Sulfate Concentration Profiles Research Articles

The effects of organic matter and anaerobic oxidation of methane on the microbial sulfate reduction in cold seeps

Cold seep sediments are dominated by intensive microbial sulfate reduction coupled to anaerobic oxidation of methane. However, the contribution proportion between this process and the role of organic matter has remained enigmatic. Here, pore water data combined with PROFILE model, fluxes of sulfate and methane concentration calculated from Fick's first law, and δ34SSO4 and δ18OSO4 of pore water sulfate were studied to reconstruct co-occurring microbial organoclastic sulfate reduction and anaerobic oxidation of methane coupled with sulfate reduction in methane seep sediments collected from South China Sea. The sulfate concentration profiles of C9 and C14 in Qiongdongnan Basin generally show quasilinear depletion with depth. Reaction-transport modeling provided close fits to concentration data. δ18OSO4 and δ34SSO4 increase fastest with sediment depth above 400 cmbsf and slowest below that depth. The values of methane flux are always lower than those of total sulfate reduction of sulfate diffusive flux at GC-10, GC-9, GC-11 and HD319 sites in Taixinan Basin. Besides, positions of sulfate methane transition zone in all study sites are approximately ~400 to 800 centimeters below seafloor. These results showed that microbial sulfate reduction in sediments is mainly controlled by intense anaerobic oxidation of methane, but there is a certain relationship with organic matter metabolism process. This emphasizes that traditional redox order of bacterial respiration is highly simplified, where, in sediments such as these seeps, all of these microbial sulfate reduction processes can occur together with complex couplings between them.

Development of a predictive kinetic model with statistically analyzed parameters for Donnan Dialysis process

Donnan Dialysis (DD) utilizes ion exchange membranes that allow the selective transport of target ions from a feed solution to a concentrated receiver solution. The kinetics of DD involves boundary layer diffusion (BLD), diffusional membrane transport (MD), and a mixed regime where both mechanisms are active. The objective of the research was to develop an automated computational model that can predict nitrate, bicarbonate, and sulfate concentration profiles and transport mechanisms in DD processes. Experiments were conducted in a batch dialyzer in sole- and multi-component feed solutions and NaCl receiver solutions.Training of the model included numerical extraction of the BLD and MD coefficients from the experimental data; determination of the statistically significant parameters that affect the DD transport; development of correlations between these parameters and the kinetic coefficients; and mapping of the ratio between the boundary layer resistance and the membrane resistance to determine the transport mechanism.The predictive model, included the kinetic equations, i.e., the Nernst-Planck formulation, mass balances, and electro-neutrality, the correlations, and the mapping of the transport mechanism. The model was verified by the experimental data used for the model training and validated by independent datasets. Very good fits between the predicted and experimental concentration profiles were obtained, and the ability to determine the transport mechanism was demonstrated.

Physicochemical Modeling of Barium and Sulfate Transport in Porous Media and Its Application in Seawater-Breakthrough Monitoring

SummarySeawater injection is widely used to improve oil recovery in offshore oil reservoirs. However, injecting seawater into reservoirs can cause many flow-assurance issues, such as scaling and reservoir souring, which are strongly related to the percentage of seawater breakthrough. Thermodynamic models have been developed to evaluate the effects of barite deposition on oil production, but the reservoir stripping effect has not been fully considered. In this study, a new model that incorporates both chemical reaction (barium and sulfate reaction) and physical reactions (ion adsorption/desorption) is developed to investigate the in-situ barite-deposition process. To the best of our knowledge, for the first time, ion adsorption/desorption is integrated by coupling the adsorption/desorption isotherm to the reservoir simulator. The barium and sulfate chemical reaction is modeled by incorporating the solubility product constant into the model. The model accuracy is verified through convergence rate tests and comparison with the coreflood experimental results. The simulation results of both barium and sulfate concentration profiles are greatly improved by integrating the ion adsorption/desorption process. The new physicochemical model is further used to investigate barite deposition under various scenarios. Simulation results indicate that most barite deposits are in the deep reservoir for the areal model. Barite that deposits in the reservoir before seawater breakthrough accounts for 45% of total barite deposition and the barite deposited during the seawater-breakthrough period makes up 54%, while the deposition during the tailing period, where the seawater fraction is larger than 95%, is negligible. For a homogeneous reservoir, the barite-deposition period at the near-wellbore area of the producer is between 30% and 65% of the seawater-breakthrough percentage, and heterogeneity leads to a broader deposition period. For vertical heterogeneous reservoirs, a considerable amount of barite forms in the wellbore, which accounts for 17% of total barite deposition. Based on the accurate simulation of barium and sulfate transport in the reservoir, barium and sulfate concentration profiles can be used to determine the seawater-breakthrough percentage and help optimize production operations that aim to mitigate flow assurance issues.

Sedimentary geochemical proxies for methane seepage at Site C14 in the Qiongdongnan Basin in the northern South China Sea

Recent studies have shown that specific geochemical characteristics of sediments can be used to reconstruct past methane seepage events. In this work, the correlation between the Sr/Ca and Mg/Ca ratios of sediment samples is analyzed and the sulfate concentration profile in Site C14 from cold-seep sediments in the Qiongdongnan Basin in northern South China Sea is obtained. The results confirmed that, sulfate at 0-247 cm below sea floor (Unit I) is mainly consumed by organic matter sulfate reduction (OSR), while sulfate at 247-655 cm (Unit II) is consumed by both the OSR and the anaerobic oxidation of methane (AOM). In addition, the bottom sediment layer is affected by weak methane seepage. The Mo and U enrichment factors also exhibit similar trends in their respective depth profiles. The responses of trace elements, including Co/Al, Ni/Al, Cr/Al and Zn/Al ratios to methane seepage allowed the study of depositional conditions and methane seepage events. Based on the results, it is speculated that the depositional conditions of Unit II changed with depth from moderate conditions of sulfidic and oxic conditions to locally anoxic conditions, and finally to suboxic conditions due to methane fluid leakage. The stable isotope values of chromium-reducible sulfide produced by AOM and those of sulfide formed by OSR in the early diagenetic environment suffered serious depletion of 34S. This was probably due to weak methane leakage, which caused the slower upward diffusion and the effect of early diagenesis on the samples. It is necessary to consider the effects of depositional environments and diagenesis on these geochemical parameters.

The influence of methane fluxes on the sulfate/methane interface in sediments from the Rio Grande Cone Gas Hydrate Province, southern Brazil

ABSTRACT: Much research has been published regarding the relation between major gas hydrate accumulations and the global carbon cycle. In this context, the determination of the sulfate/methane interface (SMI) depth is of primary importance in order to understand the dynamics of methane flux in the shallow section. This paper identifies the depth of the SMI in sediments based on sulfate and methane concentration profiles in cores recovered in the Rio Grande Cone Gas Hydrate Province, Pelotas Basin, southern Brazil. The shape of methane and sulfate concentration profiles in the sediments can be linked to the local methane flux rate as follows: (i) near linear, high upward-diffusing methane flux coupled with high sulfate diffusion from seawater; (ii) irregular, variable methane flux rates; and (iii) kink-type profile, which is indicative of variable rather than strictly high upward methane flux. The areas in which a high methane flux was identified are spatially associated with gas chimneys in sediments within pockmarks, whereas profiles with low methane flux are present in adjacent areas. These chimneys appear as acoustic blankings in seismic records and can therefore be mapped in subsurface. The wavy-like seismic reflection following the SMI coincides with the occurrence of authigenic carbonate nodules and concretions. In addition, high methane fluxes and the occurrence of concretions and nodules carbonates were correlated by stratigraphic position of the concretions bearing intervals and sulfate profiles.

External sulfate attack of cementitious materials: New insights gained through numerical modeling including dissolution/precipitation kinetics and surface complexation

In this study, a new physically and chemically coupled model taking into account multiionic diffusion, precipitation/dissolution kinetics and surface complexation is proposed to predict the ingress of sulfate ions into saturated cementitious materials. The results are compared to experimental data and numerical profiles that were previously obtained on the total sulfate concentration profile in the material. The results show good agreement between experimental and numerical results. They also show that surface complexation has to be taken into account in order to reproduce the sulfate quantity peak experimentally observed. In addition, dissolution and precipitation kinetics are indispensable to retrieve the sulfate quantity amplitude and still more the slow decay of the sulfate quantity after the peak. Finally, the model may explain the supersaturation of the sulfate concentration with respect to ettringite because of the dissolution of monosulfoaluminate and ettringite precipitation kinetics. Moreover, it shows that the supersaturation increases with the concentration of the aggressive sulfate solution without significant increase in the total quantity of sulfate in the material. These results are coherent with the crystallization pressure theory.

Acquisition of Anion Profiles and Diffusion Coefficients in the Opalinus Clay at the Mont Terri Rock Laboratory (Switzerland)

Chloride, bromide and sulphate concentration profiles have been analysed through the Opalinus Clay of Mont Terri in the framework of the Deep Borehole Experiment. Aqueous leaching and out diffusion experiments were carried out to acquire anion concentrations and estimate pore diffusion coefficients. Out diffusion technique gave consistent values of chloride and bromide compared to the concentration profiles acquired so far at the tunnel level of the Mont Terri rock laboratory. Concentrations acquired by leaching experiments show a maximum chloride concentration of 16.1 ± 1.7g/l at the basal part of Opalinus Clay which is higher than the value of 14.4 ± 1.4g/l obtained by out diffusion at the same level. This excess of chloride is likely due to dissolution of Cl− bearing minerals or release of Cl initially contained in unaccessible porosity. Bromide to chloride ratios are virtually the same as that of seawater, whereas sulphate to chloride ratios are significantly higher. Those latter are probably due to pyrite oxidation and dissolution of sulphate-bearing minerals occurring during sample collection and preparation. An anisotropy ratio of 2.4 was estimated for pore diffusion coefficient in the Opalinus Clay sandy facies.

Diffusion-reaction model of stochastically mixed sulfate in cast-in-situ piles

This study investigated the diffusion-reaction of sulfate in a cast-in-situ pile after different serving times. The diffusion equation was established based on Fick’s second law, and the model was established via the method of separation of variables by considering the appropriate initial and boundary conditions. An example of the diffusion-reaction of sulfate was compared to a traditional model. Additionally, the validity and advantages of the proposed method were verified. The results of this study showed that the discrepancy of the sulfate concentration profile increases as the radius of a cast-in-situ pile decrease. Additionally, the discrepancy was found to increase over time, which indicated that the diffusion-reaction of sulfate in a cast-in-situ pile has the characteristics of time and size effect. The initial concentration, gradient and reaction rate were shown to have significant effects on the diffusion-reaction of sulfate. The variations in the sulfate concentration profile were found to be larger at locations of high concentration and surface of pile. Thus, a higher concentration of sulfate tended to produce more sulfate consumption. Conversely, a decrease in the sulfate gradient led to a decrease in sulfate consumption proportion, even in the presence of a higher concentration of sulfate. A faster kinetic rate of chemical reactions was also found to produce higher consumption rates of sulfate, and the concentration gradient decreased concurrently.

Modeling diffusion of sulfate through concrete using mixture theory

Concrete exposed to sulfates deteriorates. Here, an attempt is made to see whether the framework of mixture theory can be used to model the changes that occur in concrete exposed to sodium sulfate. Toward this, diffusion and reaction of sulfates with concrete is modeled within the framework of mixture theory. Appropriate choices are made for the Helmholtz free energy and interaction momentum so that the process of diffusion of sulfates in concrete can be captured. As expected in mixture theory, diffusion causes deformation of the solid. The parameters in the mixture theory model are determined by comparing the steady-state concentration profile of the diffusing sodium sulfate solution and inlet velocity of the fluid with that of Fick’s solution for the same boundary conditions. An assumption is made for the mass production term to capture the reaction of sulfates with concrete. The rate constant and order of the reaction are estimated using the concentration of gypsum, calcium hydroxide and ettringite reported in the literature, for various duration of exposure of cement pastes to known concentration of sodium sulfate solution. Finally, the governing equations for the combined problem of steady-state diffusion and reaction of sulfates with concrete are presented. A numerical scheme to solve the governing equations is outlined. Long-term concentration profiles of sodium sulfate predicted by the framework agree qualitatively with the experimentally observed profile reported in the literature.

Determination of Salt Transport Properties of Sandstone: A Combined Experimental/Computational Approach

A combined experimental/computational approach is applied for the determination of material parameters describing the transport of sulphate solution in porous structure of sandstone. The investigated sandstone, frequently used in historical buildings on the Czech territory, is chosen as a typical representative of stone masonry materials. At first, the moisture and sulphate concentration profiles in simulated 1-D water and sulphate solution transport experiment are measured. Here, the moisture content is determined by the gravimetric method, the sulphate concentration is measured by the ion chromatography. As a penetrating medium, 1M Na2SO4 water solution is used. The sulphate binding isotherm is experimentally accessed using the adsorption method. The experimental data are subjected to an inverse analysis assuming a diffusion-advection mode of sulphate transport. Finally, the moisture diffusivity for transport of Na2SO4 water solution, and the sulphate diffusion coefficient are calculated as the functions of moisture content and sulphate concentration, respectively.

Pore water geochemistry at two seismogenic areas in the Sea of Marmara

AbstractWithin the Sea of Marmara, the highly active North Anatolian Fault (NAF) is responsible for major earthquakes (Mw ≥ 7), and acts as a pathway for fluid migration from deep sources to the seafloor. This work reports on pore water geochemistry from three sediment cores collected in the Gulfs of Izmit and Gemlik, along the Northern and the Middle strands of the NAF, respectively. The resulting data set shows that anaerobic oxidation of methane (AOM) is the major process responsible for sulfate depletion in the shallow sediment. In the Gulf of Gemlik, depth concentration profiles of both sulfate and alkalinity exhibit a kink‐type profile. The Sulfate Methane Transition Zone (SMTZ) is located at moderate depth in the area. In the Gulf of Izmit, the low concentrations observed near the seawater‐sediment interface for sulfate, calcium, strontium, and magnesium result from rapid geochemical processes, AOM, and carbonate precipitation, occurring in the uppermost part of the sedimentary column and sustained by free methane accumulation. Barite dissolution and carbonate recrystallization have also been identified at deeper depth at the easternmost basin of the Gulf of Izmit. This is supported by the profile of the strontium isotope ratios (87Sr/86Sr) as a function of depth which exhibits negative anomalies compared to the modern seawater value. The strontium isotopic signature also shows that these carbonates had precipitated during the reconnection of the Sea of Marmara with the Mediterranean Sea. Finally, a first attempt to interpret the sulfate profiles observed in the light of the seismic activity at both sites is presented. We propose the hypothesis that seismic activity in the areas is responsible for the transient sulfate profile, and that the very shallow SMTZ depths observed in the Gulf of Izmit is likely due to episodic release of significant amount of methane.

Enhanced methane flux event and sediment dispersal pattern in the Krishna–Godavari offshore basin: Evidences from rock magnetic techniques

Detailed rock magnetic measurements such as susceptibility (χ), susceptibility of anhysteric remanent magnetization (ΧARM), saturation isothermal remanent magnetization (SIRM), soft-isothermal remanent magnetization (soft-IRM) and hard-IRM (HIRM) were carried out on the topmost sediment samples of 73 gravity cores collected in the Krishna–Godavari (KG) offshore basin to understand the distribution of magnetic parameters and the associated diagenetic signatures related to upward methane flux. The variation in magnetic parameters in the KG offshore basin fluctuates around the average value (Χ = 105 × 10−8 m3 kg−1, ΧARM = 1.22 × 10−5 m3 kg−1, SIRM = 2040 × 10−5 Am2 kg−1, soft-IRM = 1807 and HIRM = 82) except at the location GC-07 (15.767°N, 81.814°E) where the magnetic parameters Χ, ΧARM and SIRM drop to ∼1/6th, ∼1/14th and ∼1/8th of the average near surface value indicating substantial loss in the concentration of primary magnetic iron oxide minerals. Anomalous low value of ΧARM/Χ and ΧARM/SIRM at GC-07 suggests that the magnetic remanence property is governed by coarser pseudo-single domain (PSD) or multi-domain (MD) grains. To understand the nature of the anomalous magnetic parameters at GC-07, complete downcore variation of magnetic parameters was also studied. The magnetic parameters were found to be even more depleted at all depths with susceptibility dropping (at 5.2 mbsf) to ∼1/200th of its average near surface value. Interestingly, this core shows a rapid increase in the concentration of dissolved methane from ∼0 nM at surface to ∼153 nM at around 5.2 mbsf where the sulfate concentration is also reduced to zero. The anaerobic oxidation of methane (AOM) and the sulfate methane transition zone (SMTZ) occur between ∼4.2 and 5.2 mbsf based on the present day sulfate concentration profile. The close association between the dissolution of magnetic iron oxide minerals and AOM at GC-07 suggests that the reduced magnetization is primarily due to the release of HS− by AOM. The magnetic iron oxide minerals are dissolved not only at SMTZ but also above the current depth of the SMTZ. It is likely that the methane flux was higher in the past leading to upward shift of the SMTZ, and the HS− released by AOM facilitated the dissolution of magnetic iron oxide minerals at shallower depths. Alternatively, the HS− diffusing upward from the current depth of SMTZ might have lead to dissolution at shallower depth. Therefore, additional geological/geochemical investigations are warranted at GC-07 to establish the dissolution mechanism of magnetic iron oxide minerals.The variability in different magnetic parameters within the slope basin and channels shows systematic decrease in the concentration of magnetic minerals and increase in the abundance of stable single domain (SSD) magnetic grains with water depth. The observed strong correlation between the silt fraction and the magnetic susceptibility for the slope basin and channels suggests that the variation in magnetic parameters is predominantly controlled by the sediment sorting/transport process as finer particles are deposited in deeper water.

Pore-water chemistry of sediment cores off Mahanadi Basin, Bay of Bengal: Possible link to deep seated methane hydrate deposit

Organoclastic degradation and anaerobic oxidation of methane (AOM) are known to influence sulfate, methane and bicarbonate concentrations of sediment pore-waters. In this work we have analyzed the pore-fluid composition of three piston cores (36–39 m long) collected from water depths ranging from 1429 to 1691 m. We attribute the observed linear sulfate concentration profiles and (ΔCa + ΔAlk)/ΔSO42− ratios dominantly to anaerobic oxidation of methane (AOM) induced sulfate consumption. The gas rich layers just below the base of hydrate stability zone (BGHSZ) is the possible source of the enhanced diffusive flux of biogenic methane (δ13CCH4 ranging from −99.7 to −106.3‰ VPDB). Occurrences of disseminated gas hydrate in the Mahanadi Basin within the hydrate stability zone (210–220 m thick) supports the link between linear sulfate profile, high rate of AOM and hydrate occurrence.

Gene expression in the deep biosphere

Scientific ocean drilling has revealed a deep biosphere of widespread microbial life in sub-seafloor sediment. Microbial metabolism in the marine subsurface probably has an important role in global biogeochemical cycles, but deep biosphere activities are not well understood. Here we describe and analyse the first sub-seafloor metatranscriptomes from anaerobic Peru Margin sediment up to 159 metres below the sea floor, represented by over 1 billion complementary DNA (cDNA) sequence reads. Anaerobic metabolism of amino acids, carbohydrates and lipids seem to be the dominant metabolic processes, and profiles of dissimilatory sulfite reductase (dsr) transcripts are consistent with pore-water sulphate concentration profiles. Moreover, transcripts involved in cell division increase as a function of microbial cell concentration, indicating that increases in sub-seafloor microbial abundance are a function of cell division across all three domains of life. These data support calculations and models of sub-seafloor microbial metabolism and represent the first holistic picture of deep biosphere activities.

The evolution of early diagenetic signals in Bering Sea subseafloor sediments in response to varying organic carbon deposition over the last 4.3 Ma

Transient pore-water and solid-phase signatures in deep subseafloor marine sediments, resulting from changes in both the amount and the quality of the organic matter input, are common but often difficult to interpret. We combined high-resolution pore-water and solid-phase data from Integrated Ocean Drilling Program (IODP) Expedition 323 Site U1341 (Bowers Ridge, Bering Sea) with inverse reaction-transport modeling to examine the evolution and potential preservation of diagenetic signals in these deep subseafloor sediments. We explore how these signals reflect major changes in the deposition and reactivity of organic matter to the seafloor at Bowers Ridge. Results of the inverse model approach reveal that 2.51–2.58Ma ago a high deposition flux of extremely labile organic matter, probably linked to increased surface water primary productivity, affected this site. Associated elevated organoclastic sulfate reduction rates facilitated low sulfate concentrations, the onset of methanogenesis, and consequently sulfate reduction coupled to the anaerobic oxidation of methane (AOM). Sulfate depletion caused the dissolution of biogenic barite reflected by a sedimentary interval with low Ba/Al ratios. Two sulfate–methane transition zones (SMTZs) evolved where high rates of AOM controlled sulfate consumption which was sustained by the influx of sulfate from seawater above and a deep source below. The positions of both SMTZs shifted non-synchronously over the subsequent ∼130,000yrs, until methanogenesis and AOM declined. The present-day sulfate concentration and sulfur isotope profiles still reflect the impact of the reactive organic matter pulse. They also record a period of very low reactive organic matter deposition during the middle to late Pleistocene, probably linked to very low primary productivity, resulting in little microbial carbon turnover in the sediment. Our study shows that combining biogeochemical signatures recorded in the solid-phase of deep subseafloor sediments with the analysis of transient pore-water signals by inverse reaction-transport modeling yields new insights into past deep biosphere processes and the paleoproductivity of marine basins.

Resolving Cl and SO4 Profiles in a Clay-Rich Rock Sequence

The chloride and sulfate concentration profiles in a 260 m thick clay-rich Mesozoic sediment sequence have been analyzed by various methods. Chloride data generally indicate a good consistency between different methods if anion exclusion is accounted for in leaching tests. For sulfate, however, there is an apparent inconsistency between leaching data and those obtained from the other methods, which points to the dissolution of a sulfur-bearing mineral. Traces of diagenetic gypsum seem to be a likely source, but other sulfur minerals cannot be ruled out.

Control of sulphate and methane distributions in marine sediments by organic matter reactivity

The reactivity of organic matter serving as fuel for microbial activity in buried sedimentary layers imposes a strong control on the subsurface biogeochemical zonation and rates of metabolism. To understand the consequences of different organic matter degradation rates on subsurface microbial activity and sediment pore water chemistry, sulphate and methane concentration profiles were simulated using different organic matter decay models from the literature. Results show how the decay of a more reactive pool of organic matter results in a more curved sulphate profile and deeper sulphate methane transition (SMT) with decreasing apparent initial age of the organic matter. A minimum in SMT is reached if the initial age is intermediate, commonly in the order of 104–105 years. If the initial age is very large the sulphate profile is almost linear and the sulphate methane transition is located deeper with increasing apparent initial age of the organic matter. Curved versus linear shaped sulphate profiles reflect different relative contributions of organoclastic sulphate reduction versus anaerobic methane oxidation to the total consumption of sulphate. Our model results demonstrate that the SMT can move up or down depending on organic matter reactivity and its depth is not necessarily diagnostic of total organic carbon flux. The model provides a mechanistic understanding of how the shape of the sulphate profiles and the relative contributions of organoclastic sulphate reduction versus anaerobic oxidation of methane are controlled by the kinetics of organic matter degradation.

Geochemical and geological constraints on the composition of marine sediment pore fluid: Possible link to gas hydrate deposits

Pore water sulfate consumption in marine sediments is controlled by microbially driven sulfate reduction via organo-clastic and methane oxidation processes. In this work, we present sediment pore fluid compositions of 10 long sediment cores and high resolution seismic data from the Krishna–Godavari (K–G) basin, Bay of Bengal. Our results show occurrence of transient (S and kink types) and steady state (quasi-linear) sulfate concentration profiles which are attributed partly to the anaerobic oxidation of methane (δ13CCH4: −84.8 to −100.1‰ VPDB) and organo-clastic sulfate reduction. Influence of AOM on alkalinity is evident from the presence of authigenic carbonate layers with highly depleted carbon isotope ratios in core MD161-8. The authigenic carbonates represent the paleo-SMTZs and suggest marked fluctuation in vertical methane flux. Our geophysical data show the acoustic signatures of upward fluid migration from shallow sub-surface, whereas, coring during NGHP expedition-01 confirms the presence of sub-surface gas hydrate deposits in K–G basin which can be linked to deep methane sources. The geochemical analysis suggests that shallow methane source can be attributed to high burial flux of labile organic matter due to high sedimentation rate. Sampling sites with high methane flux from the shallow gas source are characterized by quasi-linear sulfate concentration profile and a shallow sulfate methane transition zone (SMTZ) and may not be necessarily linked to deeper gas hydrate deposits. In contrast, the deep methane source results in a transient kink type sulfate profile and a deeper SMTZ. We have observed a close link between the occurrence of gas hydrate and the S/kink type sulfate profile. We interpret the short lived ‘kink’ in the sulfate profiles as a result of recent enhancement in vertical methane flux possibly driven by reactivation of fault-fractures systems which provide the conduits for fluid flow.

Anaerobic oxidation of methane at different temperature regimes in Guaymas Basin hydrothermal sediments

Anaerobic oxidation of methane (AOM) was investigated in hydrothermal sediments of Guaymas Basin based on δ(13)C signatures of CH(4), dissolved inorganic carbon and porewater concentration profiles of CH(4) and sulfate. Cool, warm and hot in-situ temperature regimes (15-20 °C, 30-35 °C and 70-95 °C) were selected from hydrothermal locations in Guaymas Basin to compare AOM geochemistry and 16S ribosomal RNA (rRNA), mcrA and dsrAB genes of the microbial communities. 16S rRNA gene clone libraries from the cool and hot AOM cores yielded similar archaeal types such as Miscellaneous Crenarchaeotal Group, Thermoproteales and anaerobic methane-oxidizing archaea (ANME)-1; some of the ANME-1 archaea formed a separate 16S rRNA lineage that at present seems to be limited to Guaymas Basin. Congruent results were obtained by mcrA gene analysis. The warm AOM core, chemically distinct by lower porewater sulfide concentrations, hosted a different archaeal community dominated by the two deep subsurface archaeal lineages Marine Benthic Group D and Marine Benthic Group B, and by members of the Methanosarcinales including ANME-2 archaea. This distinct composition of the methane-cycling archaeal community in the warm AOM core was confirmed by mcrA gene analysis. Functional genes of sulfate-reducing bacteria and archaea, dsrAB, showed more overlap between all cores, regardless of the core temperature. 16S rRNA gene clone libraries with Euryarchaeota-specific primers detected members of the Archaeoglobus clade in the cool and hot cores. A V6-tag high-throughput sequencing survey generally supported the clone library results while providing high-resolution detail on archaeal and bacterial community structure. These results indicate that AOM and the responsible archaeal communities persist over a wide temperature range.

Quantifying rates of methanogenesis and methanotrophy in Lake Kinneret sediments (Israel) using pore‐water profiles

Full seasonal sets of chemical and isotope profiles from the pore water of Lake Kinneret (Sea of Galilee, Israel) were produced to study methanogenesis and methanotrophy processes and the couplings between methane (CH4), sulfur, and iron. Sulfate is depleted within the upper 10 cm of the sediment mainly by traditional bacterial sulfate reduction by organic matter. Maximum sulfate reduction rates calculated from sulfate concentration profiles are found at the water–sediment interface (0–1 cm − 1.4 × 10−12 ± 0.2 × 10−12 mol cm−3 s−1). CH4 concentrations and modeling of dissolved inorganic carbon (DIC) and its stable carbon isotope (δ13CDIC) suggest that maximum methanogenesis rates of 2.5 × 10−13 ± 1.5 × 10−13 mol cm−3 s−1 occur at 5–12‐cm depth in the sediments, and that it ends at 20 cm. Of the produced CH4, 50–75% is converted to gas bubbles of CH4 before it reaches the bottom water. Model results suggest the occurrence of anaerobic oxidation of CH4 (AOM) in the deep sediments of the lake below the zone of methanogenesis.