The northern slope of the South China Sea is a gas-hydrate-bearing region related to a high deposition rate of organic-rich sediments co-occurring with intense methanogenesis in subseafloor environments.Anaerobic oxid...The northern slope of the South China Sea is a gas-hydrate-bearing region related to a high deposition rate of organic-rich sediments co-occurring with intense methanogenesis in subseafloor environments.Anaerobic oxidation of methane（AOM） coupled with bacterial sulfate reduction results in the precipitation of solid phase minerals in seepage sediment,including pyrite and gypsum.Abundant aggregates of pyrites and gypsums are observed between the depth of 667 and 850 cm below the seafloor（cmbsf） in the entire core sediment of HS328 from the northern South China Sea.Most pyrites are tubes consisting of framboidal cores and outer crusts.Gypsum aggregates occur as rosettes and spheroids consisting of plates.Some of them grow over pyrite,indicating that gypsum precipitation postdates pyrite formation.The sulfur isotopic values（δ^34 S） of pyrite vary greatly（from –46.6‰ to –12.3‰ V-CDT） and increase with depth.Thus,the pyrite in the shallow sediments resulted from organoclastic sulfate reduction（OSR） and is influenced by AOM with depth.The relative high abundance and δ^34 S values of pyrite in sediments at depths from 580 to 810 cmbsf indicate that this interval is the location of a paleo-sulfate methane transition zone（SMTZ）.The sulfur isotopic composition of gypsum（from–25‰ to –20.7‰） is much lower than that of the seawater sulfate,indicating the existence of a 34 S-depletion source of sulfur species that most likely are products of the oxidation of pyrites formed in OSR.Pyrite oxidation is controlled by ambient electron acceptors such as MnO2,iron（Ⅲ） and oxygen driven by the SMTZ location shift to great depths.The δ^34 S values of gypsum at greater depth are lower than those of the associated pyrite,revealing downward diffusion of 34 S-depleted sulfate from the mixture of oxidation of pyrite derived by OSR and the seawater sulfate.These sulfates also lead to an increase of calcium ions from the dissolution of calcium carbonate mineral,which will be favor to the formation of gypsum.Overall,the mineralogy and sulfur isotopic composition of the pyrite and gypsum suggest variable redox conditions caused by reduced seepage intensities,and the pyrite and gypsum can be a recorder of the intensity evolution of methane seepage.展开更多
Recently,methane seepage related to the dissociation of natural gas hydrates has attracted much attention,which has a significant impact on the study of the global carbon and nitrogen cycles.Based on the detailed geoc...Recently,methane seepage related to the dissociation of natural gas hydrates has attracted much attention,which has a significant impact on the study of the global carbon and nitrogen cycles.Based on the detailed geochemical analyses of sediments(core Q6)from the Qiongdongnan Basin,South China Sea,three methane seepage activities were identified and the exact horizons of anaerobic oxidation of methane(AOM)were defined.Furthermore,organic carbon isotopic(δ^(13)C_(TOC))levels ranged from−23.6‰–−20.6‰PDB;nitrogen isotopes(δ^(15)N_(TN))of the same sedimentary samples ranged from 1.8‰–5.3‰.We also found obvious simultaneous negative excursions of organic carbon isotopes(δ^(13)C_(TOC))and nitrogen isotopes(δ^(15)N_(TN))in the horizons of methane seepages.Compared with the normal sediments,their maximum negative excursions were 2.6‰and 2.5‰,respectively.We discuss in detail the various characteristics ofδ^(15)N_(TN) andδ^(13)C_(TOC) levels in sediments and their coupling responses to methane seepage activities.We believe that the methane seepage events changed the evolution trajectory ofδ^(15)N_(TN) andδ^(13)C_(TOC) levels in sediment records,which resulted in the simultaneous negative excursions.This phenomenon is of great significance to reveal the historical dissociation of natural gas hydrates and their influence on the deep-sea carbon and nitrogen pool.展开更多
Based on the equation of the gas flow continuity and state, Darcy law and Langmuir equation, the law of methane seepage in the wall of drainage roadway was studied. The governing equation of methane one-way seepage in...Based on the equation of the gas flow continuity and state, Darcy law and Langmuir equation, the law of methane seepage in the wall of drainage roadway was studied. The governing equation of methane one-way seepage in the seam was founded. By solving the equation, the calculation of methane seepage velocity in the coal wall was worked out. The result has really applied worth and will give beneficial references to re-lated research, it provides preventing coal and gas outbursts with theoretical gist.展开更多
Seafloor elongated depressions are indicators of gas seepage or slope instability. Here we report a sequence of slopeparallel elongated depressions that link to headwalls of sediment slides on upper slope. The depress...Seafloor elongated depressions are indicators of gas seepage or slope instability. Here we report a sequence of slopeparallel elongated depressions that link to headwalls of sediment slides on upper slope. The depressions of about 250 m in width and several kilometers in length are areas of focused gas discharge indicated by bubble-release into the water column and methane enriched pore waters. Sparker seismic profiles running perpendicular and parallel to the coast, show gas migration pathways and trapped gas underneath these depressions with bright spots and seismic blanking. The data indicate that upward gas migration is the initial reason for fracturing sedimentary layers. In the top sediment where two young stages of landslides can be detected, the slopeparallel sediment weakening lengthens and deepens the surficial fractures, creating the elongated depressions in the seafloor supported by sediment erosion due to slope-parallel water currents.展开更多
Seafloor pockmarks are important indicators of submarine methane seepages and slope instabilities.In order to promote the understanding of submarine pockmarks and their relationship with sediment instabilities and cli...Seafloor pockmarks are important indicators of submarine methane seepages and slope instabilities.In order to promote the understanding of submarine pockmarks and their relationship with sediment instabilities and climate changes,here we summarize the research results of pockmarks in the spatio-temporal distributions and shaping factors.Most of pockmarks occur along active or passive continental margins during the last 25 kyr B.P..Circular and ellipse are the most common forms of pockmarks,whereas pockmarks in a special crescent or elongated shape are indicators of slope instabilities,and ring-shape pockmarks are endemic to the gas hydrate zones.Further researches should be focused on the trigger mechanism of climate changes based on the pockmarks in the high latitudes formed during the deglaciation periods,and the role of gas hydrates in the seafloor evolution should be elucidated.In addition,the feature of pockmarks at their early stage(e.g.,developing gas chimneys and gas driving sedimentary doming)and the relations between pockmarks and mass movements,mud diapirs could be further studied to clarify the influences of rapid methane release from submarine sediments on the atmospheric carbon contents.展开更多
基金The Qingdao National Laboratory for Marine Science and Technology under contract No.QNLM2016ORP0210the National Natural Science Foundation of China under contract Nos 41306061,41473080 and 41376076the Scientific Cooperative Project by China National Petroleum Corporation and Chinese Academic of Sciences under contract No.2015A-4813
文摘The northern slope of the South China Sea is a gas-hydrate-bearing region related to a high deposition rate of organic-rich sediments co-occurring with intense methanogenesis in subseafloor environments.Anaerobic oxidation of methane（AOM） coupled with bacterial sulfate reduction results in the precipitation of solid phase minerals in seepage sediment,including pyrite and gypsum.Abundant aggregates of pyrites and gypsums are observed between the depth of 667 and 850 cm below the seafloor（cmbsf） in the entire core sediment of HS328 from the northern South China Sea.Most pyrites are tubes consisting of framboidal cores and outer crusts.Gypsum aggregates occur as rosettes and spheroids consisting of plates.Some of them grow over pyrite,indicating that gypsum precipitation postdates pyrite formation.The sulfur isotopic values（δ^34 S） of pyrite vary greatly（from –46.6‰ to –12.3‰ V-CDT） and increase with depth.Thus,the pyrite in the shallow sediments resulted from organoclastic sulfate reduction（OSR） and is influenced by AOM with depth.The relative high abundance and δ^34 S values of pyrite in sediments at depths from 580 to 810 cmbsf indicate that this interval is the location of a paleo-sulfate methane transition zone（SMTZ）.The sulfur isotopic composition of gypsum（from–25‰ to –20.7‰） is much lower than that of the seawater sulfate,indicating the existence of a 34 S-depletion source of sulfur species that most likely are products of the oxidation of pyrites formed in OSR.Pyrite oxidation is controlled by ambient electron acceptors such as MnO2,iron（Ⅲ） and oxygen driven by the SMTZ location shift to great depths.The δ^34 S values of gypsum at greater depth are lower than those of the associated pyrite,revealing downward diffusion of 34 S-depleted sulfate from the mixture of oxidation of pyrite derived by OSR and the seawater sulfate.These sulfates also lead to an increase of calcium ions from the dissolution of calcium carbonate mineral,which will be favor to the formation of gypsum.Overall,the mineralogy and sulfur isotopic composition of the pyrite and gypsum suggest variable redox conditions caused by reduced seepage intensities,and the pyrite and gypsum can be a recorder of the intensity evolution of methane seepage.
基金supported by the National Key R&D Program of China(No.2017YFC0306703)the Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory(Guangzhou)(No.GML2019 ZD0201).
文摘Recently,methane seepage related to the dissociation of natural gas hydrates has attracted much attention,which has a significant impact on the study of the global carbon and nitrogen cycles.Based on the detailed geochemical analyses of sediments(core Q6)from the Qiongdongnan Basin,South China Sea,three methane seepage activities were identified and the exact horizons of anaerobic oxidation of methane(AOM)were defined.Furthermore,organic carbon isotopic(δ^(13)C_(TOC))levels ranged from−23.6‰–−20.6‰PDB;nitrogen isotopes(δ^(15)N_(TN))of the same sedimentary samples ranged from 1.8‰–5.3‰.We also found obvious simultaneous negative excursions of organic carbon isotopes(δ^(13)C_(TOC))and nitrogen isotopes(δ^(15)N_(TN))in the horizons of methane seepages.Compared with the normal sediments,their maximum negative excursions were 2.6‰and 2.5‰,respectively.We discuss in detail the various characteristics ofδ^(15)N_(TN) andδ^(13)C_(TOC) levels in sediments and their coupling responses to methane seepage activities.We believe that the methane seepage events changed the evolution trajectory ofδ^(15)N_(TN) andδ^(13)C_(TOC) levels in sediment records,which resulted in the simultaneous negative excursions.This phenomenon is of great significance to reveal the historical dissociation of natural gas hydrates and their influence on the deep-sea carbon and nitrogen pool.
文摘Based on the equation of the gas flow continuity and state, Darcy law and Langmuir equation, the law of methane seepage in the wall of drainage roadway was studied. The governing equation of methane one-way seepage in the seam was founded. By solving the equation, the calculation of methane seepage velocity in the coal wall was worked out. The result has really applied worth and will give beneficial references to re-lated research, it provides preventing coal and gas outbursts with theoretical gist.
基金funded by the European project EUROFLEETS (Seventh Framework Programme, No. 228344)
文摘Seafloor elongated depressions are indicators of gas seepage or slope instability. Here we report a sequence of slopeparallel elongated depressions that link to headwalls of sediment slides on upper slope. The depressions of about 250 m in width and several kilometers in length are areas of focused gas discharge indicated by bubble-release into the water column and methane enriched pore waters. Sparker seismic profiles running perpendicular and parallel to the coast, show gas migration pathways and trapped gas underneath these depressions with bright spots and seismic blanking. The data indicate that upward gas migration is the initial reason for fracturing sedimentary layers. In the top sediment where two young stages of landslides can be detected, the slopeparallel sediment weakening lengthens and deepens the surficial fractures, creating the elongated depressions in the seafloor supported by sediment erosion due to slope-parallel water currents.
基金supported by the National Natural Science Foundation of China (Nos. 41606044 41906068+3 种基金 91 858208) the National Key Research and Development Program (No. 2018YFC031000303) the Taishan Scholar Special Experts Project (No. ts201712079)the Marine Geological Survey Program of China Geological Survey (No. DD20190819)
文摘Seafloor pockmarks are important indicators of submarine methane seepages and slope instabilities.In order to promote the understanding of submarine pockmarks and their relationship with sediment instabilities and climate changes,here we summarize the research results of pockmarks in the spatio-temporal distributions and shaping factors.Most of pockmarks occur along active or passive continental margins during the last 25 kyr B.P..Circular and ellipse are the most common forms of pockmarks,whereas pockmarks in a special crescent or elongated shape are indicators of slope instabilities,and ring-shape pockmarks are endemic to the gas hydrate zones.Further researches should be focused on the trigger mechanism of climate changes based on the pockmarks in the high latitudes formed during the deglaciation periods,and the role of gas hydrates in the seafloor evolution should be elucidated.In addition,the feature of pockmarks at their early stage(e.g.,developing gas chimneys and gas driving sedimentary doming)and the relations between pockmarks and mass movements,mud diapirs could be further studied to clarify the influences of rapid methane release from submarine sediments on the atmospheric carbon contents.
