Characteristic gypsum micro-sphere and granular mass were discovered by bin-ocular microscope in the gas hydrate-associated sediments at cores SO143-221 and SO143/TVG40-2A respectively on Hydrate Ridge of Cascadia mar...Characteristic gypsum micro-sphere and granular mass were discovered by bin-ocular microscope in the gas hydrate-associated sediments at cores SO143-221 and SO143/TVG40-2A respectively on Hydrate Ridge of Cascadia margin, the eastern North Pacific. XRD patterns and EPA analyses show both micro-sphere and granular mass of the crystals have the typical peaks and the typical main chemical compositions of gypsum, although their weight percents are slightly less than the others in the non-gas hydrate-associated marine regions. SEM pictures show that the gypsum crystals have clear crystal boundaries, planes, edges and cleav-ages of gypsum in form either of single crystal or of twin crystals. In view of the fact that there are meanwhile gas hydrate-associated authigenic carbonates and SO42--rich pore water in the same sediment cores, it could be inferred reasonably that the gypsums formed also authigenically in the gas hydrate-associated environment too, most probably at the interface between the down-ward advecting sulfate-rich seawater and the below gas hydrate, which spilled calcium during its formation on Hydrate Ridge. The two distinct forms of crystal intergrowth, which are the granular mass of series single gypsum crystals at core SO143/TVG40-2A and the microsphere of gypsum crystals accompanied with detrital components at core SO143-221 respectively, indicate that they precipitated most likely in different interstitial water dynamic environments. So, the distinct authigenic gypsums found in gas hydrate-associated sediments on Hydrate Ridge could also be believed as one of the parameters which could be used to indicate the presence of gas hydrate in an unknown marine sediment cores.展开更多
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 for展开更多
Authigenic gypsum crystals, along with pyrite and carbonate mineralization, predominantly calcites were noticed in distinct intervals in a 32 m long piston core, collected in the gas hydratebearing sediments in the no...Authigenic gypsum crystals, along with pyrite and carbonate mineralization, predominantly calcites were noticed in distinct intervals in a 32 m long piston core, collected in the gas hydratebearing sediments in the northern portion of the Krishna-Godavari basin, eastern continental margin of India at a water depth of 1691 m. X-ray diffraction and energy dispersive spectrum studies confirm presence of pyrite, gypsum, calcite, and other mineral aggregates. The occurrence of gypsum in such deep sea environment is intriguing, because gypsum is a classical evaporite mineral and is under saturated with respect to sea water. Sedimentological, geochemical evidences point to diagenetic formation of the gypsum due to oxidation of sulphide minerals （i.e. pyrite）. Euhedral, transparent gypsum crystals, with pyrite inclusions are cemented with authigenic carbonates, possibly indicating that they were formed authigenically in situ in the gas hydrate-influenced environment due to late burial diagenesis involving sulphate reduction and anaerobic oxidation of methane （AOM）. Therefore, the authigenic gypsums found in sediments of the Krishna-Godavari and Mahanadi offshore regions could be seen as one of the parameters to imply the presence of high methane flux possibly from gas hydrate at depth.展开更多
基金This work was supported jointly by the Key Laboratory of Submarine Geosciences, SOA (Grant No. 2002-3) the National Natural Science Foundation of China (Grant No. 40272052) China Scholarship Council (CSC).
文摘Characteristic gypsum micro-sphere and granular mass were discovered by bin-ocular microscope in the gas hydrate-associated sediments at cores SO143-221 and SO143/TVG40-2A respectively on Hydrate Ridge of Cascadia margin, the eastern North Pacific. XRD patterns and EPA analyses show both micro-sphere and granular mass of the crystals have the typical peaks and the typical main chemical compositions of gypsum, although their weight percents are slightly less than the others in the non-gas hydrate-associated marine regions. SEM pictures show that the gypsum crystals have clear crystal boundaries, planes, edges and cleav-ages of gypsum in form either of single crystal or of twin crystals. In view of the fact that there are meanwhile gas hydrate-associated authigenic carbonates and SO42--rich pore water in the same sediment cores, it could be inferred reasonably that the gypsums formed also authigenically in the gas hydrate-associated environment too, most probably at the interface between the down-ward advecting sulfate-rich seawater and the below gas hydrate, which spilled calcium during its formation on Hydrate Ridge. The two distinct forms of crystal intergrowth, which are the granular mass of series single gypsum crystals at core SO143/TVG40-2A and the microsphere of gypsum crystals accompanied with detrital components at core SO143-221 respectively, indicate that they precipitated most likely in different interstitial water dynamic environments. So, the distinct authigenic gypsums found in gas hydrate-associated sediments on Hydrate Ridge could also be believed as one of the parameters which could be used to indicate the presence of gas hydrate in an unknown marine sediment cores.
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 for
基金funded by National Gas Hydrate Program (NGHP), India
文摘Authigenic gypsum crystals, along with pyrite and carbonate mineralization, predominantly calcites were noticed in distinct intervals in a 32 m long piston core, collected in the gas hydratebearing sediments in the northern portion of the Krishna-Godavari basin, eastern continental margin of India at a water depth of 1691 m. X-ray diffraction and energy dispersive spectrum studies confirm presence of pyrite, gypsum, calcite, and other mineral aggregates. The occurrence of gypsum in such deep sea environment is intriguing, because gypsum is a classical evaporite mineral and is under saturated with respect to sea water. Sedimentological, geochemical evidences point to diagenetic formation of the gypsum due to oxidation of sulphide minerals （i.e. pyrite）. Euhedral, transparent gypsum crystals, with pyrite inclusions are cemented with authigenic carbonates, possibly indicating that they were formed authigenically in situ in the gas hydrate-influenced environment due to late burial diagenesis involving sulphate reduction and anaerobic oxidation of methane （AOM）. Therefore, the authigenic gypsums found in sediments of the Krishna-Godavari and Mahanadi offshore regions could be seen as one of the parameters to imply the presence of high methane flux possibly from gas hydrate at depth.
