Porewater Sulfide Research Articles

Sulfur isotope analysis of microcrystalline iron sulfides using secondary ion mass spectrometry imaging: Extracting local paleo-environmental information from modern and ancient sediments.

Sulfur isotope ratio measurements of bulk sulfide from marine sediments have often been used to reconstruct environmental conditions associated with their formation. In situ microscale spot analyses by secondary ion mass spectrometry (SIMS) and laser ablation multiple-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) have been utilized for the same purpose. However, these techniques are often not suitable for studying small (≤10 μm) grains or for detecting intra-grain variability. Here, we present a method for the physical extraction (using lithium polytungstate heavy liquid) and subsequent sulfur isotope analysis (using SIMS; CAMECA IMS 7f-GEO) of microcrystalline iron sulfides. SIMS sulfur isotope ratio measurements were made via Cs+ bombardment of raster squares with sides of 20-130 μm, using an electron multiplier (EM) detector to collect counts of 32 S- and 34 S- for each pixel (128 × 128 pixel grids) for between 20 and 960 cycles. The extraction procedure did not discernibly alter pyrite grain-size distributions. The apparent inter-grain variability in 34 S/32 S in 1-4 μm-sized pyrite and marcasite fragments from isotopically homogeneous hydrothermal crystals was ~ ±2‰ (1σ), comparable with the standard error of the mean for individual measurements (≤ ±2‰, 1σ). In contrast, grain-specific 34 S/32 S ratios in modern and ancient sedimentary pyrites and marcasites can have inter- and intra-grain variability >60‰. The distributions of intra-sample isotopic variability are consistent with bulk 34 S/32 S values. SIMS analyses of isolated iron sulfide grains yielded distributions that are isotopically representative of bulk 34 S/32 S values. Populations of iron sulfide grains from sedimentary samples record the evolution of the S-isotopic composition of pore water sulfide in their S-isotopic compositions. These data allow past local environmental conditions to be inferred.

Oxygen and Sulfides in Bottom Sediments of the Coastal Sevastopol Region of Crimea

Spatial variations in the distribution and fluxes of dissolved oxygen and sulfide in bottom sediments of Omega and Sevastopol bays have been studied. The results of analysis reveal that the distribution of dissolved oxygen and sulfide in pore water depends mostly on seasonal variations in the oxygen concentration in bottom water, grain size, the organic carbon content in bottom sediments, and, additionally for Sevastopol Bay, the iron content. The oxygen flux at the bottom of Sevastopol Bay is 20 times larger in winter–spring compared to that of Omega Bay. Anaerobic conditions in Sevastopol Bay sediments are observed much closer to the surface, with their subsequent development in bottom water.

Effects of Zostera marina rhizosphere and leaf detritus on the concentration and distribution of pore-water sulfide in marine sediments

The relationship between seagrasses and pore-water hydrogen sulfide in the sediment is complex due to the opposing effects of the seagrass root system, which can reduce sulfide concentration by promoting its oxidation to sulfate, and the burial of organic matter from the plant itself, which can increase sulfide concentration by stimulating anaerobic sulfate reduction. Here, field samples of pore-water sulfide were collected in areas with and without Zostera marina shoots using Diffusive Gradients in Thin-Films (DGTs) to obtain two-dimensional visualizations of sulfide concentrations within the sediment in relation to location of eelgrass detritus and the rhizosphere. To decouple the effects of leaf detritus and the eelgrass rhizosphere on sulfide, manipulative laboratory studies were conducted over 4 weeks using 10 aquaria with or without eelgrass and different quantities of eelgrass detritus planted at 4 cm and 11 cm depths. Spatial distribution of pore-water sulfide was again measured via DGT. We found high spatial variation in pore-water sulfide concentration depending on the location analyzed relative to the rhizosphere or detritus. In the field study, pore-water sulfide concentration was greater in samples with eelgrass present, and the presence of leaf detritus accounted for higher than average sulfide concentrations in the sediment. In the laboratory experiment, sulfide concentrations localized around the leaf detritus increased with higher mass of added detritus, and sediment within the rhizosphere exhibited reduced sulfide concentrations. These results suggest that on the large scale the presence of eelgrass increases sulfide concentration, but on the smaller scale the pattern varies, with higher concentrations near buried detritus and lower concentrations near root tips.

An evaluation of sedimentary molybdenum and iron as proxies for pore fluid paleoredox conditions

Iron speciation and trace metal proxies are commonly applied together in efforts to identify anoxic settings marked by the presence of free sulfide (euxinia) or dissolved iron (ferruginous) in the water column. Here, we use a literature compilation from modern localities to provide a new empirical evaluation of coupled Fe speciation and Mo concentrations as a proxy for pore water sulfide accumulation at non-euxinic localities. We also present new Fe speciation, Mo concentration, and S isotope data from the Friends of Anoxic Mud (FOAM) site in Long Island Sound, which is marked by pore water sulfide accumulation of up to 3 mM beneath oxygen-containing bottom waters. For the operationally defined Fe speciation scheme, ‘highly reactive’ Fe (FeHR) is the sum of pyritized Fe (Fepy) and Fe dominantly present in oxide phases that is available to react with pore water sulfide to form pyrite. Observations from FOAM and elsewhere confirm that Fepy/FeHR from non-euxinic sites is a generally reliable indicator of pore fluid redox, particularly the presence of pore water sulfide. Molybdenum (Mo) concentration data for anoxic continental margin sediments underlying oxic waters but with sulfidic pore fluids typically show authigenic Mo enrichments (2–25 ppm) that are elevated relative to the upper crust (1–2 ppm). However, compilations of Mo concentrations comparing sediments with and without sulfidic pore fluids underlying oxic and low oxygen (non-euxinic) water columns expose non-unique ranges for each, exposing false positives and false negatives. False positives are most frequently found in sediments from low oxygen water columns (for example, Peru Margin), where Mo concentration ranges can also overlap with values commonly found in modern euxinic settings. FOAM represents an example of a false negative, where, despite elevated pore water sulfide concentrations and evidence for active Fe and Mn redox cycling in FOAM sediments, sedimentary Mo concentrations show a homogenous vertical profile across 50 cm depth at 1 to 2 ppm. A diagenetic model for Mo provides evidence that muted authigenic enrichments are derived from elevated sedimentation rates. Consideration of a range of additional parameters, most prominently pore water Mo concentration, can replicate the ranges of most sedimentary Mo concentrations observed in modern non-euxinic settings. Together, the modern Mo and Fe data compilations and diagenetic model provide a framework for identifying paleo-pore water sulfide accumulation in ancient settings and linked processes regulating seawater Mo and sulfate concentrations and delivery to sediments. Among other utilities, identifying ancient accumulation of sulfide in pore waters, particularly beneath oxic bottom waters, constrains the likelihood that those settings could have hosted organisms and ecosystems with thiotrophy at their foundations.

Hypersalinity as a trigger of seagrass (Thalassia testudinum) die-off events in Florida Bay: Evidence based on shoot meristem O2 and H2S dynamics

The mechanisms initiating rapid, large-scale (>50 km2) seagrass die-off events globally remain elusive. Thalassia testudinum, a dominant habitat-forming tropical seagrass of the Caribbean and tropical Atlantic region, experiences recurrent die-off events in Florida Bay. Thus, T. testudinum provides an excellent case study to identify triggers of large-scale seagrass mortality. Presently, seagrass die-off events in Florida Bay are correlated to high total sulfide (H2S, HS−, S2−) concentrations in the sediment porewater (≥2000 μM), water column hypoxia, high temperature and hypersalinity (salinity > 45). Because the mortality appears to be initiated at the shoot base within the meristem, we examined the response of shoot meristematic O2 and H2S dynamics using microsensors over a 5-hour nighttime simulation. Shoots were held at salinity 35 (ambient) or 65 (hypersaline) using microsensors in intact cores with plants from the field. The rate of H2S intrusion in the dark and oxidation in the light were similar at 35 and 65 salinity. However, the tissue O2 consumption rate was significantly higher under hypersaline conditions (−11.07 ± 4.32 kPa pO2 h−1) compared to ambient (−3.93 ± 0.88 kPa pO2 h−1) in the dark. Consequently, the meristematic pO2 threshold (~1.5 kPa O2) where H2S intrusion occurred in the meristem was reached more rapidly, increasing the time H2S accumulated (1.5 to 2.8 h). Longer H2S accumulation time significantly increased maximum meristem H2S levels at 65 (536 ± 330 μM H2S) compared to 35 salinity (121 ± 62 μM H2S). These results in T. testudinum provide evidence for a triggering mechanism that links an enhanced respiratory rate under hypersaline conditions to sulfide toxicity. We propose that hypersalinity in Florida Bay, or any stressor that significantly increases nighttime respiration rates, can subject seagrasses to longer and higher concentrations of sulfide, a known phytotoxin, within the shoot meristem. This mechanism likely explains large-scale mass mortality of T. testudinum in Florida Bay during periods of high salinity and elevated porewater sulfide levels, although field experiments are required to further validate this supposition.

Recycled Archean sulfur in the mantle wedge of the Mariana Forearc and microbial sulfate reduction within an extremely alkaline serpentine seamount

The identification of microbial activity under extreme conditions is important to define potential boundaries of the habitable and uninhabitable zones of terrestrial and extraterrestrial living forms. The subseafloor regimes of serpentinite seamounts in the Mariana Forearc are among the most extreme environments for life on earth owing to the widespread presence of highly alkaline fluids with pH values greater than 12. The potential activity of sulfate-reducing microorganisms has been suggested within the South Chamorro serpentinite seamounts on the basis of depletion of sulfate and enrichment of dissolved sulfide in pore water. However, the vertical distribution of sulfate-reducing microorganisms and the origin of sulfate are still uncertain. To address these issues, we analyzed quadruple sulfur isotopes of sulfide minerals and pore water sulfate in the upper 56 m of sedimentary sequences at the summit of the S. Chamorro Seamount and those of dissolved sulfate in upwelling fluids collected as deep as 202 mbsf (meters below the seafloor) in a cased hole near the summit of the same seamount. The depth profiles of the concentrations and the δ34S and Δ33S′ values of sulfide minerals and pore water sulfate indicate microbial sulfate reduction as deep as 30 mbsf. Further, apparent isotopic fractionations (ε34) and exponents of mass dependent relationships (λ33) during sulfate reduction are estimated to be 62 ± 14‰ and 0.512 ± 0.002, respectively. The upwelling fluids show both the chlorine depletion relative to seawater and the negative δ15N values of ammonia (−4‰). Although these signatures point to dehydration of the subducting oceanic plate, the negative Δ33S′ values of sulfate (−0.16‰ to −0.26‰ with analytical errors of ±0.01‰) are unlikely to originate from surrounding modern crusts. Instead, sulfate in the upwelling fluid likely possess non-mass-dependent (NMD) sulfur. Because NMD sulfur was produced primarily in the Archean atmosphere, our results suggest that the presence of recycled Archean crust that could be incorporated into the upper mantle through subduction of Archean oceanic crusts or from the NMD-bearing OIB seamounts located in the southern margin of the Pacific Plate.

Characterization of benthic biogeochemistry and ecology at three methane seep sites on the Northern U.S. Atlantic margin

Several hundred methane seeps were recently discovered along the U.S. Atlantic margin, a passive margin without a major hydrocarbon basin. These seeps represent a compelling opportunity to compare and contrast geochemical parameters, as well as the distribution and composition of associated chemosynthetic ecosystems, across geologically distinct seeps. Specifically, this study characterized the physical, chemical, geological, and biological features at Veatch Canyon, New England, and Shallop Canyon methane seep sites using a suite of state-of-the-art sampling and communication tools: AUV Sentry, HOV Alvin, a video-equipped multicore, and a real-time telepresence connection to the Inner Space Center (ISC) at the University of Rhode Island. Water column backscatter data collected by AUV Sentry confirmed previously detected gas emission and further indicated the presence of 13 actively emitting gas seeps within the Veatch Canyon survey area, 5 within the New England survey area, and 15 within the Shallop Canyon survey area. Complementary high-resolution seafloor bathymetry and backscatter mapping data indicated highly rugose morphology at all surveyed seep sites including the presence of extensive rock outcrops and bivalve beds. Video collected by HOV Alvin provided visual confirmation of gas plumes and the presence of chemosynthetic communities, including microbial mats and mussel beds, as well as large outcrops of authigenic carbonate rock surrounding locations of discrete gas emission. Bathymodiolus was the dominant species observed at the Veatch Canyon methane seep site, while the deep-sea red crab, Chaceon quinquedens, dominated the New England and Shallop Canyon seep sites. Elevated pore water sulfide concentrations suggested stimulation of sulfate-coupled anaerobic oxidation of methane (AOM) and/or degradation of organic matter in seep sediments relative to non-seep sediments; AOM was observed directly within overlying authigenic carbonate rocks. DNA sequencing of the surface sediments revealed diverse sulfide-oxidizing bacteria similar to the taxa described previously at other methane seeps, including members of the Thiotrichales and Campylobacterales. Together, these analyses provide one of the first interdisciplinary descriptions of sites within this massive seep system, setting the stage for more targeted, hypothesis-driven investigations. In this context, telepresence facilitated interdisciplinary research between specialists at the ISC and scientists and engineers at sea, enabling researchers to transfer data and maximize research efforts with sampling tools at sea. Our findings suggest that although the geological setting is distinct, the surficial microbial and macrofaunal communities closely resemble those of previously described methane seeps.

Sea Level Rise Impacts to Coastal Marshes may be Ameliorated by Natural Sedimentation Events

Coastal wetland sustainability in the future will likely depend on the extent to which increases in sea level drive flooding duration, plant submergence, and higher salinities, and how wetlands respond to these changes. Coastal wetlands will need to grow vertically to cope with rising seas, and sedimentation, often observed following hurricane passage, could play a role. A greenhouse mesocosm experiment was conducted to investigate if the impacts of sea level rise (SLR) and elevated salinity on the productivity and resilience of Spartina alterniflora marshes could be mediated by simulated hurricane sedimentation. Overall, sedimentation ameliorated the negative impacts of moderate SLR on plant productivity and resilience. Sedimentation improved growth conditions at current and moderate increases in sea level by reducing flooding duration, which in-turn, increased soil Eh, and lowered porewater sulfide. This led to greater productivity of vegetation above- and belowground and improved plant resilience. However, at the highest sea levels, inundation stress was too great for the benefits of added sediment to be realized. Thus, it is likely that the sustainability of coastal marshes will be improved by hurricane-generated sedimentation under moderate SLR scenarios, but will see no improvement with more extreme SLR.

Transient biogeochemistry in intertidal sediments: New insights from tidal pools in Zostera noltei meadows of Arcachon Bay (France)

Several studies highlighted the occurrence of circular pools in intertidal flats of different coastal systems and their transient water chemistry over both tidal and diurnal cycles. However, little is known about (1) the response of benthic biogeochemical reactions and fluxes at the sediment-water interface over such short time scales, and (2) the role of these tidal pools in the biogeochemical functioning of coastal systems. Based on in situ microprofiles and water sampling, we investigated the dynamics of dissolved oxygen (O2), nutrients, sulfides and metals, and the associated fluxes at the sediment-water interface in tidal pools from the Arcachon Bay (Atlantic coast of France). Our integrative approach included several tidal and diurnal cycles over two different seasons in the presence and absence of Zostera noltei. The results show that water temperature and light irradiance were the main factors driving the biogeochemical functioning of the tidal pools, as they controlled the physiological activity of the microphytobenthos. Changes in light radiations induced diurnal fluctuations of O2 concentrations within surficial sediment, thus resulting in fluctuations of the O2 diffusive fluxes at the sediment-water interface and of the O2 penetration depth in sediment. At high tide, the increase in turbulence above the sediment induced the advection of oxygenated water within the first millimeters of sediment, resulting in a significant increase in porewater O2 concentrations and sediment O2 penetration depth. Porewater sulfide concentrations and apparition depth were concomitant with the O2 dynamic over both diurnal and tidal cycles, indicating that intermediate redox diagenetic processes were impacted by O2 dynamic over such short time-scale. The rapid changes in redox processes in the sediment column are confirmed by a significant flux of dissolved manganese toward the water column during nighttime. The consumption of nitrate and the release of ammonium and phosphate, associated to the mineralization of the organic matter in the surface sediment did not appeared related however to such short time cycles. The efflux of dissolved silica from the sediment was most likely associated with the enhanced dissolution of Si-bearing particles in surface sediment at higher temperatures, although silica uptake by Z. noltei was also noted. This study clearly shows that tidal pools function as natural incubators of transient biogeochemical processes. A rough assessment of the nutrient budget at the scale of the bay indicates the tidal pools may contribute significantly to the biogeochemical functioning of Arcachon Bay.

The Seagrass Holobiont and Its Microbiome.

Seagrass meadows are ecologically and economically important components of many coastal areas worldwide. Ecosystem services provided by seagrasses include reducing the number of microbial pathogens in the water, providing food, shelter and nurseries for many species, and decreasing the impact of waves on the shorelines. A global assessment reported that 29% of the known areal extent of seagrasses has disappeared since seagrass areas were initially recorded in 1879. Several factors such as direct and indirect human activity contribute to the demise of seagrasses. One of the main reasons for seagrass die-offs all over the world is increased sulfide concentrations in the sediment that result from the activity of sulfate-reducing prokaryotes, which perform the last step of the anaerobic food chain in marine sediments and reduce sulfate to H2S. Recent seagrass die-offs, e.g., in the Florida and Biscayne Bays, were caused by an increase in pore-water sulfide concentrations in the sediment, which were the combined result of unfavorable environmental conditions and the activities of various groups of heterotrophic bacteria in the sulfate-rich water-column and sediment that are stimulated through increased nutrient concentrations. Under normal circumstances, seagrasses are able to withstand low levels of sulfide, probably partly due to microbial symbionts, which detoxify sulfide by oxidizing it to sulfur or sulfate. Novel studies are beginning to give greater insights into the interactions of microbes and seagrasses, not only in the sulfur cycle. Here, we review the literature on the basic ecology and biology of seagrasses and focus on studies describing their microbiome.

The Evolution of Sulfide in Shallow Aquatic Ecosystem Sediments: An Analysis of the Roles of Sulfate, Organic Carbon, and Iron and Feedback Constraints Using Structural Equation Modeling

AbstractThe generation of elevated concentrations of sulfide in sediment pore waters that are toxic to rooted macrophytes is problematic in both marine and freshwaters. In marine waters, biogeochemical conditions that lead to toxic levels of sulfide generally relate to factors that affect oxygen dynamics or the sediment iron concentration. In freshwaters, increases in surface water sulfate have been implicated in decline of Zizania palustris (wild rice), which is important in wetlands across the Great Lakes region of North America. We developed a structural equation (SE) model to elucidate key variables that govern the evolution of sulfide in pore waters in shallow aquatic habitats that are potentially capable of supporting wild rice. The conceptual basis for the model is the hypothesis that dissimilatory sulfate reduction is limited by the availability of both sulfate and total organic carbon (TOC) in the sediment. The conceptual model also assumes that pore water sulfide concentrations are constrained by the availability of pore water iron and that sediment iron supports the supply of dissolved iron to the pore water. A key result from the SE model is that variations in three external variables (sulfate, sediment TOC, and sediment iron) contribute nearly equally to the observed variations in pore water sulfide. As a result, management efforts to mitigate against the toxic effects of pore water sulfide on macrophytes such as wild rice should approach defining a protective sulfate threshold as an exercise tailored to the geochemistry of each site that quantitatively considers the effects of ambient concentrations of sediment Fe and TOC.

Sulfide Generated by Sulfate Reduction is a Primary Controller of the Occurrence of Wild Rice (Zizania palustris) in Shallow Aquatic Ecosystems

AbstractField observations suggest that surface water sulfate concentrations control the distribution of wild rice, an aquatic grass (Zizania palustris). However, hydroponic studies show that sulfate is not toxic to wild rice at even unrealistically high concentrations. To determine how sulfate might directly or indirectly affect wild rice, potential wild rice habitat was characterized for 64 chemical and physical variables in over 100 sites spanning a relatively steep climatic and geological gradient in Minnesota. Habitat suitability was assessed by comparing the occurrence of wild rice with the field variables, through binary logistic regression. This analysis demonstrated that sulfide in sediment pore water, generated by the microbial reduction of sulfate that diffuses or advects into the sediment, is the primary control of wild rice occurrence. Water temperature and water transparency independently control the suitability of habitat for wild rice. In addition to generating phytotoxic sulfide, sulfate reduction also supports anaerobic decomposition of organic matter, releasing nutrients that can compound the harm of direct sulfide toxicity. These results are important because they show that increases in sulfate loading to surface water can have multiple negative consequences for ecosystems, even though sulfate itself is relatively benign.

Increase in Nutrients, Mercury, and Methylmercury as a Consequence of Elevated Sulfate Reduction to Sulfide in Experimental Wetland Mesocosms

AbstractMicrobial sulfate reduction (MSR) in both freshwater and marine ecosystems is a pathway for the decomposition of sedimentary organic matter (OM) after oxygen has been consumed. In experimental freshwater wetland mesocosms, sulfate additions allowed MSR to mineralize OM that would not otherwise have been decomposed. The mineralization of OM by MSR increased surface water concentrations of ecologically important constituents of OM: dissolved inorganic carbon, dissolved organic carbon, phosphorus, nitrogen, total mercury, and methylmercury. Increases in surface water concentrations, except for methylmercury, were in proportion to cumulative sulfate reduction, which was estimated by sulfate loss from the surface water into the sediments. Stoichiometric analysis shows that the increases were less than would be predicted from ratios with carbon in sediment, indicating that there are processes that limit P, N, and Hg mobilization to, or retention in, surface water. The highest sulfate treatment produced high levels of sulfide that retarded the methylation of mercury but simultaneously mobilized sedimentary inorganic mercury into surface water. As a result, the proportion of mercury in the surface water as methylmercury peaked at intermediate pore water sulfide concentrations. The mesocosms have a relatively high ratio of wall and sediment surfaces to the volume of overlying water, perhaps enhancing the removal of nutrients and mercury to periphyton. The presence of wild rice decreased sediment sulfide concentrations by 30%, which was most likely a result of oxygen release from the wild rice roots. An additional consequence of the enhanced MSR was that sulfate additions produced phytotoxic levels of sulfide in sediment pore water.

Environmental insights from high-resolution (SIMS) sulfur isotope analyses of sulfides in Proterozoic microbialites with diverse mat textures.

In modern microbial mats, hydrogen sulfide shows pronounced sulfur isotope (δ34 S) variability over small spatial scales (~50‰ over <4mm), providing information about microbial sulfur cycling within different ecological niches in the mat. In the geological record, the location of pyrite formation, overprinting from mat accretion, and post-depositional alteration also affect both fine-scale δ34 S patterns and bulk δ34 Spyrite values. We report μm-scale δ34 S patterns in Proterozoic samples with well-preserved microbial mat textures. We show a well-defined relationship between δ34 S values and sulfide mineral grain size and type. Small pyrite grains (<25μm) span a large range, tending toward high δ34 S values (-54.5‰ to 11.7‰, mean: -14.4‰). Larger pyrite grains (>25μm) have low but equally variable δ34 S values (-61.0‰ to -10.5‰, mean: -44.4‰). In one sample, larger sphalerite grains (>35μm) have intermediate and essentially invariant δ34 S values (-22.6‰ to -15.6‰, mean: -19.4‰). We suggest that different sulfide mineral populations reflect separate stages of formation. In the first stage, small pyrite grains form near the mat surface along a redox boundary where high rates of sulfate reduction, partial closed-system sulfate consumption in microenvironments, and/or sulfide oxidation lead to high δ34 S values. In another stage, large sphalerite grains with low δ34 S values grow along the edges of pore spaces formed from desiccation of the mat. Large pyrite grains form deeper in the mat at slower sulfate reduction rates, leading to low δ34 Ssulfide values. We do not see evidence for significant 34 S-enrichment in bulk pore water sulfide at depth in the mat due to closed-system Rayleigh fractionation effects. On a local scale, Rayleigh fractionation influences the range of δ34 S values measured for individual pyrite grains. Fine-scale analyses of δ34 Spyrite patterns can thus be used to extract environmental information from ancient microbial mats and aid in the interpretation of bulk δ34 Spyrite records.

Comparing in situ colorimetric DET and DGT techniques with ex situ core slicing and centrifugation for measuring ferrous iron and dissolved sulfide in coastal sediment pore waters

In productive coastal sediments the separation between different biogeochemical zones (e.g. oxic, iron(III)-reducing and sulfate-reducing) may be on the scale of millimetres. Conventional measurement techniques simply cannot resolve changes in pore water solute concentrations over such small distances. The diffusive equilibration in thin films (DET) and the diffusive gradients in thin films (DGT) techniques allow in situ determination of pore water solute concentration profiles with one-dimensional profiles and/or two-dimensional distributions on the millimetre scale. Here we compare measurements of pore water iron(II) and sulfide using conventional core sampling (slicing and centrifugation) and colorimetric DET-DGT techniques. DET-DGT samplers were deployed within replicate sediment cores from three different sites, which were processed by slicing and centrifugation following retrieval of the samplers, so that the measurements were approximately co-located. Iron(II) concentrations were determined by DET at all three sites (0.3–262 μmol L−1), while dissolved sulfide was consistently measured by DGT at one site only (0.003–112 μmol L−1). Pore water concentrations of iron(II) and sulfide determined conventionally following pore water extraction (iron(II); 0.4–88 μmol L−1 and sulfide; 0.05–36 μmol L−1), were systematically lower than the colorimetric DET and DGT measurements in the same sample. This underestimation was most likely due to the mixing of sediment from different biogeochemical zones during pore water extraction, which resulted in the precipitation of iron(II) and sulfide. This study shows that conventional pore water extraction methods can be unreliable for the determination of redox-active solutes due to artefacts associated with pore water mixing.

Intense molybdenum accumulation in sediments underneath a nitrogenous water column and implications for the reconstruction of paleo-redox conditions based on molybdenum isotopes

The concentration and isotope composition of molybdenum (Mo) in sediments and sedimentary rocks are widely used proxies for anoxic conditions in the water column of paleo-marine systems. While the mechanisms leading to Mo fixation in modern restricted basins with anoxic and sulfidic (euxinic) conditions are reasonably well constrained, few studies have focused on Mo cycling in the context of open-marine anoxia. Here we present Mo data for water column particulate matter, modern surface sediments and a paleo-record covering the last 140,000years from the Peruvian continental margin. Mo concentrations in late Holocene and Eemian (penultimate interglacial) shelf sediments off Peru range from ∼70 to 100µgg−1, an extent of Mo enrichment that is thought to be indicative of (and limited to) euxinic systems. To investigate if this putative anomaly could be related to the occasional occurrence of sulfidic conditions in the water column overlying the Peruvian shelf, we compared trace metal (Mo, vanadium, uranium) enrichments in particulate matter from oxic, nitrate-reducing (nitrogenous) and sulfidic water masses. Coincident enrichments of iron (Fe) (oxyhydr)oxides and Mo in the nitrogenous water column as well as co-variation of dissolved Fe and Mo in the sediment pore water suggest that Mo is delivered to the sediment surface by Fe (oxyhydr)oxides. Most of these precipitate in the anoxic-nitrogenous water column due to oxidation of sediment-derived dissolved Fe with nitrate as a terminal electron acceptor. Upon reductive dissolution in the surface sediment, a fraction of the Fe and Mo is re-precipitated through interaction with pore water sulfide. The Fe- and nitrate-dependent mechanism of Mo accumulation proposed here is supported by the sedimentary Mo isotope composition, which is consistent with Mo adsorption onto Fe (oxyhydr)oxides. Trace metal co-variation patterns as well as Mo and nitrogen isotope systematics suggest that the same mechanism of Mo delivery caused the ‘anomalously’ high interglacial Mo accumulation rates in the paleo-record. Our findings suggest that Fe- and nitrate-dependent Mo shuttling under nitrogenous conditions needs to be considered a possible reason for sedimentary Mo enrichments during past periods of widespread anoxia in the open ocean.

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.

Trace metal diagenesis in sulfidic sediments: Insights from Chesapeake Bay

Sediments were recovered from the mid-bay portion of the Chesapeake Bay when bottom water conditions were sulfidic. Metal concentrations (Fe, Mn, Ba, V, Ni, Cu, Co, Cd, Mo, U, and Re) were determined in both pore waters and solid phase samples, which were determined using total and nitric acid digestion (EPA 3051a) methods. V, Cr, and Ni were preferentially retained in pore waters even under sulfidic conditions that would normally result in authigenic accumulation, likely due to complexation with dissolved organic matter (DOM). Mo was efficiently removed from pore waters in the zone in which FeS phases and polysufides are available. Re was not delivered to the sediments through the Mn/Fe shuttle, so that its diffusive flux into sediments solely depends on the extent of reducing conditions.A non-steady state oxygenation event was recorded in the solid phase, with coincident solid phase enrichments in Fe, Mn, Ni, Cu and Co. Deeper enrichments in U and Mo likely reflect remobilization from the relict Fe and Mn peaks, downward diffusion into more reducing conditions, and subsequent immobilization. The absence of peaks in solid phase Re, Cr and Cd was indicative of their lack of association with Mn and Fe oxides and/or resistance of their respective authigenic phases to oxidation on these seasonal to yearly timescales. Relative changes within the solid phase profiles were captured using the acid digestion method for Fe, Ni and Co, and acid digestion concentrations were within 90% of the total concentration for Mn and Cu. The presence of excess pore water sulfide may have promoted the removal of trace metals from the detrital fraction resulting in the formation of authigenic sulfides that might become mobile during oxygenation events.

Influence of porewater sulfide on methylmercury production and partitioning in sulfate-impacted lake sediments

In low-sulfate and sulfate-limited freshwater sediments, sulfate loading increases the production of methylmercury (MeHg), a potent and bioaccumulative neurotoxin. Sulfate loading to anoxic sediments leads to sulfide production that can inhibit mercury methylation, but this has not been commonly observed in freshwater lakes and wetlands. In this study, sediments were collected from sulfate-impacted, neutral pH, surface water bodies located downstream from ongoing and historic mining activities to examine how chronic sulfate loading produces porewater sulfide, and influences MeHg production and transport. Sediments were collected over two years, during several seasons from lakes with a wide range of overlying water sulfate concentration. Samples were characterized for in-situ solid phase and porewater MeHg, Hg methylation potentials via incubations with enriched stable Hg isotopes, and sulfur, carbon, and iron content and speciation. Porewater sulfide reflected historic sulfur loading and was strongly related to the extractable iron content of sediment. Overall, methylation potentials were consistent with the accumulation of MeHg on the solid phase, but both methylation potentials and MeHg were significantly lower at chronically sulfate-impacted sites with a low solid-phase Fe:S ratio. At these heavily sulfate-impacted sites that also contained elevated porewater sulfide, both MeHg production and partitioning are influenced: Hg methylation potentials and sediment MeHg concentrations are lower, but occasionally porewater MeHg concentrations in sediment are elevated, particularly in the spring. The dual role of sulfide as a ligand for inorganic mercury (decreasing bioavailability) and methylmercury (increasing partitioning into porewater) means that elucidating the role of iron and sulfur loads as they define porewater sulfide is key to understanding sulfate's influence on MeHg production and partitioning in sulfate-impacted freshwater sediment.

Contribution of non-carbonate inorganic and organic alkalinity to total measured alkalinity in pore waters in marine sediments (Gulf of Gdansk, S-E Baltic Sea)

Various microbiological, chemical and physical processes during early diagenesis in marine sediments make the chemical composition of pore water significantly different from that of the sea water. The aim of this study was (a) to compare concentrations of total alkalinity measured using titration method (TAmeas.) with carbonate alkalinity calculated (TAcalc.) from dissolved inorganic carbon and pH, and (b) to determine the proportion of non-carbonate inorganic bases (N-CIBs) and alkalinity from organic compounds (OA) in TAmeas. in pore water in the Gulf of Gdansk. In the study, TAmeas. as well as concentrations of dissolved inorganic and organic carbon (DIC and DOC), hydrogen sulphide, sulphate, ammonium, phosphates and borate in near-bottom and pore water were measured at three sampling sites in October 2013. At all the sites, TAmeas. was 1632–14,460μmolkg−1 and increased with depth into the sediment, as did the concentrations of other parameters (excluding sulphate). Inorganic and organic compounds produced during organic matter (OM) decomposition in sediments had a considerable contribution to TAmeas. in pore water. As a result TAmeas. was up to 38% higher than TAcalc. The observed excess alkalinity (TAexcess) was from 56 to 3972μmolkg−1 and increased gradually going deeper into the sediment. TAexcess was correlated with concentration of N-CIBs (mainly hydrogen sulphide, ammonia and phosphates) and DOC present in pore water. The main source of N-CIBs in pore water was bacterial sulphate reduction during OM mineralization. The concentration of N-CIBs in pore water ranged from 85 to 1053μmoll−1 and the percentage share of N-CIBs in TAmeas. was 4–35%. The highest proportion of N-CIBs in TAmeas. was observed in the upper (0–12cm) layers of the sediment. Anoxic decomposition of OM has led to the production of OA in pore water which constituted up to 26% of TAmeas. The concentration of OA varied from 2 to 2953μmoll−1and at all sites increased with depth into the sediment. This is due to the accumulation of products of decomposition of OM in pore water, i.e. organic acids and fulvic and humic acids.