on the fractionation of sulfur isotopes by a deep-sea hydrothermal vent sulfate-reducing bacterium

The fractionation of sulfur isotopes by the thermophilic chemolithoautotrophic Thermodesulfatator indicus was explored during sulfate reduction under excess and reduced hydrogen supply, and the full temperature range of growth (40–80 °C). Fractionation of sulfur isotopes measured under reduced H 2 conditions in a fed-batch culture revealed high fractionations (24–37‰) compared to fractionations produced under excess H 2 supply (1–6‰). Higher fractionations correlated with lower sulfate reduction rates. Such high fractionations have never been reported for growth on H 2. For temperature-dependant fractionation experiments cell-specific rates of sulfate reduction increased with increasing temperatures to 70 °C after which sulfate-reduction rates rapidly decreased. Fractionations were relatively high at 40 °C and decreased with increasing temperature from 40–60 °C. Above 60 °C, fractionation trends switched and increased again with increasing temperatures. These temperature-dependant fractionation trends have not previously been reported for growth on H 2 and are not predicted by a generally accepted fractionation model for sulfate reduction, where fractionations are controlled as a function of temperature, by the balance of the exchange of sulfate across the cell membrane, and enzymatic reduction rates of sulfate. Our results are reproduced with a model where fractionation is controlled by differences in the temperature response of enzyme reaction rates and the exchange of sulfate in and out of the cell.

Microbial community structure of sandy intertidal sediments in the North Sea, Sylt-Rømø Basin, Wadden Sea

Molecular biological methods were used to investigate the microbial diversity and community structure in intertidal sandy sediments near the island of Sylt (Wadden Sea) at a site which was characterized for transport and mineralization rates in a parallel study (D. de Beer, F. Wenzhöfer, T. Ferdelman, S.E. Boehme, M. Huettel, J.E.E. van Beusekom, M.E. Böttcher, N. Musat, N. Dubilier, Transport and mineralization rates in North Sea sandy intertidal sediments, Sylt-Romo Basin, Wadden Sea, Limnol. Oceanogr. 50 (2005) 113–127). Comparative 16S rRNA sequence analysis revealed a high bacterial diversity. Most sequences retrieved by PCR with a general bacterial primer set were affiliated with Bacteroidetes, Gammaproteobacteria, Deltaproteobacteria and the Pirellula cluster of Planctomycetales. Fluorescence in situ hybridization (FISH) and slot-blot hybridization with group-specific rRNA-targeted oligonucleotide probes were used to characterize the microbial community structure over depth (0–12 cm) and seasons (March, July, October). We found high abundances of bacteria with total cell numbers up to 3×10 9 cells ml −1 and a clear seasonal variation, with higher values in July and October versus March. The microbial community was dominated by members of the Planctomycetes, the Cytophaga/Flavobacterium group, Gammaproteobacteria, and bacteria of the Desulfosarcina/Desulfococcus group. The high abundance (1.5×10 7–1.8×10 8 cells ml −1 accounting for 3–19% of all cells) of presumably aerobic heterotrophic polymer-degrading planctomycetes is in line with the high permeability, deep oxygen penetration, and the high rates of aerobic mineralization of algal biomass measured in the sandy sediments by de Beer et al. (2005). The high and stable abundance of members of the Desulfosarcina/Desulfococcus group, both over depth and season, suggests that these bacteria may play a more important role than previously assumed based on low sulfate reduction rates in parallel cores (de Beer et al., 2005).

Ore-Forming Processes in Irish-Type Carbonate-Hosted Zn-Pb Deposits: Evidencefrom Mineralogy, Chemistry, and Isotopic Composition of Sulfides at the LisheenMine

Lisheen is a strata-bound zinc-lead deposit formed during the Mississippian by replacement of hydrothermally dolomitized, grossly stratiform breccia bodies located near the base of the Carboniferous Waulsortian Limestone. It represents one of a number of carbonate-hosted massive sulfide ore deposits in the Irish ore field that, due to several unique features, have been classified as Irish type. Disseminated pyrite occurs in preore dolomite and around the margins of preore dolomite clasts within dolomite breccias. Early fine-grained sphalerite-pyrite mineralization occurs as infill of intergranular dolomite porosity. Locally, massive to semimassive iron sulfide is observed, mainly comprising pyrite with lesser marcasite. A complex polymetallic sulfide assemblage typifies the main ore stage, dominated by fine-grained disseminated, massive or colloform sphalerite and galena, with minor pyrite, chalcopyrite, arsenopyrite, tennantite, nickel- and cobalt-bearing minerals. Silver occurs in solid solution in tennantite, galena, and sphalerite. Dolomite and barite dominate the gangue, with lesser calcite. Main-stage mineralization involved the progressive replacement of preexisting iron sulfides and the dolomite breccias, initially by replacement of the breccia matrix and ultimately by replacement of clasts. Coarse crystalline sphalerite and euhedral galena crystals are generally restricted to fracture-fill mineralization or vugs within main ore-stage assemblages where they occur with euhedral dolomite and calcite. Barite intergrown with main ore-stage sulfides has δ 34S values of 14.3 to 18.1 per mil, consistent with the derivation of sulfate from coeval Carboniferous seawater. The δ 34S values for sulfides range from –44.1 to +11.8 per mil, with a mean value of –13.7 per mil, typical of the Irish ore deposits. The dominant low δ 34S signature is considered to be the result of bacterial reduction of coeval seawater sulfate. Extremely low δ 34S values, in the range of –38 to –44 per mil, are only observed in preore disseminated pyrite; such extreme fractionations are thought to be due to low bacterial sulfate reduction rates coupled with oxidative cycles in near-sea floor pore waters. Main ore-stage sulfides have δ 34S values in the range of –4 to –18 per mil, with a mode of –10 per mil, consistent with a typical bacterial fractionation from coeval seawater sulfate. Isotopic equilibrium between cogenetic sulfides is not observed. The bacteriogenic sulfur component was probably transported from bacterial colonies fringing the ore system by low-temperature brines. The δ 34S values of late ore-stage sulfides mainly range from –20.2 to +12.0 per mil, with the majority having relatively high values (mean = –3.0 ± 8.5‰, 1 σ ) interpreted as being due to the presence of a hydrothermal sulfur component, leached from the lower Paleozoic basement. For galena and sphalerite there is a general increase in δ 34S values with depth in the system, with time, and with proximity to east-west– and northwest-trending faults. These relationships suggest that input of hydrothermal sulfur from depth via fractures became increasingly important. Hydrothermal sulfur appears to be more important at Lisheen than the other major Irish deposits. Galena lead isotope analyses gave average 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of 18.183, 15.594, and 38.080, respectively. These data do not correlate with ore-stage, galena texture or δ 34S. The results are comparable to previous data from Lisheen and from Silvermines, 35 km to the west, implying a common lead source in the lower Paleozoic basement. The textural, mineral, chemical, and isotopic evidence suggests that main-stage ore was precipitated as a consequence of rapid supersaturation, caused by fluid mixing within the permeable dolomite breccias. This process involved relatively high temperature (ca. 240°C), metal-bearing solutions derived from a basement-equilibrated fluid reservoir (carrying Zn, Pb, Fe, Cd) and shallow, saline (ca. 25 wt % NaCl equiv) formation waters rich in bacteriogenic H2S. Minor metals (Cu, As, Ni, Co) are thought to have been stripped from the footwall Old Red Sandstone during hydrothermal alteration around fault conduits. The availability of abundant seawater sulfate, operation of open-system bacterial sulfate reduction, and episodic availability of free oxygen imply that ore formation cannot have occurred at significant depth below the paleosea floor. Cessation of mineralization was due to a cut-off of the sulfur-rich brine supply, possibly by deposition of impermeable hanging-wall sediments. This process of ore formation is consistent with evidence from the other economic Irish-type deposits in the ore field.

A cold chromium distillation procedure for radiolabeled sulfide applied to sulfate reduction measurements

The separation and detection of extremely small amounts of radiolabeled reduced sulfur species for the determination of very low sulfate reduction rates (SRR) with the 35SO42− radiotracer was improved by optimization of the entire distillation and detection process. By reducing the amount of background radioactivity, the threshold from which turnover of radiotracer can be detected was lowered considerably. Reduction of the background radioactivity was achieved by (1) reducing cross‐contamination between distillations by modifying the distillation setup, and (2) preventing an unidentified 35S‐containing compound that greatly contributes to the background from reaching the final trap by lowering the distillation temperature. These improvements allow the measurement of low SRR, shorter incubation times, and the use of less radiotracer. Experiments with pure sulfur minerals and a variety of sediments verified that the efficiency of the new method is equal to the hot single‐step chromium reduction method, but with a greatly improved reproducibility.

RESPONSE OF ANAEROBIC CARBON MINERALIZATION RATES TO SULFATE AMENDMENTS IN A BOREAL PEATLAND

A small body of research suggests that dissimilatory sulfate reduction can affect the carbon balance of peatlands, yet this has not been tested widely, despite the fact that peatlands contain approximately one-third of the global soil carbon pool. Here we evaluate the role of dissimilatory sulfate reduction in a site that receives low atmospheric sulfur deposition. We hypothesized that, in peatlands receiving low sulfate inputs, methane production should dominate anaerobic carbon mineralization. We further hypothesized that with sulfate amendments, anaerobic carbon mineralization could show an overall increase if terminal carbon mineralization in unamended peat is limited by an inadequate supply of electron acceptors. To test these hypotheses, we delineated six 1-m2 plots in an ombrotrophic, boreal peatland in central Alberta, Canada (Bleak Lake Bog), which receives <2 mmol S·m−2·yr−1. Three of the plots were amended with sulfate (78 mmol S·m−2·yr−1). We measured anaerobic rates of sulfate reduction, CH4 production, and CO2 production. In opposition to our hypotheses, sulfate amendments did not increase rates of sulfate reduction, increase CO2 production, or decrease CH4 production over a 2-yr period, but did increase both sulfate pool sizes and residence times. Despite low rates of sulfate reduction compared to other freshwater wetlands, daily average sulfate reduction (1.7 mmol/m2) exceeded regional annual inputs of atmospheric sulfate deposition (1.6 mmol/m2). Between 77% and 99% of reduced sulfate was incorporated into the carbon-bonded sulfur pool, which turns over slowly. The slow turnover rate and comparatively low sulfate reduction rates may be related to the low iron content of the bog peat. Additionally, sulfate reduction initially was sulfate limited in the control plots, but with sulfur amendments, the sulfate limitation was removed; sulfate reduction appeared to be limited by some other factor, possibly labile carbon. Dissimilatory sulfate reduction was more important to total anaerobic carbon mineralization than methane production, although neither process dominated overall anaerobic carbon mineralization (<2% of total). Fermentation appeared to be the dominant anaerobic carbon mineralization pathway at Bleak Lake Bog, yet the mechanisms of how this process affects the carbon balance of peatlands has never been evaluated. Overall, our results suggest that, in at least the short term, soil carbon turnover in peatlands will not be enhanced by increased atmospheric sulfur deposition. If we want to understand the controlling factors on soil carbon storage in sites like Bleak Lake Bog, we should begin to examine anaerobic carbon mineralization via fermentation pathways. Corresponding Editor: J. M. Melack

Sulfur isotope geochemistry of the Black Sea water column

We studied the isotopic composition of dissolved sulfide in the Black Sea water column during different seasons at a total of 15 stations spanning the entire basin. The isotopic composition of dissolved sulfide averaged over all depths varies between −42.0‰ and −32.6‰, ave. −39.6±1.3‰ (1 σ) (118 data points). Seasonal and spatial (open sea vs. coastal stations) differences in the δ 34S–H 2S values are not observed. Slight 34S enrichments in the sulfide isotope composition are revealed in the uppermost and the lowest parts of the anoxic water column. The upper trend is explained as (i) the effect of mixing with 34S-enriched sulfide produced near the interface by chemical oxidation with MnO 2 or O 2, (ii) small fractionation during biological sulfide oxidation, (iii) a result of a decreased isotope fractionation factor due to higher sulfate reduction rates. The lower trend is likely the result of the mixing with 34S-enriched pore water sulfide. We generated the first isotope data for sulfur intermediates in the lower part of the anoxic zone, which show values close to the isotope composition of dissolved sulfide. We hypothesize that the high isotope depletions of sulfide observed in the entire Black Sea water column are a result of low sulfate reduction rates and superimposed disproportionation reactions within the oxidative part of the sulfur cycle. Different physical and chemical mechanisms facilitating the formation and transport of sulfur intermediates in the anoxic interior are discussed.

Impact of fiddler crabs ( Uca spp.) on rates and pathways of benthic mineralization in deposited mangrove shrimp pond waste

The impact of fiddler crabs ( Uca spp.) on benthic mineralization rates and pathways in deposits of shrimp pond waste (SPW) with planted mangrove trees ( Rhizophora apiculata) were determined in the Ranong mangrove forest, Thailand. Sediment metabolism, measured as CO 2 flux, increased by 2- to 3-fold when either fiddler crabs or mangrove trees were present compared to control plots. Sulfate reduction rates (SRR) were always high and partitioning of various electron acceptors to total carbon oxidation revealed that sulfate reduction contributed by >90%, with iron reduction being important only near the sediment–water interface. However, significant iron reduction appeared down to 7 cm when bioturbation and plant roots were combined in the easily oxidizable mangrove sediment (MS), indicating that infaunal activity and plant roots were able to alter the substratum. Microprofiles around individual Uca burrows showed 46% lower SRRs in a 15-mm-thick oxidized layer around the burrows compared to the surrounding sediment. The burrow wall environment appears to be a zone of intense reoxidation of reduced compounds as indicated by low pools of reduced sulfide compounds and a high Fe(III) content. Despite the decreased SRRs near the burrow, and the introduction of Fe(III) to deeper sediment layers, fiddler crabs and mangrove trees have only limited impact on rates and partitioning of anaerobic carbon mineralization in the SPW. This lack of response was attributed to the relative small volume of sediment affected by crab activities.