Sulfate-methane Transition Zone Research Articles

Formation mechanism of authigenic gypsum in marine methane hydrate settings: Evidence from the northern South China Sea

During the last decade, gypsum has been discovered widely in marine methane hydrate-bearing sediments. However, whether this gypsum is an in-situ authigenic precipitate remains controversial. The GMGS2 expedition carried out in 2013 by the Guangzhou Marine Geological Survey (GMGS) in the northern South China Sea provided an excellent opportunity for investigating the formation of authigenic minerals and, in particular, the relationship between gypsum and methane hydrate. In this contribution, we analyzed the morphology and sulfur isotope composition of gypsum and authigenic pyrite as well as the carbon and oxygen isotopic compositions of authigenic carbonate in a drillcore from Site GMGS2–08. These methane-derived carbonates have characteristic carbon and oxygen isotopic compositions (δ13C: −57.9‰ to −27.3‰ VPDB; δ18O: +1.0‰ to +3.8‰ VPDB) related to upward seepage of methane following dissociation of underlying methane hydrates since the Late Pleistocene. Our data suggest that gypsum in the sulfate-methane transition zone (SMTZ) of this core precipitated as in-situ authigenic mineral. Based on its sulfur isotopic composition, the gypsum sulfur is a mixture of sulfate derived from seawater and from partial oxidation of authigenic pyrite. Porewater Ca2+ ions for authigenic gypsum were likely generated from carbonate dissolution through acidification produced by oxidation of authigenic pyrite and ion exclusion during methane hydrate formation. This study thus links the formation mechanism of authigenic gypsum with the oxidation of authigenic pyrite and evolution of underlying methane hydrates. These findings suggest that authigenic gypsum may be a useful proxy for recognition of SMTZs and methane hydrate zones in modern and ancient marine methane hydrate geo-systems.

Enhanced framboidal pyrite formation related to anaerobic oxidation of methane in the sulfate-methane transition zone of the northern South China Sea

Many studies have confirmed that size distributions of framboidal pyrite can be an effective indicator of bottom-water redox conditions in modern as well as ancient sedimentary environments. However, one environment in which production of framboidal pyrite has not been sufficiently studied to date is the sulfate-methane transition zone (SMTZ), in which anaerobic oxidation of methane (AOM) is coupled with microbial sulfate reduction (MSR) to enhance iron sulfide mineral precipitation (e.g., FeS2, FeS, and Fe3S4). Here, we document for the first time size distributions for pyrite framboids from the SMTZ, based on data from two sites in the methane hydrate-bearing region of the northern South China Sea. On the basis of framboid size, pyrite concentration, and sulfur isotope data, we propose new insights into the formation process of authigenic pyrite framboids within the SMTZ. We conclude that (1) Enhanced anaerobic oxidation of methane (AOM) not only plays a dominant role in the accumulation of 34S-enriched pyrite but also is responsible for formation of highly variable and sometimes exceptionally large pyrite framboids in the SMTZ; (2) Most framboids occur in ‘framboid clusters’ which commonly exhibit rod-like shapes, secondary overgrowths, heavier δ34S values, and unusual size distributions (e.g., mean size>20μm and standard deviation>3.0μm) in the SMTZ; (3) Pyrite framboids formed in the SMTZ, which have characteristics different from those formed in the sulfate reduction zone (SRZ), do not comment on redox conditions of the overlying water column; and (4) Framboid occurrences with similar characteristics in ancient marine deposits may be considered indicators of enhanced AOM and mark the former position of the SMTZ in the paleo-marine system. In addition, significant quantities of elemental sulfur were observed in the SMTZ, possibly related to anaerobic oxidation of hydrogen sulfide and fluctuations of the SMTZ.

New insights into the transport processes controlling the sulfate-methane-transition-zone near methane vents.

Over the past years, several studies have raised concerns about the possible interactions between methane hydrate decomposition and external change. To carry out such an investigation, it is essential to characterize the baseline dynamics of gas hydrate systems related to natural geological and sedimentary processes. This is usually treated through the analysis of sulfate-reduction coupled to anaerobic oxidation of methane (AOM). Here, we model sulfate reduction coupled with AOM as a two-dimensional (2D) problem including, advective and diffusive transport. This is applied to a case study from a deep-water site off Nigeria’s coast where lateral methane advection through turbidite layers was suspected. We show by analyzing the acquired data in combination with computational modeling that a two-dimensional approach is able to accurately describe the recent past dynamics of such a complex natural system. Our results show that the sulfate-methane-transition-zone (SMTZ) is not a vertical barrier for dissolved sulfate and methane. We also show that such a modeling is able to assess short timescale variations in the order of decades to centuries.

Exhumed hydrocarbon-seep authigenic carbonates from Zakynthos Island (Greece): Concretions not archaeological remains

In Zakynthos Island (Greece), authigenic cementation of marine sediment has formed pipe-like, disc and doughnut-shaped concretions. The concretions are mostly composed of authigenic ferroan dolomite accompanied by pyrite. Samples with >80% dolomite, have stable isotope compositions in two groups. The more indurated concretions have δ18O around +4‰ and δ13C values between −8 and −29‰ indicating dolomite forming from anaerobic oxidation of thermogenic methane (hydrocarbon seep), in the sulphate-methane transition zone. The outer surfaces of some concretions, and the less-cemented concretions, typically have slightly heavier isotopic compositions and may indicate that concretion growth progressed from the outer margin in the ambient microbially-modified marine pore fluids, inward toward the central conduit where the isotopic compositions were more heavily influenced by the seep fluid. Sr isotope data suggest the concretions are fossil features, possibly of Pliocene age and represent an exhumed hydrocarbon seep plumbing system. Exposure on the modern seabed in the shallow subtidal zone has caused confusion, as concretion morphology resembles archaeological stonework of the Hellenic period.

Sedimentary pyrite δ34S differs from porewater sulfide in Santa Barbara Basin: Proposed role of organic sulfur

Santa Barbara Basin sediments host a complex network of abiotic and metabolic chemical reactions that knit together the carbon, sulfur, and iron cycles. From a 2.1-m sediment core collected in the center of the basin, we present high-resolution profiles of the concentrations and isotopic compositions of all the major species in this system: sulfate, sulfide (∑H2S), elemental sulfur (S0), pyrite, extractable organic sulfur (OS), proto-kerogen S, total organic and dissolved inorganic carbon, and total and reducible iron. Below 10cm depth, the core is characterized by low apparent sulfate reduction rates (<0.01mM/yr) except near the sulfate-methane transition zone. Surprisingly, pyrite forming in shallow sediments is ∼30‰ more 34S-depleted than coexisting ∑H2S in porewater. S0 has the same strongly 34S-depleted composition as pyrite where it forms near the sediment–water interface, though not at depth. This pattern is not easily explained by conventional hypotheses in which sedimentary pyrite derives from abiotic reactions with porewater ∑H2S or from the products of S0 disproportionation. Instead, we propose that pyrite formation in this environment occurs within sulfate reducing microbial aggregates or biofilms, where it reflects the isotopic composition of the immediate products of bacterial sulfate reduction. Porewater ∑H2S in Santa Barbara Basin may be more 34S-enriched than pyrite due to equilibration with relatively 34S-enriched OS. The difference between OS and pyrite δ34S values would then reflect the balance between microbial sulfide formation and the abundance of exchangeable OS. Both OS and pyrite δ34S records thus have the potential to provide valuable information about biogeochemical cycles and redox structure in sedimentary paleoenvironments.

Stable isotope patterns of coexisting pyrite and gypsum indicating variable methane flow at a seep site of the Shenhu area, South China Sea

Aggregates of authigenic pyrite and gypsum were found at site HS373 in the Shenhu seepage area, South China Sea. Most of the pyrite occurs as tubes consisting of framboids. Gypsum aggregates are mainly represented by rosettes and some of them grew over pyrite, revealing that gypsum precipitation postdated pyrite formation. At seeps, extensive sulfate-driven anaerobic oxidation of methane (AOM) commonly leads to the accumulation of 34S enriched sulfide, resulting in the formation of 32S depleted pyrite. The sulfur isotopic composition of pyrite at site HS373 reveals a great variability of 75.5‰ in δ34S values, spanning from −37.6‰ to 37.9‰ (V-CDT) with a distinct trend towards higher values with increasing depth. This trend indicates an increase of the effect of sulfate-driven AOM with depth, whereas pyrite at shallow depth apparently resulted mostly from organiclastic sulfate reduction. To date, reports on coexisting pyrite and gypsum in seep environments are limited, and therefore the mechanism of gypsum formation is insufficiently understood. This is the first time that the sulfur and oxygen stable isotope composition of gypsum has been analyzed to understand its formation at seeps. The δ34S values of gypsum vary from −24.5‰ to 0.9‰, revealing a similar trend with depth like pyrite. The δ18Ogypsum values range from 5.3‰ to 12.3‰ (V-SMOW), with a scattered trend to lower values with depth. The combined evidence reveals that gypsum formation was linked to the establishment of more oxidizing conditions, probably caused by waning seepage. This led to the oxidation of pyrite and the associated increase in pore water sulfate and acidity, which favored carbonate mineral dissolution, providing the calcium ions required for gypsum formation. The observed isotopic variability of gypsum suggests that oxidation occurred by different mechanisms at different depth, whereby pyrite oxidation was dominated by aerobic processes at shallow depth involving dissolved molecular oxygen, and by anaerobic processes at greater depth involving electron acceptors like ferric iron. Taken together, the stable isotope compositions of pyrite and gypsum suggest that the redox conditions within the studied sediments were variable. Such variability was controlled by complex factors, among which changes in seepage activity have been instrumental for the vertical displacement of the sulfate methane transition zone.

Numerical modeling study of mineralization induced by methane cold seep at the sea bottom

In order to investigate the response of authigenic minerals to gas hydrate geo-systems, the biogeochemical processes and its induced mineralization were predicted by employing the comprehensive reactive transport modeling approach. Based on the available data extracted from the northern continental slope area of the South China Sea, a 1-D vertical column model was developed. Three cases with different upward methane flux rates and three cases with different mineral compositions, i.e., a total of six cases were designed to investigate the effects of variations in the depth of sulfate methane transition zone (SMTZ) and in the mineral composition on the formation of authigenic minerals. The simulation results indicate that the SMTZ depth influenced by both the upward methane flux rate and the initial composition played an important role in the formation of authigenic minerals. The AOM reaction is intensive at the interface, and the precipitation amount of calcite is large, which is mainly controlled by AOM. When the methane leakage rate is 20 times higher than the base case, aragonite starts to precipitate. During the simulation, oligoclase, k-feldspar, smectite-Na, smectite-Ca, chlorite dissolved. Our study specific to this area as a starting point may provide a quantitative approach for investigating carbonate and pyrite formation in hydrate-bearing sediments accounting for methane oxidation and sulfate reduction. The method presented here and the model built in this study can be used for other sites with similar conditions. In addition, this study may serve as an indication for the potential natural gas hydrate reservoir in depth, and is also significant for marine carbon and sulfur cycle.

Transcriptional analysis of sulfate reducing and chemolithoautotrophic sulfur oxidizing bacteria in the deep subseafloor.

Sulfate reducing bacteria (SRB) oxidize a significant proportion of subseafloor organic carbon, but their metabolic activities and subsistence mechanisms are poorly understood. Here, we report in depth phylogenetic and metabolic analyses of SRB transcripts in the Peru Margin subseafloor and interpret these results in the context of sulfate reduction activity in the sediment. Relative abundance of overall SRB gene transcripts declines strongly whereas relative abundance of ribosomal protein transcripts from sulfate reducing δ-Proteobacteria peak at 90 m below seafloor (mbsf) within a deep sulfate methane transition zone. This coincides with isotopically heavy δ(34) S values of pore water sulfate (70‰), indicating active subseafloor microbial sulfate reduction. Within the shallow sulfate reduction zone (0-5 mbsf), a transcript encoding the beta subunit of dissimilatory sulfite reductase (dsrB) was related to Desulfitobacterium dehalogenans and environmental sequences from Aarhus Bay (Denmark). At 159 mbsf we discovered a transcript encoding the reversely operating dissimilatory sulfite reductase α-subunit (rdsrA), with basal phylogenetic relation to the chemolithoautotrophic SUP05 Group II clade. A diversity of SRB transcripts involved in cellular maintenance point toward potential subsistence mechanisms under low-energy over long time periods, and provide a detailed new picture of SRB activities underlying sulfur cycling in the deep subseafloor.

A relict sulfate–methane transition zone in the mid-Devonian Marcellus Shale

A barium-enriched interval of Marcellus Shale (Middle Devonian Oatka Creek Formation) from a core in Chenango County, NY contains ∼100μm diameter ellipsoidal grains with variable mineralogical compositions between pure barite and pure pyrite endmembers. Petrographic characterization and in-situ sulfur isotope analysis by Secondary Ion Mass Spectrometry (SIMS) was performed to better understand the diagenetic conditions under which these grains form and are preserved in the shale. Textural relationships suggest partial to complete pseudomorphic replacement of ellipsoidal barite by pyrite. Spatially, the ellipsoidal grains are concentrated in discrete layers parallel to original bedding and intervals within these layers often contain grains with similar degrees of replacement. The fraction of barite replaced by pyrite between these intervals can vary significantly, which is remarkable considering these intervals are separated by stratigraphic distances on the order of mm to cm in the shale (depths equivalent to deposition over 10’s–1000’s of years).The mean δ34S of barite and pyrite in ellipsoidal grains is 63.3±3.6‰ and 2.2±3.0‰, respectively, indicating that the grains are authigenic. Mass balance calculations based on density and stoichiometric differences between barite and pyrite indicate that reduction of sulfate from barite alone cannot be the sole source of sulfur in the replaced grains: only ∼23% of sulfur in pyrite comes from the dissolution of barite while the remainder derives from an additional source with δ34S=−17.6±1.3‰. We suggest that pseudomorphic replacement of barite led first to the formation of greigite (Fe3S4), where one mole of sulfur was provided by barite and the other three moles of sulfur were contributed by FeS(aq); the latter formed by reaction of Fe2 + with sulfide from microbial sulfate reduction. Transformation of greigite to pyrite occurred via the sulfur addition and/or iron loss pathways. These observations suggest the following mechanism for the replacement of barite by pyrite in the ellipsoidal barite grains: (1) burial of authigenic barite below the sulfate–methane transition zone (SMTZ), and (2) partial to complete dissolution of the grain and concomitant precipitation of greigite (and its subsequent transformation to pyrite) in the presence of pore water depleted in sulfate and enriched in FeS(aq) and polysulfides. We suggest that closely-spaced intervals containing different barite to pyrite ratios may reflect fine-scale temporal shifts or fluctuations in the position of the SMTZ due to variable rates of methanogenesis and/or sedimentation during diagenesis.

Paleo‐methane emissions recorded in foraminifera near the landward limit of the gas hydrate stability zone offshore western Svalbard

AbstractWe present stable isotope and geochemical data from four sediment cores from west of Prins Karls Forland (ca. 340 m water depth), offshore western Svalbard, recovered from close to sites of active methane seepage, as well as from shallower water depths where methane seepage is not presently observed. Our analyses provide insight into the record of methane seepage in an area where ongoing ocean warming may be fueling the destabilization of shallow methane hydrate. The δ13C values of benthic and planktonic foraminifera at the methane seep sites show distinct intervals with negative values (as low as −27.8‰) that do not coincide with the present‐day depth of the sulfate methane transition zone (SMTZ). These intervals are interpreted to record long‐term fluctuations in methane release at the present‐day landward limit of the gas hydrate stability zone (GHSZ). Shifts in the radiocarbon ages obtained from planktonic foraminifera toward older values are related to methane‐derived authigenic carbonate overgrowths of the foraminiferal tests, and prevent us from establishing the chronology of seepage events. At shallower water depths, where seepage is not presently observed, no record of past methane seepage is recorded in foraminifera from sediments spanning the last 14 ka cal BP (14C‐AMS dating). δ13C values of foraminiferal carbonate tests appear to be much more sensitive to methane seepage than other sediment parameters. By providing nucleation sites for authigenic carbonate precipitation, foraminifera thus record the position of even a transiently stable SMTZ, which is likely to be a characteristic of temporally variable methane fluxes.

Genomic reconstruction of a novel, deeply branched sediment archaeal phylum with pathways for acetogenesis and sulfur reduction

Marine and estuary sediments contain a variety of uncultured archaea whose metabolic and ecological roles are unknown. De novo assembly and binning of high-throughput metagenomic sequences from the sulfate-methane transition zone in estuary sediments resulted in the reconstruction of three partial to near-complete (2.4-3.9 Mb) genomes belonging to a previously unrecognized archaeal group. Phylogenetic analyses of ribosomal RNA genes and ribosomal proteins revealed that this group is distinct from any previously characterized archaea. For this group, found in the White Oak River estuary, and previously registered in sedimentary samples, we propose the name 'Thorarchaeota'. The Thorarchaeota appear to be capable of acetate production from the degradation of proteins. Interestingly, they also have elemental sulfur and thiosulfate reduction genes suggesting they have an important role in intermediate sulfur cycling. The reconstruction of these genomes from a deeply branched, widespread group expands our understanding of sediment biogeochemistry and the evolutionary history of Archaea.

Microbial methanogenesis in the sulfate-reducing zone of surface sediments traversing the Peruvian margin

Abstract. We studied the concurrence of methanogenesis and sulfate reduction in surface sediments (0–25 cm below sea floor) at six stations (70, 145, 253, 407, 990 and 1024 m) along the Peruvian margin (12° S). This oceanographic region is characterized by high carbon export to the seafloor creating an extensive oxygen minimum zone (OMZ) on the shelf, both factors that could favor surface methanogenesis. Sediments sampled along the depth transect traversed areas of anoxic and oxic conditions in the bottom-near water. Net methane production (batch incubations) and sulfate reduction (35S-sulfate radiotracer incubation) were determined in the upper 0–25 cm b.s.f. of multiple cores from all stations, while deep hydrogenotrophic methanogenesis (&gt; 30 cm b.s.f., 14C-bicarbonate radiotracer incubation) was determined in two gravity cores at selected sites (78 and 407 m). Furthermore, stimulation (methanol addition) and inhibition (molybdate addition) experiments were carried out to investigate the relationship between sulfate reduction and methanogenesis.Highest rates of methanogenesis and sulfate reduction in the surface sediments, integrated over 0–25 cm b.s.f., were observed on the shelf (70–253 m, 0.06–0.1 and 0.5-4.7 mmol m−2 d−1, respectively), while lowest rates were discovered at the deepest site (1024 m, 0.03 and 0.2 mmol m−2 d−1, respectively). The addition of methanol resulted in significantly higher surface methanogenesis activity, suggesting that the process was mostly based on non-competitive substrates – i.e., substrates not used by sulfate reducers. In the deeper sediment horizons, where competition was probably relieved due to the decrease of sulfate, the usage of competitive substrates was confirmed by the detection of hydrogenotrophic activity in the sulfate-depleted zone at the shallow shelf station (70 m).Surface methanogenesis appeared to be correlated to the availability of labile organic matter (C ∕ N ratio) and organic carbon degradation (DIC production), both of which support the supply of methanogenic substrates. A negative correlation between methanogenesis rates and dissolved oxygen in the bottom-near water was not obvious; however, anoxic conditions within the OMZ might be advantageous for methanogenic organisms at the sediment-water interface.Our results revealed a high relevance of surface methanogenesis on the shelf, where the ratio between surface to deep (below sulfate penetration) methanogenic activity ranged between 0.13 and 105. In addition, methane concentration profiles indicated a partial release of surface methane into the water column as well as consumption of methane by anaerobic methane oxidation (AOM) in the surface sediment. The present study suggests that surface methanogenesis might play a greater role in benthic methane budgeting than previously thought, especially for fueling AOM above the sulfate–methane transition zone.

Vertical distribution of archaeal communities in cold seep sediments from the jiulong methane reef area in the south China sea

Jiulong methane reef, a large cold seep area on the continental slope of the South China Sea (SCS), was characterized by chemosynthetic fauna, bacterial mats, methane-derived carbonate structures, and a shallow depth of sulfate-methane transition zone (SMTZ) in gravity cores. To characterize the microbial diversity and variation in correlation to methane venting in the cold seep area, the archaeal community in sediments of the gravity Core CL11 was analyzed, using the Illumina MiSeq sequencing system. A total of 96,917 valid sequences were obtained and classified into 275 OTUs. Taxonomic analysis showed that all the archaeal sequences belonged to the phyla of Crenarchaeota (69.41%) and Eurarchaeota (25.26%). Vertical distribution of the archaeal communities showed that the Marine Benthic Group B (MBG-B) was the predominant group (58.92%) in the topmost sediment of the core, while percentages of the South African Gold Mine Euryarchaeotal Group (SAGMEG) sequences increased with depth. Our findings indicate that the archaeal community in the cold seep of Jiulong Methane Reef is unique and provides us new insights into the archaeal community composition of the cold seep area.

Organic matter cycling across the sulfate-methane transition zone of the Santa Barbara Basin, California Borderland

Consumption of sulfate (SO42−) in the sulfate-methane transition zone (SMTZ) has often been considered to be due solely to anaerobic oxidation of methane (AOM). However, recent studies show SO42− fluxes into the SMTZ that exceed methane (CH4) fluxes, thereby challenging this conceptual model. Co-occurrence of organoclastic SO42− reduction (oSR) with AOM in the SMTZ has been hypothesized to be the cause for this flux imbalance, but conclusive evidence is lacking. To address this knowledge gap, we investigated organic matter cycling in the SMTZ of the organic-rich sediments of the Santa Barbara Basin, California Borderland, and examined the occurrence of oSR within this zone using bulk solute profiles and Δ14C and δ13C values of selected carbon pools. We also tested the hypothesis that the SMTZ acts as an oxidation front not just for CH4, but also for dissolved organic carbon (DOC) that is produced below the SMTZ and migrates upward. Mass balance calculations for the SMTZ based on reaction stoichiometry and Δ14C and δ13C values of associated carbon fluxes indicate that ∼35–45% of total SO42− reduction in the SMTZ occurs via oSR, with the remainder attributable to AOM. The δ13C value of net DOC production is distinct from that of the fraction of bulk POC undergoing degradation, suggesting that pore-water DOC represents a compositionally unique slice of the metabolizable POC pool. DOC diffusing upward at 450cm is virtually free of 14C and contain low levels of short-chain organic acids. Radiocarbon mass balance shows that >30% of this pre-aged, and presumably refractory, DOC is removed from the pore waters within or immediately below the SMTZ. Although the SMTZ does not appear to be a major net DOC oxidation front, these results show that DOC dynamics provide unique insights into organic matter processing in these subsurface sediments.

Variance and potential niche separation of microbial communities in subseafloor sediments off Shimokita Peninsula, Japan.

Subseafloor pelagic sediments with high concentrations of organic matter form habitats for diverse microorganisms. Here, we determined depth profiles of genes for SSU rRNA, mcrA, dsrA and amoA from just beneath the seafloor to 363.3 m below the seafloor (mbsf) using core samples obtained from the forearc basin off the Shimokita Peninsula. The molecular profiles were combined with data on lithostratigraphy, depositional age, sedimentation rate and pore-water chemistry. The SSU rRNA gene tag structure and diversity changed at around the sulfate-methane transition zone (SMTZ), whereas the profiles varied further with depth below the SMTZ, probably in connection with the variation in pore-water chemistry. The depth profiles of diversity and abundance of dsrA, a key gene for sulfate reduction, suggested the possible niche separations of sulfate-reducing populations, even below the SMTZ. The diversity and abundance patterns of mcrA, a key gene for methanogenesis/anaerobic methanotrophy, suggested a stratified distribution and separation of anaerobic methanotrophy and hydrogenotrophic or methylotrophic methanogensis below the SMTZ. This study provides novel insights into the relationships between the composition and function of microbial communities and the chemical environment in the nutrient-rich continental margin subseafloor sediments, which may result in niche separation and variability in subseafloor microbial populations.

The indicative effect of structures of archaeal communities at deep-water sediment cores on natural gas hydrate: A case study from Station 973-4 in the Southwest Taiwan Basin, South China Sea

The formation and decomposition of submarine natural gas hydrate (hereinafter hydrate for short) are greatly influenced by methanogenesis and anaerobic methane oxidation of microorganisms, so it is necessary to conduct further analysis on the metabolic mechanism of archaeal communities and its indicative effect on hydrate reservoir formation. In this paper, the structures of archaeal communities at the cores of Station 973-4 in the Southwest Taiwan Basin of the South China Sea were studied by means of 16S rDNA molecular biological technologies, and then were compared with those at hydrate occurrence and non-occurrence zones in the Japan Sea and the Shenhu area of the South China Sea. It is shown that Methanosarcina is the predominant community at the cores from Station 973-4, with a percentage of 50% in the surface zone, 46.1% in the sulfate–methane transition zone (SMTZ) and 66.7% in the deep zone. Methanomicrobiales is the secondary predominant community, with a percentage of 28.3% in the surface zone, 30.7% in SMTZ and 11.1% in the deep zone. The percentage changing tendency of the above-mentioned communities are closely related with the variation of organic matter content and compositions during the early diagenetic process. It is concluded that the structures of archaeal communities are in accordance with the analysis results of bacteria, geochemistry and mineralogy, indicating the occurrence of hydrate at the bottom of the cores from Station 973-4. In addition, the structures of archaeal communities are characterized by diversity, so attention shall be paid to the synergy between geochemical parameters and geologic characteristics, so that the indicative effect on hydrate reservoir formation will be presented more accurately.

Coupled pyrite concentration and sulfur isotopic insight into the paleo sulfate–methane transition zone (SMTZ) in the northern South China Sea

The sulfate–methane transition zone (SMTZ) is an important diagenetic redox boundary within marine sediments where the anaerobic oxidation of methane (AOM), coupled with bacterial sulfate reduction, can promote sulfur isotopic enrichments in several solid phase minerals including pyrite (FeS2). Authigenic pyrite can form in concentrated abundances within the SMTZ and as such, can be used as a proxy to identify paleo-SMTZs. This study uses enrichments in 34S and anomalously high abundances of authigenic pyrites in 287 samples from the northern South China Sea (SCS) to determine the paleo-SMTZ. The pyrite samples were collected from sediment cores acquired at three sites, each of which are known to be located in natural gas hydrate-bearing regions. We assess the relative abundances of authigenic pyrites, the types of pyrite morphologies recovered in the cored sediments, and the sulfur isotopic values of recovered pyrite samples using two methods: (1) handpicked sample analysis using a binocular microscope, and (2) the chromium reduction method. Our results show that pyrite concentrations and sulfur isotopic compositions exhibit synchronous fluctuations, particularly from 6.8 m below seafloor (mbsf) to 8.4 mbsf at all three study sites. There is a significant increase in the occurrence of rod-like pyrite morphology within this key interval. We define the position of the paleo-SMTZ by the presence of anomalously high accumulations of pyrites at greater than 5.0wt.% using the handpicking method or greater than 0.5wt.% via the chromium reduction method, along with positive Δδ34S excursions greater than 10.0‰ VCDT. We discovered a regional paleo-SMTZ that is shallower than the modern SMTZ, suggesting a previous period of elevated methane flux from depth, possibly related to widespread gas hydrate dissociation.

U-Th isotope constraints on gas hydrate and pockmark dynamics at the Niger delta margin

The application of uranium-thorium dating methods to authigenic carbonates provides unique constraints on the temporal evolution of methane seeps at ocean margins. In this study, we report U-Th isotope measurements for carbonate breccias collected from within a hydrate-bearing pockmark located at the Niger Delta margin. These concretions were extracted from a carbonate-rich layer in the upper two meters of a sediment core (N2-KS-44; ~1200m water depth), well above the present-day sulphate-methane transition zone (about 3m depth) and the presence of gas hydrates in the sediment. The stratigraphy of core N2-KS-44 was established by tuning its downcore Al/Ti profile to a well-dated nearby reference core, and carbonate 230Th/U ages were calculated using isochron methods.Our results indicate that a major event of aragonite precipitation occurred between about 13 and 2.5ka at the studied location. Comparison of sediment accumulation rates at both core N2-KS-44 and the nearby reference site suggests that the initial stage of carbonate precipitation, between 13 and 10ka, was associated with sediment winnowing, probably related to intense fluid seepage. In contrast, our data indicate that sedimentation rates rapidly increased within the pockmark after 7ka. In agreement with the presence of carbonate breccias exhibiting U-Th ages older than their corresponding stratigraphic age, this observation would suggest that sediment reworking took place after that time, possibly caused by erosion of the surrounding sediment within the pockmark.We hypothesize that the period of carbonate formation between 13 and 2.5ka was related to an upward migration of gas-hydrate reservoirs to the near seafloor environment. After this pulse of enhanced fluid flow, the diminution of methane fluxes at the base of the local gas-hydrate occurrence zone would have led to hydrate dissolution in sub-surface sediments and pockmark formation, thereby explaining the progressive increase in sedimentation rates, the absence of recent carbonate concretions and the deepening of the sulphate-methane transition zone at site N2-KS-44 inferred from pore water data. Overall, these results provide further constraints about the relationship between gas hydrate dynamics and the evolution of pockmarks at ocean margins through time.

Hydrocarbon sources of cold seeps off the Vesterålen coast, northern Norway

We investigated active methane seeps in a water depth of 200m in the Hola area off the coast of Vesterålen, northern Norway, to assess (1) hydrocarbon sources, (2) migration pathways and (3) the influence of hydrocarbon seepage on sediment pore water and water column chemistry. The seepage area is characterised by the presence of gas flares in the water column as revealed by hydro acoustic surveys and elevated methane concentrations of up to 42nM ca. 5m above the seafloor. Pore water analyses of three gravity cores from the seepage area show varying depths of the sulphate–methane-transition zone (SMTZ) between 80cm and >250cm indicating spatially heterogeneous methane ascent. The isotopic composition of methane (δ13C from −40‰ to −63‰ and δ2H from −191‰ to −225‰) and δ13C depth profiles of methane and dissolved inorganic carbon show that the hydrocarbons are predominantly of thermogenic origin, consistent with δ13C values of C2 to C4 hydrocarbons. Isotope data also indicate considerable biodegradation of propane. Seismic profiles from the study area reveal major faults and steeply dipping unconformities between the basement and overlying Mesozoic sedimentary rocks. We propose that these act as migration pathways for the hydrocarbons from late Jurassic to early Cretaceous source rocks.

Pyritization induced by anaerobic oxidation of methane (AOM) – An example from the upper devonian shale succession, western New York, USA

The diagenetic history of the Upper Devonian Rhinestreet Shale of the western New York region of the Appalachian basin appears to reflect the upward diffusion of methane through these deposits shortly after their accumulation. The most obvious diagenetic signature of this process is numerous horizons of authigenic calcium carbonate concretions interpreted to reflect mineralization induced by the anaerobic oxidation of methane (AOM) focused along the sulfate-methane transition (SMT) zone. Additionally, a 2-m-thick section of the lower Rhinestreet preserves a diagenetic record consistent with AOM in the form of two pyritiferous stratigraphic horizons. The stratigraphically lowest horizon is a 2-cm-thick layer containing abundant 34S-enriched (11.8 and 18.4‰ V-CDT) coarsely crystalline pyrite. The thinness of this interval suggests that AOM-related diagenesis remained fixed at this stratigraphic position for a brief period of time before these deposits were buried below the SMT or the SMT shifted position within the sediment column. A second paleo-SMT appears to have stabilized 60 to 80 cm above the 2-cm-thick pyritiferous horizon, meter-scale carbonate concretions being the most obvious evidence of this episode of AOM. More than this, though, the apparently Fe-limited nature of the host black shale induced the downward diffusion of 34S-enriched hydrogen sulfide produced by AOM resulting in the precipitation of 34S-enriched (23.1 to 27.3‰ V-CDT) pyrite below the SMT. The maximum depth of diffusion of the sulfidization front was limited by the amount of hydrogen sulfide produced within and near the SMT as well as by the availability of reactive iron in underlying deposits. The focusing of AOM-related diagenesis at the two stratigraphic positions was likely caused by non-steady-state sedimentation. However, it is impossible to decipher relative timing of formation of the two paleo-SMTs.