Anaerobic Oxidation Of Methane Zone Research Articles

Analysing seasonal variations of methane oxidation processes coupled with denitrification in a stratified lake using stable isotopes and numerical modeling

Since the discovery of nitrite (n-damo) and nitrate dependent anaerobic oxidation of methane in 2006, evidence has arisen about the occurrence of this process in freshwater ecosystems. Anaerobic oxidation of methane (AOM) linked with denitrification has been proposed to be a previously overlooked sink of methane in freshwater environments. However, the occurrence and relevance of AOM coupled with denitrification in methane removal and, therefore, mitigating methane emissions in the environment is so far largely unquantified.We investigated methane oxidation processes in the water column at the seasonally stratified lake Fohnsee located in Southern Germany to determine the seasonal dynamics and to quantitatively describe the contribution of anaerobic and aerobic oxidation of methane removing CH₄ from the water column throughout nearly one year. Vertical concentration profiles and corresponding stable isotope ratios of methane (δ13C) and nitrate (δ15N and δ18O), together with dissolved oxygen concentrations were measured, and a numerical model was developed to evaluate whether anaerobic oxidation of methane coupled with denitrification is a key biogeochemical process at lake Fohnsee. Our data set together with the results of the numerical model revealed a redox zone within the water column where both denitrification and anaerobic oxidation of methane are linked, contributing to approximately 70% of methane removal between June and September. Therefore, AOM linked with denitrification represents the dominant methane sink in the water column of lake Fohnsee during summer stratification.The data patterns also showed that the anaerobic oxidation of methane zone was located near the lake bottom in May, and moved upwards following the seasonal vertical displacement of the oxycline and the availability of nitrate. This redox dynamic within the water column of lake Fohnsee had also an effect on the relevance of aerobic and anaerobic oxidation of methane removing methane from the water column during the year and the formation of a bacterial sulfate reduction zone close to the lake sediments.

Formation of carbonate concretions in surface sediments of two mud mounds, offshore Costa Rica: a stable isotope study

The surface sediments of two mud mounds (‘‘Mound 11’’ and ‘‘Mound 12’’) offshore southwest Costa Rica contain abundant authigenic carbonate concretions dominated by high-Mg calcite (14–20 mol-% MgCO3). Pore fluid geochemical profiles (sulfate, sulfide, methane, alkalinity, Ca and Mg) indicate recent carbonate precipitation within the zone of anaerobic oxidation of methane (AOM) at variable depths. The current location of the authigenic carbonate concretions is, however, not related to the present location of the AOM zone, suggesting mineral precipitation under past geochemical conditions as well as changes in the flow rates of upward migrating fluids. Stable oxygen and carbon isotope analysis of authigenic carbonate concretions yielded d18Ocarbonate values ranging between 34.0 and 37.7 % Vienna standard mean ocean water (VSMOW) and d13Ccarbonate values from -52.2 to -14.2 % Vienna Pee Dee belemnite (VPDB). Assuming that no temperature changes occurred during mineral formation, the authigenic carbonate concretions have been formed at in situ temperature of 4–5 °C. The d18Ocarbonate values suggest mineral formation from seawater-derived pore fluid (d18Oporefluid = 0 % VSMOW) for Mound 12 carbonate concretions but also the presence of an emanating diagenetic fluid (d18Oporefluid &5 %) in Mound 11. A positive correlation between d13Ccarbonate and d18Ocarbonate is observed, indicating the admixing of two different sources of dissolved carbon and oxygen in the sediments of the two mounds. The carbon of these sources are (1) marine bicarbonate (d13Cporefluid &0 %) and (2) bicarbonate which formed during the AOM (d13Cporefluid &-70 %). Furthermore, the d18Oporefluid composition, with values up to ?4.7 % Vienna standard mean ocean water (VSMOW), is interpreted to be affected by the presence of emanating, freshened and boronenriched fluids. Earlier, it has been shown that the origin of 18O-enriched fluids are deep diagenetic processes as it was indicated by the presence of methane with thermogenic signature (d13CCH4 = -38 %). A combination of present geochemical data with geophysical observations indicates that Mounds 11 and 12 represent a single fluid system interconnected by deep-seated fault(s).

Anaerobic oxidation of methane in hypersaline cold seep sediments

Life in hypersaline environments is typically limited by bioenergetic constraints. Microbial activity at the thermodynamic edge, such as the anaerobic oxidation of methane (AOM) coupled to sulphate reduction (SR), is thus unlikely to thrive in these environments. In this study, carbon and sulphur cycling was investigated in the extremely hypersaline cold seep sediments of Mercator mud volcano. AOM activity was partially inhibited but still present at salinity levels of 292 g L(-1) (c. eightfold sea water concentration) with rates of 2.3 nmol cm(-3) day(-1) and was even detectable under saturated conditions. Methane and evaporite-derived sulphate comigrated in the ascending geofluids, which, in combination with a partial activity inhibition, resulted in AOM activity being spread over unusually wide depth intervals. Up to 79% of total cells in the AOM zone were identified by fluorescence in situ hybridization (FISH) as anaerobic methanotrophs of the ANME-1. Most ANME-1 cells formed monospecific chains without any attached partner. At all sites, AOM activity co-occurred with SR activity and sometimes significantly exceeded it. Possible causes of these unexpected results are discussed. This study demonstrates that in spite of a very low energy yield of AOM, microorganisms carrying this reaction can thrive in salinity up to halite saturation.

Geochemical paleogradient in pore waters controlled by AOM recorded in an Oligocene laminated limestone from the Outer Carpathians

A 20-cm thick laminated limestone of Oligocene age from the Outer Carpathians is composed mainly of coccoliths and authigenic calcite. δ 13C values unequivocally indicate that the authigenic calcite precipitated as a result of methane oxidation. The δ 13C and δ 18O values change gradually from the base to the top of the bed from − 38.0‰ to − 17.6‰ and from − 1.1‰ to − 5.7‰, respectively. This trend reflects a geochemical gradient which developed in the zone of anaerobic oxidation of methane (AOM) due to the consumption of sulfate by methane. This gradient was related to an inverse relationship between the concentrations of both compounds in the pore waters of the AOM zone. Methane was the source of isotopically light carbon. It was accompanied by heavy oxygen-containing water liberated from decomposition of gas hydrates. The methane-bearing fluids migrated upwards to the base of the sulfate reduction (SR) zone, where methane was oxidized (CH 4 + SO 4 2 − → HCO 3 − + HS − + H 2O) and the AOM zone developed. The bicarbonate produced in this reaction was significantly depleted in 13C and 16O. In the SR zone organic matter was oxidized by sulfate: 2CH 2O + SO 4 2 − → H 2S + 2HCO 3 −. The bicarbonate released in that reaction was the source of isotopically heavier C and lighter O. The proportion of bicarbonate produced by oxidation of methane to bicarbonate produced by oxidation of organic matter decreased upwards. The resultant isotopic composition of bicarbonate dissolved in the pore waters changed upwards towards higher δ 13C and lower δ 18O values and has been recorded in the limestone's isotopic composition.

Evidence for anaerobic oxidation of methane in sediments of a freshwater system (Lago di Cadagno)

Anaerobic oxidation of methane (AOM) has been investigated in sediments of a high alpine sulfate-rich lake. Hot spots of AOM could be identified based on geochemical and isotopic evidence. Very high fractionation of methane (α=1.031) during oxidation was observed in the uppermost sediment layers, where methane is oxidized most likely with sulfate-containing bottom waters. However, we could not exclude that other electron acceptors such as iron, or manganese might also be involved. Light carbon isotope values (δ¹³C = -10‰ vs. Vienna Pee Dee Belemnite [VPDB]) of sedimentary carbonates at 16-20 cm sediment depth are indicative of a zone where methane was oxidized and the resulting bicarbonate ions were used for carbonate precipitation. 16S rRNA gene analysis revealed the presence of sequences belonging to the marine benthic groups B, C, and D and to the recently described clade of AOM-associated archaea (AAA). Catalyzed reporter deposition-FISH analysis revealed a high abundance of Deltaproteobacteria, especially of free-living sulfate-reducing bacteria of the Desulfosarcina/Desulfococcus branch of Deltaproteobacteria in the AOM zone. Here, loose aggregations of AAA cells were found, suggesting that AAA might be responsible for oxidation of methane in Lake Cadagno sediments.

Pathways and regulation of carbon, sulfur and energy transfer in marine sediments overlying methane gas hydrates on the Opouawe Bank (New Zealand)

This study combines sediment geochemical analysis, in situ benthic lander deployments and numerical modeling to quantify the biogeochemical cycles of carbon and sulfur and the associated rates of Gibbs energy production at a novel methane seep. The benthic ecosystem is dominated by a dense population of tube-building ampharetid polychaetes and conspicuous microbial mats were unusually absent. A 1D numerical reaction-transport model, which allows for the explicit growth of sulfide and methane oxidizing microorganisms, was tuned to the geochemical data using a fluid advection velocity of 14 cm yr −1. The fluids provide a deep source of dissolved hydrogen sulfide and methane to the sediment with fluxes equal to 4.1 and 18.2 mmol m −2 d −1, respectively. Chemosynthetic biomass production in the subsurface sediment is estimated to be 2.8 mmol m −2 d −1 of C biomass. However, carbon and oxygen budgets indicate that chemosynthetic organisms living directly above or on the surface sediment have the potential to produce 12.3 mmol m −2 d −1 of C biomass. This autochthonous carbon source meets the ampharetid respiratory carbon demand of 23.2 mmol m −2 d −1 to within a factor of 2. By contrast, the contribution of photosynthetically-fixed carbon sources to ampharetid nutrition is minor (3.3 mmol m −2 d −1 of C). The data strongly suggest that mixing of labile autochthonous microbial detritus below the oxic layer sustains high measured rates of sulfate reduction in the uppermost 2 cm of the sulfidic sediment (100–200 nmol cm −3 d −1). Similar rates have been reported in the literature for other seeps, from which we conclude that autochthonous organic matter is an important substrate for sulfate reducing bacteria in these sediment layers. A system-scale energy budget based on the chemosynthetic reaction pathways reveals that up to 8.3 kJ m −2 d −1 or 96 mW m −2 of catabolic (Gibbs) energy is dissipated at the seep through oxidation reactions. The microorganisms mediating sulfide oxidation and anaerobic oxidation of methane (AOM) produce 95% and 2% of this energy flux, respectively. The low power output by AOM is due to strong bioenergetic constraints imposed on the reaction rate by the composition of the chemical environment. These constraints provide a high potential for dissolved methane efflux from the sediment (12.0 mmol m −2 d −1) and indicates a much lower efficiency of (dissolved) methane sequestration by AOM at seeps than considered previously. Nonetheless, AOM is able to consume a third of the ascending methane flux (5.9 mmol m −2 d −1 of CH 4) with a high efficiency of energy expenditure (35 mmol CH 4 kJ −1). It is further proposed that bioenergetic limitation of AOM provides an explanation for the non-zero sulfate concentrations below the AOM zone observed here and in other active and passive margin sediments.

Methane gas-phase dynamics in marine sediments: A model study

A one-dimensional reactive transport model including mass, momentum and volume conservation for the solid, aqueous, and gaseous phases is developed to explore the fate of free methane gas in marine sediments. The model assumes steady–state compaction for the solid phase in addition to decoupled gas and aqueous phase transport, instigated by processes such as buoyancy, externally impressed flows and compaction. Chemical species distributions are governed by gas advection, dissolved advection and diffusion as well as by reaction processes, which include organoclastic sulfate reduction, methanogenesis and anaerobic oxidation of methane (AOM). The model is applied to Eckernforde Bay, a shallow-water environment where acoustic profiles confirm a widespread occurrence of year-round biogenic free methane gas within the muddy regions of the sediment, and where subsurface methanogenesis, overlaid by a zone of AOM has been reported. The model results reveal that, under steady-state conditions, upward gas migration is an effective methane transport mechanism from oversaturated to undersaturated intervals of the sediment. Furthermore, sensitivity tests show that when methanogenesis rates increase, the gas flux to the AOM zone becomes progressively more important and may reach values comparable to those of the aqueous methane diffusive flux. Nevertheless, the model also proves that the gas transport rates always remain smaller than the removal rates by combined gaseous methane dissolution and oxidation. Consequently, for the range of environmental conditions investigated here, the AOM zone acts as an efficient subsurface barrier for both aqueous and gaseous methane, preventing methane escape from the sediments to the water column.

Anaerobic oxidation of methane (AOM) in marine sediments from the Skagerrak (Denmark): I. Geochemical and microbiological analyses

The organic rich sediments of the Skagerrak contain high quantities of shallow gas of mostly biogenic origin that is transported to the sediment surface by diffusion. The sulfate methane transition zone (SMTZ), where anaerobic oxidation of methane (AOM) and sulfate reduction occur, functions as a methane barrier for this upward diffusing methane.To investigate the regulation of AOM and sulfate reduction rates (SRR) and the controls on the efficiency of methane consumption, pore water concentrations, and microbial rates of AOM, sulfate reduction and methanogenesis were determined in three gravity cores collected along the slope of the Norwegian Trench in the Skagerrak. SRR occurred in two distinct peaks, at the sediment surface and the SMTZ, the latter often exceeding the peak AOM rates that occurred at the bottom of the SMTZ. Highest rates of both AOM and SRR were observed in a core from a pockmark, where advective methane transport occurred, generating high methane and sulfate fluxes. But even at this site with a shallow SMTZ, the entire flux of methane was oxidized below the sediment surface. AOM, SRR and methanogenesis seem to be closely associated and strongly regulated by sulfate concentrations, which were, in turn, regulated by the methane flux. Rate measurements of SRR, AOM and methanogenesis revealed a tight coupling of these processes. Bicarbonate-based methanogenesis occurred at moderate sulfate concentrations (>5mM) above the AOM zone but seemed to be inhibited in the depth where AOM occurred. The unbalanced stoichiometry of AOM and SRR in the SMTZ was more pronounced in rate measurements than in methane and sulfate fluxes, and seemed more likely be related to enhanced SRR in this zone than an underestimation of methane fluxes.

Diagenetic formation of greigite and pyrrhotite in gas hydrate marine sedimentary systems

Mineral magnetic results and electron microscope observations from gas hydrate-bearing marine sediments cored at southern Hydrate Ridge during Ocean Drilling Program Leg 204 (Sites 1244 to 1252, Cascadia Margin, offshore Oregon) demonstrate that authigenic greigite and pyrrhotite formed as a byproduct of microbially-mediated diagenetic reactions in the sulphate, the anaerobic oxidation of methane (AOM), and the methanic/gas hydrate zones. Geochemical conditions favourable for formation and preservation of greigite and pyrrhotite appear to be a limited source of sulphide, whether it derives from microbially-driven sulphate reduction in the sulphate zone, in the AOM zone or in deep sediments undergoing AOM, so that pyritization reactions are not driven to completion. Our results indicate that rock magnetic identification of greigite and pyrrhotite should be useful for detecting ancient gas hydrate systems in the marine sedimentary record, because it can enable rapid screening of ancient sediments for potential horizons where methane and disseminated gas hydrates might have occurred. Formation of authigenic greigite and pyrrhotite at different depths within the gas hydrate stability zone also implies that the magnetization of the host sediments will have been acquired at variable times, which is likely to compromise paleomagnetic results from greigite- and pyrrhotite-bearing marine sediments.

Methane emission and consumption at a North Sea gas seep (Tommeliten area)

Abstract. The Tommeliten seepage area is part of the Greater Ekofisk area, which is situated above the Tommeliten Delta salt diapir in the central North Sea (56°29.90' N, 2°59.80' E, Norwegian Block 1/9, 75 m water depth). Here, cracks in a buried marl horizon allow methane to migrate into overlying clay-silt and sandy sediments. Hydroacoustic sediment echosounding showed several venting spots coinciding with the apex of marl domes where methane is released into the water column and potentially to the atmosphere. In the vicinity of the gas seeps, sea floor observations showed small mats of giant sulphide-oxidizing bacteria above patches of black sediments as well as carbonate crusts, which are exposed 10 to 50 cm above seafloor forming small reefs. These Methane-Derived Authigenic Carbonates (MDACs) contain 13C-depleted, archaeal lipids indicating previous gas seepage and AOM activity. High amounts of sn2-hydroxyarchaeol relative to archaeol and low abundances of biphytanes in the crusts give evidence that ANaerobic MEthane-oxidising archaea (ANME) of the phylogenetic cluster ANME-2 were the potential mediators of Anaerobic Oxidation of Methane (AOM) at the time of carbonate formation. Small pieces of MDACs were also found subsurface at about 1.7 m sediment depth, associated with the AOM zone. This zone is characterized by elevated AOM and Sulphate Reduction (SR) rates, increased concentrations of 13C-depleted tetraether derived biphytanes, and specific bacterial Fatty Acids (FA). Further biomarker and 16S rDNA based analyses of this horizon give evidence that AOM is mediated by archaea belonging to the ANME-1b group and Sulphate Reducing Bacteria (SRB) most likely belonging to the Seep-SRB1 cluster. The zone of active methane consumption was restricted to a distinct horizon of about 20 cm. Concentrations of 13C-depleted lipid biomarkers (e.g. 500 ng g-dw−1 biphythanes, 140 ng g-dw−1 fatty acid ai-C15:0), cell numbers (1.5×108 cells cm−3), AOM and SR rates (3 nmol cm−3 d−1) in the Tommeliten AOM zone are 2–3 orders of magnitude lower compared to AOM zones of highly active deep water cold seeps such as Hydrate Ridge or the Gulf of Mexico.

Environmental control on anaerobic oxidation of methane in the gassy sediments of Eckernförde Bay (German Baltic)

We investigated the effect of seasonal environmental changes on the rate and distribution of anaerobic oxidation of methane (AOM) in Eckernforde Bay sediments (German Baltic Sea) and identified organisms that are likely to be involved in the process. Surface sediments were sampled during September and March. Field rates of AOM and sulfate reduction (SR) were measured with radiotracer methods. Additional parameters were determined that potentially influence AOM, i.e., temperature, salinity, methane, sulfate, and chlorophyll a. Methanogenesis as well as potential rates of AOM and aerobic oxidation of methane were measured in vitro. AOM changed seasonally within the upper 20 cm of the sediment, with rates being between 1 and 14 nmol cm(-3) d(-1). Its distribution is suggested to be controlled by oxygen and sulfate penetration, temperature, as well as methane supply, leading to a shallow AOM zone during the warm productive season and to a slightly deeper AOM zone during the cold winter season. Rising methane bubbles apparently fed AOM above the sulfate-methane transition. Methanosarcinales-related anaerobic methanotrophs (ANME-2), identified with fluorescence in situ hybridization, is suggested to mediate AOM in Eckerntorde Bay. These archaea are known also from other marine methane-rich locations. However, they were not directly associated with sulfate-reducing bacteria. AOM is possibly mediated solely by these archaea that show a mesophilic physiology according to the seasonal temperature changes in Eckernforde Bay.