Methane-rich Fluids Research Articles

A new method for the U–Th dating of a carbonate chimney deposited during the last glaciation in the northern Okinawa Trough, East China Sea

Authigenic carbonates from cold seep systems, such as chimney and crust carbonate deposits, are important archives of biogeochemical processes associated with the migration and seepage of methane-rich fluids and seepages on the seafloor. Here we use a new U–Th dating method to precisely determine the ages of the authigenic carbonates of a chimney in the Okinawa Trough (OT), East China Sea. The method uses a series of heavy liquids (HL, a mixture of tribromomethane and alcohol with a density of 2.71–2.80 g/cm 3 ) to separate subsamples of carbonate contaminated by detrital material. The contents of relevant elements (Al, Rb, Zr, Nb, and rare earth elements) of the subsamples reveal that the separation of the mineral phase was successful. Five samples from along the radius of a well-structured chimney (PES-1) from station D2-5 in the OT were analyzed and yielded U–Th ages of 28.8 ± 0.4 ka, 26.1 ± 0.5 ka, 24.4 ± 0.7 ka, 22.5 ± 0.6 ka and 20.1 ± 0.6 ka, from the exterior to the interior of the chimney section, and AMS 14 C dating of the same samples produced the similar results. The agreement between the 230 Th/U and AMS 14 C ages suggests that the application of heavy liquid methods in U–Th dating can be very successful, and it reduces the influence of detrital contamination. Therefore, the approach has a great potential space for studying the evolutionary history of authigenic carbonates in cold seep systems. Moreover, the new 230 Th/U ages show that the carbonate chimney was formed during the Last Glacial Maximum (LGM), which corresponding to the large sea-level fall in the East China Sea. Together with information on biogeochemical processes, the results will provide valuable geochronological information for the future study of active fluid venting on continental margins. • Carbonates chimney from metals-rich sediments can be precisely dated with U-series. • Heavy liquids were used to separate subsamples of seep impure carbonate. • Multiple-point HL regressions provide the more reliable chronology. • U–Th ages between 28.8 and 20.1 ka point to sea-level low stands during the LGM.

Geochemistry of clumped isotopologues of CH4 within fluid inclusions in Alpine tectonic quartz fissures

Petroleum-bearing fluid inclusions are small encapsulations of oil and gas that persist in the rock record long after their parent fluids have moved on. The chemical and isotopic compositions of inclusions offer a unique way to investigate hydrocarbon formation pathways in deep Earth environments and in ancient geological times. Here we use stable isotope compositions of methane, including clumped isotopologues 13CH3D and 12CH2D2, to evaluate the formation conditions of methane within fluid inclusions in Alpine tectonic quartz fissures. We observe that this CH4 is relatively 13C and 2H enriched (δ13C = −26 to −39‰; δ2H = −126 to −146‰), consistent with the formation by advanced catagenesis of organic matter, and has clumped isotope compositions indicating intramolecular isotopic equilibrium at catagenetic to low-grade (sub-greenschist) metamorphic temperatures (TΔ13CH3D: 120-300°C; TΔ12CH2D=2120-260°C). Our findings, combined with an analysis of the geographic distributions of inclusion-bearing fissures and fluid inclusion PVT behaviors, provide direct evidence of reservoirs of thermogenic methane trapped beneath the Alps during Mid-Miocene tectonic nappe emplacement (c.a. 25 to 15 Ma). The gas was sequestered in a concentrated form that likely favored the accumulation of up to 50,000-150,000 Megatons (Mt) of CH4 across the Helvetic domain in Switzerland. Following the subsequent tectonic uplift during the period 17 to 10 Ma, we suggest that the trapped methane-rich fluids migrated to shallower depths along faults, leading to the release of Alpine metamorphic methane into near-surface aquifers and the atmosphere at rates of 0.01 to 0.03 Mt⋅y−1.

Miocene Seep-Carbonates of the Northern Apennines (Emilia to Umbria, Italy): An Overview

The natural emission of methane-rich fluids from the seafloor, known as cold seepage, is a widespread process at modern continental margins. The studies on present-day cold seepages provide high-resolution datasets regarding the fluid plumbing system, biogeochemical processes in the sediment, seafloor seepage distribution and ecosystems. However, the long-term (hundreds of thousands to millions of years) evolution of cold seepage remains elusive. The identification and study of outcrop analogous now exposed on land represent a valuable method for better understanding the effects of geological processes and climate forcing on the development of cold seepage systems. Here, we provide an overview on Miocene seep-carbonate deposits of the northern Apennines (from Emilia to the Umbria-Marchean sector, Italy), based on decades of field research integrated with detailed sedimentological and geochemical investigations. We report a total of 13 seep-carbonate outcrops, which formed in three different structural settings of the paleo-accretionary wedge corresponding to wedge-top basins, outer slope and intrabasinal highs at the deformational front. We discuss the recurring lithostratigraphic occurrence of seep deposits and the main compositional features (carbonate facies, carbon and oxygen stable isotopes) in order to interpret the seepage dynamics, duration and infer the contribution of methane-rich fluids released by paleo-gas hydrates. The datasets presented in this study represent a valuable complete record of cold seepage spanning ~12 Myr, that can be used to better understand factors controlling the regional-scale spatial and temporal evolution of cold seepage systems at modern active continental margins.

Active Anaerobic Archaeal Methanotrophs in Recently Emerged Cold Seeps of Northern South China Sea.

Cold seep ecosystems are developed from methane-rich fluids in organic rich continental slopes, which are the source of various dense microbial and faunal populations. Extensive studies have been conducted on microbial populations in this unique environment; most of them were based on DNA, which could not resolve the activity of extant organisms. In this study, RNA and DNA analyses were performed to evaluate the active archaeal and bacterial communities and their network correlations, particularly those participating in the methane cycle at three sites of newly developed cold seeps in the northern South China Sea (nSCS). The results showed that both archaeal and bacterial communities were significantly different at the RNA and DNA levels, revealing a higher abundance of methane-metabolizing archaea and sulfate-reducing bacteria in RNA sequencing libraries. Site ROV07-01, which exhibited extensive accumulation of deceased Calyptogena clam shells, was highly developed, and showed diverse and active anaerobic archaeal methanotrophs (ANME)-2a/b and sulfate-reducing bacteria from RNA libraries. Site ROV07-02, located near carbonate crusts with few clam shell debris, appeared to be poorly developed, less anaerobic and less active. Site ROV05-02, colonized by living Calyptogena clams, could likely be intermediary between ROV07-01 and ROV07-02, showing abundant ANME-2dI and sulfate-reducing bacteria in RNA libraries. The high-proportions of ANME-2dI, with respect to ANME-2dII in the site ROV07-01 was the first report from nSCS, which could be associated with recently developed cold seeps. Both ANME-2dI and ANME-2a/b showed close networked relationships with sulfate-reducing bacteria; however, they were not associated with the same microbial operational taxonomic units (OTUs). Based on the geochemical gradients and the megafaunal settlements as well as the niche specificities and syntrophic relationships, ANMEs appeared to change in community structure with the evolution of cold seeps, which may be associated with the heterogeneity of their geochemical processes. This study enriched our understanding of more active sulfate-dependent anaerobic oxidation of methane (AOM) in poorly developed and active cold seep sediments by contrasting DNA- and RNA-derived community structure and activity indicators.

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.

Methane seepage patterns during the middle Pleistocene inferred from molybdenum enrichments of seep carbonates in the South China Sea

Seepage of methane-rich fluids, are widespread along the continental margins of the world. Intense seepages of methane may lead to the emission of this powerful greenhouse gas into the hydrosphere and atmosphere with implications in the understanding of past dramatic marine environment changes and global warming. However, euxinic environments with sulfidic bottom water as a result of vigorous seepages at ancient cold seeps remain unidentified. Here we report trace element geochemistry and associated mineralogical and carbon and oxygen isotopic data in seep carbonates collected from long drill cores in the South China Sea. Three distinct stages of methane seepages events probably caused by the dissociation of gas hydrates are discernible. Carbonates formed in stage 2 during the Marine Isotope Stage 5 (MIS5) are mainly occurred as aragonite and show extremely high molybdenum (Mo) contents (up to 118 μg/g), and strong enrichments of authigenic Mo in relative to uranium. These results suggest the presence of intermittent or even permanent euxinic conditions generated by sulfidic bottom water that result from intensive methane seepages. In contrast, mineral compositions and trace element patterns indicate that the carbonates before and after the MIS 5 (stages 1 and 3) form within sulfidic conditions restricted to pore water under relatively lower methane fluxes. Our results highlight extremely intense methane seepages that may potentially result in localized euxinic environments. The inferred euxinic environments imply more amount of massive methane release into the water column than a previous estimation, which was triggered by large-scale dissociation of gas hydrates in the South China Sea during MIS 5.

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

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

Origin of the gypsum veins associated with the diagenetic dolomite nodules from the Neogene Marls of South-East Spain

The late Neogene marls from the Lorca, Murcia and Vera basins in S-E. Spain contain abundant dolomite nodules that were formed due to intense methane-rich fluid migration. The pathways for these fluids are evidenced by dense networks of fractures that are crossing the sedimentary layers and eventually the dolomite nodules. The fractures are generally filled by secondary fibrous gypsum that form veins of a few cm thick.The oxygen and sulfur isotopic compositions of the gypsum veins exhibit wide ranges (−2.2 < δ18O‰ VSMOW <6.7; −22.3 < δ34S‰ VCDT <10.5) that are far away from the average δ values of Tortonian-Messinian gypsum from the basins of Lorca, Fortuna and Sorbas (δ18O = 13.8 ± 2.3‰; δ34S = 21.3 ± 1.0‰) precipitated from the late Neogene seawater sulfate. These low δ values of the gypsum veins clearly indicate their diagenetic origin that would have resulted from sulfide oxidation with 18O-depleted water. The isotopic compositions of the diagenetic dolomite nodules close to the gypsum veins (−0.7 < δ18O‰ VPDB <3.7; −11.9 < δ13C‰ VPDB <7.3) are indicative of their formation in methane-rich fluids. In this context, the formation of sulfide is related to the bacterial sulfate reduction that is associated with the anaerobic oxidation of methane. During this process, pyrite coprecipitated with dolomite, either within the dolomite framework or within conduits where the fluids were seeping. The pyrite oxidation occurred later during diagenesis when more or less oxygenated groundwaters circulated within the fractures during the sealevel drawdown of the Messinian Salinity Crisis and after the regional uplift of the sedimentary deposits in relation with the Betic cordillera tectonics.

New constraints on the source, composition, and post-emplacement modification of kimberlites from in situ C–O–Sr-isotope analyses of carbonates from the Benfontein sills (South Africa)

Primary carbonates in kimberlites are the main CO2 carriers in kimberlites and thus can be used to constrain the original carbon and oxygen-isotope composition of kimberlite melts and their deep mantle sources. However, the contribution of syn- and post-emplacement processes to the modification of the C–O-isotope composition of kimberlites is yet to be fully constrained. This study aims to shed new light on this topic through a detailed textural, compositional (major and trace elements), and in situ C–O–Sr isotopic characterisation of carbonates in the Benfontein kimberlite sills (Kimberley, South Africa). Our multi-technique approach not only reveals the petrographic and geochemical complexity of carbonates in kimberlites in unprecedented detail, but also allows identification of the processes that led to their formation, including: (1) magmatic crystallisation of Sr-rich calcite laths and groundmass; (2) crystallisation of late groundmass calcite from hydrothermal fluids; and (3) variable degrees of crustal contamination in carbonate-rich diapirs and secondary veins. These diapirs most likely resulted from a residual C–O–H fluid or carbonate melt with contributions from methane-rich fluids from the Dwyka shale wall rock, leading to higher 87Sr/86Sr and δ18O, but lower δ13C values than in pristine magmatic calcite. Before coalescing into the diapiric segregations, these fluids/melts also variably entrained early formed calcite laths and groundmass phases. Comparison between in situ and bulk-carbonate analyses confirms that O isotopic analyses of bulk carbonates from kimberlite rocks are not representative of the original isotopic signature of the kimberlite magma, whereas bulk C-isotope compositions are similar to those of the pristine magmatic carbonates. Calcite laths and most groundmass grains at Benfontein preserve isotopic values (δ18O = 6–8‰ and δ13C = − 4 to − 6‰), similar to those of unaltered carbonatites worldwide, which, therefore, probably correspond to those of their parental melts. This narrow range suggests kimberlite derivation from a mantle source with little contribution from recycled crustal material unless the recycled material had isotopic composition indistinguishable from typical mantle values.

Stable and clumped isotope characterization of authigenic carbonates in methane cold seep environments

Cold seep environments are characterized by methane-rich fluid migration and discharge at the seafloor. These environments are also intimately linked to microbial communities, which oxidize methane anaerobically, increase alkalinity and promote authigenic carbonate precipitation. We have analyzed a suite of methane-derived authigenic carbonate (MDAC) crusts from the North and Barents Sea using stable and clumped isotopes (δ13C, δ18O, δ44Ca, and Δ47) to characterize the sources of fluids as well as the environment of carbonate authigenesis. We additionally assess the potential of MDACs as a Δ47-based paleotemperature archive.The MDACs occur as three main textural-mineralogic types: micritic Mg-calcite cements, micritic aragonite cements and cavity filling aragonite cements. We find that micritic Mg-calcite cements have low δ13CVPDB values (−30 to −47‰), high δ44CaSW values (−0.4 to −0.8‰), and Δ47-temperatures (0–6 °C) consistent with shallow sub-seafloor precipitation in isotopic equilibrium. Micritic aragonite cements and cavity filling aragonite cements both have a wider range in δ13CVPDB values (−18 to −58‰), lower δ44CaSW values (−0.8 to −1.6‰) and a larger range in Δ47-based apparent temperatures (–2 – 25 °C) with samples displaying equilibrium and disequilibrium clumped isotope values.The range in apparent temperatures as well as δ44CaSW values seen in the aragonite MDACs suggest two kinetic processes: a kinetic isotope effect (KIE) due to the incomplete equilibration of carbon and oxygen isotopes among DIC species from the different sources of DIC (i.e., seawater, methane-sourced DIC and DIC residual to CO2 degassing or diffusion) and a KIE due to a fast, irreversible precipitation affecting the cations, particularly Ca, bound to carbonate mineral. Our results improve the understanding of kinetic effects on clumped isotope temperatures in MDACs and demonstrate how the multi-isotopic approach combined with textural-mineralogic criteria can be used to identify MDACs for accurate paleotemperature reconstructions.

A new approach to discern the hydrocarbon sources (oil vs. methane) of authigenic carbonates forming at marine seeps

Numerous marine hydrocarbon seeps have been discovered in the past three decades, the majority of which are dominated by methane-rich fluids. However, an increasing number of modern oil seeps and a few ancient oil-seep deposits have been recognized in recent years. Oil seepage exerts significant control on the composition of the seep-dwelling fauna and may have impacted the marine carbon cycle through geological time to a greater extent than previously recognized. Yet, distinguishing oil-seep from methane-seep deposits is difficult in cases where δ13Ccarb values are higher than approximately −30‰ due to mixing of different carbon sources. Here, we present a comparative study of authigenic carbonates from oil-dominated (site GC232) and methane-dominated (site GC852) seep environments of the northern Gulf of Mexico, aiming to determine the geochemical characteristics of the two types of seep carbonates. We analyzed (1) major and trace element compositions of carbonates, (2) total organic carbon (TOC), total nitrogen (TN) and carbon isotope (δ13CTOC) of residue after decalcification, (3) sulfur isotope signatures of chromium reducible sulfur (CRS, δ34SCRS) and residue after CRS extraction (δ34STOS), as well as (4) sulfur contents (TOS) of residue after CRS extraction. Carbonates from the studied oil seep are dominated by aragonite and exhibit lower δ34SCRS values, suggesting carbonate precipitation close to the sediment surface. In addition, oil-seep carbonates are characterized by higher TOC and TOS contents and higher TOC/TN ratios, as well as less negative δ13CTOC values compared to methane-seep carbonates, probably reflecting a contribution of residual crude oil enclosed in oil-seep carbonates. Very low δ13CTOC values (as low as −68.7‰, VPDB) and low TOC/TN ratios of methane-seep carbonates indicate that the enclosed organic matter is derived mainly from the biomass of methanotrophic biota. This study presents new geochemical data that will allow the discrimination of oil-seep from methane-seep deposits. Although some of the geochemical patterns are likely to be affected by late diagenesis, if applied with caution, such patterns can be used to discern the two end-member types of seepage – oil seeps and methane seeps – in the geological record.

Fluid transport and reaction processes within a serpentinite mud volcano: South Chamorro Seamount

Natural fluids with a pH (25 °C) up to 12.3 were collected from a sub-seafloor borehole observatory (Ocean Drilling Program (ODP) Hole 1200C) on South Chamorro Seamount, a serpentinite mud volcano in the Mariana forearc. We used systematic differences in the chemical compositions of pore waters from drilling operations during ODP Leg 195 and borehole fluids collected subsequently from Hole 1200C to define two endmember solutions, one of which was a sulfate-rich fluid with a methane concentration of 50 mM that ascends from the subduction channel and the other was a low-sulfate fluid. The sequence of sample collection and fluid compositions constrain subsurface hydrologic conditions. Deep-sourced, sulfate- and methane-rich, sterile fluids from the subduction channel can reach the seafloor unchanged within the central conduit, whereas other fluid pathways likely intersect the pelagic sediment that underlies the serpentinite mud volcano, providing potentially suitable conditions and inoculum for microbial anaerobic oxidation of methane (AOM). These AOM-affected, low-sulfate fluids also make it to the seafloor where they discharge. The source of the sulfate- and methane-rich fluid in the subduction channel is attributed to abiotic methane production fueled by hydrogen production from serpentinization and carbonate dissolution. This methane production includes a mechanism to raise the pH above values from serpentinization alone. Results from South Chamorro Seamount represent an end member along a transect defined by the distance from the trench. Results from this site are applied to other serpentinite mud volcanoes along this transect to speculate on likely chemical conditions within shallower and cooler portions of the subduction channel.

Trace element systematics in cold seep carbonates and associated lipid compounds

Seeping of methane-rich fluids at submarine cold seeps drives intense microbial activity and precipitation of authigenic carbonates. Some trace elements play an important role in the biogeochemical processes operating at cold seeps, especially as specific enzymatic co-factors related to methanogenesis and the anaerobic oxidation of methane (AOM). However, it is unclear whether microbial trace metal utilization can be traced by the geochemical composition of seep carbonates. In this study, we analyzed a series of authigenic carbonate samples recovered from various seep settings worldwide and report for the first time trace element concentrations for total lipid fractions, combined with biomarker analyses and determination of elemental abundances in associated inorganic mineral phases (carbonate phases, sulfides, organic compounds and detrital fractions). Our results indicate marked enrichments of Co, Ni, Cu, Mo and W in the archaeal and bacterial lipids associated with authigenic carbonates, which can all be ascribed to previously identified enzymatic pathways. In addition to the microbial communities involved in AOM, which most likely control specific lipid-bound enrichments of Co, Ni, Mo and W in seep carbonates, Cu was found to display higher concentrations in the lipid fractions extracted from a few authigenic carbonate samples formed closer to the sediment-water interface, hence possibly related to the presence of aerobic methane-oxidizing bacterial assemblages in the near seafloor environment. While the above mentioned trace metals are relatively enriched in all studied inorganic and organic fractions, the very low W concentrations measured in carbonate phases, combined with their pronounced enrichment in associated lipid fractions and inferred microbial requirement, suggest that tungsten depletion in pore waters could possibly act as a limiting factor on AOM at cold seeps. Finally, two other trace elements (Li and Ti) also displayed particular enrichments in studied lipid fractions, which, despite no reported evidence, could possibly indicate that they are also involved as metalloenzymes in microbial methane oxidation processes at cold seeps.

Sediment geochemical study of hydrocarbon seeps in Isfjorden and Mohnbukta: a comparison between western and eastern Spitsbergen, Svalbard

Methane is the most widespread volatile hydrocarbon and one of the most potent greenhouse gases. Marine sediments form the largest methane reservoir, from where large quantities of methane from gas-saturated sediments are released into the sea water and into the shallow shelf seas, through the water column almost unaltered into the atmosphere. Craters on the sea floor known as pockmarks are often related to seepage of methane-rich fluids originating either from shallow (microbial) or deep (thermogenic) sources. This paper presents the geochemical characteristics of migrating hydrocarbon gas from selected pockmarks and surrounding seabed in Isfjorden and Mohnbukta in western and eastern Spitsbergen, respectively. Collected gas samples, including wet gas fraction, were analyzed for methane concentration and methane carbon stable isotope ratio. Mixtures of microbial and thermogenic methane, together with higher homologies were found in the pockmarks as well as in the adjacent undisturbed seafloor, suggesting that the seepage activity in both areas is currently on the same level. Although the methane concentration profiles and methane stable carbon isotope ratios in Isfjorden and Mohnbukta show signs of biological oxidation and elevated concentrations of ethane/propane indicate input from deeper thermogenic sources. The gas composition and methane carbon stable isotope ratio from Isfjorden and Mohnbukta areas show similar trends, most likely due to originating in the same Mesozoic organic-rich marine mudstone.

An Areal Assessment of Subseafloor Carbon Cycling in Cold Seeps and Hydrate-Bearing Areas in the Northern South China Sea

Gas hydrates, acting as a dynamic methane reservoir, store methane in the form of a solid phase under high-pressure and low-temperature conditions and release methane through the sediment column into seawater when they are decomposed. The seepage of methane-rich fluid (i.e., cold hydrocarbon seeps) fuels the chemosynthetic biota-inhabited surface sediments and represents the major pathway to transfer carbon from sediments to the water column. Generally, the major biogeochemical reactions related to carbon cycling in the anoxic marine sediments include organic matter degradation via sulfate reduction (OSR), anaerobic oxidation of methane (AOM), methanogenesis (ME), and carbonate precipitation (CP). In order to better understand the carbon turnover in the cold seeps and gas hydrate-bearing areas of the northern South China Sea (SCS), we collected geochemical data of 358 cores from published literatures and retrieved 37 cores and corresponding pore water samples from three areas of interest (i.e., Xisha, Dongsha, and Shenhu areas). Reaction-transport simulations indicate that the rates of organic matter degradation and carbonate precipitation are comparable in the three areas, while the rates of AOM vary over several orders of magnitude (AOM: 8.3-37.5 mmol·m-2·yr-1 in Dongsha, AOM: 12.4-170.6 mmol·m-2·yr-1 in Xisha, and AOM: 9.4-30.5 mmol·m-2·yr-1 in Shenhu). Both the arithmetical mean and interpolation mean of the biogeochemical processes were calculated in each area. Averaging these two mean values suggested that the rates of organic matter degradation in Dongsha (25.7 mmol·m-2·yr-1) and Xisha (25.1 mmol·m-2·yr-1) are higher than that in Shenhu (12 mmol·m-2·yr-1) and the AOM rate in Xisha (135.2 mmol·m-2·yr-1) is greater than those in Dongsha (27.8 mmol·m-2·yr-1) and Shenhu (17.5 mmol·m-2·yr-1). In addition, the rate of carbonate precipitation (32.3 mmol·m-2·yr-1) in Xisha is far higher than those of the other two regions (5.3 mmol·m-2·yr-1 in Dongsha, 5.8 mmol·m-2·yr-1 in Shenhu) due to intense AOM sustained by gas dissolution. In comparison with other cold seeps around the world, the biogeochemical rates in the northern SCS are generally lower than those in active continental margins and special environments (e.g., the Black sea) but are comparable with those in passive continental margins. Collectively, ~2.8 Gmol organic matter was buried and at least ~0.82 Gmol dissolved organic and inorganic carbon was diffused out of sediments annually. This may, to some extent, have an impact on the long-term deep ocean carbon cycle in the northern SCS.

Effects of oceanographic changes on controlling the stability of gas hydrates and the formation of authigenic carbonates at mud volcanoes and seepage sites on the Iberian margin of the Gulf of Cadiz

The Gulf of Cadiz is characterized by extensive active and inactive fluid escape structures which manifest as mud volcanoes, diapiric ridges, pockmarks and cold seeps. The high methane content and the presence of gas hydrates in the shallow sediments of the most active fluid escape structures indicate that these are areas of preferential migration and escape of fluids enriched in hydrocarbons and methane. Extensive fields of methane-derived authigenic carbonates are found at fluid escape structures along the upper and mid-continental slope, where the Mediterranean Outflow water is in direct contact with the seafloor, at water depths generally <1400 m. These methane-derived authigenic carbonates occur in much higher abundance at these shallower depths than in the fluid escape structures of the deeper (>1400 m) parts of the basin. The estimated U/Th ages of the authigenic carbonates, suggest that they were probably formed during discrete episodes of gas hydrate activity that may have occurred as a result of rapid climatic changes (such as the termination of the last glacial stage.) Numerical modelling indicates that seafloor temperature variations, associated with glacial/interglacial cycles and the changes of the Mediterranean Outflow pathway, could have restricted or eliminated the gas hydrate stability zone at the mud volcanoes shallower than 1400 m, resulting in intense fluxes of methane-rich fluids to the shallow sediments or even the seafloor. This fluid flux was recorded by the microbially mediated precipitation of authigenic carbonates.

Biogeochemical processes at the Krasniy Yar seepage area (Lake Baikal) and a comparison with oceanic seeps

The expulsion of sedimentary, methane-rich fluids to bottom waters is a widespread process in Lake Baikal (eastern Siberia), resulting in deep water cold seep systems comparable in size and frequency to those of oceanic, high-productivity continental margins. Little is known, however, about how biogeochemical processes in Baikal cold seeps compare with those of oceanic cold seeps. In this paper, we present new pore water chemistry data from the Krasniy Yar seepage area located on the slope near the Selenga river delta. We compare biogeochemical processes deduced from these pore water chemical profiles with processes prevalent at oceanic cold seeps of highly productive continental margins. This comparison allows to draw the following conclusions: (1) in sediments not affected by seepage the fresh water mass of Lake Baikal results in a very low relative importance of the nitrogenous and sulfidic geochemical zones compared to the ocean; (2) diagenetic processes involving silicate minerals are, however, similar in Lake Baikal and the ocean; (3) fluid advection rates in cold seep sediments are similar in Lake Baikal and ocean systems but (4) the deep methane flux of Baikal seeps is mitigated by reaction with O2, and possibly Mn(IV) and Fe(III) oxides, whereas in oceanic sediments the main methane-consuming process is the anaerobic oxidation of methane with sulfate. Lake Baikal cold seep sediments are therefore nearly devoid of authigenic carbonate minerals and have a reduced capacity to decrease the deep methane flux.

Authigenic carbonate formation revealed by lipid biomarker inventory at hydrocarbon seeps: A case study from the Okinawa Trough

Abstract Authigenic carbonates were recovered from the northern Okinawa Trough at 540–700 m water depth. Development of microbial communities and seepage dynamics driving the precipitation of authigenic carbonates remains poorly constrained, even though the source of methane-rich fluids, formation of Fe-rich carbonates, and the potential driving forces were previously reported. Here, petrologic observations, stable carbon and oxygen isotopic compositions, mineralogy, and lipid inventories of authigenic carbonates from the northern Okinawa Trough were analyzed. The carbonate minerals were comprised predominantly of aragonite, high-magnesium calcite, and siderite. The presence of molecular fossils diagnostic for anaerobic methane oxidizing archaea (ANME) and sulfate-reducing bacteria (SRB) with pronounced 13C depletions (as low as −134‰), together with highly negative δ13Ccarb values (as low as −55.8‰), suggest that the carbonates precipitated from anaerobic oxidation of methane (AOM) with microbial methane as the predominant carbon source. The difference between measured and calculated δ18Ocarb values (based on 0‰ of sea water δ18O vs V-SMOW) was higher than +3‰ in one of the carbonates (GGD16), reflecting an origin from gas hydrate dissociation. Most of the carbonates revealed δ18O offsets between measured and calculated values within +3‰, or a lower measured δ18O than calculated value, suggesting a mixture of methane-derived carbonates (low δ13C/high δ18O) and detrital origin carbonates (high δ13C/low δ18O). Biomarker patterns varied significantly among samples. A suite of 13C-depleted biomarkers indicated the predominance of an ANME-1 assemblage for all samples, suggesting carbonates formed at low to medium methane flux. The predominance of ANME-1, abundant detrital minerals, and allochthonous biomarkers, as well as low carbonate contents indicate that most samples formed at greater depth within the sediment column. Only one carbonate sample in this study is interpreted to have precipitated at relatively shallower depth, as indicated by the occurrence of crocetane, dominance of aragonite, and higher overall carbonate content.

Using chemical compositions of sediments to constrain methane seepage dynamics: A case study from Haima cold seeps of the South China Sea

Cold seeps frequently occur at the seafloor along continental margins. The dominant biogeochemical processes at cold seeps are the combined anaerobic oxidation of methane and sulfate reduction, which can significantly impact the global carbon and sulfur cycles. The circulation of methane-rich fluids at margins is highly variable in time and space, and assessing past seepage activity requires the use of specific geochemical markers. In this study, we report multiple sedimentary proxy records for three piston gravity cores (QDN-14A, QDN-14B, and QDN-31) from the Haima seep of the South China Sea (SCS). By combining total organic carbon (TOC), total inorganic carbon (TIC), total nitrogen (TN), total sulfur (TS), acid insoluble carbon and sulfur isotope (δ13Corganic carbon and δ34Sacid-insoluble), and δ34S values of chromium reducibility sulfur (δ34SCRS), as well as carbon isotopes of TIC (δ13CTIC) in sediments, our aim was to provide constraints on methane seepage dynamics in this area. We identified three sediment layers at about 260–300 cm, 380–420 cm and 480–520 cm sediment depth, characterized by particular anomalies of low δ13CTIC values and high TS content, high TS and CRS contents, and high δ34Sacid-insoluble and δ34SCRS values, respectively. On this basis, we propose that these sediment horizons correspond to distinct methane release events preserved in the sediment record. While the exact mechanisms accounting for the presence (or absence) of these particular geochemical signals in the sediment are not known, we propose that they correspond to variations in methane flux and their duration through time. Overall, our results suggest that sedimentary carbon and sulfur and their isotopes are useful tracers for better understanding of methane seepage dynamics over time.

Methane seepage inferred from pore water geochemistry in shallow sediments in the western slope of the Mid-Okinawa Trough

Abstract Widely distributed cold seeps are crucial sources of carbon to the seawater, whereas anaerobic oxidation of methane (AOM) and precipitation of authigenic carbonate might change the compositions of the methane-rich fluids and thus reduce the outputs of dissolved carbon to seawater. In this work, we analyze the pore water compositions of four gravity cores with high methane concentration abnormalities in pore water or in overlying seawater in the western slope of the Mid-Okinawa Trough. For the northern research area, active weak methane seepage through the seafloor is identified in the vicinity of mud volcanoes (sites C01 and C10), and strong emissions of gas and fluids may occur in the central mud volcanoes. In a submarine canyon at the south, C23 site exhibits rarely methane seepage, while high rate transportation of methane from deep sediments and associated AOM are identified at site C25 where small vertical faults and gas-bearing layers are developed at the surrounding. AOM at site C25 is indicated by (1) quasi-linearly decrease of sulfate concentrations with depths, (2) intense increases of dissolved inorganic carbon (DIC) concentrations and (3) significantly negative δ13C values (vary from −7.2 to −26.3‰) of DIC. Organoclastic degradation in the shallow sediments is not obviously recognized in the study area because of the low concentrations of NH4+, as well as negligible sulfate consumption at three of the four sites. Our data reveal that the cold systems are one of the DIC sources (the fluxes are ca. 11.2 mmol/m2/yr) to the seawater in the Okinawa Trough, which can be tracked by high depletion of 13C (δ13CDIC ranges from −7.2 to −26.3‰ VPDB).