Sulfate-methane Transition Research Articles

The impact of diffusive transport of methane on pore-water and sediment geochemistry constrained by authigenic enrichments of carbon, sulfur, and trace elements: A case study from the Shenhu area of the South China Sea

Sulfate-driven anaerobic oxidation of methane (SD-AOM) controls methane release from marine sediments to the ocean. A variety of authigenic precipitates within sediments has been widely used to identify past occurrences of SD-AOM. However, the lack of a systematic evaluation of the formation conditions of these precipitates sometimes impedes the recognition of SD-AOM in past environments characterized by diffusive transport of methane. Carbon, sulfur, and trace element geochemistry of pore-water and sediments was investigated at a site affected by upward methane diffusion in the Shenhu area of the South China Sea. Here, the sulfate-methane transition zone (SMTZ) is located ~7.6 m below the seafloor based on sulfate and methane concentrations. The slope of δ34S vs. δ18O values of sulfate is consistent with diffusive transport of methane. Concentration and isotope profiles of pore-water species point to diffusive rather than advective transport of solutes. Enhanced sulfate reduction inferred from δ34S values of sulfate agrees with the local occurrence of relatively abundant, 34S-enriched iron sulfide minerals. The time required to produce the observed authigenic iron sulfides around the SMTZ is estimated to be ~1.1 ka based on the amount of iron sulfide minerals and the present diffusive flux of sulfate. No enrichment of authigenic carbonate, barium (Ba), and molybdenum (Mo) was detected in the studied sediments. This observation is consistent with the calculation that the current fluxes of pore-water calcium, magnesium, Ba, and Mo are too low to allow for authigenic enrichments. The low fluxes are largely controlled by diffusion, which facilitates the formation of 34S-enriched iron sulfide minerals. The observed enrichment patterns – unlike those of sediments affected by advective seepage – are expected to be prevalent in modern and ancient continental-margin sediments, and may contribute to the identification of past methane-rich zones and overlying SD-AOM zones as sinks for methane in the geological record.

Environmental evolution of the East China Sea inner shelf and its constraints on pyrite sulfur contents and isotopes since the last deglaciation

To better understand the depositional constraints on the sulfate reduction process, we present a set of sulfur isotope data for hand-picked pyrites (δ34Spyr) from the coarse fraction (63–250 μm) of bulk sediments, coupled with the contents of carbonate, total organic carbon (TOC) and nitrogen (TN), as well as δ13C values of the TOC, from sediments of core EC2005 on the inner shelf of the East China Sea (ECS) since 16.4 ka. Our results show that core sediments were deposited in a terrestrial environment before 13.1 ka (stage I) when pyrite rarely occurred. During stages II (13.1–12.3 ka) and III (12.3–12.1 ka) seawater began to influence the core location, forming a tidal flat environment where abundant pyrites are well preserved. Then the inner shelf of the ECS was fully flooded, and two intervals with high (stage IV:12.1–6.0 ka; stage VI: 5.2–1.5 ka) and low sedimentation rate (stage V: 6.0–5.2; stage VII: 1.5 ka-present) are identified respectively. When compared with previous δ34S values of chromium-reducible sulfur (δ34SCRS) derived from the bulk sediments, δ34Spyr values are usually higher, indicating the macroscopic pyrites form in the later stages of sedimentary diagenesis when ongoing sulfate reduction has distilled porewater sulfate to a high degree. Considering the wide occurrence of biogenic shallow gas (dominated by CH4) in these sediments, sulfate-driven anaerobic oxidation of methane (SO4-AOM) and organoclastic sulfate reduction (OSR) are expected to be two competitive pathways for sulfate reduction. We propose that the changes in sedimentation rate controls the pace of migration of the sulfate methane transition zone (SMTZ). When sedimentation rate is low, the SMTZ may be fixed in a certain depth (e.g., sediments deposited in stage III and V), where H2S is produced by SO4-AOM. In contrast, when sedimentation rate is very high (> 1 cm/yr), the SMTZ would be deep, and H2S is mostly produced by OSR. Further, our results imply that sulfur diagenesis in recent (since 1.5 ka) ECS mud sediments might be influenced by physical reworking (e.g., typhoon) and/or biological activity (bioturbation), resulting in partial reoxidation of pyrite crystals in fluid mud. Our findings therefore demonstrate that pyrite sulfur isotope compositions (bulk δ34SCRS & hand-picked δ34Spyr) are sensitive to local environmental evolution and the combined use of these two indicators can provide new insights into pathway of sulfate reduction in unsteady environments.

The Effect of Early Diagenesis in Methanic Sediments on Sedimentary Magnetic Properties: Case Study From the SE Mediterranean Continental Shelf

Microbial respiration in marine sediment can affect the magnetic properties of the sediment through a complicated interplay between reductive dissolution and authigenic precipitation of iron-bearing magnetic minerals. However, a direct link between the main diagenetic zones in the upper sedimentary column and sedimentary magnetic properties using high resolution multi parameter profiles has been demonstrated only in few studies. Here, we directly correlate early diagenetic processes and sedimentary magnetism using a composite high-resolution sedimentary record of pore water chemistry, solid phase chemical measurements and mineral-magnetic parameters. Measurements along the profiles include the entire redox cascade, from the water-sediment interface, down through the deep methanic zone, on a six-meter sediment core collected from the Southern Eastern Mediterranean continental shelf. The uppermost part of the sediment core, associated with oxic, nitrous, manganous, ferruginous, and sulfate reduction zones, is characterized by high ferrous iron and sulfate concentrations and high values of the measured magnetic parameters (susceptibility, Isothermal remanent magnetization (IRM) and Anhysteretic remanent magnetization (ARM)). This layer is underlain by a sulfate-methane transition zone (SMTZ) that shows a significant decrease in magnetic parameters due to the dissolution of magnetic minerals. Below the SMTZ, the methanic zone has been assumed to be magnetically inactive under steady-state conditions. However, we observe in the upper methanic zone an increase in microbial iron reduction, coupled to an abrupt increase in magnetic parameters. Our data indicate that the observed increase in the magnetic signal is related to the precipitation of authigenic magnetic minerals. These diagenetic changes should be considered when interpreting paleomagnetic data, and highlight the potential to use high-resolution magnetic data as a proxy for identifying diagenetic processes.

АУТИГЕННЫЕ МИНЕРАЛЫ В ДОННЫХ ОСАДКАХ СИПОВЫХ ОБЛАСТЕЙ МОРЯ ЛАПТЕВЫХ

Метановые сипы являются широко распространенным явлением, наблюдаемым на шельфах и континентальныхсклонах внутренних и окраинных морей по всему миру, в том числе и в море Лаптевых. Ключевымибиогеохимическими процессами, протекающими в донных осадках этих районов, являются анаэробное окисление метана в сочетании с бактериальной сульфатредукцией. Оба этих процесса контролируют образованиеспецифической аутигенной минерализации. Целью настоящей работы являлось изучение аутигенных минералов донных осадков с аномально высокими концентрациями метана, отобранных на двух сиповых участках в северо-восточной части моря Лаптевых, для определения признаков их идентификации в древних осадочных породах.Приведены результаты литологических и минералогических исследований донных осадков. Было установлено, что основными аутигенными минералами в исследованных образцах донных осадков, отобранных с двух сиповых участков в северо-восточной части моря Лаптевых, являются магнезиальный кальцит, гипс и пирит. Разная специфика аутигенной минерализации, предположительно, указывает на различия в режимах миграции метан-содержащих флюидов на этих участках. Временное снижении интенсивности просачивания метана в пределах «восточного сипа» способствовало насыщению поровой воды ионами SO42– и Ca2+ и, как следствие, осаждению гипса. Близповерхностное положение сульфат-метановой транзитной зоны в «западном сипе», обусловленное высокими потоками метана, благоприятствовало осаждению магнезиального кальцита в верхних горизонтах донных осадков. Присутствие пирита в осадках как восточного, так западного сиповых участков является свидетельством активности процесса бактериальной сульфат-редукции при анаэробном окислении метана.

Pore-water dissolved inorganic carbon sources and cycling in the shallow sediments of the Haima cold seeps, South China Sea

Abstract Pore-water geochemical compositions provide the insights into seafloor cold seep activities and biogeochemical processes of methane-derived carbon in sediments. In this study, six sediment push cores were collected by manned submersible at the center and edge of the active Haima cold seeps, and a nearby background site. Pore-water compositions of the SO42−, Ca2+, Mg2+, Sr2+, NH4+, dissolved inorganic carbon (DIC), and δ13C-DIC at each sediments core were measured. The very shallow depths of sulfate–methane transition zone (SMTZ) suggest that high methane flux and significant anaerobic oxidation of methane (AOM) activities occur in the shallow sediments. Reaction-transport modelling results show that the relative contributions of AOM and organoclastic sulfate reduction (OSR) consuming sulfate are 84.1% and 15.9% at site HM-2-6, and 94.6% and 5.4% at site HM-3-3, respectively. Furthermore, extremely low δ13C-DIC values at the central sites HM-2-6 (−45.67‰) and HM-3-3 (−53.87‰) indicate an external methane-derived DIC. Based on the δ13C mass balance, we quantify the proportions of pore-water DIC derived from organic matter degradation and AOM as 24.7% and 65.3% at site HM-2-6, and 9.3% and 80.7% at site HM-3-3, respectively. An increase in pore-water DIC concentration leads to carbonate precipitation. Plots of Sr/Ca vs. Mg/Ca ratios reveal that the carbonate phases in sediments are dominantly high Mg-calcite in this region. Our study demonstrates that AOM can serve as a dominant biogeochemical process through incorporation of methane-derived carbon into dissolved and solid inorganic carbon phases, significantly influencing DIC cycling within the shallow sediments of the Haima cold seep area.

Methyl-compounds driven benthic carbon cycling in the sulfate-reducing sediments of South China Sea.

Methane is a potent greenhouse gas; methane production and consumption within seafloor sediments has generated intense interest. Anaerobic oxidation of methane (AOM) and methanogenesis (MOG) primarily occur at the depth of the sulfate-methane transition zone or underlying sediment respectively. Methanogenesis can also occur in the sulfate-reducing sediments through the utilization of non-competitive methylated compounds; however, the occurrence and importance of this process are not fully understood. Here, we combined a variety of data, including geochemical measurements, rate measurements and molecular analyses to demonstrate the presence of a cryptic methane cycle in sulfate-reducing sediments from the continental shelf of the northern South China Sea. The abundance of methanogenic substrates as well as the high MOG rates from methylated compounds indicated that methylotrophic methanogenesis was the dominant methanogenic pathway; this conclusion was further supported by the presence of the methylotrophic genus Methanococcoides. High potential rates of AOM were observed in the sediments, indicating that methane produced in situ could be oxidized simultaneously by AOM, presumably by ANME-2a/b as indicated by 16S rRNA gene analysis. A significant correlation between the relative abundance of methanogens and methanotrophs was observed over sediment depth, indicating that methylotrophic methanogenesis could potentially fuel AOM in this environment. In addition, higher potential rates of AOM than sulfate reduction rates at in situ methane conditions were observed, making alternative electron acceptors important to support AOM in sulfate-reducing sediment. AOM rates were stimulated by the addition of Fe/Mn oxides, suggesting AOM could be partially coupled to metal oxide reduction. These results suggest that methyl-compounds driven methane production drives a cryptic methane cycling and fuels AOM coupled to the reduction of sulfate and other electron acceptors.

A geophysical, geochemical and microbiological study of a newly discovered pockmark with active gas seepage and submarine groundwater discharge (MET1-BH, central Gulf of Gdańsk, southern Baltic Sea)

High-resolution bathymetric data were collected with a multi-beam echosounder in the southern part of the Baltic Sea (region MET1, Gulf of Gdańsk) revealing the presence of a 10 m deep and 50 m in diameter pockmark (MET1-BH) on the sea bottom (78.7 m). To date, no such structures have been observed to reach this size in the Baltic Sea. The salinity of the near-bottom water in the pockmark was about 2 PSU (about 31.22 mmol/l Cl−), which clearly indicated the presence of a submarine groundwater discharge (SGD). Water column, sediments and the seabed structure were investigated in the MET1-BH area using various hydroacoustic devices: multi-beam and splitbeam echosounders and a sub-bottom profiler. Geochemical analyses of sediment pore waters (CH4, Cl−, Br−, F−, SO42−, Ca2+, Mg2+, K+, Na+, ∑H2S, dP, dSi, NH4+, DIC, DOC) and microbiological analysis of sediments (16S rRNA) were performed. The content of CH4 and CO2 in the outflowing gas and its origin (δ13C-CH4 and δ2D-CH4) were determined. Hydroacoustic data showed that gas was emitted intensively from the inside of the structure. The height and intensity of the gas flares varied depending on the hydrostatic pressure. The gas contained 76.1% of CH4, 17.6% of CO2 and 0.39% of He. Methane source was microbial. Geophysical investigation revealed the presence of dislocations in sub-surface sediment layers in the MET1 region, which could have created a passage for groundwater and gas. Geochemical analyses pointed to intensive processes of organic matter decomposition in this area, active methanogenesis in the surface sediment layer, lack of the sulphate-methane transition, and freshwater seepage at a depth of ~88 m (bottom of the pockmark), probably from Upper Cretaceous deposits. The Prokaryota composition, atypical for marine surface sediments, resulted from the combination of freshwater and high organic matter content, and reflected active in situ methanogensis.

Sulfate-Methane Transition Depths and Its Implication for Gas Hydrate

The biological removal of CH4 by methanotrophic (CH4 -oxidizing) archaea always occurs at a distinct zone which is known as sulfate-methane transition zone (SMTZ). It is an important indication for high methane flux and gas hydrate occurrence. In this study, we collected pore-water data from Southern China Sea, Carolina Rise and Blake Ridge to analyze the relationship between CH4, SO42−concentration and depth. We found that below the SMTZ, the methane concentration increases continuously with depth and sulfate concentration decrease linearly to zero. In addition, the geochemical data taken from all these sites show that SMTZ is relatively shallow (less than 20m), which may indicate that these areas have high methane flux.

Microbial diversity in fracture and pore filling gas hydrate-bearing sediments at Site GMGS2-16 in the Pearl River Mouth Basin, the South China Sea

Gas hydrate systems are unique habitats for microorganisms in deep marine sediments, and gas hydrate reservoirs can be divided into focused high-flux and distributed low-flux gas hydrate systems. Little is known about the microbial distribution patterns in these systems, especially in the South China Sea (SCS). In this study, a macroscopic fracture filling gas hydrate zone (GHZ) and a microscopic pore filling GHZ, used as analogs to focused high-flux gas hydrate systems and distributed low-flux gas hydrate systems, were sampled and microbial diversity was investigated at Site GMGS2-16 in the northern SCS during the GMGS2 gas hydrate expedition based on high-throughput 16S rRNA gene sequencing. Compared to previous studies of the gas hydrate-bearing sites, high microbial diversity was observed in the sulfate methane transition zone (SMTZ) at Site GMGS2-16, and ANME-1b, a clade of anaerobic methanotrophic archaea (ANME), may mainly perform anaerobic oxidation of methane (AOM) in partnership with Desulfarculaceae and Desulfobacteraceae. For the GHZ (fracture-fill GHZ and pore-fill GHZ) and non-GHZ (SMTZ and Other, Other refers to the depth of 31.0-179.5mbsf), the bacterial richness indices (Shannon and inverse Simpson) revealed higher diversity in the GHZ than in the non-GHZ, while the archaeal richness indices showed a contrary pattern. Non-metric multidimensional scaling ordination revealed no significant differences in archaeal communities between the GHZ and non-GHZ, while differences between these zones were discovered in bacterial communities (P < .05), as the relative abundances of Bacilli and Clostridia (Phylum Firmicutes), Bacteroidia (Phylum Bacteroidetes), Alphaproteobacteria, Acidobacteria were significantly higher in the GHZ samples than in the non-GHZ samples. For the fracture-fill GHZ and pore-fill GHZ, the archaeal richness indices showed a higher diversity in the fracture-fill GHZ than in the pore-fill GHZ. High relative abundance of Hadesarchaea were found in the GHZ, especially in the pore-fill GHZ (accounting for 92.0–94.3% of the archaeal sequences), which might indicate Hadesarchaea play a vital role in the biogeochemical processes. For bacteria, the diversity indices were similar between the fracture-fill GHZ and the pore-fill GHZ, and no significant differences in bacterial communities were discovered between the two zones. Our study has provided new information for understanding the microbial diversity of the gas hydrate systems.

Synsedimentary to early diagenetic rejuvenation of barite-sulfides ore deposits: Example of the Triassic intrakarstic mineralization in the Lodève basin (France)

The exhumed Lodève Basin (Hérault, France) provides a rich suite of outcrops showing diagenetic Ba–Pb–Fe–Cu fronts trapped in karst system in Cambrian dolomites during the Triassic post-rift exhumation of the basin. The sedimentological analysis on 10 sites in the basin reveals that barites-sulfides fronts formed during humid-arid climate fluctuations and the emplacement of a shallow lake environment. The fabric of ore deposits, the microthermometry of fluid inclusions entrapped within barites and the strontium/sulfur isotopic compositions of barite-sulfides associations indicate two distinct groups of mineralizations, Type I and Type II, which are contemporaneous but resulting from different processes. The synsedimentary mineralization of the Type I, the presence of only primary single-phase liquid fluid inclusions within barite crystals and the gradual increase of δ34S for both barites and chalcopyrites with depth (from −7 to +18.9‰ V-CDT) suggest ore precipitation close to the vadose zone under bacterial sulfate reduction (BSR) in a confined sulfate-rich playa lake aquifer. The similar 87Sr/86S ratios between barites and the overlying Triassic evaporites indicate that the barium and strontium derived directly from the overlying sulfate-rich lake. For the Type II, the high homogenization temperature of fluid inclusions entrapped within barite (modal Th between 60 and 80 °C) and the association with hydrocarbon markers, confirm the participation of deeper basinal brines in addition to downward percolating sulfates derived from the lake environment. The high positive values of δ34S for both barites and sulfides are typical of a precipitation linked to the combined action between anaerobic oxidation of methane and sulfate reduction (AOM-SR) at the sulfate-methane transition zone (SMTZ) during hydrocarbon migration. Similar 87Sr/86Sr ratios between Middle Triassic barites and previous Late Permian barites confirm that the source of metals precipitated at the SMTZ originated from the dissolution of anterior ore deposits located in the sulfate-depleted zone. This study links very shallow metallogenesis processes to reworking of MVT ore deposits by the action of sulfate-reducing bacteria around hydrocarbon seeps in a karstic environment.

Impact of iron release by volcanic ash alteration on carbon cycling in sediments of the northern Hikurangi margin

We present geochemical data collected from volcanic ash-bearing sediments on the upper slope of the northern Hikurangi margin during the RV SONNE SO247 expedition in 2016. Gravity coring and seafloor drilling with the MARUM-MeBo200 allowed for collection of sediments down to 105 meters below seafloor (mbsf). Release of dissolved Sr2+ with isotopic composition enriched in 86Sr (87Sr/86Sr minimum = 0.708461 at 83.5 mbsf) is indicative of ash alteration. This reaction releases other cations in the 30-70 mbsf depth interval as reflected by maxima in pore-water Ca2+ and Ba2+ concentrations. In addition, we posit that Fe(III) in volcanogenic glass serves as an electron acceptor for methane oxidation, a reaction that releases Fe2+ measured in the pore fluids to a maximum concentration of 184 μM. Several lines of evidence support our proposed coupling of ash alteration with Fe-mediated anaerobic oxidation of methane (Fe-AOM) beneath the sulfate-methane transition (SMT), which lies at ∼7 mbsf at this site. In the ∼30-70 mbsf interval, we observe a concurrent increase in Fe2+ and a depletion of CH4 with a well-defined decrease in δ13C-CH4 values indicative of microbial fractionation of carbon. The negative excursions in δ13C values of both DIC and CH4 are similar to that observed by sulfate-driven AOM at low SO42− concentrations, and can only be explained by the microbially-mediated carbon isotope equilibration between CH4 and DIC. Mass balance considerations reveal that the iron cycled through the coupled ash alteration and AOM reactions is consumed as authigenic Fe-bearing minerals. This iron sink term derived from the mass balance is consistent with the amount of iron present as carbonate minerals, as estimated from sequential extraction analyses. Using a numerical modeling approach we estimate the rate of Fe-AOM to be on the order of 0.4 μmol cm−2 yr−1, which accounts for ∼12% of total CH4 removal in the sediments. Although not without uncertainties, the results presented reveal that Fe-AOM in ash-bearing sediments is significantly lower than the sulfate-driven CH4 consumption, which at this site is 3.0 μmol cm−2 yr−1. We highlight that Fe(III) in ash can potentially serve as an electron acceptor for methane oxidation in sulfate-depleted settings. This is relevant to our understanding of C-Fe cycling in the methanic zone that typically underlies the SMT and could be important in supporting the deep biosphere.

Butyrate Conversion by Sulfate-Reducing and Methanogenic Communities from Anoxic Sediments of Aarhus Bay, Denmark.

The conventional perception that the zone of sulfate reduction and methanogenesis are separated in high- and low-sulfate-containing marine sediments has recently been changed by studies demonstrating their co-occurrence in sediments. The presence of methanogens was linked to the presence of substrates that are not used by sulfate reducers. In the current study, we hypothesized that both groups can co-exist, consuming common substrates (H2 and/or acetate) in sediments. We enriched butyrate-degrading communities in sediment slurries originating from the sulfate, sulfate–methane transition, and methane zone of Aarhus Bay, Denmark. Sulfate was added at different concentrations (0, 3, 20 mM), and the slurries were incubated at 10 °C and 25 °C. During butyrate conversion, sulfate reduction and methanogenesis occurred simultaneously. The syntrophic butyrate degrader Syntrophomonas was enriched both in sulfate-amended and in sulfate-free slurries, indicating the occurrence of syntrophic conversions at both conditions. Archaeal community analysis revealed a dominance of Methanomicrobiaceae. The acetoclastic Methanosaetaceae reached high relative abundance in the absence of sulfate, while presence of acetoclastic Methanosarcinaceae was independent of the sulfate concentration, temperature, and the initial zone of the sediment. This study shows that there is no vertical separation of sulfate reducers, syntrophs, and methanogens in the sediment and that they all participate in the conversion of butyrate.

Dissolved Inorganic Carbon Pump in Methane-Charged Shallow Marine Sediments: State of the Art and New Model Perspectives

Methane transport from subsurface reservoirs to shallow marine sediment is characterized by unique biogeochemical interactions significant for ocean chemistry. Sulfate-Methane Transition Zone (SMTZ) is an important diagenetic front in the sediment column that quantitatively consumes the diffusive methane fluxes from deep methanogenic sources toward shallow marine sediments via sulfate-driven anaerobic oxidation of methane (AOM). Recent global compilation from diffusion-controlled marine settings suggests methane from below and sulfate from above fluxing into the SMTZ at an estimated rate of 3.8 Tmol yr-1 and 5.3 Tmol yr-1 respectively, and wider estimate for methane flux ranges from 1—19 Tmol yr-1. AOM converts the methane carbon to dissolved inorganic carbon (DIC) at the SMTZ. Organoclastic sulfate reduction (OSR) and deep-DIC fluxes from methanogenic zones contribute additional DIC to the shallow sediments. Here, we provide a quantification of 8.7 Tmol yr-1 DIC entering the methane-charged shallow sediments due to AOM, OSR, and the deep-DIC flux (range 6.4–10.2 Tmol yr-1). Of this total DIC pool, an estimated 6.5 Tmol yr-1 flows toward the water column (range: 3.2–9.2 Tmol yr-1), and 1.7 Tmol yr-1 enters the authigenic carbonate phases (range: 0.6–3.6 Tmol yr-1). This summary highlights that carbonate authigenesis in settings dominated by diffusive methane fluxes is a significant component of marine carbon burial, comparable to ~15% of carbonate accumulation on continental shelves and in the abyssal ocean, respectively. Further, the DIC outflux through the SMTZ is comparable to ~20% of global riverine DIC flux to oceans. This DIC outflux will contribute alkalinity or CO2 in different proportions to the water column, depending on the rates of authigenic carbonate precipitation and sulfide oxidation and will significantly impact ocean chemistry and potentially atmospheric CO2. Settings with substantial carbonate precipitation and sulfide oxidation at present are contributing CO2 and thus to ocean acidification. Our synthesis emphasizes the importance of SMTZ as not only a methane sink but also an important diagenetic front for global DIC cycling. We further underscore the need to incorporate a DIC pump in methane-charged shallow marine sediments to models for coastal and geologic carbon cycling.

Geochemical signatures of intense episodic anaerobic oxidation of methane in near-surface sediments of a recently discovered cold seep (Kveithola trough, NW Barents Sea)

At cold seep environments, the detection of large 34S enrichments within bulk sulphide minerals has been frequently used as a proxy to infer the vertical migrations of the sulphate-methane transition zone (SMTZ). Processes related to the oxidative part of the sulphur cycle can alter the original stable sulphur isotope composition (δ34S) of the solid sulphur phases, in particular at the sediment-water interface. Identification of paleo and present positions of methane fronts in the sedimentary records requires the use of multiple geochemical proxies. Authigenic enrichment of Mo has been demonstrated to represent a valid and durable tracer of the past SMTZ depth fluctuations and of the intensities of the related methane seepages. This study is based on two closely-spaced short cores collected from the Main Drift of the Kveithola trough, a glacially-carved depression located in northwestern Barents Sea. Active fluid seepage has been recently reported in this area and a gas flare was specifically observed in proximity of the sampling sites where the cores object of our study were retrieved. In order to collect evidences of the possible occurrence of anaerobic oxidation of methane (AOM) at the sediment-water interface of the investigated area and infer the entity of the associated methane flux, we combined the analyses of reduced sulphur species δ34S, total organic carbon and redox-sensitive elements. The negative δ34S values within the extracted solid sulphur phases (up to −49.1‰ for pyritic sulphur) show that organoclastic sulphate reduction (OSR) coupled with disproportionation of sulphur intermediates are the only active processes in the near-surface sediments. However, moderate to strong enrichments of Mo detected in the relatively organic carbon poor intervals of both cores and the lack of concurrent enrichments of Co, Cu, Ni, V and Zn, usually associated to OSR-dominated environment, suggested that the sulphidic conditions favouring the Mo enrichments were produced by AOM. Therefore, we infer that high methane flux events characterized the drift area of the Kveithola, occasionally moving upward the SMTZ and thus inducing intense AOM in proximity of the sediment-water interface of this part of the trough. Our results confirmed the validity of sedimentary Mo as an indicator of the occurrence and intensity of past methane seepages.

Influence of electron acceptor availability and microbial community structure on sedimentary methane oxidation in a boreal estuary

Methane is produced microbially in vast quantities in sediments throughout the world’s oceans. However, anaerobic oxidation of methane (AOM) provides a near-quantitative sink for the produced methane and is primarily responsible for preventing methane emissions from the oceans to the atmosphere. AOM is a complex microbial process that involves several different microbial groups and metabolic pathways. The role of different electron acceptors in AOM has been studied for decades, yet large uncertainties remain, especially in terms of understanding the processes in natural settings. This study reports whole-core incubation methane oxidation rates along an estuarine gradient ranging from near fresh water to brackish conditions, and investigates the potential role of different electron acceptors in AOM. Microbial community structure involved in different methane processes is also studied in the same estuarine system using high throughput sequencing tools. Methane oxidation in the sediments was active in three distinct depth layers throughout the studied transect, with total oxidation rates increasing seawards. We find extensive evidence of non-sulphate AOM throughout the transect. The highest absolute AOM rates were observed below the sulphate-methane transition zone (SMTZ), strongly implicating the role of alternative electron acceptors (most likely iron and manganese oxides). However, oxidation rates were ultimately limited by methane availability. ANME-2a/b were the most abundant microbial phyla associated with AOM throughout the study sites, followed by ANME-2d in much lower abundances. Similarly to oxidation rates, highest abundances of microbial groups commonly associated with AOM were found well below the SMTZ, further reinforcing the importance of non-sulphate AOM in this system.

Preservation of 34S-enriched sulfides in fossil sulfate-methane transition zones: new evidence from Miocene outcrops of the northern Apennines (Italy)

We provide new evidence of the preservation of 34S-enriched signals in methane seep-impacted sediments from two onshore Miocene outcrops located in the northern Apennines (Italy). Selected outcrops include methane-derived authigenic carbonates (MDAC) with δ13C composition between − 42.3 and − 18.2‰. MDACs contain chemosynthetic clams and abundant pyrite indicative of formation close to or within a shallow sulfate-methane transition zone (SMTZ). This study aims to evaluate the relative contributions of background organic matter mineralization and anaerobic oxidation of methane (AOM) to sulfate consumption and to gain insight into the transport process (i.e., diffusion, advection) controlling the depth of the SMTZ. Host sediments were investigated by CHN elemental analysis coupled with total sulfur (TS) and sulfur isotopic measurements on bulk samples. Total organic carbon (TOC) measurements reveal a consistent and low amount of organic carbon (TOC 0‰ and up to + 17.1‰), which is characteristic of sulfides precipitated in association with AOM. We propose that advection of methane-rich fluids was responsible for maintaining the shallow depth of the studied paleo-SMTZs. AOM at paleo-SMTZ positions in the investigated seep-impacted sediments resulted in excess bicarbonate and sulfide production, favoring solid-phase MDAC and iron sulfide precipitation.

Sulfidization processes in seasonally hypoxic shelf sediments: A study off the West coast of India

Continental shelves are the major sites for organic carbon burial and sulfate reduction. Organic carbon degradation and sulfidization along continental shelves are expected to play a significant role in global carbon-sulfur-iron (C–S–Fe) cycles. Here we report sediment pore-water chemistry and iron-sulfur speciation in two short cores SSK-42/9 and 10 collected off Malvan from the seasonally hypoxic shelf region off the west coast of India (WCI) at water depths of 30 and 13 m respectively. Concentration profiles of pore-water SO 4 2− , NH 4 + , DIC, ΣHS − and depth-integrated sulfate reduction rates (J SO4 ) suggest a variable influence of sedimentation rates, the composition of organic matter (marine and terrestrial) and anaerobic oxidation of methane (AOM) on the pore-fluid chemistry. Chromium reducible sulfur (CRS) and organic-bound sulfur (OBS) are the detectable solid-phase sulfur species in the studied sediment cores. Sulfur content and isotope ratios of chromium reducible sulfur (CRS) and organic-bound sulfur (OBS) phases produced via HS − /S x 2− pathways (sulfidization and sulfurization) show contrasting profiles in SSK42/9 and 10, apparently influenced by the availability of reactive iron, the relative significance of early and late diagenetic processes, source of OBS (detritus of marine origin and sulfurized organic molecules) and diffusion of ΣHS − produced via AOM across the SMTZ. The marked influence of AOM-driven sulfate reduction and the diffusion of isotopically enriched hydrogen sulfide across the sulfate-methane transition zone (SMTZ) is apparent from the δ 34 S CRS profiles in SSK-42/10. Sediment TOC/TS ratios recorded in this study are significantly less than that of average modern marine surface sediments (2.8:1) underlying oxygenated water and this is attributed to the enhanced sulfidization and burial of iron-sulfur minerals in the seasonally hypoxic regions. Since the inner shelf of WCI experiences seasonal alternation from normoxia to hypoxia which may have a profound influence on the benthic community structure, nature, and depth of bioturbation/bioirrigation, we propose that the impact of these processes need to be studied at a significantly higher resolution for better understanding C–S–Fe biogeochemical cycle. • The seasonally hypoxic shelf sediments play important role in C–S–Fe biogeochemical cycle. • Depth-integrated sulfate reduction rates off the west coast of India, show marked spatial variation. • δ 34 S CRS and δ 34 S OBS profiles show marked spatial variability and contrast with δ 34 HS .

Unique Authigenic Mineral Assemblages and Planktonic Foraminifera Reveal Dynamic Cold Seepage in the Southern South China Sea

Many cold seeps and gas hydrate areas have not been discovered beside the Beikan basin in the southern South China Sea (SCS), and their characteristics and histories also remain poorly known. Here we describe authigenic minerals and the carbon and oxygen isotopic composition of planktonic foraminifera Globigerinoides ruber from sediment core 2PC, recovered from the gas hydrate zone of the Nansha Trough, southern SCS, to elucidate its history of dynamic cold seepage. We infer that the occurrence of authigenic gypsum crystals and pyrite concretions, and anomalously negative δ13C values of Globigerinoides ruber, reflect paleo-methane seepage. Two major methane release events were identified, based on remarkable excursions in foraminifera δ13C at depths of 150–250 cm and 350–370 cm. Euhedral gypsum crystals and tubular pyrite concretions co-occur with extremely negative planktonic foraminifera δ13C values, indicating a shift in the sulfate methane transition zone and a change in the methane flux. Our data suggest that authigenic mineral assemblages and δ13C values of planktonic foraminifera provide a valuable tool in elucidating the characteristics of dynamic methane seepage in a marine environment.

A rock magnetic perspective of gas hydrate occurrences in a high-energy depositional system in the Krishna-Godavari basin, Bay of Bengal

We conducted a detailed rock magnetic study complemented by sedimentological and mineralogical methods on a 177.2-m-long sediment core of Hole NGHP-01-14A to constrain the influence of high-energy depositional environment on the magnetic mineral diagenesis and formation of gas hydrates in the Krishna-Godavari (K-G) basin, Bay of Bengal. Five sediment magnetic zones were identified based on the downcore variation in rock magnetic parameters. The magnetic mineralogies comprised of detrital titanomagnetite, diagenetically formed magnetic iron sulfides and their mixture. A distinct magnetically enhanced zone (Z-3) close to a bottom simulating reflector (BSR) is dominated by higher magnetite content, sediment bulk density, and grain size and probably represents several sand-rich sediment layers formed as a result of short-duration intense sedimentation events and got rapidly buried and provided conducive environment for the formation of gas hydrate deposits at the studied site. The concurrence of gas hydrates and high gas saturations within the sand-rich layers (Z-3) suggests that the high-energy depositional environment had a larger control over hydrate formation at Hole NGHP-01-14A. Two magnetic iron sulfide bearing sediment intervals in Z-1b and Z-1c below sulfate-methane transition zone (SMTZ) suggest that their formation is controlled by microbially mediated diagenetic reactions fuelled by presence of gas hydrates and probably represents the fossil gas hydrate zones. We propose that the rock magnetic variations in the studied sediment core at Hole NGHP-01-14A is controlled by both differential loading of detrital magnetic minerals as well as hydrate-induced late diagenetic magnetic iron sulfide formation.

A Quantitative Assessment of Methane-Derived Carbon Cycling at the Cold Seeps in the Northwestern South China Sea

Widespread cold seeps along continental margins are significant sources of dissolved carbon to the ocean water. However, little is known about the methane turnovers and possible impact of seepage on the bottom seawater at the cold seeps in the South China Sea (SCS). We present seafloor observation and porewater data of six push cores, one piston core and three boreholes as well as fifteen bottom-water samples collected from four cold seep areas in the northwestern SCS. The depths of the sulfate–methane transition zone (SMTZ) are generally shallow, ranging from ~7 to &lt;0.5 mbsf (meters below seafloor). Reaction-transport modelling results show that methane dynamics were highly variable due to the transport and dissolution of ascending gas. Dissolved methane is predominantly consumed by anaerobic oxidation of methane (AOM) at the SMTZ and trapped by gas hydrate formation below it, with depth-integrated AOM rates ranging from 59.0 and 591 mmol m−2 yr−1. The δ13C and Δ14C values of bottom-water dissolved inorganic carbon (DIC) suggest discharge of 13C- and 14C-depleted fossil carbon to the bottom water at the cold seep areas. Based on a two-endmember estimate, cold seeps fluids likely contribute 16–26% of the bottom seawater DIC and may have an impact on the long-term deep-sea carbon cycle. Our results reveal the methane-related carbon inventories are highly heterogeneous in the cold seep systems, which are probably dependent on the distances of the sampling sites to the seepage center. To our knowledge, this is the first quantitative study on the contribution of cold seep fluids to the bottom-water carbon reservoir of the SCS, and might help to understand the dynamics and the environmental impact of hydrocarbon seep in the SCS.