Taixinan Basin Research Articles

Geomorphological features and formation mechanism of the Taixinan canyon‐channel system in the north‐eastern South China Sea

During the formation and evolution of the South China Sea, a series of multiscale submarine geomorphologies have been produced in the continental margin. The obvious submarine canyons and channels are widely distributed in the continental shelf and slope of the South China Sea. In the Taixinan Basin, several submarine canyons and channels termed as the Taixinan canyon‐channel system (TCCS) are distributed between the active and passive continental margins. Based on acquired ship‐borne multi‐beam bathymetry data in this study and the global GEBCO 2023 bathymetric dataset, we identify and define nine submarine canyons and seven submarine channels in the Taixinan Basin. The TCCS consists of the Dongsha, Taiwan, Jiulong, West Penghu, Penghu, Kaoping, Shoushan, Kaohsiung and the Fangliao canyons and submarine channels. The detailed geomorphological features of different submarine canyons and channels within the TCCS are analysed and summarized using multi‐beam bathymetry data and seismic reflection profiles across canyons. Based on the slope variations of the continental margin and the effects of turbidity currents and bottom currents on canyon, we propose a three‐stage evolutionary model of the TCCS. In the initial formation stage of canyon, the initial erosional grooves were created by tectonic activity on the continental slope and it represents the foundation of submarine canyons. During the growth and development stage, the submarine canyons are further evolved and the canyons began to deepen and widen from the continental slope to the deep‐water areas. It shows the weak erosion and sediment infilling within the canyons in this stage. On the northern continental slope of the South China Sea, continuous transportation and erosion of sediments led to the initial formation of grooves and it becomes the embryonic stage of submarine channels. The present stage of the TCCS was formed when the initial grooves on the continental slope have further developed and rebuilt under the erosion by the turbidity current and the scouring by the bottom current. In the last stage, the intense erosion by the turbidity current is supported by sediment waves around the submarine canyons and the migration of canyons is suggested by the cyclic steps formed within some canyons.

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

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

Possible links with methane seepage and gas hydrate dynamics inferred from authigenic barite records in the northern south china sea

Numerous methane seepage events occurred in periods of low or falling sea level since 330 ka BP, which is attributed to decrease in hydrostatic pressure and subsequent gas hydrate dissociation in the northern South China Sea (SCS). The seepage intensity likely decrease due to gas hydrate stabilization once there was a relatively high-stand sea level. However, there are few geochemical records of decline in upward methane flux in the northern South China Sea. Here, combing porewater and solid-phase analyses, the geochemical cycling of barium was investigated in two piston cores from sites HD109 and HD319 within two areas with inferred gas hydrate occurrence in the Taixinan Basin of the northern SCS, in order to track the net decrease in the upward methane flux and to estimate the total duration time of these events in the studied sediments. The results indicate that there are four intervals with barium enrichments in the sediment section overlying the occurrent sulfate-methane transition zone (SMTZ) at both cores, suggesting the SMTZs have downward migrated through time. Based on the excess barium contents and the diffusive Ba2+ fluxes above the current SMTZ, we estimate the total time for barium accumulation at both cores is about ten thousand years. It is suggested that some methane seepage events temporarily enhance the upward flux of methane, inducing anaerobic oxidation of methane and associated SMTZ close to the sediment surface before the Holocene. After the most intensive seepage event ceased in the post-glacial period, the upward methane flux decreased and the SMTZ migrated downward gradually, preserving enrichments of diagenetic barite. Overall, these new data confirm the episodic decrease in upward methane flux recorded by authigenic barite after the last glacial maximum, which is likely related to the stabilization of underlying gas hydrate reservoir. This study may fill in the gap of the geochemical records of the variations in methane seepage and gas hydrate system during the post-glacial period in the northern SCS.

Gas Hydrate Accumulation and Occurrence Associated with Cold Seep Systems in the Northern South China Sea: An Overview

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.

Cenozoic Sea‐land Transition and its Petroleum Geological Significance in the Northern South China Sea

AbstractThe process of Cenozoic sea‐land changes in the northern South China Sea (SCS) controlled the sedimentary filling pattern and played an important role in the petroleum geological characteristics of the northern marginal sedimentary basins. Under the control of the opening process of the SCS, the northern SCS Cenozoic transgression generally showed the characteristics of early in the east and late in the west, and early in the south and late in the north. The initial transgression occurred in the Eocene in the Taixinan Basin (TXNB) of the eastern SCS, while the transgression occurred until the Pliocene in the Yinggehai Basin (YGHB) of the western SCS. International Ocean Discovery Program (IODP) expeditions (Expeditions 367/368) revealed that the initial transgression of the SCS basin occurred at approximately 34 Ma, which was the initial opening time of the SCS. The period of drastic changes in the sedimentary environment caused by large‐scale transgression corresponded to the opening time of the southwestern subbasin (approximately 23 Ma), which also represented the peak of the spreading of the SCS. The sea‐land transition process controls the distribution of alternating continent‐marine facies, marine facies source rocks and reservoirs in the basins. The marine facies source rocks of the basins in the northern SCS have a trend of gradually becoming younger from east to west, which is consistent with the regional process of gradual transgression from east to west. Regional sea‐level changes were comprehensively influenced by SCS opening and global sea‐level changes. These processes led to the early development in the east and south and late development in the west and north for the carbonate platform in the SCS. Carbonate platforms form another type of “self‐generating and self‐accumulating” oil‐gas reservoir in the northern SCS. The sea‐land transition controlled the depositional filling patterns of different basins and laid the foundation of marine deposits for oil and gas resources. The source‐reservoir‐cap assemblage in the northern SCS was controlled horizontally by provenance supply and sedimentary environmental changes caused by sea‐land transition and vertically by the tectonic evolution of the SCS and regional sea‐level changes.

Porewater geochemical and lipid biomarker signatures for anaerobic methane oxidation in the hydrate-bearing system from the Taixinan Basin, the South China Sea

Abstract Anaerobic oxidation of methane (AOM) is one of the most important biogeochemical processes associated with hydrate dissociation and carbon cycling. Though AOM has been extensively investigated, the variations of its microorganisms and lipid biomarkers are still not fully understood. In this study, the porewater geochemical and lipid biomarker evidence for AOM was systematically investigated in the hydrate-bearing system from the Taixinan Basin, the South China Sea, aiming to better expound the vertical variations of AOM-related biomarker signatures and causes of the weak lipid biomarker patterns. Accordingly, the sulfate-methane interface (SMI) depth of 1,000 cmbsf and onset of AOM were determined on the basis of the vertical profiles of porewater parameters. The occurrence of 13C-depleted 2,6,10,15,19-pentamethylicosane (PMI) with the δ13C ratios of −77.7‰ to −131.4‰ near the SMI are in accord with the porewater parameters, despite the absence of archaeol and sn2-hydroxyarchaeol and low abundances of PMI. Meanwhile, the strongly 13C-enriched iso-C15:0 and anteiso-C15:0 fatty acids suggest that non-methane hydrocarbons or oil derivatives are speculated as the probable carbon sources for sulfate-reducing bacteria due to the presence of unresolved complex mixtures (UCMs) and odd–even predominance (OEP) values (about 1.0). The weak in situ signal of AOM-related biomarkers may be attributed to the low-flux methane supply, and dilution effect from the high input of allochthonous and autochthonous organic matters. The great difference in biomarkers between the Taixinan Basin and previously reported sites should be further clarified on the basis of more extensive studies in various AOM-related settings.

Zone of metal-driven anaerobic oxidation of methane is an important sink for phosphorus in the Taixinan Basin, South China Sea

Phosphorus (P) is an important nutrient and has an important impact on global primary productivity. Presently, the dynamics of the P cycle in low sulfate, anoxic settings are largely unknown. In this study, the adsorption–desorption behavior of Fe-bound P, organic-P, loosely-bund P, detrital apatite P, and refractory P was described in the sulfate and methane transition zone (SMTZ) at site 973–4 in the cold-seep area of the Taixinan Basin, South China Sea. Results showed that the Fe-bound P and organic-P were influenced by sulfate-driven anaerobic oxidation of methane (SD-AOM) and metal-driven anaerobic oxidation of methane (MD-AOM), and subsequently, the Fe-bound P and organic-P were released as HPO42− into the pore water. Meanwhile, significant increases were detected in authigenic carbonate fluorapatite (PCFA) and vivianite within the MD-AOM zone, which indicate that this zone is an important P-sink area. The MD-AOM plays an important role in the formation of PCFA and vivianite because MD-AOM provide the necessary HPO42− and Fe2+ for the formation of these minerals. Our findings have major implications for understanding the P cycle in low sulfate, anoxic settings and are important for understanding the influence of MD-AOM on the phosphate cycle.

Geochemical insights into contribution of petroleum hydrocarbons to the formation of hydrates in the Taixinan Basin, the South China Sea

Methane hydrate in the South China Sea (SCS) has extensively been considered to be biogenic on the basis of its δ13C and δD values. Although previous efforts have greatly been made, the contribution of thermogenic oil/gas has still been underestimated. In this study, biomarkers and porewater geochemical parameters in hydrate-free and hydrate-bearing sediments in the Taixinan Basin, the SCS have been measured for evaluating the contribution of petroleum hydrocarbons to the formation of hydrate deposits via a comparative study of their source inputs of organic matters, environmental conditions, and microbial activities. The results reveal the occurrence of C14–C16 branched saturated fatty acids (bSFAs) with relatively high concentrations from sulfate-reducing bacteria (SRBs) in hydrate-bearing sediments in comparison with hydrate-free sediments, which is in accord with the positive δ13C values of dissolved inorganic carbon (DIC), increasing methane concentrations, decreasing alkalinity, and concentration fluctuation of ions (Cl−, Br−, SO42−, Ca2+, and Mg2+). These data indicate the relatively active microbial activities in hydrate-bearing sediments and coincident variations of environmental conditions. Carbon isotope compositions of bSFAs (−34.0‰ to −21.2‰), n-alkanes (−34.5‰ to −29.3‰), and methane (−70.7‰ to −69.9‰) jointly demonstrate that SRBs might thrive on a different type of organic carbon rather than methane. Combining with numerous gas/oil reservoirs and hydrocarbon migration channels in the SCS, the occurrence of unresolved complex mixtures (UCMs), odd-even predominance (OEP) values (about 1.0), and biomarker patterns suggest that petroleum hydrocarbons from deep oil/gas reservoirs are the most probable carbon source. Our new results provide significant evidence that the deep oil/gas reservoirs may make a contribution to the formation of methane hydrate deposits in the SCS.

The multiple sulphur isotope fingerprint of a sub-seafloor oxidative sulphur cycle driven by iron

Oxidative sulphur cycling is pervasive in marine sediments, replenishing the oxidised sulphur reservoir via re-oxidation of sulphide. An active, yet cryptic, sulphur cycle has been proposed to operate at depth beneath the sulphate-methane transition (SMT), fuelled by simultaneous sulphide oxidation and sulphate reduction under low-sulphate conditions. The existence of a cryptic sulphur cycle, however, is centred on porewater and genetic data that have little, to no, preservation potential, and thus are rarely accessible from the geological record. The absence of a suitable archive has hindered our ability to reconstruct the operation and importance of the cryptic sulphur cycle through space and time. To overcome this obstacle, and to develop a better understanding of the oxidative sulphur cycle in the deep biosphere, we have determined the abundance and triple sulphur isotope composition (Δ33S and δ34S) of both elemental sulphur and pyrite extracted from sediments recovered from the methane prone Taixinan Basin, South China Sea. Here, multiple sulphur isotope systematics of pyrite clearly reveal a tiering, with organoclastic sulphate reduction succumbing to sulphate-driven anaerobic oxidation of methane at depth. Importantly, a negative Δ33S-δ34S correlation was found at the periphery of the SMT that requires repeated and sustained iron-driven sulphide oxidation with concomitant disproportionation of the elemental sulphur product. We conclude that minor sulphur isotopes may provide a unique lens to resolve the cryptic sulphur cycle, allowing the importance of the deep biosphere to be evaluated over geological timescales. In turn, a better understanding of the cryptic sulphur cycle remains central to testing hypotheses linking major elemental cycles and diverse microbial activities that persist under the energy-limited conditions that typify the deep biosphere.

Plate tectonic control on the formation and tectonic migration of Cenozoic basins in northern margin of the South China Sea

The tectonic evolution history of the South China Sea (SCS) is important for understanding the interaction between the Pacific Tectonic Domain and the Tethyan Tectonic Domain, as well as the regional tectonics and geodynamics during the multi-plate convergence in the Cenozoic. Several Cenozoic basins formed in the northern margin of the SCS, which preserve the sedimentary tectonic records of the opening of the SCS. Due to the spatial non-uniformity among different basins, a systematic study on the various basins in the northern margin of the SCS constituting the Northern Cenozoic Basin Group (NCBG) is essential. Here we present results from a detailed evaluation of the spatial-temporal migration of the boundary faults and primary unconformities to unravel the mechanism of formation of the NCBG. The NCBG is composed of the Beibu Gulf Basin (BBGB), Qiongdongnan Basin (QDNB), Pearl River Mouth Basin (PRMB) and Taixinan Basin (TXNB). Based on seismic profiles and gravity-magnetic anomalies, we confirm that the NE-striking onshore boundary faults propagated into the northern margin of the SCS. Combining the fault slip rate, fault combination and a comparison of the unconformities in different basins, we identify NE-striking rift composed of two-stage rifting events in the NCBG: an early-stage rifting (from the Paleocene to the Early Oligocene) and a late-stage rifting (from the Late Eocene to the beginning of the Miocene). Spatially only the late-stage faults occurs in the western part of the NCBG (the BBGB, the QDNB and the western PRMB), but the early-stage rifting is distributed in the whole NCBG. Temporally, the early-stage rifting can be subdivided into three phases which show an eastward migration, resulting in the same trend of the primary unconformities and peak faulting within the NCBG. The late-stage rifting is subdivided into two phases, which took place simultaneously in different basins. The first and second phase of the early-stage rifting is related to back-arc extension of the Pacific subduction retreat system. The third phase of the early-stage rifting resulted from the joint effect of slab-pull force due to southward subduction of the proto-SCS and the back-arc extension of the Pacific subduction retreat system. In addition, the first phase of the late-stage faulting corresponds with the combined effect of the post-collision extension along the Red River Fault and slab-pull force of the proto-SCS subduction. The second phase of the late-stage faulting fits well with the sinistral faulting of the Red River Fault in response to the Indochina Block escape tectonics and the slab-pull force of the proto-SCS.

Evolution of gas hydrates inventory and anaerobic oxidation of methane (AOM) after 40ka in the Taixinan Basin, South China Sea

Gas hydrates are the largest carbon sink on the earth and have an important impact on global climate and the environment. We studied theδ34S,δ13C, major trace elements and P –Fe dates at station 973-4 which acquired from the cold seep area of the Taixinan Basin. The results showed that during the Last Glacial Maximum (LGM), Younger Dryas (YD), and the Last Deglaciation, Gas hydrate inventory (GHI) was significantly reduced. And during the LGM, anaerobic oxidation of methane played an important role in preventing methane from leaking into seawater and the atmosphere. Because that there was no acidification of seawater or increase in PCO2. Meanwhile, rapid rate of sediment accumulation caused by isobaric flow provided accumulated material for Anaerobic Oxidation of Methan (AOM) during the LGM. The rapid rate of sediment accumulation also created a good environment for the formation of PCFA and vivianite below the sulfate methane transition zone (SMTZ). And according to changes of elements with depth, we conclude that the effect of methane anaerobic oxidation on redistribution of elemental P–Fe is divided into three processes: SMTZ rapid movement (material accumulation); SMTZ gradually stable; and SMTZ fixation process. In this paper, the effects of sedimentary evolution and hydrate evolution on AOM are studied in detail, which is of great significance for further understanding of anaerobic oxidation of methane.

Characteristics of Authigenic Minerals around the Sulfate-Methane Transition Zone in the Methane-Rich Sediments of the Northern South China Sea: Inorganic Geochemical Evidence.

Sediments at marine cold seep areas provide potential archives of past fluid flow, which allow insights into the evolution of past methane seepage activities. However, signals for anaerobic oxidation of methane (AOM) might be obscured in bulk sediments in cold-seep settings due to several factors, especially flood and turbidite deposition. Comprehensive inorganic data were gathered in this study to explore the availability of related records at cold seeps and to provide insights into the evolution of past methane seepage activities. Sediments collected from the site 973-4 in the Taixinan Basin on the northern slope of the South China Sea were characterized in terms of total carbon and sulfur, δ13C values of total organic carbon (δ13CTIC), δ34S values of chromium reducible sulfur (δ34SCRS), and foraminiferal oxygen and carbon isotopes. The results confirmed a strong correlation between formation of authigenic minerals and AOM. Moreover, the 34S enrichments and abundant chromium reducible sulfur (CRS) contents in the authigenic sulfides in the sulfate–methane transition zone (SMTZ) within 619–900 cm below seafloor (cmbsf) reflected past high methane fluxes supported by constant methane seepages. Lithological distribution and AMS (Accelerator Mass Spectra) 14C dating of planktonic foraminifera show that the turbidite (~35.14 ka) was related to a foraminifera-rich interval (Unit II: 440-619 cmbsf) and increased carbonate productivity during the last glacial maximum (LGM). Enrichment of Mo and U was observed accompanied by low contents of nutrient metals (Al, Ti, V, Ni, Fe, Mn, and Cu) in Unit II. The foraminifera-rich interval (Unit II) of cold seep sediments was probably linked to the phenomenon of inconsecutive sedimentary sequence due to the turbidites, which resulted in the lack of Fe, Mn, and Ba enrichment. There is no U enrichment but only Mo enrichment within Unit III, which might be related to H2S produced by AOM during the methane seepages. Based on the above results, it can be speculated that this area has experienced multiple-episodes of methane seep events. Further exploration of AOM should focus on the risks of rapid deposition, especially the impact of turbidity current on sediments.

Multibeam water column data research in the Taixinan Basin: Implications for the potential occurrence of natural gas hydrate

A multi beam sonar survey is carried out in the continental slope of the Taixinan Basin to obtain submarine topographic and water column data. The data are processed to obtain water column images. Anomalous water column images, displaying plume characteristics, are found in gas hydrate enriched areas in the Taixinan Basin. This indicates the presence of natural gas resources in the Taixinan Basin. The multibeam sonar system is shown to provide an accurate and effective approach for detecting sub-sea gas hydrate.

Genetic types of gas hydrates in China

Abstract Researches were carried out on the origin of gas hydrate samples from the tundra in the Qilian Mountain, Pearl River Mouth Basin in the northern South Sea and the continental slope of Taixinan Basin in China. Gases of the gas hydrate samples from the Jurassic Jiangcang Formation in the Muli County in Qilian Mountain are mainly of oil-derived origin, characterized by self-generation and self-preservation. δ13C1 values range from −52.7‰ to −35.8‰, and the δ13C2 values vary from −42.3‰ to −29.4‰. There was a small amount of coal-derived gases, which might source from the coal-bearing Middle-Jurassic Muli Formation with δ13C1 of −35.7‰ – −31.3‰ and δ13C2 of −27.5‰ – −25.7‰. Gases of the gas hydrate samples from the Pearl River Mouth Basin and Taixinan Basin are dominated by bacterial origin of carbonate reduction, with δ13C1 of −74.3‰ – −56.7‰ and δD1 of −226‰ – −180‰. A trace amount of thermogenic gases were also found in these basins with δ13C1 of −54.1‰ – −46.2‰. This study combined the geochemical data of gas hydrates from 20 areas (basins) in the world, and concluded that thermogenic gases of the gas hydrates in the world can be either of coal-derived or oil-derived origin, but dominated by oil-derived origin. A small amount of coal-derived gas was also found in the Qilian Mountain in China and the Vancouver Island in Canada. The coal-derived gas has relatively heavy δ13C1 ≥ −45‰ and δ13C2 > −28‰, while the oil-derived gas has δ13C1 from −53‰ – −35‰ and δ13C2

Geophysical evidence for gas hydrate accumulation related to methane seepage in the Taixinan Basin, South China Sea

Seafloor features, seismic amplitude anomaly, geophysical attributes, and chemosynthetic community are different in the methane seepage areas of Taixinan Basin, South China Sea. We integrate the new acquired seafloor images with seismic data to present the differences of three cold seeps which are associated with fluid flows. These three cold seeps include two inactive sites (CS02-8 and CS02-9) and one active site (Site F). At site CS02-8 area, the seafloor carbonate and buried carbonate layer are identified from core samples at Site GMGS2-08. BSR is interrupted by the upward migration of fluids from the base of gas hydrate stability zone (BGHSZ) and gas hydrate-bearing sediments show pull-up seismic reflections below the buried carbonate layer. At site CS02-9 area, near seafloor carbonate was found at hole GMGS2-09B where a 62 m-high pinnacle and gas hydrate were also found from the core samples. The LWD data at hole 09A indicate that free gas is present below the depth of 64 m without gas hydrate. The average saturations of free gas estimated from P-wave velocity are about 3% with a patchy distribution and 0.3% with homogenous distribution respectively, which is accord with the enhanced reflection below BGHSZ. At Site F area, the exposed gas hydrate, carbonate and flourishing chemosynthetic communities are detected using ROV with a high amplitude seismic reflection at seafloor. The BSR is interrupted by a chimney structure from the seismic data which indicates that the fluids are migrating vertically and rise to the seafloor as gas bubbles. The geophysical evidence at these three seep sites show that pore-filling gas hydrates with saturations ranging from 10% to 40% are widely distributed above the BSR. However, massive or vein gas hydrates only occur around cold seeps.

Geochemical and microbial characters of sediment from the gas hydrate area in the Taixinan Basin, South China Sea

The Taixinan Basin is one of the most potential gas hydrate bearing areas in the South China Sea and abundant gas hydrates have been discovered during expedition in 2013. In this study, geochemical and microbial methods are combinedly used to characterize the sediments from a shallow piston Core DH_CL_11 (gas hydrate free) and a gas hydrate-bearing drilling Core GMGS2-16 in this basin. Geochemical analyses indicate that anaerobic oxidation of methane (AOM) which is speculated to be linked to the ongoing gas hydrate dissociation is taking place in Core DH_CL_11 at deep. For Core GMGS2-16, AOM related to past episodes of methane seepage are suggested to dominate during its diagenetic process; while the relatively enriched δ 18O bulk-sediment values indicate that methane involved in AOM might be released from the “episodic dissociation” of gas hydrate. Microbial analyses indicate that the predominant phyla in the bacterial communities are Firmicutes and Proteobacteria (Gammaproteobacteria and Epsilonproteobacteria), while the dominant taxa in the archaeal communities are Marine_Benthic_Group_B (MBGB), Halobacteria, Thermoplasmata, Methanobacteria, Methanomicrobia, Group C3 and MCG. Under parallel experimental operations, comparable dominant members (Firmicutes and MBGB) are found in the piston Core DH_CL_11 and the near surface layer of the long drilling Core GMGS2-16. Moreover, these members have been found predominant in other known gas hydrate bearing cores, and the dominant of MBGB has even been found significantly related to gas hydrate occurrence. Therefore, a high possibility for the existing of gas hydrate underlying Core DH_CL_11 is inferred, which is consistent with the geochemical analyses. In all, combined geochemical and microbiological analyses are more informative in characterizing sediments from gas hydrate-associated areas in the South China Sea.

Contribution of thermogenic organic matter to the formation of biogenic gas hydrate: Evidence from geochemical and microbial characteristics of hydrate-containing sediments in the Taixinan Basin, South China Sea

High-saturation (40–100%), microbial gas hydrates have been acquired by expedition GMGS2 from the Taixinan Basin. In this study, geochemical and microbial features of hydrate-containing sediments from the drilling cores (GMGS2-09 and GMGS2-16) were characterized to explore their relationships with gas hydrate formation. Results showed that the average TOC content of GMGS2-09 and GMGS2-16 were 0.45% and 0.63%, respectively. They could meet the threshold for in situ gas hydrate formation, but were not available for the formation of high-saturation gas hydrates. The dominant members of Bacteria at the class taxonomic level were Alphaproteobacteria, Bacilli, Bacteroidia, Epsilonproteobacteria and Gammaproteobacteria, and those in Archaea were Marine_Benthic_Group_B (MBGB), Miscellaneous_Crenarchaeotic_Group (MCG), Group C3, Methanomicrobia and Methanobacteria. Indicators of microbes associated with thermogenic organic matter were measured. These include: (1) most of the dominant microbes had been found dominant in other gas hydrates bearing sediments, mud volcanos as well as oil/coal deposits; (2) hydrogenotrophic methanogens and an oilfield-origin thermophilic, methylotrophic methanogen were found dominant the methanogen community; (3) hydrocarbon-assimilating bacteria and other hyperthermophiles were frequently detected. Therefore, thermogenic signatures were inferred existed in the sediments. This deduction is consistent with the interpretation from the seismic reflection profiles. Owing to the inconsistency between low TOC content and gas hydrates with high saturation, secondary microbial methane generated from the bioconversion of thermogenic organic matters (oil or coal) was speculated to serve as enhanced gas flux for the formation of high-saturation gas hydrates. A preliminary formation model of high-saturation biogenic gas hydrates was proposed, in which diagenesis processes, tectonic movements and microbial activities were all emphasized regarding to their contribution to gas hydrates formation. In short, this research helps explain how microbial act and what kind of organic matter they use in forming biogenic gas hydrates with high saturations.

Seismic stratigraphy and tectono-sedimentary framework of the Pliocene to recent Taixinan foreland basin in the northeastern continental margin, South China Sea

The Pliocene to recent Taixinan basin is a unique foreland basin built on the northeastern part of the northern passive margin of the South China Sea (SCS). We have used multichannel seismic profiles tied to well controls from ODP Leg 184 to investigate the tectonic and sedimentary characteristics of the foreland basin. We defined three seismic sequences, dated respectively to the Pliocene (5.33–2.5 Ma), early Quaternary (2.5–1.0 Ma), and late Quaternary (1.0 Ma–present). They represent three stages of evolution of the foreland basin. We have recognized seven types of seismic facies, which are parallel-to-subparallel, progradational, fill-type, divergent mounded, wavy, lenticular, and chaotic facies, and are interpreted as hemipelagic deposits, deltas, submarine canyon fills, levees, sediment waves, submarine fans, and mass transport deposits, respectively. Seismic facies analysis indicates that sedimentation within the foreland basin has been dominated by turbidity currents and the other gravity transport processes. Tectonically, the foreland basin consists of three structural zones: an eastern wedge-top, a central foredeep, and a western forebulge zones. Different from a typical foreland basin, however, the basin extends in the northeast–southwest direction, which is oblique to the north–south-striking Taiwan orogenic zone, but parallel to the northern SCS passive margin, where the basin is hosted, suggesting that the foreland basin is significantly influenced by the development of the passive margin. In addition, the basin displays a distinctive inverted-triangle-shaped downstream-converging sediment dispersal system instead of ideal transverse or longitudinal drainage systems common in a typical foreland basin. We have suggested that the Pliocene to recent Taixinan basin is an atypical foreland basin, which was formed as a flexural response of tectonic loading by the Taiwan orogenic wedge, but strongly affected by its passive continental margin background.

Similarity and differentiation between the East China Sea Shelf Basin and Cenozoic basins in the northeast South China Sea

The East China Sea Shelf Basin (ECSSB), the Pearl River Mouth Basin (PRMB) and the Taixinan Basin (TXNB) in the northern continental margin of the South China Sea (SCS) are important oil‐ and gas‐bearing basins on the Western Pacific Continental Margin. During the Paleocene to Late Miocene, their strata can be compared, and the lithofacies were continuous from the TXNB, via the Taixi Basin (TXB) to the ECSSB. The lithologies mainly consist of interbedded shale, sandstone and mudstone layers. In addition, these basins have similar tectonic and sedimentation features. The three basins had similar marine‐terrigenous facies in the Paleocene and marine–lacustrine–fluvial facies during the Eocene and Late Miocene. The basins experienced several coeval tectonic movements and episodes, and they developed a series of NE‐ and NNE‐trending faults during the Paleocene and Eocene, which controlled the structure of the basins. During the Early Oligocene and Middle Miocene, they developed a series of NW‐ and WNW‐trending strike‐slip faults, reverse folds and flower‐like strike‐slip faults. However, there were obvious differences in sedimentary and tectonic evolution since the Late Miocene. The TXNB and TXB developed marine facies after the Late Miocene, while in the Quaternary, open marine facies replaced the Pliocene terrigenous facies in the ECSSB. Since the Late Miocene, the south of the ECSSB developed into a subsidence stage, and fault activity stopped. The TXB and TXNB developed some inverse structures and then developed into a thermal subsidence episode after the Dongsha Movement. Thus, this paper proposes that the ECSSB and the Cenozoic basins in the SCS were originally a unified basin and then subsequently separated into two basins as a result of the indentation of the Philippine Sea Plate (PSP) and the arc–continent collision between the Luzon Arc and the Eurasian Plate. Copyright © 2016 John Wiley & Sons, Ltd.

Comprehensive Geophysical Research on the Deep Structure of Northeastern South China Sea

AbstractThe complex crustal structure of northeastern South China Sea and neighboring areas, especially the structure of the land‐sea transition zone, has been a hot subject of the lithospheric structure research of South China Sea. In this paper we construct the density model along two profiles based on gravity data and constraints from Pn wave tomography and deep seismic sounding. The result of 2.5D density modeling supports the point of view that a high velocity layer exists in the lower crust of the area from Qiongyue Uplift to Luzon island arc; and the fitting result to the Taixinan Basin indicates that the crust in the southern depressed region should belong to the transitional type. Although the tectonic feature of the northeastern shelf of South China Sea is generally of volcanic type, the structures of profile AA' and BB' are different, which indicates the lateral variation and the complexity of crustal structure in the northeastern margin of South China Sea.