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Boron and nitrogen co-doped carbon dots (BN-CDs) were synthesized by one-pot hydrothermal carbonization. Then the as-prepared CDs was combined with alginate and polyethyleneimine to fabricate the multi-layered coating on the cotton fabric by Layer-by-Layer (LbL) self-assembly method. The CDs containing coating emitted visible blue light under 365 nm UV lamp. Subsequently, the UV protection property, thermal stability, flame retardancy and wash durability of the CDs treated cotton fabric were investigated. The ultraviolet protection factors of pure cotton fabric and cotton fabric treated with 12 bilayers CDs consisting coating were 6.48 and 64.10, respectively, which indicated that the synthesized CDs was an efficient UV absorber. In TGA test, more char residues at 700°C were observed for CDs consisting coatings treated cotton fabrics under nitrogen atmosphere. After B, N co-doped CDs were enriched in the PEI/alginate multi-layered coating, the PHRR of cotton-CDs-12BL were reduced 15% and TPHRR of cotton-CDs-12BL were improved 26.4°C in microscale combustion calorimeter test. This result indicated that B, N co-doped CDs presented potential application as a flame retardant on surface of cotton fiber.

Anaerobic oxidation of methane (AOM) coupled with reduction of metal oxides is supposed to be a globally important bioprocess in marine sediments. However, the responsible microorganisms and their contributions to methane budget are not clear in deep sea cold seep sediments. Here, we combined geochemistry, muti-omics and numerical modeling to study metal-dependent AOM in methanic cold seep sediments in the northern continental slope of the South China Sea. Geochemical data based on methane concentrations, carbon stable isotope, solid-phase sediment analysis and pore water measurements indicate the occurrence of anaerobic methane oxidation coupled to metal oxides reduction in the methanic zone. The 16S rRNA gene amplicons and transcripts, along with metagenomic and metatranscriptomic data suggest that diverse ANME groups actively mediated methane oxidation in the methanic zone either independently or in syntrophy with e.g. ETH-SRB1 as potential metal reducers. Modeling results suggest that the estimated rates of methane consumption via Fe-AOM and Mn-AOM were both 0.3 μmol cm-2yr-1, which account for ∼3% of total CH4removal in sediments. Overall, our results highlight metal-driven anaerobic oxidation of methane as an important methane sink in methanic cold seep sediments.

Studying deep-water cold seep systems is of great significance to gas hydrate exploration due to their close relationship. Various cold seep systems and related gas hydrate accumulations have been discovered in the northern South China Sea in the past three decades. Based on high-resolution seismic data, subbottom profiles, in situ submergence observations, deep drilling and coring, and hydrate gas geochemical analyses, the geological and geophysical characteristics of these cold seep systems and their associated gas hydrate accumulations in the Qiongdongnan Basin, the Shenhu area, the Dongsha area, and the Taixinan Basin have been investigated. Cold seep systems are present in diverse stages of evolution and exhibit various seabed microgeomorphic, geological, and geochemical features. Active cold seep systems with a large amount of gas leakage, gas plumes, and microbial communities and inactive cold seep systems with authigenic carbonate pavements are related to the variable intensity of the gas-bearing fluid, which is usually derived from the deep strata through mud diapirs, mud volcanoes, gas chimneys, and faults. Gas hydrates are usually precipitated in cold seep vents and deeper vertical fluid migration pathways, indicating that deep gas-bearing fluid activities control the formation and accumulation of gas hydrates. The hydrocarbons collected from cold seep systems and their associated gas hydrate reservoirs are generally mixtures of biogenic gas and thermogenic gas, the origin of which is generally consistent with that of deep conventional gas. We also discuss the paragenetic relationship between the gas-bearing fluid and the seafloor morphology of cold seeps and the deep-shallow coupling of gas hydrates, cold seeps, and deep petroleum reservoirs. It is reasonable to conclude that the deep petroleum systems and gas-bearing fluid activity jointly control the development of cold seep systems and the accumulation of gas hydrates in the northern South China Sea. Therefore, the favorable areas for conventional oil and gas enrichment are also prospective areas for exploring active cold seeps and gas hydrates.

A flame retardant and hydrophobic coating was prepared by layer-by-layer self-assembly on cotton fabric. The coated fabric was treated with polyethylenimine, hypophosphorous acid modified chitosan,...

Methane is a potent greenhouse gas; methane production and consumption within seafloor sediments has generated intense interest. Anaerobic oxidation of methane (AOM) and methanogenesis (MOG) primarily occur at the depth of the sulfate-methane transition zone or underlying sediment respectively. Methanogenesis can also occur in the sulfate-reducing sediments through the utilization of non-competitive methylated compounds; however, the occurrence and importance of this process are not fully understood. Here, we combined a variety of data, including geochemical measurements, rate measurements and molecular analyses to demonstrate the presence of a cryptic methane cycle in sulfate-reducing sediments from the continental shelf of the northern South China Sea. The abundance of methanogenic substrates as well as the high MOG rates from methylated compounds indicated that methylotrophic methanogenesis was the dominant methanogenic pathway; this conclusion was further supported by the presence of the methylotrophic genus Methanococcoides. High potential rates of AOM were observed in the sediments, indicating that methane produced in situ could be oxidized simultaneously by AOM, presumably by ANME-2a/b as indicated by 16S rRNA gene analysis. A significant correlation between the relative abundance of methanogens and methanotrophs was observed over sediment depth, indicating that methylotrophic methanogenesis could potentially fuel AOM in this environment. In addition, higher potential rates of AOM than sulfate reduction rates at in situ methane conditions were observed, making alternative electron acceptors important to support AOM in sulfate-reducing sediment. AOM rates were stimulated by the addition of Fe/Mn oxides, suggesting AOM could be partially coupled to metal oxide reduction. These results suggest that methyl-compounds driven methane production drives a cryptic methane cycling and fuels AOM coupled to the reduction of sulfate and other electron acceptors.

Fluid flow patterns at cold seeps provide insights into the mechanism and influence of methane emission into the ocean, which is critical in its environmental impact assessment. Here, we report pore fluid compositions of three ~8 m long piston cores (QDN-14A, QDN-14B and R1) collected from the newly-discovered active Haima cold seeps on the northwestern slope of the South China Sea. Reaction-transport models were further applied to quantify related biogeochemical processes and to reveal the patterns of fluid flow. Extremely low δ13C values (< -52‰) of dissolved inorganic carbon (DIC) near the sulfate-methane transition in the three cores suggest that anaerobic oxidation of methane is the predominant biogeochemical process. The presence of small pieces of gas hydrates along with negative anomalies of porewater chloride and sodium concentrations reflects gas hydrate dissociation. Nearly invariable concentrations of sulfate, DIC, and calcium on a meter-scale were observed in the uppermost part of the sediment cores QDN-14A and QDN-14B. This irrigation-like feature is inferred to result from enhanced methane flux in QDN-14A and QDN-14B. We infer that lateral migration of methane-rich fluids from R1 site to QDN-14A and QDN-14B sites together with upward migrated methane is responsible for the enhanced methane flux. This speculation is supported by the occurrence of gas hydrates which might have clogged the fluid channel in the seepage center (R1) and driven the transportation of methane-bearing fluid along a coarser sediment layer in surrounding sediments (QDN-14A and QDN-14B). The proposed scenario is further demonstrated using a non-steady-state modeling that reconstructed the porewater irrigation-like feature assuming an increased methane flux. The modeling result predicts that gas-hydrate formation in core R1 started at least 150 yr B.P. The proposed fluid flow pattern within a localized seep site may have a great implication for understanding the heterogeneity of sedimentary records.

Authigenic carbonates retrieved from sites ROV1 and ROV2 of the Haima hydrocarbon seeps of the South China Sea at approximately 1390 m water depth were studied using lipid biomarker analyses. Abundant molecular fossils of anaerobic methane oxidizing archaea (ANME) and sulfate-reducing bacteria (SRB) with strong 13C-depletions (δ13C values as low as −126‰), in combination with low δ13Ccarbonate values (−42.7‰ to −36.8‰), provide evidence that anaerobic oxidation of methane (AOM) was the major process driving the precipitation of the studied seep carbonates. The extremely low δ13C values of archaeal biomarkers confirm that biogenic methane was the main carbon source, but the seepage of accessory crude oil is also suggested by the presence of unresolved complex mixtures in the hydrocarbon fractions. A suite of 13C-depleted biomarkers indicate the predominance of ANME-1/DSS consortia at both sites, which indicates that the studied carbonates formed during low to medium methane flux. Somewhat higher contents of archaeal biomarkers and their stronger 13C-depletion at site ROV2 probably reflect at least temporarily higher seepage intensities than at site ROV1. Abundant bacterial dialkyl glycerol diethers (DAGEs), revealing a large offset of their δ13C values compared to SRB-derived terminally branched fatty acids, were possibly produced by distinct SRB species other than members of the DSS cluster. The encountered hopanoids are attributed to aerobic methanotrophic bacteria based on their moderate 13C-depletion. The application of molecular fossils in combination with their compound-specific isotope signatures is an efficient tool to reconstruct the composition of seepage fluids and seepage intensities.

Authigenic carbonates and pyrite associated with sulfate-driven anaerobic oxidation of methane (AOM) at methane seeps provide archives to explore the biogeochemical processes involved and seepage dynamics over time. The wide range and extremely high δ34S values of pyrite (δ34Spy) have been used to trace the AOM-related processes. However, the detailed mechanism for this phenomenon is not well understood. We propose that the characteristics of δ34Spy were mainly controlled by the competition between sulfate reduction and sulfate supply, as well as the redox condition. To test this hypothesis, we investigated Sr/Ca and Mg/Ca ratios, trace element compositions, pyrite contents and sulfur isotopic compositions in seep carbonates from Site F and Haima in the northern South China Sea. Calcite and aragonite contents were distinguished through the Sr/Ca and Mg/Ca ratios. The data show that aragonites are always associated with relatively low δ34Spy values compared to calcites. The Mo contents show a good correlation with pyrite contents in calcites and aragonites, and the slope in aragonites is larger than that in calcites. This relationship indicates that the aragonite precipitated in a relatively open system with higher Mo availability. Thus, we conclude that sulfides with low δ34S values formed at high supply of sulfate under the relatively open system with respect to diffusive replenishment of sulfate, where the carbonate precipitation occurred close to the seafloor due to a strong methane flux. Under vigorous methane flux simultaneously, the high potential of less anoxic conditions, which could limit the additional pyrite accumulation and/or favor the microbial disproportionation, could also be the cause of the low δ34Spy, as supported by samples from the Haima sites. Evidence for this assumption is based on the occurrence of bivalve shells and less enrichment in As and Sb. Conversely, the positive δ34Spy values result from near to complete exhaustion of dissolve sulfate via AOM within a deeper sulfate-methane transition zone, where Mo is less available. The combination of a detailed elemental study of authigenic carbonates with sulfur isotopic composition of sulfide minerals in carbonates is a promising tool for reconstructing the dynamics of seep intensities at modern and, potentially, geological seep sites.

Paraescarpia echinospica is a conspicuous annelid living in the cold seeps and hydrothermal vents of the Western Pacific region and relying on their endosymbiont bacteria as a source of energy and organic carbon. We report the complete mitochondrial genome of P. echinospica, which is 15,280 bp in length, containing 13 protein-coding genes, two ribosomal RNA genes, 22 tRNA genes and a putative control region. The overall base composition is AT-biased. The control region contains repeated nucleotide motifs. Phylogenetic analyses of the concatenated mitochondrial genes strongly support a sister relationship of P. echinospica with a clade containing Escarpia and Seepiophila.

The mean circulation pattern and its mechanism over Caiwei Guyot (1308~5600 m) in the Northwest Pacific were studied utilizing three years of in‐situ data. A deep anticyclonic cap was found to enclose the entire guyot from its bottom up to a depth of 728 m, which is comprised of a stable but highly asymmetric anticyclonic circulation at the foot and a bottom‐trapped anticyclonic circulation over the summit. On the slope, the circulation is complex with a dominant anticyclonic circulation near the bottom and a weak cyclonic circulation at ~2200 m. The anticyclonic cap intensity over the summit is significantly modulated by the time‐varying impinging flow. An intensified cold ring above the summit edge was observed at Caiwei Guyot, which differs from the cold domes observed over traditional conic seamounts. Further analysis suggests that the impinging flow is primarily responsible for the cap formation, and the M2 tide‐seamount interaction plays a secondary role. The anticyclonic cap may play a role in the local geological distribution.

This study presents new data for the identification of the source and assessment of the thermal maturity of oils based on the diamondoid indices of oils from the Tazhong and Luntai uplifts in the Tarim Basin in northwest China. The oil samples were divided into three groups according to biomarker characteristics and the abundance and distribution of diamondoids. Group I oils are located along the Tazhong No. 1 fault zone, which contained abundant diamondoids and are of high thermal maturity, suggesting they are late-charged hydrocarbons that were derived from a Middle–Upper Ordovician source. Group II oils are mainly located in blocks close to the Tazhong No. 1 fault zone and are dominated by early formed hydrocarbons from Cambrian–Lower Ordovician source rocks. Group III lacustrine oils comprise relatively low concentrations of diamondoids compared with group I and group II oils. Group III oils were sourced from Jurassic or possibly Triassic units and are hosted by the low-relief Yingmaili section of the Luntai uplift. The thermal maturity of the oils in each group was evaluated using diamondoid parameters; a few group I oils exhibit intense thermal cracking. To estimate the extent of oil cracking using the concentrations of diamondoids in oils, this study proposes a practical approach that facilitates the determination of baseline 4- and 3-methyldiamantane concentrations. Application of this method indicates that the Tarim Basin marine oils contain a baseline concentration of approximately 69 ppm.

Chemosymbiotic bivalves harboring endosymbiotic, chemotrophic bacteria have been investigated from a variety of hydrocarbon seeps worldwide. It has been shown that carbon, nitrogen, and sulfur isotopic compositions of the animal soft body parts are excellent indicators for evaluating energy transfer and food sources for the respective deep-sea habitats. However, recognition of chemosymbiosis has proven to be difficult for bivalves that dwelled at ancient seeps due to the lack of soft tissue. Here, we investigated δ13C, δ15N, and δ34S signatures of the tissue (mantle) and the shell organic matrix (SOM) of the same specimens of three bathymodiolin mussel species with different chemotrophic symbionts (methanotrophs in Bathymodiolus platifrons and B. childressi and thiotrophs in B. aduloides) and one vesicomyid clam (Calyptogena sp.) from a variety of hydrocarbon seeps from the South China Sea and the Gulf of Mexico. The data obtained demonstrate that all seep bivalves regardless of species or locations reveal overall small differences in δ13C (≤+4‰), δ15N (≤+1‰), and δ34S (≤+5‰) values between SOM and mantle (∆SOM-mantle) of the specimens. Relatively larger ∆SOM-mantle for δ13C values (as high as +10‰) in B. platifrons and larger ∆SOM-mantle for δ34S values (up to 16‰) in B. aduloides and Calyptogena sp. might be due to different symbionts in their gills. Since SOM can be extracted from fossil bivalve shells, the proxy can be used as a fingerprint of chemosynthesis-based food chains, although its utility will depend on the quality of preservation of the shell organic matter. Despite this uncertainty, the new proxy has great potential to reconstruct energy flow through different types of chemosynthesis-based ecosystems.

The size-fractionated biomass and primary production of phytoplankton, and the influence of environmental factors on it were studied in the Dongsha natural gas hydrate zone of the northern South China Sea in May 2013. Low nutrient, low chlorophyll a (Chl a) and primary productivity characteristics were found in these waters. The phenomena of subsurface Chl a maximum layers (SCMLs) and primary production maximum layers (SPMLs) were observed in the Dongsha waters. There were significant differences in the size-fractionated biomass and primary production that showed picophytoplankton>nanophytoplankton>microphytoplankton in terms of biomass and degree of contribution to production. Vertical biomass distribution indicated there were considerable differences among different phytoplankton within the euphotic zone (Zeu) in spring. For example, microphytoplankton was distributed evenly in the euphotic layer and nanophytoplankton was mainly distributed in the subsurface or in the middle of the euphotic layer, while picophytoplankton was mainly distributed in the middle or bottom of the euphotic layer. Smaller cell size and larger relative surface area allow picophytoplankton to benefit from nutrient competition and to hold a dominant position in the tropical oligotrophic waters of low latitudes. There was a positive correlation between size-fractionated biomass and temperature with pH and a negative correlation between size-fractionated biomass and silicate with phosphate. There was a positive correlation between size-fractionated primary production and temperature and a negative correlation between size-fractionated biomass and salinity with phosphate. Phosphate was an important factor influencing the size structure of phytoplankton. Meanwhile, irradiation and the euphotic layer were more important in regulating the vertical distribution of size-fractionated phytoplankton in the Dongsha natural gas hydrate zone.

Sulfate-driven anaerobic oxidation of methane (SD-AOM) and organoclastic sulfate reduction (OSR) are the two processes that consume sulfate in methane-rich marine sediments. Determining the relative contribution of each process to overall sulfate reduction is critical to assess the global carbon and sulfur cycle in marine settings. The analysis of sulfur and oxygen isotope compositions of porewater sulfate (δ18OSO4 and δ34SSO4) provides crucial insight into sulfate reduction pathways and the dynamics of the sedimentary sulfur cycle. The δ18OSO4/δ34SSO4 ratio is governed by net sulfate reduction rates and has been widely used to distinguish SD-AOM from OSR. Furthermore, δ18OSO4/δ34SSO4 slopes are believed to vary with changing methane flux. To enhance the applicability of δ18OSO4 vs. δ34SSO4 plots for the tracing of SD-AOM activity and methane flux, we investigate sulfate sulfur and oxygen isotopes of porewaters from three piston cores collected from the Haima seeps of the South China Sea. Reaction-transport modeling previously indicated that sulfate consumption was dominated by SD-AOM under changing methane fluxes (1126 mmol m−2 yr−1 to 26 mmol m−2 yr−1). Our new data demonstrate an increase of the initial δ18OSO4/δ34SSO4 slope when methane flux decreases. An initial slope of 0.43 corresponds to a methane flux of 194 mmol m−2 yr−1, the lowest currently known methane flux producing a small δ18OSO4/δ34SSO4 slope diagnostic for SD-AOM in sediments close to the seafloor. When methane flux decreases to 26 mmol m−2 yr−1, a small slope (0.48) is only observed in deeper sediment layers. Our results also reveal an increase of δ18OSO4/δ34SSO4 slopes with decreasing SD-AOM rates. We estimate that net sulfate reduction rates can be as low as 10−9 mol cm−3 yr−1 to produce slopes smaller than 0.5 in SD-AOM-dominated settings. In OSR-dominated settings, δ18OSO4/δ34SSO4 slopes tend to be significantly higher than 0.5, while net sulfate reduction rates tend to be lower than 10−5 mol cm−3 yr−1. This study reveals that quantification of methane flux is required to utilize the full potential of δ18OSO4 vs. δ34SSO4 plots in the endeavor of tracing SD-AOM activity and the dynamics of the sulfur cycle in methane-rich environments.

Authigenic carbonates recovered from two newly discovered active cold seeps on the northwestern slope of the South China Sea have been studied using petrography, mineralogy, stable carbon and oxygen isotopic, as well as trace element compositions, together with AMS 14C ages of shells of seep-dwelling bivalves to unravel fluid sources, formation conditions, and seepage dynamics. The two seeps (ROV1 and ROV2), referred to as ‘Haima seeps’ herein, are approximately 7 kilometers apart, and are typified by abundant carbonate rocks represented bycrusts and nodules. Aragonite and high-Mg calcite are the main carbonate minerals. Based on low δ13Ccarbonate values ranging from −43.0‰ to −27.5‰ (V-PDB) methane is apparently the predominant carbon source of seep carbonates. The corresponding δ18O values, varying from 2.5‰ to 5.8‰ (V-PDB), mostly are higher than calculated values representing precipitation in equilibrium with seawater (2.5‰ to 3.8‰), which probably reflects past destabilization of locally abundant gas hydrates. In addition, we found that carbonates with bivalve shells are generally aragonite-dominated, and bear no barium enrichment but uranium enrichments, reflecting shallow formation depths close to the seafloor. In contrast, carbonate crusts without bivalve shells and nodules contain more calcite, and are characterized by major molybdenum enrichment and different degrees of barium enrichment, agreeing with precipitation at greater depth under strictly anoxic conditions. AMS 14C ages suggest that a major episode of carbonate precipitation occurred between 6.1 ka and 5.1 ka BP at the Haima seeps, followed by a possibly subordinate episode from approximately 3.9 ka to 2.9 ka BP. The common occurrence of dead bivalves at both sites indicates that chemosynthesis-based communities flourished to a greater extent in the past, probably reflecting a decline of seepage activity in recent times. Overall, these results confirm that authigenic carbonates from gas hydrate-bearing areas can provide insight into long-term seepage dynamics and the genesis and fate of marine gas hydrate reservoirs.

The archaeal phylum Bathyarchaeota, which is composed of a large number of diverse lineages, is widespread and abundant in marine sediments. Environmental factors that control the distribution, abundance, and evolution of this largely diversified archaeal phylum are currently unclear. In this study, a new pair of specific primers that target the major marine subgroups of bathyarchaeotal 16S rRNA genes was designed and evaluated to investigate the distribution and abundance of Bathyarchaeota in marine sediments. The abundance of Bathyarchaeota along two sediment cores from the deep-sea sediments of South China Sea (SCS, each from the Dongsha and Shenhu area) was determined. A strong correlation was found between the bathyarchaeotal abundance and the content of total organic carbon (TOC), suggesting an important role of Bathyarchaeota in organic matter remineralization in the sediments of SCS. Furthermore, diversity analysis revealed that subgroups Bathy-2, Bathy-8 and Bathy-10 were dominant bathyarchaeotal members of the deep-sea sediments in the SCS. Bathy-8 was found predominantly within the reducing and deeper sediment layers, while Bathy-10 occurred preferentially in the oxidizing and shallower sediment layers. Our study lays a foundation for the further understanding of the ecological functions and niche differentiation of the important but not well-understood sedimentary archaeal group. This article is protected by copyright. All rights reserved.

Sediments at marine methane seep sites provide potential archives of past fluid flow that serve to explore seepage activities over time. Three gravity cores (D-8, D-F, and D-7) were collected from seep sites on the northern slope of the South China Sea where gas hydrates were drilled in the subsurface. Various carbon and sulfur contents, δ13C values of total inorganic carbon (δ13CTIC), δ34S values of chromium reducible sulfur (δ34SCRS), trace element contents, grain size, and AMS 14C dating of planktonic Foraminifera in the sediments were determined to explore the availability of related proxies at seeps and to trace past methane seepage activities. Evidence for the presence of methane seepage and consequently anaerobic oxidation of methane comes from the occurrence of 13C-depleted authigenic carbonate nodules (δ13C values as low as −49‰) discovered at an interval of 150–200 cm in core D-7. This finding is supported by high S/C ratios and molybdenum enrichment in the same interval. However, low contents of CRS and negative δ34SCRS values are present. It is suggested to reflect a transient methane seepage event, which continued for about 1 ka based on the 14C ages. Cores D-8 and D-F have δ13CTIC values close to zero, low S/C ratios and CRS contents, negative δ34SCRS values, and no trace element enrichment, suggesting a negligible impact of methane-seepage on the sediments. The negative δ34SCRS values of the studied seep-impacted and background sediments suggest that the application of δ34SCRS alone as a proxy to identify AOM-related process may be insufficient. Sediment carbon-sulfur-trace element systematics and 14C ages used here have the potential to be a promising tool to recognize transient methane seepages and constrain their timescales.

Three series of laboratory vaporization experiments were conducted to investigate the carbon isotope fractionation of low molecular weight hydrocarbons (LMWHs) during their progressive vaporization. In addition to the analysis of a synthetic oil mixture, individual compounds were also studied either as pure single phases or mixed with soil. This allowed influences of mixing effects and diffusion though soil on the fractionation to be elucidated. The LMWHs volatilized in two broad behavior patterns that depended on their molecular weight and boiling point. Vaporization significantly enriched the 13C present in the remaining components of the C6–C9 fraction, indicating that the vaporization is mainly kinetically controlled; the observed variations could be described with a Rayleigh fractionation model. In contrast, the heavier compounds (n-C10–n-C12) showed less mass loss and almost no significant isotopic fractionation during vaporization, indicating that the isotope characteristics remained sufficiently constant for these hydrocarbons to be used to identify the source of an oil sample, e.g., the specific oil field or the origin of a spill. Furthermore, comparative studies suggested that matrix effects should be considered when the carbon isotope ratios of hydrocarbons are applied in the field.

Integrated investigations have revealed abundant resources of gas hydrates on the northern slope of the South China Sea (SCS). Regarding the gas hydrate research of northern SCS, the gas hydrate related environment problem such as seabed landslides were also concentrated on in those areas. Based on 2D seismic data and sub-bottom profiles of the gas hydrate areas, submarine landslides in the areas of Qiongdongnan, Xisha, Shenhu, and Dongsha have been identified, characterized, and interpreted, and the geophysical characteristics of the northern SCS region investigated comprehensively. The results show 6 major landslides in the gas hydrate zone of the northern SCS and 24 landslides in the Shenhu and Dongsha slope areas of the northern SCS. The landslide zones are located mainly at water depths of 200&ndash, 3000 m, and they occur on the sides of valleys on the slope, on the flanks of volcanoes, and on the uplifted steep slopes above magmatic intrusions. All landslides extend laterally towards the NE or NEE and show a close relationship to the ancient coastline and the steep terrain of the seabed. We speculate that the distribution and development of submarine landslides in this area has a close relationship with the tectonic setting and sedimentary filling characteristics of the slopes where they are located. Seismic activity is the important factor controlling the submarine landslide in Dongsha area, but the important factor controlling the submarine landslides in Shenhu area is the decomposition of natural gas hydrates.

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 of various origins is widespread in the sediments of continental margins, representing a key component of the carbon cycle in the ocean. Its occurrence in shallow marine sediments can be linked to the gas hydrate reservoir in the deep sedimentary column. However, this type of methane is difficult to track due to complex reactions occurred during its long distance migration. Porewater geochemical analyses of two ∼7 m piston cores (D17-15 and W01-16) combined with a reaction-transport model were applied to quantify methane-related biogeochemical processes and to explore the linkage of shallow methane to gas hydrate reservoir in the hydrate drilling areas of Shenhu and Dongsha in the South China Sea (SCS). The model results revealed that anaerobic oxidation of methane (AOM) is the predominant biogeochemical process in both cores, consuming approximately 85% and 74% of sulfate in the modeled domain, respectively. The crossplot of dissolve inorganic carbon (DIC) accumulation versus sulfate depletion of the system suggests that methane consumed during the AOM is mainly originated from external methane sources rather than local methanogenesis. Using a δ13CDIC mass balance calculation together with model-derived fractions of different DIC sources, we estimated that the δ13C value of the external methane in core D17-15 is −66.6‰, which is in accordance with the previous reported δ13C values of hydrate-bound methane in the area. These results suggest that methane consumed in shallow sediments in the hydrate drilling areas of the SCS is mainly originated from local gas hydrate reservoir. The proposed approach has the potential to be used to differentiate the sources of methane, which will provide constraints on dynamics of methane in gas hydrate-bearing marine sediments.

The magnesium (Mg) isotopic composition of marine carbonate has been used as a proxy to constrain the geochemical composition of seawater and to trace the global Mg cycle. Sulfate-driven anaerobic oxidation of methane (SD-AOM) is a prominent process that facilitates the formation of marine authigenic carbonate. Yet the understanding of the impact of SD-AOM on the Mg isotope composition of authigenic carbonate under natural conditions is limited. Here we present the analysis of two tubular methane-seep carbonates from the South China Sea. The tubular carbonates reveal a trend of decreasing δ13C values and increasing carbonate contents from the periphery (i.e., the outer surface of the carbonate) to the inner portion (i.e., close to the inner surface of the carbonate pipes). The tubular carbonates are interpreted as former fluid conduits, indicating intense and lasting seepage that apparently increased with time. The observed variability in composition probably results from changes in the intensity of SD-AOM and correlating carbonate precipitation rates. Trends of enrichment of middle rare earth elements (MREEs), molybdenum (Mo), and uranium (U) from the periphery inward indicate more reducing formation conditions toward the inner surface of the former conduit. Along the same transects, δ26Mg values decrease from periphery to inner portion in one tubular carbonate (−2.63‰ to −3.24‰), while δ26Mg values are close to constant for the second tubular carbonate (−3.42‰ to −3.37‰); such patterns highlight the complexity of Mg isotope fractionation during seep carbonate formation. A positive correlation between δ26Mg and δ13C values and a negative correlation between δ26Mg and Mg/Ca molar ratios suggests that enhanced SD-AOM facilitates the incorporation of light Mg isotopes into the carbonate lattice. However, such assumption contradicts with the fact that higher carbonate precipitation rates reduce Mg isotope fractionation due to incomplete dehydration of Mg ions. It is hypothesized that hydrogen sulfide produced by SD-AOM may enlarge the extent of fractionation by weakening the hydration of Mg. If this interpretation is correct, the effect of sulfide on the fractionation of Mg isotopes can even offset or exceed the effect of precipitation rate. Our study indicates that SD-AOM has an effect on the Mg isotope fractionation during carbonate precipitation. Similar effects may have occurred in geological periods with more widespread carbonate authigenesis. A better understanding of the factors affecting the Mg isotopic fractionation during marine carbonate authigenesis is needed to efficiently use this archive of paleoenvironments and element cycling.

Marine sediments represent the largest reservoir of methane on Earth. Anaerobic oxidation of methane (AOM) mediated by anaerobic methanotrophic archaea (ANME) and associated sulfate-reducing bacteria is the main process controlling methane emission from marine sediments. Recent laboratory studies have revealed that ANME or AOM consortia can mediate nitrogen (N) fixation and ammonium assimilation. These processes are known to preferentially utilize 14N rather than the 15N isotope. However, geochemical fingerprints of microbial nitrogen uptake in marine sediments are lacking, limiting our understanding of its role in modern and ancient sedimentary environments. Here we report prominent negative excursions of bulk sediment δ15N values (up to −5‰) at sulfate-methane transitions (SMTs) where ANME show peak abundances in sediments at four sites of the Haima seeps of the South China Sea. Such δ15N excursions are restricted to SMTs, suggesting that 15N depletion results from ANME-mediated nitrogen uptake. We posit that this process is ubiquitous at the SMT – a reaction front in coastal and continental margin sediments worldwide – resulting in similar isotope patterns. Negative δ15N excursions are consequently a promising proxy to track AOM in the geological record and to better constrain carbon and nitrogen cycling in ancient sedimentary environments.

Cold hydrocarbon seepage is a frequently observed phenomenon along continental margins worldwide. However, little is known about the impact of seeping fluids on the geochemical cycle of redox-sensitive elements. Pore waters from four gravity cores (D-8, D-5, D-7, and D-F) collected from cold-seep sites of the northern South China Sea were analyzed for SO42−, Mg2+, Ca2+, Sr2+, dissolved inorganic carbon (DIC), δ13CDIC, dissolved Fe, Mn, and trace elements (e.g. Mo, U). The sulfate concentration–depth profiles, δ13CDIC values and (ΔDIC+ΔCa2++ΔMg2+)/ΔSO42− ratios suggest that organoclastic sulfate reduction (OSR) is the dominant process in D-8 core. Besides OSR, anaerobic oxidation of methane (AOM) is partially responsible for depletion of sulfate at D-5 and D-7 cores. The sulfate consumption at D-F core is predominantly caused by AOM. The depth of sulfate–methane interface (SMI) and methane diffusive flux of D-F core are calculated to be ~7 m and 0.035 mol m−2 yr−1, respectively. The relatively shallow SMI and high methane flux at D-F core suggest the activity of gas seepage in this region. The concentrations of dissolved uranium (U) were inferred to decrease significantly within the iron reduction zone. It seems that AOM has limited influence on the U geochemical cycling. In contrast, a good correlation between the consumption of sulfate and the removal of molybdenum (Mo) suggests that AOM has a significantly influence on the geochemical cycle of Mo at cold seeps. Accordingly, cold seep environments may serve as an important potential sink in the marine geochemical cycle of Mo.

Sulfate-driven anaerobic oxidation of methane (SD-AOM) controls methane release from marine sediments to the ocean. A variety of authigenic precipitates within sediments has been widely used to identify past occurrences of SD-AOM. However, the lack of a systematic evaluation of the formation conditions of these precipitates sometimes impedes the recognition of SD-AOM in past environments characterized by diffusive transport of methane. Carbon, sulfur, and trace element geochemistry of pore-water and sediments was investigated at a site affected by upward methane diffusion in the Shenhu area of the South China Sea. Here, the sulfate-methane transition zone (SMTZ) is located ~7.6 m below the seafloor based on sulfate and methane concentrations. The slope of δ34S vs. δ18O values of sulfate is consistent with diffusive transport of methane. Concentration and isotope profiles of pore-water species point to diffusive rather than advective transport of solutes. Enhanced sulfate reduction inferred from δ34S values of sulfate agrees with the local occurrence of relatively abundant, 34S-enriched iron sulfide minerals. The time required to produce the observed authigenic iron sulfides around the SMTZ is estimated to be ~1.1 ka based on the amount of iron sulfide minerals and the present diffusive flux of sulfate. No enrichment of authigenic carbonate, barium (Ba), and molybdenum (Mo) was detected in the studied sediments. This observation is consistent with the calculation that the current fluxes of pore-water calcium, magnesium, Ba, and Mo are too low to allow for authigenic enrichments. The low fluxes are largely controlled by diffusion, which facilitates the formation of 34S-enriched iron sulfide minerals. The observed enrichment patterns – unlike those of sediments affected by advective seepage – are expected to be prevalent in modern and ancient continental-margin sediments, and may contribute to the identification of past methane-rich zones and overlying SD-AOM zones as sinks for methane in the geological record.

The gas hydrate deposit in Shenhu, Northern South China Sea (NSCS) was one of the main sources for deep-water hydrocarbon and hydrate. Based on field observations on a gas hydrate deposit among 20 × 20 km2 at Shenhu of the northern continental slope of SCS in May (before drilling) and September (after drilling) in 2015, environmental baseline, comparisons of phytoplankton community, biomass and potential environmental drivers were studied. Results indicated the deposit area was a typical low-nutrients low-chlorophyll a (LNLC) environment, characterized by low phytoplankton abundance and diversity. The maximum of abundance and Chl a were fixed at 75 m before and after drilling, but their decrease was larger from 0.53 mg/m3 to 0.40 mg/m3 and 1218 cells/L to 843 cells/L with non-significance found, respectively. After drilling, ammonium decreased significantly (2.13 μmol/L to 0.20 μmol/L), along with nitrite, salinity and dissolved methane, while pH increased significantly (8.10–8.19). Phytoplankton community was composed of 97 taxa of 47 genera in 4 classes, including diatoms, dinoflagellates, cyanobacteria and chrysophyte. The dominant diatom species before and after drilling remained the same, which included Navicula parva, Pseudonitzchia circumpora and Fragilariopsis doliolus. Dominant dinoflagellates shifted from the cosmopolitan Scrippsiella trochoidea to the warm-water adapted Gyrodinium flavum. The diazotrophic cyanobacterium Trichodesmium thiebaultii become more apparent in higher occurrence and abundance due to alleviation from lower-pH inhibition on nitrogen fixation. Redundancy analysis and Spearman correlation analysis revealed that the dominant species and Chl a were mainly associated to pH, DO and nitrite. The X-bar control chart indicated that a constrained maximum problem of phytoplankton abundance in particular layers (50 m, 75 m and 100 m). We clearly documented in the revisited stations (SH3–W2 and SH3–W14), the decrease of abundance and Chl a and the increase of pH and DO were evident in each sampling layer accompanied with lower dissolved methane associated to in situ anoxic or aerobic microniches activities, but the correlations with dissolved methane were not strong.

We analyzed the data obtained from field observations on a gas hydrate drilling area in Dongsha of northern South China Sea (SCS) in middle May (before drilling) and early October (after drilling) in 2013. The variation in the phytoplankton communities and biomass as well as the impacts of environmental factors including dissolved methane was studied. Results indicated that the gas hydrate drilling area in Dongsha, SCS exhibited a typical low-nutrients low-chlorophyll a (LNLC) environment accompanied with low phytoplankton abundance. A total of 103 taxa belonging to 52 genera of 5 classes were identified, with diatoms and dinoflagellates dominating the community. Both phytoplankton abundance and chlorophyll a (Chl a) were highest at the subsurface maximum layer. The subsurface chlorophyll maximum (SCM) before and after drilling were stabilized at 75 m (0.30 ± 0.06 mg/m3 and 0.51 ± 0.29 mg/m3, respectively), while the subsurface maximum of abundance after drilling went deeper to 75 m (604.17 ± 313.22 cells/L) from the surface (707.14 ± 243.98 cells/L) before drilling. After drilling, phosphate and Chl a increased significantly, but no significant differences were observed on abundance. Dominant species of diatoms were basically constant with dinoflagellates becoming more apparent in higher occurrence and abundance, while Cyanophyta was diverse after drilling. Redundancy analysis (RDA) and Spearman's correlation analysis both indicated that temperature, pH and phosphates were major factors causing fluctuation in phytoplankton community structure, while dissolved methane had non-significant impact directly. We clearly found both abundance and Chl a increased in particular water layers (between 50 and 75 m) and at stations (DS06, DS08 and DS15) where dissolved methane concentrations were also abnormally high. This study appeared to partly coincide with the findings of natural oil seeps in the Gulf of Mexico, which assumed that the turbulence from the natural oil and gas leaking zone could raise the bottom water through the rising bubbles and bring cold nutrient rich waters to the thermocline from the deep seeps. This plume-generated upwelling could then fuel a bottom-up effect on the photosynthetic species in the upper pelagic waters within the euphotic zone.

On the basis of the results of simulation experiments, now we better understand the contribution of high carbon number hydrocarbons to diamondoid generation during thermal pyrolysis of crude oil and its sub-fractions (saturated, aromatic, resin, and asphalene fractions). However, little is known about the effect of volatile components in oil on diamondoid genera- tion and diamondoid indices due to the lack of attention to these components in experiments. In this study, the effect of volatile components in oil on diamondoid generation and maturity indices was investigated by the pyrolysis simulation experiments on a normal crude oil from the HD23 well of the Tarim Basin and its residual oil after artificial volatilization, combined with quantitative analysis of diamondoids. The results indicate that the volatile components (≤nC12) in oil have an obvious contri- bution to the generation of adamantanes, which occurs mainly in the early stage of oil cracking (EasyRo