Methanogenic Zone Research Articles

Organoclastic sulfate reduction in the sulfate-methane transition of marine sediments

Abstract Sulfate and methane diffuse vertically along opposed concentration gradients into the sulfate-methane transition in subsurface marine sediments where anaerobic oxidation of methane with sulfate takes place. The stoichiometry of this process is 1:1, yet the calculated sulfate flux often exceeds the calculated methane flux. Our aim was to determine whether organoclastic sulfate reduction, fueled by the oxidation of buried organic matter, explains this excess sulfate flux into the sulfate-methane transition. We analyzed data for sulfate, methane and sulfate reduction from eight sites in Danish coastal waters. Fluxes of sulfate and methane were determined by diffusion-reaction modeling whereas sulfate reduction rates were determined by 35S radiotracer experiments at high depth resolution. The sulfate reduction data showed that the organic carbon mineralization rates followed a log-log linear depth trend throughout the sulfate zone. Extrapolation of this power law trend down into the sulfate-methane transition showed organic carbon oxidation corresponded to 14–59% of the total sulfate flux into the zone and in average explained 82% of the excess sulfate consumption above the 1:1 stoichiometry. A part of the excess sulfate was consumed directly by organoclastic sulfate reduction while a part drove a cryptic methane cycle by which the organic matter was degraded to methane and the methane concurrently oxidized to CO2 with sulfate. We extrapolated the same power law trend down through the methanogenic zone and found that this provided a good estimate of total methanogenesis in the sediment column when compared to the upward flux of methane.

Formate and Hydrogen as Electron Shuttles in Terminal Fermentations in an Oligotrophic Freshwater Lake Sediment.

The energetic situation of terminal fermentations in methanogenesis was analyzed by pool size determinations in sediment cores taken in the oligotrophic Lake Constance, Germany. Distribution profiles of fermentation intermediates and products were measured at three different water depths (2, 10, and 80 m). Methane concentrations were constant below 10 cm of sediment depth. Within the methanogenic zone, concentrations of formate, acetate, propionate, and butyrate varied between 1 and 40 μM, and hydrogen was between 0.5 and 5 Pa. From the distribution profiles of the fermentation intermediates, Gibbs free energy changes for their interconversion were calculated. Pool sizes of formate and hydrogen were energetically nearly equivalent, with -5 ± 5 kJ per mol difference of free energy change (ΔG) for a hypothetical conversion of formate to hydrogen plus CO2 The ΔG values for conversion of fatty acids to methanogenic substrates and their further conversion to methane and CO2 were calculated with hydrogen and with formate as intermediates. Syntrophic propionate oxidation reached energetic equilibrium with formate as the sole electron carrier but was sufficiently exergonic if at least some of the electrons were transferred via hydrogen. The energetic consequences of formate versus hydrogen transfer in secondary and methanogenic fermentations indicate that both carrier systems are probably used simultaneously to optimize the energy yields for the partners involved.IMPORTANCE In the terminal steps of methane formation in freshwater lake sediments, fermenting bacteria cooperate syntrophically with methanogens and homoacetogens at minimum energy increments via interspecies electron transfer. The energy yields of the partner organisms in these cooperations have so far been calculated based mainly on in situ hydrogen partial pressures. In the present study, we also analyzed pools of formate as an alternative electron carrier in sediment cores of an oligotrophic lake. The formate and hydrogen pools appeared to be energetically nearly equivalent and are likely to be used simultaneously for interspecies electron transfer. Calculations of reaction energies of the partners involved suggest that propionate degradation may also proceed through the Smithella pathway, which converts propionate via butyrate and acetate to three acetate residues, thus circumventing one energetically difficult fatty acid oxidation step.

Conductive Particles Enable Syntrophic Acetate Oxidation between Geobacter and Methanosarcina from Coastal Sediments

ABSTRACTCoastal sediments are rich in conductive particles, possibly affecting microbial processes for which acetate is a central intermediate. In the methanogenic zone, acetate is consumed by methanogens and/or syntrophic acetate-oxidizing (SAO) consortia. SAO consortia live under extreme thermodynamic pressure, and their survival depends on successful partnership. Here, we demonstrate that conductive particles enable the partnership between SAO bacteria (i.e., Geobacter spp.) and methanogens (Methanosarcina spp.) from the coastal sediments of the Bothnian Bay of the Baltic Sea. Baltic methanogenic sediments were rich in conductive minerals, had an apparent isotopic fractionation characteristic of CO2-reductive methanogenesis, and were inhabited by Geobacter and Methanosarcina. As long as conductive particles were delivered, Geobacter and Methanosarcina persisted, whereas exclusion of conductive particles led to the extinction of Geobacter. Baltic Geobacter did not establish a direct electric contact with Methanosarcina, necessitating conductive particles as electrical conduits. Within SAO consortia, Geobacter was an efficient [13C]acetate utilizer, accounting for 82% of the assimilation and 27% of the breakdown of acetate. Geobacter benefits from the association with the methanogen, because in the absence of an electron acceptor it can use Methanosarcina as a terminal electron sink. Consequently, inhibition of methanogenesis constrained the SAO activity of Geobacter as well. A potential benefit for Methanosarcina partnering with Geobacter is that together they competitively exclude acetoclastic methanogens like Methanothrix from an environment rich in conductive particles. Conductive particle-mediated SAO could explain the abundance of acetate oxidizers like Geobacter in the methanogenic zone of sediments where no electron acceptors other than CO2 are available.

Energy Gradients Structure Microbial Communities Across Sediment Horizons in Deep Marine Sediments of the South China Sea.

The deep marine subsurface is a heterogeneous environment in which the assembly of microbial communities is thought to be controlled by a combination of organic matter deposition, electron acceptor availability, and sedimentology. However, the relative importance of these factors in structuring microbial communities in marine sediments remains unclear. The South China Sea (SCS) experiences significant variability in sedimentation across the basin and features discrete changes in sedimentology as a result of episodic deposition of turbidites and volcanic ashes within lithogenic clays and siliceous or calcareous ooze deposits throughout the basin's history. Deep subsurface microbial communities were recently sampled by the International Ocean Discovery Program (IODP) at three locations in the SCS with sedimentation rates of 5, 12, and 20 cm per thousand years. Here, we used Illumina sequencing of the 16S ribosomal RNA gene to characterize deep subsurface microbial communities from distinct sediment types at these sites. Communities across all sites were dominated by several poorly characterized taxa implicated in organic matter degradation, including Atribacteria, Dehalococcoidia, and Aerophobetes. Sulfate-reducing bacteria comprised only 4% of the community across sulfate-bearing sediments from multiple cores and did not change in abundance in sediments from the methanogenic zone at the site with the lowest sedimentation rate. Microbial communities were significantly structured by sediment age and the availability of sulfate as an electron acceptor in pore waters. However, microbial communities demonstrated no partitioning based on the sediment type they inhabited. These results indicate that microbial communities in the SCS are structured by the availability of electron donors and acceptors rather than sedimentological characteristics.

Significance of stable carbon isotope trends in carbonate concretions formed in association with anaerobic oxidation of methane (AOM), Middle and Upper Devonian shale succession, western New York State, USA

Carbonate concretions are concentrated along discrete stratigraphic horizons within Middle and Upper Devonian carbonaceous black and organic-deficient gray shale of western New York State. Textural characteristics preserved throughout studied concretions, including a clay grain microfabric typical of flocculated clay and spherical algal cysts, are consistent with a model entailing the formation of low-density, compaction-resistant calcium carbonate masses at shallow burial depth, within the zone of bacterial sulfate reduction. Burial of nascent concretions to the sulfate methane transition zone (SMTZ) was accompanied by the nearly complete infilling of porosity, the diagenetic consequence of the anaerobic oxidation of methane (AOM). Each concretionary horizon, then, defines the position of the SMTZ stabilized during an episode of reduced sedimentation rate. Most analyzed concretions (75%) of the Middle Devonian Marcellus Formation into the Upper Devonian Gowanda Formation display center-to-edge profiles of increasing bulk δ13C; far fewer concretions are characterized by radial profiles of decreasing δ13C. The common radial profiles of increasing δ13C can be attributed to AOM-related kinetic carbon isotope fractionation under diagenetic conditions of limited methane, which favors the production of dissolved inorganic carbon of increasing δ13C. Less common concretions displaying center-to-edge profiles of diminishing δ13C may record the anaerobic oxidation of more strongly 13C-depleted methane produced by carbon dioxide reduction, methanogenesis related to the degradation of organic matter within and near the SMTZ, or AOM back flux, an enzyme-mediated equilibrium isotope effect. These processes are favored by sulfate-limited diagenetic conditions more typical of the lower SMTZ and immediately underlying upper methanogenic zone (MEZ). Contradictory isotope profiles of texturally similar concretions can be accounted for by diagenesis associated with an extended SMTZ (ESMTZ) comprising the methane-limited SMTZ and the underlying sulfate-deficient upper MEZ. Vertical shifts of the diagenetic horizons of the ESMTZ induced by variable methane flux would have juxtaposed concretions displaying seemingly incompatible isotopic histories.

Identification of groundwater redox process induced by landfill leachate based on sensitive factor method.

Landfill site is a significant source of groundwater pollution. To ensure that the groundwater contamination of landfills can be controlled and repaired scientifically, the identification of groundwater pollution process is needed. On the basis of biogeochemical process of leachate pollutants in the groundwater environment, a sensitive factor method for the identification of groundwater redox process from landfills was established in this research. The method encompasses four phases, including sensitive factors selection, redox zone characterization, weight calculation, and redox zone identification. In the sensitive factor index system employed here, five indicators involving dissolved oxygen (DO), nitrite, Fe2+, sulfide, and CO2 were selected. The boundary of each redox zones was determined by the quantitative method, and the weight of each indicator was calculated by combined weight method. This method was applied to a landfill site in the northeast of China. The result showed that there were five redox zones that appeared in pollution plume, including methanogenic zone (MGZ), sulfate reduction zone (SRZ), iron reduction zone (IRZ), nitrate reduction zone (NRZ), and oxygen reduction zone (ORZ). The results were consistent with the actual situation of the site. The sensitive factor method was scientific and effective to identify the groundwater redox process in landfill and can provide reference data related to investigation and remediation of groundwater pollution in landfill sites.

Methane-metabolizing microbial communities in sediments of the Haima cold seep area, northwest slope of the South China Sea.

Cold seeps are widespread chemosynthetic ecosystems in the deep-sea environment, and cold seep microbial communities of the South China Sea are poorly constrained. Here we report on the archaeal communities, particularly those involved in methane metabolization, in sediments of a newly discovered cold seep (named 'Haima') on the northwest slope of the South China Sea. Archaeal diversity, abundance and distribution were investigated in two piston cores collected from a seep area (QDN-14B) and a non-seep control site (QDN-31B). Geochemical investigation of the QDN-14B core identified an estimated sulfate-methane transition zone (Estimated SMTZ) at 300-400 cm below sea floor (cmbsf), where a high abundance of anaerobic methane-oxidizing archaea (ANME) occurred, as revealed by analysis of the 16S rRNA gene and the gene (mcrA) encoding the α-subunit of the key enzyme methyl-coenzyme M reductase. ANME-2a/b was predominant in the upper and middle layers of the estimated SMTZ, whereas ANME-1b outcompeted ANME-2 in the sulfate-depleted bottom layers of the estimated SMTZ and the methanogenic zone. Fine-scale phylogenetic analysis further divided the ANME-1b group into three subgroups with different distribution patterns: ANME-1bI, ANME-1bII and ANME-1bIII. Multivariate analyses indicated that dissolved inorganic carbon and sulfate may be important factors controlling the composition of the methane-metabolizing community. Our study on ANME niche separation and interactions with other archaeal groups improves our understanding of the metabolic diversity and flexibility of ANME, and the findings further suggest that ANME subgroups may have evolved diversified/specified metabolic capabilities other than syntrophic anaerobic oxidation of methane coupled with sulfate reduction in marine sediments.

Hydrocarbon Degradation in Caspian Sea Sediment Cores Subjected to Simulated Petroleum Seepage in a Newly Designed Sediment-Oil-Flow-Through System.

The microbial community response to petroleum seepage was investigated in a whole round sediment core (16 cm length) collected nearby natural hydrocarbon seepage structures in the Caspian Sea, using a newly developed Sediment-Oil-Flow-Through (SOFT) system. Distinct redox zones established and migrated vertically in the core during the 190 days-long simulated petroleum seepage. Methanogenic petroleum degradation was indicated by an increase in methane concentration from 8 μM in an untreated core compared to 2300 μM in the lower sulfate-free zone of the SOFT core at the end of the experiment, accompanied by a respective decrease in the δ13C signal of methane from -33.7 to -49.5‰. The involvement of methanogens in petroleum degradation was further confirmed by methane production in enrichment cultures from SOFT sediment after the addition of hexadecane, methylnapthalene, toluene, and ethylbenzene. Petroleum degradation coupled to sulfate reduction was indicated by the increase of integrated sulfate reduction rates from 2.8 SO42-m-2 day-1 in untreated cores to 5.7 mmol SO42-m-2 day-1 in the SOFT core at the end of the experiment, accompanied by a respective accumulation of sulfide from 30 to 447 μM. Volatile hydrocarbons (C2–C6 n-alkanes) passed through the methanogenic zone mostly unchanged and were depleted within the sulfate-reducing zone. The amount of heavier n-alkanes (C10–C38) decreased step-wise toward the top of the sediment core and a preferential degradation of shorter (<C14) and longer chain n-alkanes (>C30) was seen during the seepage. This study illustrates, to the best of our knowledge, for the first time the development of methanogenic petroleum degradation and the succession of benthic microbial processes during petroleum passage in a whole round sediment core.

Microbial Community Response to Simulated Petroleum Seepage in Caspian Sea Sediments.

Anaerobic microbial hydrocarbon degradation is a major biogeochemical process at marine seeps. Here we studied the response of the microbial community to petroleum seepage simulated for 190 days in a sediment core from the Caspian Sea using a sediment-oil-flow-through (SOFT) system. Untreated (without simulated petroleum seepage) and SOFT sediment microbial communities shared 43% bacterial genus-level 16S rRNA-based operational taxonomic units (OTU0.945) but shared only 23% archaeal OTU0.945. The community differed significantly between sediment layers. The detection of fourfold higher deltaproteobacterial cell numbers in SOFT than in untreated sediment at depths characterized by highest sulfate reduction rates and strongest decrease of gaseous and mid-chain alkane concentrations indicated a specific response of hydrocarbon-degrading Deltaproteobacteria. Based on an increase in specific CARD-FISH cell numbers, we suggest the following groups of sulfate-reducing bacteria to be likely responsible for the observed decrease in aliphatic and aromatic hydrocarbon concentration in SOFT sediments: clade SCA1 for propane and butane degradation, clade LCA2 for mid- to long-chain alkane degradation, clade Cyhx for cycloalkanes, pentane and hexane degradation, and relatives of Desulfobacula for toluene degradation. Highest numbers of archaea of the genus Methanosarcina were found in the methanogenic zone of the SOFT core where we detected preferential degradation of long-chain hydrocarbons. Sequencing of masD, a marker gene for alkane degradation encoding (1-methylalkyl)succinate synthase, revealed a low diversity in SOFT sediment with two abundant species-level MasD OTU0.96.

Early overpressuring in organic-rich shales during burial: evidence from fibrous calcite veins in the Lower Jurassic Shales-with-Beef Member in the Wessex Basin, UK

Field, petrographic and geochemical analysis of fibrous calcite veins in the Lower Jurassic Shales-with-Beef Member in the Wessex Basin was conducted to investigate the formation mechanism of the veins. Bedding-parallel fibrous calcite veins, including beef veins and tabular cone-in-cone structures, are widespread in the black shales. The calcite veins consist of subvertical fibrous crystals and a dark median zone. The median zone contains scattered clays, pyrite microcrystals, skeletal fragments and amorphous organic matter. The veins exhibit moderate carbon isotope values, ranging from −1.515 to 2.732‰. The oxygen isotope composition ranges from −8.872 to −4.521‰, which is possibly too negative to reflect the primary porewater oxygen isotope signatures and indicates a porewater modification. It is interpreted that the veins mainly derive carbonates from seawater inorganic carbon and bioclasts. The veins formed as closed-system hydraulic fractures in overpressured cells during sediment degassing in the methanogenic zone. The shale beds with a high total organic carbon content could have generated abundant CO 2 , which may have resulted in either the cementing of the pores in the matrix or overpressure buildup. The skeletal fragments provide a control on the spatial distribution of veins as nuclei for calcite precipitation from supersaturated pore fluids.

Unraveling signatures of biogeochemical processes and the depositional setting in the molecular composition of pore water DOM across different marine environments

Dissolved organic matter (DOM) in marine sediment pore waters derives largely from decomposition of particulate organic matter and its composition is influenced by various biogeochemical and oceanographic processes in yet undetermined ways. Here, we determine the molecular inventory of pore water DOM in marine sediments of contrasting depositional regimes with ultrahigh-resolution mass spectrometry and complementary bulk chemical analyses in order to elucidate the factors that shape DOM composition. Our sample sets from the Mediterranean, Marmara and Black Seas covered different sediment depths, ages and a range of marine environments with different (i) organic matter sources, (ii) balances of organic matter production and preservation, and (iii) geochemical conditions in sediment and water column including anoxic, sulfidic and hypersaline conditions. Pore water DOM had a higher molecular formula richness than overlying water with up to 11,295 vs. 2114 different molecular formulas in the mass range of 299–600Da and covered a broader range of element ratios (H/C=0.35–2.19, O/C=0.03–1.19 vs. H/C=0.56–2.13, O/C=0.15–1.14). Formula richness was independent of concentrations of DOC and TOC. Near-surface pore water DOM was more similar to water column DOM than to deep pore water DOM from the same core with respect to formula richness and the molecular composition, suggesting exchange at the sediment–water interface. The DOM composition in the deeper sediments was controlled by organic matter source, selective decomposition of specific DOM fractions and early diagenetic molecule transformations. Compounds in pelagic sediment pore waters were predominantly highly unsaturated and N-bearing formulas, whereas oxygen-rich CHO-formulas and aromatic compounds were more abundant in pore water DOM from terrigenous sediments. The increase of S-bearing molecular formulas in the water column and pore waters of the Black Sea and the Mediterranean Discovery Basin was consistent with elevated HS- concentrations reflecting the incorporation of sulfur into biomolecules during early diagenesis. Sulfurization resulted in an increased average molecular mass of DOM and higher formula richness (up to 5899 formulas per sample). In sediments from the methanogenic zone in the Black Sea, the DOM pool was distinctly more reduced than overlying sediments from the sulfate-reducing zone. Bottom and pore water DOM from the Discovery Basin contained the highest abundances of aliphatic compounds in the entire dataset; a large fraction of abundant N-bearing formulas possibly represented peptide and nucleotide formulas suggesting preservation of these molecules in the life inhibiting environment of the Discovery Basin. Our unique data set provides the basis for a comprehensive understanding of the molecular signatures in pore water DOM and the turnover of sedimentary organic matter in marine sediments.

Geochemical and microbial context of the gassy sediments in the Ría de Vigo (NW of Spain)

Multidisciplinary data coming from high-resolution seismic profiles, gravity core analysis (sedimentology and geochemistry), radiocarbon dating and metagenomics has been used to establish the stratigraphic framework as well as the geochemical and microbial context of the gassy sediments in the Ría de Vigo. Gas appears within the muddy sediments that compound the youngest Holocene seismic unit of the infilling of the ria and is mainly methane, generated by microbial decomposition in a rich organic matter depositional environment. Within the ria, the biogeochemical zonation is affected by the quality, quantity and age of the buried organic matter. The 16S ribosomal RNA gene amplicon clone libraries sequencing suggests that prokaryotic composition may be linked to the sediment type. Uncultured archaea common to the marine sub-sea floor of the Miscellaneous Crenarchaeotal Group (MCG) and Marine Benthic Group-D (MBGD) are dominant in these organic-rich estuarine and shallow marine sediments. The presence of Methanomicrobia and ANME in the sulphate-methane transition zone (SMTZ) from the estuarine gassy area indicates anaerobic oxidation of methane (AOM) in these sediments. Desulfobacteraceae are also homogeneously distributed along the sediment columns, even in the methanogenic zone, suggesting the existence of a cryptic sulphur cycle driven by iron under the SMTZ. The older sediments retrieved from the non-gassy outer area of the ria contain less organic matter and appear to represent mainly a sulphate-reducing environment where SMTZ is not yet reached at the bottom of the sediment core. Methanogens of various affiliations and traces of methane are detected in this sulphate-reducing environment, with an unusual increase in concentration towards the sediment surface. Microbial methanogenesis in the sulphate-reducing zone of surface sediments, a process recently described in organic-rich sediments able to supply methanogenic substrates could reconcile these observations.

A new model of the formation of Pennsylvanian iron carbonate concretions hosting exceptional soft-bodied fossils in Mazon Creek, Illinois.

Preservation of Pennsylvanian-aged (307Ma) soft-bodied fossils from Mazon Creek, Illinois, USA, is attributed to the formation of siderite concretions, which encapsulate the remains of terrestrial, freshwater, and marine flora and fauna. The narrow range of positive δ34 S values from pyrite in individual concretions suggests microenvironmentally limited ambient sulfate, which may have been rapidly exhausted by sulfate-reducing bacteria. Tissue of the decaying carcass was rapidly encased by early diagenetic pyrite and siderite produced within the sulfate reduction and methanogenic zones of the sediment, with continuation of the latter resulting in concretion cementation. Cross-sectional isotopic analyses (δ13 C and δ18 O) and mineralogical characterization of the concretions point to initiation of preservation in high porosity proto-concretions during the early phases of microbially induced decay. The proto-concretion was cemented prior to compaction of the sediments by siderite as a result of methanogenic production of 13 C-rich bicarbonate-which varies both between Essex and Braidwood concretions and between fossiliferous and unfossiliferous concretions. This work provides the first detailed geochemical study of the Mazon Creek siderite concretions and identifies the range of conditions allowing for exceptional soft-tissue fossil formation as seen at Mazon Creek.

Controls on subsurface methane fluxes and shallow gas formation in Baltic Sea sediment (Aarhus Bay, Denmark)

Shallow gas accumulates in coastal marine sediments when the burial rate of reactive organic matter beneath the sulfate zone is sufficiently high and the methanogenic zone is sufficiently deep. We investigated the controls on methane production and free methane gas accumulation along a 400m seismo-acoustic transect across a sharp transition from gas-free into gas-bearing sediment in Aarhus Bay (Denmark). Twelve gravity cores were taken, in which the pore water was analyzed for inorganic solutes while rates of organic carbon mineralization were measured experimentally by 35SO42− radiotracer method. The thickness of organic-rich Holocene mud increased from 5 to 10m along the transect concomitant with a shallowing of the depth of the sulfate-methane transition from >4m to 2.5m. In spite of drastic differences in the distribution of methane and sulfate in the sediment along the transect, there were only small differences in total mineralization, and methanogenesis was only equivalent to about 1% of sulfate reduction. Shallow gas appeared where the mud thickness exceeded 8–9m. Rates of methanogenesis increased along the transect as did the upward diffusive flux of methane. Interestingly, the increase in the sedimentation rate and Holocene mud thickness had only a modest direct effect on methanogenesis rates in deep sediments. This increase in methane flux, however, triggered a shallowing of the sulfate-methane transition which resulted in a large increase in methanogenesis at the top of the methanogenic zone. Thus, our results demonstrate a positive feedback mechanism that causes a strong enhancement of methanogenesis and explains the apparently abrupt appearance of gas when a threshold thickness of organic-rich mud is exceeded.

Hydrogen Utilization Potential in Subsurface Sediments.

Subsurface microbial communities undertake many terminal electron-accepting processes, often simultaneously. Using a tritium-based assay, we measured the potential hydrogen oxidation catalyzed by hydrogenase enzymes in several subsurface sedimentary environments (Lake Van, Barents Sea, Equatorial Pacific, and Gulf of Mexico) with different predominant electron-acceptors. Hydrogenases constitute a diverse family of enzymes expressed by microorganisms that utilize molecular hydrogen as a metabolic substrate, product, or intermediate. The assay reveals the potential for utilizing molecular hydrogen and allows qualitative detection of microbial activity irrespective of the predominant electron-accepting process. Because the method only requires samples frozen immediately after recovery, the assay can be used for identifying microbial activity in subsurface ecosystems without the need to preserve live material. We measured potential hydrogen oxidation rates in all samples from multiple depths at several sites that collectively span a wide range of environmental conditions and biogeochemical zones. Potential activity normalized to total cell abundance ranges over five orders of magnitude and varies, dependent upon the predominant terminal electron acceptor. Lowest per-cell potential rates characterize the zone of nitrate reduction and highest per-cell potential rates occur in the methanogenic zone. Possible reasons for this relationship to predominant electron acceptor include (i) increasing importance of fermentation in successively deeper biogeochemical zones and (ii) adaptation of H2ases to successively higher concentrations of H2 in successively deeper zones.

Redox zones stratification and the microbial community characteristics in a periphyton bioreactor

Bioremediation techniques based on microorganisms have been widely applied to treat polluted surface water, but the efficiencies have been limited, especially in deep and static waters. Microbial aggregates, known as periphyton, were introduced into a tank bioreactor to improve pollutants removal and a periphyton bioreactor with an 84cm column was built to investigate microbe–wastewater interactions. Periphyton greatly improved water quality and produced a distinct stratification in the water column into five redox zones with slight overlaps. From top to bottom these were: oxygen reduction, nitrate reduction, iron reduction, sulfate reduction and methanogenic zone. Periphyton communities had high species diversities (767–947 OTUs) with the facultative zone (middle layer) having higher species richness and functional diversity than the aerobic (top layer) and anaerobic zones (bottom layer). A good knowledge of interactions between periphyton and water column stratification could benefit from integration of periphyton to improve bioremediation of deep and static water.

Using iron speciation in authigenic carbonates from hydrocarbon seeps to trace variable redox conditions

Seepage of hydrocarbon-rich fluids out of the marine sedimentary column is characterized by temporal changes of flow intensity and resultant spatially variable redox conditions. Authigenic carbonates at marine hydrocarbon seeps provide excellent geological and geochemical archives that serve to explore seepage dynamics over time. In this study, we investigated the potential of Mössbuaer spectroscopy and Fe contents of seep-related authigenic carbonates from the Congo Fan, the Gulf of Mexico, and the Black Sea for reconstructing past redox conditions and fluid seepage activity at cold seeps. The Fe speciation observed by Mössbauer spectroscopy and Fe contents suggest that (1) the Congo Fan carbonates precipitated in a sulfidic environment, (2) the formation conditions of seep carbonates were variable at the Gulf of Mexico seep site, ranging from oxic to suboxic and anoxic and even spanning into the methanogenic zone, and (3) the stratified water column of the Black Sea or suboxic condition resulted in low Fe contents of Black Sea carbonates. The study reveals that Fe speciation can provide constraints on the wide range of redox conditions that imprinted seep carbonates during the life span of seepage. Similarly, Mössbauer spectroscopy – particularly when used in combination with the analysis of redox-sensitive elements – is a promising tool to trace variable redox conditions in marine paleoenvironments other than seeps.

Diagenesis of phosphatic hardgrounds in the Monterey Formation: A perspective from bulk and clumped isotope geochemistry

Understanding the authigenesis of carbon­ ate fluorapatite through isotopic geochem­ istry can yield important information on fundamental geologic processes occurring on continental margins around the world. This is particularly true for phosphatic hardgrounds, which are often found in regions of upwell­ ing, but of which questions remain about the initial formation and subsequent diagen­ esis. Here, we apply standard isotopes (δ 13 C, δ 18 O) alongside the novel clumped isotope (Δ47) used in this study for the first time to reconstruct the temperature of formation of carbonate ions within the lattice of sedimen­ tary carbonate fluorapatite. We investigated phosphatic hardgrounds of Miocene age (12.7–10.8 Ma) sampled at El Capitan State Beach in the Monterey Formation. The range of isotopic signatures observed is between +1.5‰ and +8.0‰ for δ 13 C relative to the Vienna Peedee belemnite (VPDB) standard and –9.5‰ and –6.0‰ VPDB for δ 18 O, and val­ ues range between 0.599‰ and 0.615‰ for Δ47. The enriched δ 13 C and depleted δ 18 O signa­ tures are suggestive of recrystallization within the methanogenic zone. Clumped isotope geo­ chemistry further constrains this transforma­ tion as having taken place at a temperature of 61–66 °C ± 5 °C, in line with previous esti­ mates for maximum burial of the Monterey Formation based on the silica phase transi­ tion. The calculated δ 18 O for the connate fluid shows an expected range for seawater composition for the Miocene, suggesting only minor contribution of silica­derived oxygen to the δ 18 O of carbonate fluorapatite. The combined conventional and clumped isotope data set also points out that methanogenesis took place deeper within the sediment in the middle Miocene than at present day within the Santa Barbara Basin. This study fur­ thers our understanding of phosphogenesis and potential links to burial processes in the Monterey Formation, and it shows for the first time that the clumped isotope paleother ­ mometer could be used to under stand funda­ mental geochemical processes in authigenic sedimentary phosphates.

Habitat mapping of cold seeps associated with authigenic mineralization (Paola Ridge, southern Tyrrhenian Sea): combining seafloor backscatter with biogeochemistry signals

Mineralogical, petrographic and stable isotope analyses were carried out on sediment and rock samples collected from a deep-sea cold seep province in the Paola Ridge (southeastern Tyrrhenian Sea). The results, coupled with the interpretation of the seafloor backscatter, constrained both the biogeochemical zonation and the spatial distribution of the cold seep habitats in the area. These have proved to change in depth in a range of few meters and laterallywithin narrow zones. The mud volcanoes, characterized by a high backscatter signature, are the site of vigorous gas venting and, in the subsurface, show a rapid transition from the oxic sea water interface toward the methane-sulfate transition zone in the sediments. Intermediate backscatter typifies areas where free venting is hampered by the presence of mudflows at the seafloor. These conditions favor: i) the oxidation of sulfides near the seafloor, ii) the precipitation of siderites a few meters below the seafloor and iii) the formation of sulfides deeper in the sub-seafloor. Faults are likely candidates to act as conduits for sulfates and metal oxides that juxtapose different redox environments. Siderites precipitated in the fast and low venting sites showed enrichment in δ13C and δ18O, which are compatible with their precipitation in the methanogenic zone. The heavy-oxygen isotopic compositions of the siderites are possibly related to the dissociation of gas hydrates, which have not been mapped so far by seismic data in the study area. Mud diapirism is characterized by low backscatter seafloor, large fields of pockmarks and is dissected bynormal faults. In coincidence with the normal faults, authigenic calcites and aragonites are present at or very close to the seafloor. They have the typical isotopic signature indicating formation during sulfate-dependent microbially-mediated anaerobic oxidation of methane.They are associated with Lucinoma borealis, the youngest being dated 640-440 BP. This suggests that the seepage activity in the mud diapirs was likely clogged by either carbonates or activity of the faults only very recently.

Sulfate reduction and methane oxidation activity below the sulfate-methane transition zone in Alaskan Beaufort Sea continental margin sediments: Implications for deep sulfur cycling

Two ∼6m long sediment cores were collected along the ∼300m isobath on the Alaskan Beaufort Sea continental margin. Both cores showed distinct sulfate-methane transition zones (SMTZ) at 105 and 120cm below seafloor (cmbsf). Sulfate was not completely depleted below the SMTZ but remained between 30 and 500μM. Sulfate reduction and anaerobic oxidation of methane (AOM) determined by radiotracer incubations were active throughout the methanogenic zone. Although a mass balance could not explain the source of sulfate below the SMTZ, geochemical profiles and correlation network analyses of biotic and abiotic data suggest a cryptic sulfur cycle involving iron, manganese and barite. Inhibition experiments with molybdate and 2-bromoethanesulfonate (BES) indicated decoupling of sulfate reduction and AOM and competition between sulfate reducers and methanogens for substrates. While correlation network analyses predicted coupling of AOM to iron reduction, the addition of manganese or iron did not stimulate AOM. Since none of the classical archaeal anaerobic methanotrophs (ANME) were abundant, the involvement of unknown or unconventional phylotypes in AOM is conceivable. The resistance of AOM activity to inhibitors implies deviation from conventional enzymatic pathways. This work suggests that the classical redox cascade of electron acceptor utilization based on Gibbs energy yields does not always hold in diffusion-dominated systems, and instead biotic processes may be more strongly coupled to mineralogy.