Anoxic Basin Research Articles

Detection of Transient Denitrification During a High Organic Matter Event in the Black Sea

AbstractN2 production by denitrification can occur in anoxic water or potentially inside organic particles. Here we compare data from the Black Sea, a permanently anoxic basin, during two organic matter regimes: suspended particulate organic matter concentrations were high in the oxycline after the spring bloom in March 2005 compared to lower organic matter concentrations in June 2005, May and October 2007, July 2008, and May 2001. For all cruises, N2 gas had a maximum in the suboxic zone (O2 < 10 μmol/L). During the high organic matter event (March 2005), an additional shallower N2 gas and δ15N‐N2 maxima occurred above the suboxic zone in the oxycline where oxygen concentrations were 30–50 μmol/L. Examination of 16S rRNA indicated that anammox bacteria were not present in the oxycline. The δ15N of biologically produced N2 in the oxycline in March 2005 was significantly enriched (+7‰ to +38‰), not depleted, as would be expected from water column fractionation. A simple diffusion calculation indicated that ammonium produced from remineralization inside particles could be oxidized to nitrate and then completely consumed by denitrification inside the particle. In this calculation, half of denitrified N atoms originated from organic N [δ15N = 11‰] and half of N atoms originated from ambient nitrate [δ15N = 5‰–7‰], producing enriched δ15N‐N2 values. We suggest that denitrifiers were active in microzones inside particulates in hypoxic waters above the suboxic zone of the Black Sea. Denitrification in particles may also explain previous data from the oxycline above ocean oxygen deficient zones.

Effect of large magnetotactic bacteria with polyphosphate inclusions on the phosphate profile of the suboxic zone in the Black Sea.

The Black Sea is the world's largest anoxic basin and a model system for studying processes across redox gradients. In between the oxic surface and the deeper sulfidic waters there is an unusually broad layer of 10-40 m, where neither oxygen nor sulfide are detectable. In this suboxic zone, dissolved phosphate profiles display a pronounced minimum at the upper and a maximum at the lower boundary, with a peak of particulate phosphorus in between, which was suggested to be caused by the sorption of phosphate on sinking particles of metal oxides. Here we show that bacterial polyphosphate inclusions within large magnetotactic bacteria related to the genus Magnetococcus contribute substantially to the observed phosphorus peak, as they contain 26-34% phosphorus compared to only 1-5% in metal-rich particles. Furthermore, we found increased gene expression for polyphosphate kinases by several groups of bacteria including Magnetococcaceae at the phosphate maximum, indicating active bacterial polyphosphate degradation. We propose that large magnetotactic bacteria shuttle up and down within the suboxic zone, scavenging phosphate at the upper and releasing it at the lower boundary. In contrast to a passive transport via metal oxides, this bacterial transport can quantitatively explain the observed phosphate profiles.

Turbidite deposition and diagenesis in the southwestern Black Sea: Implications for biogeochemical cycling in an anoxic basin

The biogeochemical cycles of iron, phosphorus and sulfur are intimately linked and the fate of these elements is highly redox-dependent. Under anoxic conditions, iron is reduced to Fe(II), for an important part driven by reaction with sulfide. Reduction and sulfidation diminish the affinity of iron for phosphorus, thereby affecting sedimentary phosphorus retention. The coupled cycling of iron-phosphorus-sulfur as a function of redox conditions thereby helps control nutrient availability and primary productivity in marine systems. The Black Sea is the world's largest permanently stratified basin with a strong gradient from oxic surface waters to anoxic and strongly sulfidic deep waters, and is therefore well-suited to investigate redox-dependent changes in coupled iron-phosphorus-sulfur cycling. The presence of sulfide in the deep Black Sea alters the chemical speciation of iron (Fe) and phosphorus (P) in particulate matter in the water column and sediment. These alterations and their impact are poorly constrained for turbidites, which are deposits formed by mass transport from the continental slope into the deep basin of the Black Sea through turbidity currents. Here, we compare the geochemistry of sediments on the southwestern (SW) Black Sea slope overlain by oxygenated waters (234 m below sea surface, mbss) with that of a sediment record with turbidite intervals from the sulfidic SW deep basin (2169 mbss). The aims were to investigate the potential of SW slope sediment as source material for turbidites, and to assess the geochemical impact of reaction with sulfide. The bulk chemistry (Al, CaCO3, Fe) of the SW deep Black Sea turbidite intervals was similar to that of SW Black Sea slope sediments (234 m water depth), suggesting that the latter are source material for deep SW Black Sea turbidites. The source sediment is characterized by strong enrichments in highly reactive Fe(III) and Mn phases, high rates of sulfate reduction and the presence of methane. Similar total Fe contents in the slope and deep turbidite sediments suggest that Fe is efficiently retained in the sediment. In contrast, Mn is depleted during down-slope transport and P contents in the turbidite intervals are about 50% lower than in the slope sediment, indicating significant P release during and after turbidite deposition. Calculations using the obtained data and conservative estimates of the areal extent of turbidite deposition in the SW Black Sea indicate that turbidity currents have the potential to impact coupled Fe-P-S biogeochemical cycling by boosting Fe input, dissolved P accumulation, sulfide oxidation and FeS2 burial in the deep Black Sea.

The genetic diversity and evolution of diatom-diazotroph associations highlights traits favoring symbiont integration.

Diatom diazotroph associations (DDAs) are a widespread marine planktonic symbiosis between several diatom genera and di-nitrogen (N2)-fixing bacteria. Combining single cell confocal microscopy observations and molecular genetic approaches on individual field collected cells, we determined the phylogenetic diversity, distribution and evolution of the DDAs. Confocal analyses coupled with 3-D imaging re-evaluated the cellular location of DDA symbionts. DDA diversity was resolved by paired gene sequencing (18S rRNA and rbcL genes, 16S rRNA and nifH genes). A survey using the newly acquired sequences against public databases found sequences with high similarity (99–100%) to either host (18S rRNA) or symbiont (16S rRNA) in atypical regions for DDAs (high latitudes, anoxic basin and copepod gut). Concatenated phylogenies were congruent for the host and cyanobacteria sequences and implied co-evolution. Time-calibrated trees dated the appearance of N2 fixing planktonic symbiosis from 100–50Mya and were consistent with the symbiont cellular location: symbioses with internal partners are more ancient. An ancestral state reconstruction traced the evolution of traits in DDAs and highlight that the adaptive radiation to the marine environment was likely facilitated by the symbiosis. Our results present the evolutionary nature of DDAs and provide new genetic and phenotypic information for these biogeochemically relevant populations.

The thermal structure of the anoxic trough in Lake Untersee, Antarctica

AbstractLake Untersee is a perennially ice-covered Antarctic lake that consists of two basins. The deepest basin, next to the Anuchin Glacier is aerobic to its maximum depth of 160 m. The shallower basin has a maximum depth of 100 m, is anoxic below 80 m, and is shielded from convective currents. The thermal profile in the anoxic basin is unusual in that the water temperature below 50 m is constant at 4°C but rises to 5°C between 70 m and 80 m depth, then drops to 3.7°C at the bottom. Field measurements were used to conduct a thermal and stability analysis of the anoxic basin. The shape of the thermal maximum implies two discrete locations of energy input, one of 0.11 W m-2 at 71 m depth and one of 0.06 W m-2 at 80 m depth. Heat from microbial activity cannot account for the required amount of energy at either depth. Instead, absorption of solar radiation due to an increase in water opacity at these depths can account for the required energy input. Hence, while microbial metabolism is not an important source of heat, biomass increases opacity in the water column resulting in greater absorption of sunlight.

The Role of Reactive Iron in the Preservation of Terrestrial Organic Carbon in Estuarine Sediments

AbstractTo better understand the role of reactive Fe (FeR) in the preservation of sedimentary organic carbon (SOC) in estuarine sediments, we examined specific surface area, grain size composition, total OC (TOC), lignin phenols, FeR, FeR‐associated OC (Fe‐OC) and lignin phenols (Fe‐lignin), and δ13C of FeR‐associated OC (δ13CFe‐OC) in surface sediments of the Changjiang Estuary and adjacent shelf. An estimated 7.4 ± 3.5% of the OC was directly bound with FeR in the Changjiang Estuary and adjacent shelf. Unusually low TOC/specific surface area loadings and Fe‐OC/Fe ratios in mobile muds suggest that frequent physical reworking may reduce FeR binding with OC, with selective loss of marine OC. More depleted 13CFe‐OC relative to 13C of TOC (13Cbulk) in deltaic regions and mobile muds showed that FeR was largely associated with terrestrial OC, derived from extensive riverine OC and Fe inputs. A higher proportion of hematite in the mobile muds compared to the offshore samples indicated that Fe oxides are likely subjected to selective sorting and/or become mature during long‐term sediment transport. When considering the percentage of Fe‐OC to SOC and SOC burial rates in different marine environments (e.g., nondeltaic shelf, anoxic basins, slope, and deep sea), our findings suggest that about 15.6 ± 6.5% of SOC is directly bound to FeR on a global scale, which is lower than the previous estimation (~21.5%). This work further supports the notion of a Rusty Sink where, in this case, FeR plays an important role in the preservation and potential transport of terrestrial OC in the marine environment.

Palynostratigraphic, palynofacies, organic geochemical and palaeoenvironmental analysis of the Silurian Tanezzuft Formation in the Ghadames Basin of north-west Libya

Two palynofacies associations are documented from the Silurian Tanezzuft Formation in the Ghadames Basin. These are characteristic of the basal ‘Hot Shale’ and the overlying deposits, referred to here as the Cold Shale. The former reflects deposition in distal suboxic anoxic conditions and is dominated by highly oil-prone amorphous organic matter (AOM) typical of deposition in generally anoxic, restricted marine basins. Only a few acritarchs, prasinophyte algae and chitinozoans occur in association: virtually no spores or cryptospores were recorded. Thick-walled prasinophytes are most numerous in this part of the Tanezzuft Formation in both the Ghadames and Murzuq basins, suggesting enhanced surface water productivity. Deposition took place after the melting of the Late Ordovician ice sheets, which led to a major marine transgression. The palynofacies recorded from the overlying ‘Cold Shale’ deposits indicate deposition in distal shelf and basin conditions that were also relatively anoxic. They contain more palynomorphs, especially acritarchs, and generally less AOM. The phytoplankton assemblages are dominated by simple and thin-walled prasinophyte algae (leiospheres), suggesting dysoxic–anoxic conditions. Overall the middle and the upper parts of the Tanezzuft Formation are regarded as being deposited in distal dysoxic–anoxic shelf, distal dysoxic–oxic shelf and distal suboxic–anoxic basin respectively. Because chitinozoans are very rare, age determinations of the samples investigated are based mainly on acritarchs. The Hot Shale is dated as early-mid Rhuddanian (early Llandovery) whereas the rest of the formation is considered to late Rhuddanian–Telychian in age. As documented previously from other samples of the Tanezzuft Formation in both the Ghadames and Murzuq basins, the Hot Shale has a very high TOC content and excellent source potential for liquid hydrocarbons, whereas rest of the formation is less rich in organic matter with larger terrestrial and oxidized components and hence reduced potential for sourcing hydrocarbons (both oil and gas).

Fraction-specific rare earth elements enable the reconstruction of primary seawater signatures from iron formations

Rare earth element (REE) abundances in iron formations (IFs) provide key evidence to track the redox evolution of the Precambrian oceans. Deriving information about processes and sources from IFs is complicated by the variable roles of diagenetic dissolution and precipitation that modify primary depositional signals in the rocks. Iron formations are typically mineralogically heterogeneous with varying amounts of carbonate (ankerite + siderite), oxide (predominantly magnetite) and silicate (chert and Fe-silicates). To date, little is known about the respective contributions of these fractions to the bulk-rock REE signal, and which fraction most accurately records primary seawater signals. Here we evaluate an optimized sequential extraction scheme for REE analysis in the ca. 2.46 Ga Asbestos Hills IFs from the Griqualand-West Basin in South Africa. These rocks were deposited immediately before the Great Oxidation Event and are potentially an important archive for large-scale redox changes in the (near) surface environment.To fully evaluate our method, we measured lanthanide elements plus ytrrium (REY) in individual minerals, in bulk rock samples and in three separate fractions using acetate, oxalate and total digestion sequential extractions. The extraction techniques were then verified using a mass balance approach, the first time that this has been comprehensively applied to IFs. Similarities between the carbonate fraction data and modern seawater, coupled with strong measurement reproducibility and systematic stratigraphic variability suggests that carbonate most closely tracks the primary seawater composition. The Asbestos Hills IFs show distinctive REY signatures with strongly variable (Yb/Pr)SN ratios, positive La anomalies, positive and constant Y/Ho, weakly positive Eu anomalies and a lack of Ce anomalies in their carbonate fraction. Stratigraphic variations in these parameters suggest deposition in a stratified fully anoxic ocean, which became shallower over time. The silicate fraction was influenced by various amounts of allochthonous material (detrital and volcanic ash), although some minerals including greenalite, minnesotaite and stilpnomelane may have tracked seawater REE variability. The oxide fraction was found to be the most REY-depleted and therefore susceptible to allochthonous influences as well as diagenetic controls. It shows a systematic MREE enrichment that has not been described in IFs before, but has been seen in sub-oxic, ferrous environments. An increase in the ratio of carbonate to oxide minerals ongoing upwards through the Asbestos Hills IF, from deep to shallow water conditions, most likely reflected an increase in dissimilatory iron reduction close to the margin of the anoxic basin.

Baltic Sea diazotrophic cyanobacterium is negatively affected by acidification and warming

Nitrogen fixation is a key source of nitrogen in the Baltic Sea which counteracts nitrogen loss processes in the deep anoxic basins. Laboratory and field studies have indicated that single-strain nitrogen-fixing (diazotrophic) cyanobacteria from the Baltic Sea are sensitive to ocean acidification and warming, two drivers of marked future change in the marine environment. Here, we enclosed a natural plankton community in twelve indoor mesocosms (volume ~1400 L) and manipulated pCO2 to yield six CO2 treatments with two different temperature treatments (16.6°C and 22.4°C, pCO2 range = 360 – 2030 μatm). We followed the filamentous, heterocystous diazotrophic cyanobacteria community (Nostocales, primarily Nodularia spumigena) over four weeks. Our results indicate that heterocystous diazotrophic cyanobacteria may become less competitive in natural plankton communities under ocean acidification. Elevated CO2 had a negative impact on Nodularia sp. biomass, which was exacerbated by warming. Our results imply that Nodularia sp. may contribute less to new nitrogen inputs in the Baltic Sea in future.

Trace Dissolved Oxygen Removal Using a Hydrocyclone to Enhance Denitrification

AbstractDissolved oxygen (DO) is entrained by mixed liquid recirculation (MLR) into the anoxic basin in the pre‐denitrification biological nitrogen removal (PDBNR) process and should be removed to enhance denitrification. A laboratory‐scale PDBNR reactor with an anoxic basin divided into two isometric components was built to investigate the effect of a hydrocyclone on DO removal from the MLR and the subsequent denitrification. The denitrification rate of the mixture of influent and hydrocyclone‐treated nitrified wastewater increased with increasing DO and was higher than that of the wastewater in a conventional anoxic component. More total nitrogen was removed in the anoxic component due to the application of the hydrocyclone in the MLR of a laboratory‐scale reactor and a full‐scale anoxic/aerobic reactor.

Thallium isotopes reveal protracted anoxia during the Toarcian (Early Jurassic) associated with volcanism, carbon burial, and mass extinction

For this study, we generated thallium (Tl) isotope records from two anoxic basins to track the earliest changes in global bottom water oxygen contents over the Toarcian Oceanic Anoxic Event (T-OAE; ∼183 Ma) of the Early Jurassic. The T-OAE, like other Mesozoic OAEs, has been interpreted as an expansion of marine oxygen depletion based on indirect methods such as organic-rich facies, carbon isotope excursions, and biological turnover. Our Tl isotope data, however, reveal explicit evidence for earlier global marine deoxygenation of ocean water, some 600 ka before the classically defined T-OAE. This antecedent deoxygenation occurs at the Pliensbachian/Toarcian boundary and is coeval with the onset of initial large igneous province (LIP) volcanism and the initiation of a marine mass extinction. Thallium isotopes are also perturbed during the T-OAE interval, as defined by carbon isotopes, reflecting a second deoxygenation event that coincides with the acme of elevated marine mass extinctions and the main phase of LIP volcanism. This suggests that the duration of widespread anoxic bottom waters was at least 1 million years in duration and spanned early to middle Toarcian time. Thus, the Tl data reveal a more nuanced record of marine oxygen depletion and its links to biological change during a period of climatic warming in Earth's past and highlight the role of oxygen depletion on past biological evolution.

Microbial ecology of deep-sea hypersaline anoxic basins.

Deep hypersaline anoxic basins (DHABs) are unique water bodies occurring within fractures at the bottom of the sea, where the dissolution of anciently buried evaporites created dense anoxic brines that are separated by a chemocline/pycnocline from the overlying oxygenated deep-seawater column. DHABs have been described in the Gulf of Mexico, the Mediterranean Sea, the Black Sea and the Red Sea. They are characterized by prolonged historical separation of the brines from the upper water column due to lack of mixing and by extreme conditions of salinity, anoxia, and relatively high hydrostatic pressure and temperatures. Due to these combined selection factors, unique microbial assemblages thrive in these polyextreme ecosystems. The topological localization of the different taxa in the brine-seawater transition zone coupled with the metabolic interactions and niche adaptations determine the metabolic functioning and biogeochemistry of DHABs. In particular, inherent metabolic strategies accompanied by genetic adaptations have provided insights on how prokaryotic communities can adapt to salt-saturated conditions. Here, we review the current knowledge of the diversity, genomics, metabolisms and ecology of prokaryotes in DHABs.

Life on the edge: active microbial communities in the Kryos MgCl2-brine basin at very low water activity.

The Kryos Basin is a deep-sea hypersaline anoxic basin (DHAB) located in the Eastern Mediterranean Sea (34.98°N 22.04°E). It is filled with brine of re-dissolved Messinian evaporites and is nearly saturated with MgCl2-equivalents, which makes this habitat extremely challenging for life. The strong density difference between the anoxic brine and the overlying oxic Mediterranean seawater impedes mixing, giving rise to a narrow chemocline. Here, we investigate the microbial community structure and activities across the seawater-brine interface using a combined biogeochemical, next-generation sequencing, and lipid biomarker approach. Within the interface, we detected fatty acids that were distinctly 13C-enriched when compared to other fatty acids. These likely originated from sulfide-oxidizing bacteria that fix carbon via the reverse tricarboxylic acid cycle. In the lower part of the interface, we also measured elevated rates of methane oxidation, probably mediated by aerobic methanotrophs under micro-oxic conditions. Sulfate reduction rates increased across the interface and were highest within the brine, providing first evidence that sulfate reducers (likely Desulfovermiculus and Desulfobacula) thrive in the Kryos Basin at a water activity of only ~0.4 Aw. Our results demonstrate that a highly specialized microbial community in the Kryos Basin has adapted to the poly-extreme conditions of a DHAB with nearly saturated MgCl2 brine, extending the known environmental range where microbial life can persist.

Mercury in the Black Sea: New Insights From Measurements and Numerical Modeling.

Redox conditions and organic matter control marine methylmercury (MeHg) production. The Black Sea is the world's largest and deepest anoxic basin and is thus ideal to study Hg species along the extended redox gradient. Here we present new dissolved Hg and MeHg data from the 2013 GEOTRACES MEDBlack cruise (GN04_leg2) that we integrated into a numerical 1‐D model, to track the fate and dynamics of Hg and MeHg. Contrary to a previous study, our new data show highest MeHg concentrations in the permanently anoxic waters. Observed MeHg/Hg percentage (range 9–57%) in the anoxic waters is comparable to other subsurface maxima in oxic open‐ocean waters. With the modeling we tested for various Hg methylation and demethylation scenarios along the redox gradient. The results show that Hg methylation must occur in the anoxic waters. The model was then used to simulate the time evolution (1850–2050) of Hg species in the Black Sea. Our findings quantify (1) inputs and outputs of HgT (~31 and ~28 kmol yr−1) and MeHgT (~5 and ~4 kmol yr−1) to the basin, (2) the extent of net demethylation occurring in oxic (~1 kmol yr−1) and suboxic water (~6 kmol yr−1), (3) and the net Hg methylation in the anoxic waters of the Black Sea (~11 kmol yr−1). The model was also used to estimate the amount of anthropogenic Hg (85–93%) in the Black Sea.

Modeling the carbon isotope signatures of methane and dissolved inorganic carbon to unravel mineralization pathways in boreal lake sediments

Vertical profiles of the concentration and isotopic composition (δ13C) of methane (CH4) and dissolved inorganic carbon (DIC), as well as of ancillary parameters, were obtained in the top 25 cm of a sediment column in a seasonally anoxic basin from an oligotrophic boreal lake. Modeling the profiles of CH4 and DIC concentrations and those of their δ13C signatures with reaction-transport equations allowed us to determine the organic matter (OM) degradation rates according to various reactions and to constrain the in situ isotopic fractionation factors and diffusivity coefficients of CH4 and DIC. This exercise reveals inter alia that (i) CH4 production occurs below a depth of 5 cm, with the highest production rate between 5 and 7.5 cm depth, (ii) all CH4 is produced through hydrogenotrophy, and (iii) methanogenesis yields a production rate of CH4 about three times greater than that of DIC. This latter observation indicates either that fermentation of OM is not the exclusive source of H2 sustaining hydrogenotrophy, or that the commonly assumed model molecule CH2O does not adequately represent the fermenting OM, since its fermentation yields identical rates of CH4 and DIC production. The porewater profiles of Fe and SO42- suggest that some H2 may be produced during the reoxidation of reduced sulfur by Fe(III), but the rate of H2 production via this process, if active, would be insignificant in comparison to that required to sustain the estimated rate of hydrogenotrophy. We deduce that the imbalance between CH4 and DIC production rates is rather due to the fermentation of organic substrates that are more reduced than CH2O, i.e., having a negative average carbon oxidation state (COS). From the constraints on reaction rates and on fermentation pathways imposed by the δ13C data, we infer that the organic substrate fermenting between depths of 5 and 7.5 cm should have a COS of −1.87. We thus submit that CH4 is produced in the sediments of the seasonally anoxic basin of our boreal lake through hydrogenotrophy coupled to the fermentation of reduced organic substrates that can be represented by a mixture of fatty acids (e.g. C16H32O2; COS of −1.75) and fatty alcohols (e.g., C16H34O; COS of −2.00). This study emphasizes the importance of characterizing the sedimentary OM undergoing mineralization in order to improve diagenetic model predictions of CH4 cycling in boreal lakes and of its significance in climate change.

Recent Advances in Halophilic Protozoa Research.

Most research on microorganisms adapted to hypersaline habitats has focused on Archaea and Bacteria, with microbial eukaryotes receiving much less attention. Over the past 15yr, our knowledge of phagotrophic microbial eukaryotes, i.e. protozoa, from hypersaline habitats has greatly improved through combinations of microscopy, molecular phylogenetics, environmental sequencing, transcriptomics and growth experiments. High salinity waters from salterns, other landlocked water masses and deep hypersaline anoxic basins contain unique and diverse halophilic protozoan assemblages. These have the potential to exert substantial grazing pressure on prokaryotes and other eukaryotes. They represent many separate evolutionary lineages; species of Heterolobosea, Bicosoecida, and Ciliophora have been most intensively characterized, with several proven to be extreme (or borderline extreme) halophiles. Transcriptomic examinations of the bicosoecid Halocafeteria (and the heteroloboseid Pharyngomonas) indicate that high-salt adaptation is associated with a subtle shift in protein amino acid composition, and involves the differential expression of genes participating in ion homeostasis, signal transduction, stress management, and lipid remodeling. Instances of gene duplication and lateral transfer possibly conferring adaptation have been documented. Indirect evidence suggests that these protozoa use "salt-out" osmoadaptive strategies.

Understanding the Dynamics of the Oxic‐Anoxic Interface in the Black Sea

AbstractThe Black Sea, the largest semienclosed anoxic basin on Earth, can be considered as an excellent natural laboratory for oxic and anoxic biogeochemical processes. The suboxic zone, a thin interface between oxic and anoxic waters, still remains poorly understood because it has been undersampled. This has led to alternative concepts regarding the underlying processes that create it. Existing hypotheses suggest that the interface originates either by isopycnal intrusions that introduce oxygen or the dynamics of manganese redox cycling that are associated with the sinking of particles or chemosynthetic bacteria. Here we reexamine these concepts using high‐resolution oxygen, sulfide, nitrate, and particle concentration profiles obtained with sensors deployed on profiling floats. Our results show an extremely stable structure in density space over the entire basin with the exception of areas near the Bosporus plume and in the southern areas dominated by coastal anticyclones. The absence of large‐scale horizontal intrusive signatures in the open‐sea supports a hypothesis prioritizing the role of biogeochemical processes.

Salt and Methane Generation Initiated by Membrane Polarisation

The existence of deep marine hypersaline anoxic basins (DHAB) has been well-documented starting with the MedRiff Project in the Eastern Mediterranean. We suppose that there is analogy between the recent and ancient DHABs. This premise allows us to hypothesize that some methane accumulations in geological reservoirs may have been generated by historical euryhaline bacteria. The extreme life conditions of the bacteria and the facieses, as found in currently existing supersaturated salt brines DHABs, may have also existed in the geological past. Since salt basins overlap some of the most productive gas provinces, this article aims to introduce a new approach to salt and methane generation. It highlights the need to reconsider the classical approach to salt and methane generation due to new observations. Hereby we describe a new mechanism for DHAB generation due to membrane polarization. These phenomena generate a surface on which seawater of normal salinity meets the underneath brine of high salinity, and there is no diffusion between them. Hence we presume that non-crystalized, over-pressured, salty brine is the appropriate material to trap and host methane. Following overburden by deposited basin sediments, this viscous, gas-saturated brine can be an engine for diapir formation, which is prior to the crystalline phase. This new idea redefines our search for salt and methane deposits yet it requires further research and consideration, along with the new approach of salt diapir formation in specific salt basins.

Antimony(III) sulfide complexes in aqueous solutions at 30 °C: A solubility and XAS study

Stibnite solubility experiments and ambient temperature X-ray absorption spectroscopy (XAS) measurements of antimony in solution were undertaken to determine the aqueous speciation of Sb(III) in sulfide solutions having reduced sulfur concentrations from 0.001 to 0.1molkg−1S2−total over the pH range of 3.5 to 12. At 30°C, stibnite solubility curve was defined by a scheme of five species: Sb2S42−, HSb2S4−, H2Sb2S52−, H3SbS2O, and Sb(OH)3(aq). Equilibrium constants were determined for the following heterogeneous solubility reactions:Sb2S3s+HS−⇋Sb2S42−+H+Sb2S3s+HS−⇋HSb2S4−Sb2S3s+2HS−⇋H2Sb2S52−0.5Sb2S3s+0.5HS−+1.5H++H2Ol⇋H3SbS2O0.5Sb2S3s+3H2Ol⇋SbOH3aq+1.5HS−+1.5H+The Sb K-edge XAS measurements of antimony in alkaline (pH=10.9 to 12) sulfide solutions gave average first shell coordination environments for strongly alkaline solutions that were consistent the speciation model derived from solubility experiments (i.e., Sb2S42− and Sb(OH)3(aq)). XAS data enabled the elimination of a speciation model involving only Sb-S monomers at strongly alkaline pH. Antimony speciation in near neutral to strongly alkaline pH's is dominated by dimeric antimony sulfide complexes at sulfide concentrations>0.001molkg−1S2−total. Calculations of stibnite solubility and aqueous antimony speciation for conditions in anoxic marine basins indicate that sulfide species comprise a significant part of the total dissolved Sb(III) when molkg−1S2−total>~0.00005molkg−1 and that stibnite is close to saturation in some anoxic basins.

A multi‐year time‐series of N2O dynamics in a seasonally anoxic fjord: Saanich Inlet, British Columbia

AbstractWe present an 8‐yr time‐series of monthly water column N2O measurements from Saanich Inlet, a seasonally anoxic fjord in southern British Columbia. We document seasonal and inter‐annual variability in N2O concentrations driven by physical and biological forcing, and examine the relationship between N2O and O2 concentrations across the redoxcline in this system. Near‐surface N2O concentrations were typically supersaturated, and increased with depth to a maximum near the oxic‐anoxic transition. Genes associated with both the nitrification and denitrification pathways were widely distributed throughout the water column, suggesting the potential for simultaneous N2O production from nitrification and incomplete denitrification. Maximum N2O concentrations in Saanich Inlet were similar to other anoxic basins, but lower than those previously documented in coastal upwelling and open ocean oxygen minimum zones. N2O and O2 were inversely correlated throughout the water column, except below 10 μmol O2 L−1 where net N2O consumption appears to occur. In surface waters, maximum N2O concentrations and sea–air fluxes occurred during late summer/early fall, when O2 levels in the oxycline were low. Annual deep basin renewals often persisted over multiple months, supplying O2, , and N2O to the deep basin, with N2O subsequently declining to undetectable levels (< 0.4 nmol N2O L−1) by late spring. An unusually weak renewal was observed during 2009, possibly linked to reduced upwelling off the coast of British Columbia during El Niño. Our results can be used as a baseline to identify the effect of longer‐term changes in Saanich Inlet, such as deoxygenation and its effect on N2O concentrations.