scholarly journals A Quantitative Assessment of Methane-Derived Carbon Cycling at the Cold Seeps in the Northwestern South China Sea

Minerals ◽  
2020 ◽  
Vol 10 (3) ◽  
pp. 256 ◽  
Author(s):  
Junxi Feng ◽  
Niu Li ◽  
Min Luo ◽  
Jinqiang Liang ◽  
Shengxiong Yang ◽  
...  
Keyword(s):  
South China Sea ◽  
South China ◽  
Bottom Water ◽  
Cold Seep ◽  
Anaerobic Oxidation Of Methane ◽  
Cold Seeps ◽  
Dissolved Carbon ◽  
China Sea ◽  
Seafloor Observation ◽  
Gas Hydrate Formation

Widespread cold seeps along continental margins are significant sources of dissolved carbon to the ocean water. However, little is known about the methane turnovers and possible impact of seepage on the bottom seawater at the cold seeps in the South China Sea (SCS). We present seafloor observation and porewater data of six push cores, one piston core and three boreholes as well as fifteen bottom-water samples collected from four cold seep areas in the northwestern SCS. The depths of the sulfate–methane transition zone (SMTZ) are generally shallow, ranging from ~7 to <0.5 mbsf (meters below seafloor). Reaction-transport modelling results show that methane dynamics were highly variable due to the transport and dissolution of ascending gas. Dissolved methane is predominantly consumed by anaerobic oxidation of methane (AOM) at the SMTZ and trapped by gas hydrate formation below it, with depth-integrated AOM rates ranging from 59.0 and 591 mmol m−2 yr−1. The δ13C and Δ14C values of bottom-water dissolved inorganic carbon (DIC) suggest discharge of 13C- and 14C-depleted fossil carbon to the bottom water at the cold seep areas. Based on a two-endmember estimate, cold seeps fluids likely contribute 16–26% of the bottom seawater DIC and may have an impact on the long-term deep-sea carbon cycle. Our results reveal the methane-related carbon inventories are highly heterogeneous in the cold seep systems, which are probably dependent on the distances of the sampling sites to the seepage center. To our knowledge, this is the first quantitative study on the contribution of cold seep fluids to the bottom-water carbon reservoir of the SCS, and might help to understand the dynamics and the environmental impact of hydrocarbon seep in the SCS.

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

Geofluids ◽  
2021 ◽  
Vol 2021 ◽  
pp. 1-24
Author(s):  
Wei Zhang ◽  
Jinqiang Liang ◽  
Qianyong Liang ◽  
Jiangong Wei ◽  
Zhifeng Wan ◽  
...  
Keyword(s):  
South China Sea ◽  
South China ◽  
Gas Hydrates ◽  
Gas Hydrate ◽  
Northern South China Sea ◽  
Cold Seep ◽  
Cold Seeps ◽  
Associated Gas ◽  
China Sea ◽  
Gas Bearing

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.

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

2019 ◽  
Vol 16 (13) ◽  
pp. 2299 ◽  
Author(s):  
Daidai Wu ◽  
Tiantian Sun ◽  
Rui Xie ◽  
Mengdi Pan ◽  
Xuegang Chen ◽  
...  
Keyword(s):  
South China Sea ◽  
Transition Zone ◽  
South China ◽  
Total Carbon ◽  
Cold Seep ◽  
Anaerobic Oxidation Of Methane ◽  
Authigenic Minerals ◽  
China Sea ◽  
Methane Seepage ◽  
The Impact

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.

scholarly journals The Diversity, Composition, and Putative Functions of Gill-Associated Bacteria of Bathymodiolin Mussel and Vesicomyid Clam from Haima Cold Seep, South China Sea

2020 ◽  
Vol 8 (11) ◽  
pp. 1699
Author(s):  
Juan Ling ◽  
Hongxiang Guan ◽  
Lihua Liu ◽  
Jun Tao ◽  
Jie Li ◽  
...  
Keyword(s):  
South China Sea ◽  
South China ◽  
Phylogenetic Diversity ◽  
High Throughput Sequencing ◽  
Cold Seep ◽  
Rrna Gene ◽  
Cold Seeps ◽  
Element Cycling ◽  
China Sea ◽  
Vesicomyid Clam

The Haima cold seep, which is one of the two active cold seeps in the South China Sea, is known for its great ecological importance. The seep bivalves are assumed to depend mainly on their bacterial symbiosis for survival and growth. However, information on the bacterial diversity, composition, and putative function of gill-associated of dominant dwelling animals in Haima cold seep remain elusive. Herein, we adopted a high-throughput sequencing of 16S rRNA gene amplicons, and function prediction methods (Functional Annotation of Prokaryotic Taxa (FAPROTAX) and Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICURUSTs)) to purposely illustrate the taxonomic and phylogenetic diversity, composition, and putative functions of the symbionts in bathymodiolin mussel Gigantidas haimaensis (Bivalvia: Mytilidae: Gigantidas) and vesicomyid clam Archivesica marissinica (Bivalvia: Glossoidea: Vesicomyidae). The predominant microbes of both species were Proteobacteria and Gammaproteobacteria on the phylum and class level, respectively. The taxonomic and phylogenetic diversity of gill microbial communities in G. haimaensis were significantly different from those in A. marissinica (p < 0.05). Nine functional groups, including seven carbon-related biogeochemical groups, were identified through the FAPROTAX analysis. However, the most dominant groups for G. haimaensis and A. marissinica were both chemoheterotrophic. G. haimaensis and A. marissinica shared many pathways, however, 16 obtained Kyoto Encyclopedia of Genes and Genomes (KEGG) orthologous groups (42.11%) significantly differed between the two species (p < 0.05). These findings would provide insight into the functions of microbes in the element cycling and energy flow as well as the host-symbiont relationship of bivalves in the Haima cold seep environment.

scholarly journals Distinct Bottom-Water Bacterial Communities at Methane Seeps With Various Seepage Intensities in Haima, South China Sea

2021 ◽  
Vol 8 ◽  
Author(s):  
Xiaopeng Li ◽  
Zehan Dai ◽  
Pengfei Di ◽  
Junxi Feng ◽  
Jun Tao ◽  
...  
Keyword(s):  
South China Sea ◽  
Bacterial Community ◽  
Water Column ◽  
South China ◽  
Bottom Water ◽  
Methane Content ◽  
Cold Seeps ◽  
Community Structures ◽  
Methane Seeps ◽  
China Sea

Methane seeps are chemosynthetic ecosystems in the deep-sea environment. Microbial community structures have been extensively studied in the seepage-affected sediments and investigation in the water column above the seeping sites is still lacking. In this study, prokaryotic communities in the bottom water about 50 cm from the seabed at methane seeps with various seepage intensities in Haima, South China Sea were comparatively studied by using 16S ribosomal RNA gene sequencing. These sites were assigned based on their distinct methane content levels and seafloor landscapes as the non-seepage (NS) site, low-intensity seepage (LIS) site, and high-intensity seepage (HIS) site. The abundances of the dominant phyla Proteobacteria, Bacteroidetes, and Actinobacteria differed significantly between NS and the two seepage sites (p &lt; 0.05). Alpha diversity differed among the three sites with the HIS site showing the lowest community diversity. Principal component analysis revealed highly divergent bacterial community structures at three sites. Many environmental variables including temperature, alkalinity, pH, methane, dissolved organic carbon (DOC), and inorganic nutrients were measured. Redundancy analysis indicated that methane content is the key environmental factor driving bacterial community variation (p = 0.001). Linear discriminant analysis effect size analysis identified various differentially enriched genera at the LIS and HIS sites. Phylogenetic analysis revealed close phylogenetic relationship among the operational taxonomic units of these genera with known oil-degrading species, indicating oil seepage may occur at the Haima cold seeps. Co-occurrence networks indicated that the strength of microbial interactions was weakest at the HIS site. This study represents a comprehensive comparison of microbial profiles in the water column of cold seeps in the SCS, revealing that the seepage intensity has a strong impact on bacterial community dynamics.

scholarly journals Using Time-Series Videos to Quantify Methane Bubbles Flux from Natural Cold Seeps in the South China Sea

Minerals ◽  
2020 ◽  
Vol 10 (3) ◽  
pp. 216
Author(s):  
Pengfei Di ◽  
Dong Feng ◽  
Jun Tao ◽  
Duofu Chen
Keyword(s):  
South China Sea ◽  
South China ◽  
Release Rate ◽  
The South China Sea ◽  
Cold Seep ◽  
Time Interval ◽  
Cold Seeps ◽  
The South ◽  
China Sea ◽  
Migration Pathways

Natural cold seeps are an important source of methane and other greenhouse gases to the ocean and atmosphere in the marine environment. Accurate quantification of methane bubble fluxes from cold seeps is vital for evaluating their influence on the global methane budget and climate change. We quantified the flux of gas bubbles released from two natural cold seep sites in the South China Sea: one seep vent in the Haima cold seeps (1400 m depth) and three seep vents at Site F (1200 m depth). We determined bubble diameter, size distribution, and bubble release rate using image processing techniques and a semiautomatic bubble-counting algorithm. The bubble size distributions fit well to log-normal distribution, with median bubble diameters between 2.54 mm and 6.17 mm. The average bubble diameters and release rates (4.8–26.1 bubbles s−1) in Site F was lower than that in Haima cold seeps (22.6 bubbles s−1), which may be attributed to a variety of factors such as the nature of the gas reservoir, hydrostatic pressure, migration pathways in the sediments, and pore size. The methane fluxes emitted at Haima cold seeps (12.6 L h−1) and at Site F (4.9 L h−1) indicate that the Haima and Site F cold seeps in the South China Sea may be a source of methane to the ocean. However, temporal variations in the bubble release rate and the geochemical characteristics of the seeps were not constrained in this study due to the short observational time interval.

scholarly journals Production of Labile Protein-Like Dissolved Organic Carbon Associated With Anaerobic Methane Oxidization in the Haima Cold Seeps, South China Sea

2021 ◽  
Vol 8 ◽  
Author(s):  
Tingcang Hu ◽  
Min Luo ◽  
Yunping Xu ◽  
Shanggui Gong ◽  
Duofu Chen
Keyword(s):  
Organic Carbon ◽  
South China Sea ◽  
Dissolved Organic Carbon ◽  
South China ◽  
Northern South China Sea ◽  
Anaerobic Oxidation Of Methane ◽  
Cold Seeps ◽  
Biological Index ◽  
Fluorescence Excitation ◽  
China Sea

Cold seeps where methane-rich fluids escape from the seafloor generally support enormous biomass of chemosynthetic organisms and associated fauna. In addition to transporting a great amount of methane toward the seafloor, cold seeps also contribute to the aged, dissolved organic carbon (DOC) pool in the deep ocean. Here, two sediment cores from the “Haima cold seeps,” northern South China Sea and a nearby reference core were analyzed for pore-water sulfate and DOC concentrations, δ13C of DOC, and optical properties of dissolved organic matter (DOM). High DOC concentrations (0.9–3.7 mM) accompanied by extremely low δ13C values (−43.9 to −76.2‰) suggest the conversion of methane into sedimentary DOC pool in the seep sediments. Parallel factor analysis (PARAFAC) of the fluorescence excitation-emission matrices shows higher fluorescent intensities of labile protein-like components (C2 and C4) and lower fluorescent intensities of refractory humic-like components (C1 and C3) in the seep cores compared to the reference core. The intensity of C2 is positively correlated with DOC concentrations and δ13C-DOC in the seep sediments, suggesting that the labile protein-like DOM was produced by the anaerobic oxidation of methane (AOM). Moreover, low humification index (HIX) and high biological index (BIX) values also indicate intensified production of relatively labile DOM with lower degradation degree in the seep cores compared to the reference core. Hence, we highlight that methane-derived DOC may serve as important carbon and energy sources for heterotrophic microbial communities due to its relatively labile nature.

6. Ecosystems of cold seeps in the South China Sea

2017 ◽  
Author(s):  
Mingyang Niu ◽  
Qianyong Liang ◽  
Dong Feng ◽  
Fengping Wang
Keyword(s):  
South China Sea ◽  
South China ◽  
The South China Sea ◽  
Cold Seeps ◽  
The South ◽  
China Sea
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