Sulfate-methane Transition Research Articles

Microbial metabolism and adaptations in Atribacteria-dominated methane hydrate sediments.

Gas hydrates harbour gigatons of natural gas, yet their microbiomes remain understudied. We bioprospected 16S rRNA amplicons, metagenomes, and metaproteomes from methane hydrate-bearing sediments under Hydrate Ridge (offshore Oregon, USA, ODP Site 1244, 2-69 mbsf) for novel microbial metabolic and biosynthetic potential. Atribacteria sequences generally increased in relative sequence abundance with increasing sediment depth. Most Atribacteria ASVs belonged to JS-1-Genus 1 and clustered with other sequences from gas hydrate-bearing sediments. We recovered 21 metagenome-assembled genomic bins spanning three geochemical zones in the sediment core: the sulfate-methane transition zone, the metal (iron/manganese) reduction zone, and the gas hydrate stability zone. We found evidence for bacterial fermentation as a source of acetate for aceticlastic methanogenesis and as a driver of iron reduction in the metal reduction zone. In multiple zones, we identified a Ni-Fe hydrogenase-Na+ /H+ antiporter supercomplex (Hun) in Atribacteria and Firmicutes bins and in other deep subsurface bacteria and cultured hyperthermophiles from the Thermotogae phylum. Atribacteria expressed tripartite ATP-independent transporters downstream from a novel regulator (AtiR). Atribacteria also possessed adaptations to survive extreme conditions (e.g. high salt brines, high pressure and cold temperatures) including the ability to synthesize the osmolyte di-myo-inositol-phosphate as well as expression of K+ -stimulated pyrophosphatase and capsule proteins.

Diagenetic Controls on the Formation of the Anarraaq Clastic-Dominated Zn-Pb-Ag Deposit, Red Dog District, Alaska

AbstractThe Anarraaq clastic-dominated (CD) Zn-Pb-Ag deposit (Red Dog district, Alaska, USA) has an inferred mineral resource of 19.4 Mt at 14.4% Zn, 4.2% Pb, and 73 g/t Ag and is spatially associated with a separate ~1 Gt barite body. This study presents new cross sections and petrographic evidence from the Anarraaq area. The barite body, previously shown to have formed in a shallow subsurface environment akin to a methane cold seep, contains multiple generations of barite with locally abundant calcite masses, which are discordant to sedimentary laminae, and is underlain by an interval of massive pyrite containing abundant framboids and radiolarians. Calcite and pyrite are interpreted to have formed by methane-driven diagenetic alteration of host sediment at the sulfate-methane transition (SMT). The sulfide deposit contains two zones of Zn-Pb mineralization bounded by faults of unknown displacement. The dominant hydrothermal minerals are marcasite, pyrite, sphalerite, quartz, and galena. The presence of hydrothermal pseudomorphs after barite, early pyrite resembling diagenetic pyrite associated with the barite body, and hydrothermal quartz and sphalerite filling voids formed by dissolution of carbonate all suggest that host sediment composition and origin was similar to that of the barite body prior to hydrothermal mineralization. Rhenium-osmium isochron ages of Ikalukrok mudstone (339.1 ± 8.3 Ma), diagenetic pyrite (333.0 ± 7.4 Ma), and hydrothermal pyrite (334.4 ± 5.3 Ma) at Anarraaq are all within uncertainty of one another and of an existing isochron age (~338 Ma) for the Main deposit in the Red Dog district. This indicates that the Anarraaq deposit formed soon after sedimentation and that hydrothermal activity was approximately synchronous in the district. The initial Os composition of the Anarraaq isochrons (0.375 ± 0.019–0.432 ± 0.025) is consistent with contemporaneous seawater, indicating that a mantle source was not involved in the hydrothermal system. This study highlights the underappreciated but important role of early, methane-driven diagenetic processes in the paragenesis of some CD deposits and has important implications for mineral exploration.

Molybdenum isotope composition of seep carbonates – Constraints on sediment biogeochemistry in seepage environments

Methane seepage causes significant changes in the local diagenetic environment and impacts the marine sedimentary cycles of sulfur, carbon, and molybdenum (Mo), which are intimately linked to each other. Molybdenum isotopic compositions were determined for authigenic carbonate nodules and bulk sediments of a 230-m long sediment core collected at a methane seep site in the South China Sea. Authigenic carbonate nodules occur at different sediment depths and display low δ13C values around −50‰ revealing that carbon predominantly derived from biogenic methane. Moreover, seep carbonate nodules are substantially enriched in Mo (Al-normalized enrichment factors MoEF between 27 and 70), and display nearly constant δ98Mo values around an average of +1.9‰ (relative to NIST3134 = +0.25‰), close to the Mo isotopic composition of modern seawater (+2.3‰). Linear positive correlations between δ98Mo values and pyrite sulfur contents and Mo contents were identified for seep carbonate samples. This implies that Mo scavenging into pyrite is likely controlled by hydrogen sulfide availability derived from sulfate-driven anaerobic oxidation of methane (SD-AOM). The essentially constant and high δ98Mo values suggest that Mo species were nearly quantitatively incorporated into the authigenic carbonates under highly sulfidic conditions, probably at times when the sulfate-methane transition zones (SMTZ) was situated at shallow sediment depth at times of high seepage activity. In contrast, bulk sediments that exhibit typical marine δ13C values close to 0‰ (versus VPDB), hence devoid of methane-derived seep carbonates, are characterized by low Mo contents (MoEF = ~0.9). Their lower δ98Mo values mostly ranging from +0.2 to +0.8‰, in accord with low Mo contents, likely reflect input of detrital Mo. Bulk sediments bearing seep carbonate nodules display variable δ13C values lower than −5‰ and are characterized by a wide range in Mo contents (MoEF from 1.5 to 45.3) and δ98Mo values (+0.7 to +3.0‰). The high δ98Mo values of some sediment samples, relative to the seep carbonate signature, might indicate an additional accumulation of a 98Mo enriched pool of molybdenum during later SD-AOM at a deeper SMTZ postdating initial seep carbonate formation; such 98Mo enrichment was probably caused by preceding preferential removal of lighter Mo isotopes from pore fluids. Our results indicate that Mo isotope systematics of seep deposits can provide constraints on the dynamics of the biogeochemical cycling of carbon, sulfur, and trace elements in methane seep environments. A potential for authigenic carbonates to archive the Mo isotopic composition of seawater is indicated, but will have to be assessed in future studies.

Using multi-proxy approach to constrain temporal variations of methane flux in methane-rich sediments of the southern South China Sea

Methane seeps are widespread on the seafloor in continental margins. However, the responses of methane flux and diagenetic processes at cold seeps to drastic environmental changes in the geological history are still poorly understood. Here, we report the geochemical composition of porewaters and sediments in a sediment core (CL3A) collected from methane-rich sediments in the southern South China Sea, in order to assess the variations in upward methane flux since the last glacial period. A sediment horizon at depths of ca. 510–660 cm below seafloor just above the current sulfate-methane transition zone (SMTZ) is identified, which shows geochemical anomalies including high total inorganic carbon (TIC) and total sulfur (TS) contents, extremely negative δ13CTIC values and positive sulfur isotopic ratios of acid insoluble sulfur (δ34Sacid-insoluble). We propose that the diagenetic records of sulfate-driven anaerobic oxidation of methane within a paleo-SMTZ were recorded by the accumulation of authigenic carbonate and iron sulfide within this zone. Such geochemical pattern implies a fixation of the SMTZ at a certain depth for a prolonged time, which is estimated as ~13,000 years. The stagnation of the SMTZ is probably attributed to a large decline in sedimentation rate during the transition of last glacial/interglacial period, which is supported by the 14C ages throughout the core. Moreover, a non-steady-state modelling for the concave-up depth profiles of porewater species suggests that a recent enhancement in upward methane flux has lasted for at least 550 years. In summary, this study suggests that the fixation of SMTZ during the glacial/interglacial transitions in dynamic sedimentation regimes can lead to the accumulation of large amount of authigenic carbonate and iron sulfide in continental margins and should be considered in global carbon and sulfur biogeochemical cycles.

Non-evaporitic gypsum formed in marine sediments due to sulfate-methane transition zone fluctuations and mass transport deposits in the northern South China Sea

Gypsum (CaSO4·2H2O) of non-evaporitic origins has been observed in marine sedimentary environments over the past decade. Our understanding of related diagenetic processes has improved by analyzing the sulfur and oxygen isotopic compositions of gypsum, but, a comprehensive study of the isotopic composition of authigenic gypsum precipitated in methane-rich marine environments has not been performed to elucidate their implications for the sulfur cycle (e.g., sulfide oxidation). Furthermore, there are no studies of authigenic gypsum associated with submarine mass transport deposits, which are common sedimentary features on continental slopes. In this study, we analyzed the sulfur and oxygen isotopic compositions of gypsum and the sulfur isotopic composition of pyrite in drill cores collected from methane hydrate-bearing sites GMGS2–08, GMGS2–16, and GMGS4-W02B in the northern South China Sea to determine the formation mechanisms of authigenic gypsum. The stable sulfur isotopic analyses of gypsum and co-existing pyrite revealed that sulfate produced by pyrite oxidation contributed to gypsum precipitation. The low oxygen isotopic composition values of gypsum (−2.6 to 4.5‰ Vienna Standard Mean Ocean Water) suggest that pyrite is oxidized by metal oxides under anaerobic conditions, incorporating water oxygen, whose isotopic composition was modified by gas hydrate formation, into the produced sulfate. As pyrite oxidation also releases protons (H+) into the porewater, calcium concentrations may be elevated by the dissolution of carbonate minerals. At sites GMGS2–08 and GMGS2–16, authigenic gypsum was precipitated within paleo sulfate-methane transition zones, indicating that gypsum formation is probably associated with the downward migration of the sulfate-methane transition zone, which causes anaerobic pyrite oxidation at its original site of formation. However, at site GMGS4-W02B, most of the gypsum was distributed in a mass transport deposit characterized by a high abundance of reworked foraminiferas between the two paleo sulfate-methane transition zones. Given that seawater sulfate contributes more (>85%) than pyrite oxidation to the gypsum sulfate at site GMGS4-W02B, it is believed that authigenic gypsum formation is associated with mass transport deposits, which can rapidly trap overlying seawater sulfate within the pore space of the newly deposited sediments, combined with the sulfate and calcium from pyrite oxidation, leading to a local gypsum supersaturation.

Impact of methane seepage dynamics on the abundance of benthic foraminifera in gas hydrate bearing sediments: New insights from the South China Sea

The stable isotopic composition of benthic foraminifera tests is widely used to trace past methane release events. However, gaps in our knowledge on the ecology of benthic foraminifera at methane seeps impede the utility of test composition as proxy of methane emission histories and the influence of redox zonation on the local fauna. Here, we use sediment geochemistry and foraminiferal assemblages of a seep area of the South China Sea to reveal that seepage duration and intensity affect benthic foraminifera. Taxa tolerating eutrophic conditions like Uvigerina thrive at times of reduced seepage when aerobic methanotrophy is favored. In contrast, intense methane seepage leads to high sulfide levels, and a positioning of the sulfate-methane transition zone close to the seafloor. At these times, bottom waters may become oxygen depleted and benthic foraminifera will decline. This study provides (1) the means to understand how seepage dynamics control the distribution of benthic foraminifera and (2) new constraints on the use of foraminifera as paleoceanographic proxies, including possible mechanisms of mass mortality in the geological past.

Fluid conduits formed along burrows of giant bivalves at a cold seep site, Sounthern Taiwan

At a Pliocene methane seep site in Taiwan characterized by numerous tubular carbonate bodies and bivalves, a half meter-long dolomitic concretion containing a lucinid bivalve Anodontia goliath reveals the impact of chemosymbiotic bivalves on fluid migration through shallow sediments. The concretion consists of a cortex encasing a central channel filled with sparite and varying amounts of brecciated material derived from the channel walls. The studied bivalve is preserved with the hinge upward. When restored to life position, the central channel is connected to the anterior side of the bivalve. In the lower longer segment, the channel is connected to the posterior sector of the bivalve. The channels are interpreted as the anterior tube for supply of respiration water and the posterior tube for mining sulphide generated in the sulfate-methane transition zone of the sediment. The dolomitic cortex surrounding the central channel is characterized by δ13C values as low as −29‰ VPDB, indicating that the rate of anaerobic oxidation of methane was enhanced around the conduit. At the base of the central channel, upward soft sediment deformation structures likely resulted from the dragging effect when fluid migrated out of the sediment and entered the channel. Brecciation of the channel walls indicates pulsating seepage during the incipient stages of cementation. The burrow, thus, acted as a highly permeable conduit channeling upstreaming methane-charged fluid for a substantial period of time. Therefore, burrows can be efficient in pre-determining the formation of fluid conduits at seep sites, having potentially a large impact on fluid mixing and diagenetic reaction rates in sediments, and ultimately the amount of methane released. The reported bivalve with its conserved burrow fossilized as seep carbonate concertion provides direct evidence that fluid conduits form along animal burrows.

The impact of seasonal sulfate–methane transition zones on methane cycling in a sulfate‐enriched freshwater environment

AbstractLake Willersinnweiher located in south‐western Germany is a small eutrophic gravel pit lake fed by sulfate‐enriched groundwater. The aim of this study was to investigate the total methane (CH4) mass balance of Lake Willersinnweiher with a particular focus on the interaction of carbon and sulfur cycling within the lake sediments and the redoxcline of the water column. Our results show that Lake Willersinnweiher permanently releases CH4 to the atmosphere throughout the whole year 2018 at rates ranging from 5 to 120 mol d−1. Sediment data show the presence of intense anaerobic oxidation of CH4 in the upper sediment layers during early summer. Here, CH4 is most likely consumed via sulfate in sulfate–methane transition zones (SMTZs) that have been observed for a few specific freshwater environments only. Seasonal dynamics in biogeochemical processes trigger the non‐steady state conditions within the sediments and the CH4 consumption in the SMTZs. In parallel, CH4 released from the sediments is completely consumed by aerobic oxidation processes in the redoxcline indicated by minimum CH4 concentrations with high δ13C–CH4 values. This zone acts as an effective barrier, minimizing CH4 release into the surface water and the atmosphere and thus CH4 oversaturation along with near‐atmospheric isotopic composition indicate the presence of an additional CH4 source in the epilimnion of Lake Willersinnweiher.

Deciphering the geochemical link between seep carbonates and enclosed pyrite: A case study from the northern South China sea

Sulfate-driven anaerobic oxidation of methane (SD-AOM) is an important microbial process at methane seeps, which consumes most of the methane migrating from sediments and can lead to the formation of seep carbonate and pyrite. Geochemical proxies archived in seep carbonates have been long used to uncover the nature of seepage. Recently, the sulfur isotopic composition of enclosed pyrite shows ideal prospects for targeting the seepage dynamics, but it remains challenging due to a possible decoupled precipitation of carbonate and pyrite. In order to decipher the geochemical link between seep carbonates and enclosed pyrite, multiple geochemical proxies of both carbonate (major elements, rare earth elements, δ13C) and pyrite (contents and δ34Spy) were analyzed from two seep sites of the northern South China Sea. Multiple carbonate phases were identified via X-ray diffraction and transmission electron microscopy, including microcrystalline aragonite and botryoidal aragonite from Haima seeps, and micritic high-magnesium calcite (HMC), weakly ordered dolomite and botryoidal aragonite from site TVG11. The δ13C values for carbonates from both sites exhibit very light carbon isotopic signatures (from −59.2 to −35.8‰), confirming that the carbon source is mainly derived from the oxidation of biogenic methane. For pyrite enclosed in carbonates, their contents and sulfur isotopic compositions (δ34Spy) reveal generally positive correlations with Mg/Ca ratios, where HMC and dolomite show higher pyrite contents and δ34Spy values, but microcrystalline aragonite and botryoidal aragonite display lower pyrite contents and negative δ34Spy values. This is consistent with our knowledge that HMC and dolomite tend to precipitate at a deeper sulfate-methane transition zone (SMTZ) where the replenishment of seawater sulfate is limited, whereas microcrystalline aragonite and botryoidal aragonite tend to form at shallow SMTZ with enhanced seawater sulfate supplement. Thus, the δ34S values of pore water sulfate greatly affect the resulting sulfur isotopic composition of pyrite, i.e., the higher δ34Spy values in HMC and dolomite are attributed to the 34S enrichment in pore water sulfate at deeper SMTZ. This is supported by SIMS results that pyrite entrapped in botryoidal aragonite displays lower δ34SSIMS values than those in HMC. Our results suggest that geochemical proxies archived in both carbonate and pyrite point to the similar sedimentary environments, suggesting an intrinsic link between these two archives in our study sites.

Depth profiles of geochemical features, geochemical activities and biodiversity of microbial communities in marine sediments from the Shenhu area, the northern South China Sea.

The biogeochemical processes, anaerobic oxidation of methane (AOM) and methanogenesis, control methane emission and create distinct geochemical profiles with depth in marine sediments. Correlating the capacities and biodiversity of the microbial communities in marine sediments remains challenging. We therefore investigated the geochemical constituents and the capabilities and diversity of microbial communities in sediments at different depths in two cores from the Shenhu area in the northern South China Sea, which is characterized by underlying gas hydrates. The geochemical features, sulfate concentration decreased linearly and the acid volatile sulfur accumulated from 4m below the seafloor (mbsf) to the bottom, indicating significant sulfate reduction. However, the methane concentration was relatively low and showed irregular trends, indicating that our study cores did not reach the sulfate-methane transition zone (SMTZ). Nevertheless, incubation experiments showed that the microbial groups in sediments performed AOM and methanogenesis in the region where sulfate decreased linearly above the SMTZ. We mapped the diversity and abundance of microbial communities in sediments with depth using high-throughput sequencing. A small proportion of known methanogens (<0.3%) may have been responsible for the methanogenesis during incubation. No classical archaeal anaerobic methanotroph (ANME) sequences were detected across all samples; only a small amount of SEEP-SRB1 were detected, and their abundance did not increase with increasing depth. Thus, unknown or unconventional phylotypes may have participated in AOM during the incubation, and the dominant phylum Bathyarchaeota or the small number of detected methanogens are the most likely performers of AOM.

The evolution of early diagenetic processes at the Mozambique margin during the last glacial-interglacial transition

The Mozambique continental margin experienced large variations in sedimentation rates, primarily due to re-routing of sediment deposition from the Zambezi River during the last glacial-Holocene transition. As changes in sediment accumulation and organic matter deposition impose a strong control on the formation of authigenic minerals in the sediment, the distribution of these minerals may reflect the regional paleoenvironmental and paleoclimatic evolution. Combining geochemical analyses of porewaters and sediments with a reactive transport modeling approach, we reconstruct the depositional history and its effect on pyrite formation and other biogeochemical transformations at a site on the Mozambique margin over the past 27 kyr. Fitting the model to match the observed geochemical patterns, most importantly authigenic pyrite, allowed for the reconstruction of past sulfate-methane transition zone depth, which migrated in response to changes in the sediment accumulation and organic matter deposition. Changes in sediment deposition quickly affected organoclastic sulfate reduction and associated pyrite formation, but the effect on anaerobic methane oxidation and subsequent pyrite formation occurred with a lag on the order of thousands of years. Model results reveal a transition from high diagenetic reaction rates representative of near-shore depositional environments during the late glacial maximum, to a setting typical of offshore sediments with low reaction rates at the present day. Notably, the remnants of methane and dissolved iron pools produced in the past still shape the diagenetic processes at and below the sulfate-methane transition zone today. Since deglacial shelf-flooding and corresponding changes in sediment deposition occurred along continental margins worldwide, our analysis highlights the important role of non-steady state diagenesis in continental margin sediments and its relevance for paleoceanographic interpretation of sediment cores experiencing strong variations in sediment input.

Anthropogenic and Environmental Constraints on the Microbial Methane Cycle in Coastal Sediments.

Large amounts of methane, a potent greenhouse gas, are produced in anoxic sediments by methanogenic archaea. Nonetheless, over 90% of the produced methane is oxidized via sulfate-dependent anaerobic oxidation of methane (S-AOM) in the sulfate-methane transition zone (SMTZ) by consortia of anaerobic methane-oxidizing archaea (ANME) and sulfate-reducing bacteria (SRB). Coastal systems account for the majority of total marine methane emissions and typically have lower sulfate concentrations, hence S-AOM is less significant. However, alternative electron acceptors such as metal oxides or nitrate could be used for AOM instead of sulfate. The availability of electron acceptors is determined by the redox zonation in the sediment, which may vary due to changes in oxygen availability and the type and rate of organic matter inputs. Additionally, eutrophication and climate change can affect the microbiome, biogeochemical zonation, and methane cycling in coastal sediments. This review summarizes the current knowledge on the processes and microorganisms involved in methane cycling in coastal sediments and the factors influencing methane emissions from these systems. In eutrophic coastal areas, organic matter inputs are a key driver of bottom water hypoxia. Global warming can reduce the solubility of oxygen in surface waters, enhancing water column stratification, increasing primary production, and favoring methanogenesis. ANME are notoriously slow growers and may not be able to effectively oxidize methane upon rapid sedimentation and shoaling of the SMTZ. In such settings, ANME-2d (Methanoperedenaceae) and ANME-2a may couple iron- and/or manganese reduction to AOM, while ANME-2d and NC10 bacteria (Methylomirabilota) could couple AOM to nitrate or nitrite reduction. Ultimately, methane may be oxidized by aerobic methanotrophs in the upper millimeters of the sediment or in the water column. The role of these processes in mitigating methane emissions from eutrophic coastal sediments, including the exact pathways and microorganisms involved, are still underexplored, and factors controlling these processes are unclear. Further studies are needed in order to understand the factors driving methane-cycling pathways and to identify the responsible microorganisms. Integration of the knowledge on microbial pathways and geochemical processes is expected to lead to more accurate predictions of methane emissions from coastal zones in the future.

ANME-1 archaea may drive methane accumulation and removal in estuarine sediments.

ANME-1 archaea subsist on the very low energy of anaerobic oxidation of methane (AOM). Most marine sediments shift from net AOM in the sulfate methane transition zone (SMTZ) to methanogenesis in the methane zone (MZ) below it. In White Oak River estuarine sediments, ANME-1 comprised 99.5% of 16S rRNA genes from amplicons and 100% of 16S rRNA genes from metagenomes of the Methanomicrobia in the SMTZ and 99.9% and 98.3%, respectively, in the MZ. Each of the 16 ANME-1 OTUs (97% similarity) had peaks in the SMTZ that coincided with peaks of putative sulfate-reducing bacteria Desulfatiglans sp. and SEEP-SRB1. In the MZ, ANME-1, but none of the putative sulfate-reducing bacteria or cultured methanogens, increased with depth. Our meta-analysis of public data showed only ANME-1 expressed methanogenic genes during both net AOM and net methanogenesis in an enrichment culture. We conclude that ANME-1 perform AOM in the SMTZ and methanogenesis in the MZ of White Oak River sediments. This metabolic flexibility may expand habitable zones in extraterrestrial environments, since it enables greater energy yields in a fluctuating energetic landscape.

Deeply Buried Authigenic Carbonates in the Qiongdongnan Basin, South China Sea: Implications for Ancient Cold Seep Activities

Cold seep carbonates are important archives of pore water chemistry and ancient methane seepage activity. They also provide an important contribution to the global carbon sink. Seep carbonates at three sediment layers (3.0, 52.1, and 53.6 mbsf) were collected at site W08B in the Qiongdongnan Basin of the South China Sea. This study investigated the mineralogy, microstructure, stable carbon and oxygen isotopes, trace elements, and U-Th dates of these carbonates to identify the relationship between methane flux and authigenic carbonate precipitation. The results showed that the δ13C and δ18O values of all carbonates are similar, indicating that the carbon source for shallow carbonates and deep carbonates has remained constant over time and included biogenic and thermogenic methane. Although carbonates were found in three sediment layers, the two main stages of methane seepage events were discernible, which was likely caused by the dissociation of gas hydrates. The first methane seep took place at 131.1–136.3 ka BP. During a dramatic drop in the sea level, the seep carbonate at 52.1 mbsf formed at 136.3 ka BP through the anaerobic oxidation of methane (AOM). The carbonate at 53.6 mbsf resulted from the vertical downward movement of the sulfate-methane transition zone with decreasing methane flux at 131.1 ka BP. This is the reason for the age of carbonates at 52.1 mbsf being older than the age of carbonates at 53.6 mbsf. The second methane seep took place at 12.2 ka BP. Shallow carbonate formed at that time via AOM and is now located at 3 mbsf. Moreover, thin-section photomicrographs of deep carbonate mainly consisted of matrix micrite and biological debris and acicular aragonite occurred as vein cement filling the pore spaces between the matrix micrite. The acicular aragonite was mainly influenced by the timing of the carbonate precipitation of minerals. This research identified a long history of methane seep activity reflected by the vertical distribution of carbonates.

A novel authigenic magnetite source for sedimentary magnetization

AbstractWe report a novel authigenic nanoscale magnetite source in marine methane seep sediments. The magnetite occurs in large concentrations in multiple horizons in a 230 m sediment core with gas hydrate–bearing intervals. In contrast to typical biogenic magnetite produced by magnetotactic bacteria and dissimilatory iron-reducing bacteria, most particles have sizes of 200–800 nm and many are aligned in distinctive structures that resemble microbial precipitates. The magnetite is interpreted to be a byproduct of microbial iron reduction within methanic sediments with rapidly changing redox conditions. Iron sulfides that accumulated at a shallow sulfate-methane transition zone were oxidized after methane seepage intensity decreased. The alteration process produced secondary iron (oxyhydr)oxides that then became a reactive iron source for magnetite authigenesis when methane seepage increased again. This interpretation is consistent with 13C depletion in coexisting carbonate nodules. The authigenic magnetite will record younger paleomagnetic signals than surrounding sediments, which is important for paleomagnetic interpretations in seep systems. The microbial and possibly abiotic processes that caused these magnetic minerals to form at moderate burial depths remain to be determined.

Hydrocarbon-seep deposits in the lower Permian Angie Formation, Central Lhasa Block, Tibet

Methane seepage along continental margins has been linked to climatic warming, yet it is uncertain whether such seepage took place during the Permian, a period that witnessed a transition from an icehouse to a greenhouse world. To address this issue, we investigated calcareous concretions and layers within mudstones/shales of the Kungurian Angie Formation in the Xainza area, central Tibet. A combination of petrographic, mineralogical, elemental, and carbon‑sulfur isotopic analyses provide strong evidence for a hydrocarbon-seep origin of these carbonates, with thermogenic methane and crude oil being the most likely carbon sources. Our dataset also indicates that, although fueled by a common deep-buried hydrocarbon reservoir, these layers and concretions experienced different biogeochemical processes due to variable rates of cold-seep fluid flux combined with persistently low sulfate concentrations in bottom waters of the Bangong-Nujiang Tethyan Ocean. A biogeochemical switch from sulfate-dependent to iron oxide-coupled anaerobic oxidation of methane in the sulfate-methane transition zone, likely triggered by sulfate exhaustion, was inferred to affect carbonate layer formation near the sediment-water interface. If true, this switch, together with common vent structures, would have resulted in escape of large amounts of methane directly to the ocean-atmosphere system. The hydrocarbon seeps in the central Lhasa Block was approximately coeval with rapid climate warming in the peri-Gondwanan region during the early Kungurian. Although further investigations are required in the future, this study demonstrates a possible causal relationship between regional tectonic evolution, hydrocarbon seepage, and global climate change.

Assessing methane cycling in the seep sediments of the mid-Okinawa Trough: Insights from pore-water geochemistry and numerical modeling

The methane-enriched fluids in cold seeps are likely to crystallize as gas hydrates and serve as crucial sources of carbon to seawater. In this research, we analyzed the pore-water composition in terms of CH4, dissolved inorganic carbon (DIC), Cl−, Br−, SO42−, Na+, Mg2+, Ca2+, Sr2+, and NH4+, and the δ13CDIC, δ13CCH4, and δDCH4 values of two gravity cores and six remotely operated vehicle (ROV) video-guided push cores retrieved from fault scarps and dome-like structures (DSs) on the western slope of the mid-Okinawa Trough. In addition, a reaction–transport model was applied to quantify the methane fluxes and related biogeochemical processes. Active seepage of biogenic (δ13CCH4 ~ –70‰ V-PDB) and thermogenic (δ13CCH4 = −40‰ to −56‰ V-PDB) methane was identified on fault scarps and dome structures, respectively. Methane seepage was controlled by the transport and dissolution of the ascending gas rather than by clay dehydration or gas hydrate dissociation-induced fluid advection. The high methane concentrations and shallow sulfate–methane transition zones (SMTZs; between 0.1 and 0.4 mbsf) at sites R3-C2, R4-C4, and R6-C1 suggest strong methane seepage at three of the four studied DS (the highest gas dissolution rates are 6450, 1475, and 515 mmol m−2yr−1). Site GC08, located along a fault scarp, exhibits a moderate methane seepage; the SMTZ is located at ~2.5mbsf, and the rate of anaerobic oxidation of methane (AOM) is 130 mmol m−2yr−1. The methane migrating from depth is mainly consumed by AOM. However, ~12%–66% of the methane released from the two most intensive seep sites escapes to the water column. The precipitation of high–Mg calcite (at all sites) and aragonite (only at site R3-C2) has fixed 27%–50% (average = 39%) of the DIC. Therefore, the carbon outputs to the water column have been reduced. In the study area, the area-weighted seafloor CH4 and DIC fluxes are ~30 and 20 mmol m−2yr−1, respectively. Together, they correspond to ~7%–14% of the organic carbon burial rate, indicating that sediments could not be simply regarded as a stable carbon sink because they provide methane and a certain amount of DIC to the water column. Our findings contribute to the results of the ongoing efforts in understanding carbon cycling in submarine cold seep systems.

Methane transport and sources in an Arctic deep-water cold seep offshore NW Svalbard (Vestnesa Ridge, 79°N)

We investigate the uppermost 60 cm of sediment in active pockmarks of a deep-water methane seep site from Vestnesa Ridge offshore NW Svalbard. Using video guided core sampling with a remotely operated vehicle we collected push cores directly from bacterial mats within two active pockmarks, Lunde and Lomvi. Pore water analyses show very shallow sulphate methane transition zones and transport-reaction modelling suggests a considerable amount of dissolved methane passing through the sediment water interface due to upwards advection of an aqueous fluid not previously reported from Vestnesa Ridge. In addition, we show that the amount of methane that bypasses the benthic methane filter greatly increases with higher aqueous fluid advection rate. Recent changes in methane flux are evident from lipid biomarker, seep carbonate, and δ13C-organic carbon profiles in both pockmarks. Hydrocarbons at this cold seep site are supplied both by deep thermogenic sources from below the gas hydrate stability zone but also to a significant degree by microbial methanogenesis which dominates the signature in our shallow sediment cores with δ13C–CH4 values as low as −77‰.

The Formation of Authigenic Carbonates at a Methane Seep Site in the Northern Part of the Laptev Sea

Authigenic carbonates from cold seeps are unique archives for studying environmental conditions, including biogeochemical processes associated with methane-rich fluid migration through the sediment column. The aim of this research was to study major oxide, mineralogical, and stable isotopic compositions of cold-seep authigenic carbonates collected in the northern part of the Laptev Sea. These carbonates are represented by Mg-calcite with an Mg content of 2% to 8%. The δ13C values range from −27.5‰ to −28.2‰ Vienna Peedee belemnite (VPDB) and indicate that carbonates formed due to anaerobic oxidation of methane, most likely thermogenic in origin. The authigenic pyrite in Mg-calcite is evidence of sulfate reduction during carbonate precipitation. The δ18O values of carbonates vary from 3.5‰ to 3.8‰ VPDB. The calculated δ18Ofluid values show that pore water temperature for precipitated Mg-calcite was comparable to bottom seawater temperature. The presence of authigenic carbonate in the upper horizons of sediments suggests that the sulfate–methane transition zone is shallowly below the sediment–water interface.

Effects of postglacial seawater intrusion on sediment geochemical characteristics in the Romanian sector of the Black Sea

Pore water and sediment geochemistry in the western Black Sea were investigated on long Calypso piston core samples. Using this type of coring device facilitates the recovery of the thick sediment record necessary to analyze transport-reaction processes in response to the postglacial sea-level rise and intrusion of Mediterranean salt water 9 ka ago, and thus, to better characterize key biogeochemical processes and process changes in response to the shift from lacustrine to marine bottom water composition. Complementary data indicate that organic matter degradation occurs in the upper 15 m of the sediment column. However, sulfate reduction coupled with Anaerobic Methane Oxidation (AOM) is the dominant electron-accepting process and characterized by a shallow Sulfate Methane Transition Zone (SMTZ). Net silica dissolution, total alkalinity (TA) maxima and carbonate peaks are found at shallow depths. Pore water profiles clearly show the uptake of K+, Mg2+ and Na + by, and release of Ca2+ and Sr2+ from the heterogeneous lacustrine sediments, which is likely controlled by chemical reactions of silicate minerals and changes in clay mineral composition. Iron (Fe2+) and manganese (Mn2+) maxima largely coincide with Ca2+ peaks and suggest a close link between Fe2+, Mn2+ and Ca2+ release. We hypothesize that the Fe2+ maxima below the SMTZ result from deep Fe3+ reduction linked to organic matter degradation, either driven by DOC escaping from the shallow sulfate reduction zone or slow degradation of recalcitrant POC. The chemical analysis of dissolved and solid iron species indicates that iron is essentially associated with clay minerals, which suggests that microbial iron reduction is influenced by clay mineral composition and bioavailability of clay mineral-bound Fe(III). Overall, our study suggests that postglacial seawater intrusion plays a major role in shaping redox zonation and geochemical profiles in the lacustrine sediments of the Late Quaternary.