| Methane hydrate(gas hydrate)is a huge potential clean energy resource.The successful experiment exploiting drilling operation of methane hydrate in the Shenhu area of the northern South China Sea,marks human one step away from large-scale commercial exploitation of methane hydrates.The energy status and influence of methane hydrates has been well noticed,but the effect of methane hydrate on the environment does not seem to be fully appreciated by humans.The large-scale decomposition of methane hydrates in geological history has led to global warming,ocean acidification,and even global extinctions.Under normal circumstances,the methane leakage from the sea floor,as known as "cold spring" still can be observed from all over the world.The upward-moving methane encountered sulfate in sulfate methane transition zone(SMTZ),result in anaerobic oxidation of methane coupling with sulfate reduction(AOM-SR).This reaction occurred in the early diagenesis of sediments,shared the porewater sulfate with the reaction of organic matter oxidation driven sulfate reduction.Thus,in order to research the upward-moving methane consumption process,it is key important to understand the source to pool migration of element sulfur in sediment.From the above,research on the trigger of the methane hydrate decomposition and source to pool migration of element sulfur in sediment of methane hydrate area has very significant value to study the potential events of large-scale methane hydrates decomposition and possible environmental disaster caused by it.This research was jointly supported by the National Nature Science Foundation of China "Research on the biogeochemical cycling pattern of sediment iron manganese in the methane hydrate area of Northern South China Sea(project number:41773078)" and "Research on the sulfate-methane system prosumer mechanism and its interface(SMI)characteristics of the sediments in the northern South China Sea(project number:40976035)",and "Research on biomarkers and molecular biology of methanogenesis and anaerobic oxidation of methane in gas hydrate system(project number:41276046)",and National Major Fundamental Research and Development Project "Basic study on gas hydrate accumulation laws and exploitation in the South China Sea(No.2009CB219501)" and other projects.We chose the gravity piston core 973-4 near the "Kowloon Methane Reef" of Southwest Taiwan Basin in Northern South China Sea to proceed a series of geological,isotopic geochemical,element geochemical and mineralogical experiments,combined with the geological,geochemical and geophysical research data of South China Sea conduct comprehensive analysis and discussion.The major outcomes are as followings:(1)The sediments deposited in core 973-4 since 44.5ka B.P.are mainly argillaceous silt terrigenous,most likely from the Zengwen and Kaoping River,Taiwan Island.The deposition rate in this area is stable at approximately 31.36 cm/ka.There is a turbidity event happened at about 14ka B.P.This event led to the sediments resuspension and redeposition,reverse the 455?885 cm interval stratum.The turbidity caused the carbonate content increased from 5.01%to 30.19%.Combined with core observation result,that there were foraminifera shell enrichment intervals at 450 cm and 510 cm to be founded.(2)The PCA analysis results of major and trace elements of sediment of core 973-4 show that the variance contribution rate of component one accounted for 70.93%of the total variance contribution,and two of the obvious negative loading Ca and Sr were caused by the redeposition which caused by turbidity current events.Component two accounts for 12.39%of total variance contribution,mainly for the Ba and Rb element combination,these two elements within the SMTZ belt distribution has been,"Rb front" phenomenon may be associated with barium in the barite homogeneous classes like.Enrichment factor(EF)analysis found that four elements(Cu,Ni,Zn,and Li)in core 973-4,abnormal enrichment,for Cu EF is 187.43,and the Ni is 8.89.The high elements EF indicates there is a high salinity(enriched Cu,Ni,Zn,and Li)fluid from deep layer that injected to this area.(3)The methane hydrate in this area is keep decomposition until14.0ka B.P.During the last 14.0ka B.P.,due to the sea level rising,the methane hydrate decomposition scale gradually reduced.However,the carbonate isotope of bulk sediment in core 973-4 indicates that in the period of Younger Dryer event(12ka B.P.),8200 events(8.2ka B.P.)and Pulleniatina obliauiloculata minimum event(3.5ka B.P.),the methane hydrate decomposition suddenly increased.(4)The extremely negative ?18 Ocarb value in 90cm(-4.11%?),250cm(-3.18‰)and 370cm(-3.80‰)coupled with enhanced methane hydrate decomposition indicates that there is mud volcano eruption synchronized with methane hydrate decomposition in this area.The key factor of formation the methane hydrate and the mud volcano is the high deposition rate in this area,and the sea level changes controlled the methane hydrate decomposition and the mud volcano eruption.The primary cause of synchronized formation and release activities is the same controlling factor.(5)The pyrite content increased rapidly from 0.09%to 0.72%,meanwhile the(Feox+Femag)/FeHR dropped from 0.67 to 0.33,shows that the iron element source of authigenic pyrite formation in this area are iron hydroxide/oxide(Feox)and magnetite(Femag),and prefer to use the Feox instead of Femag.In the interface of upper and lower part of SMTZ which has the highest sulfate reduction rate and highest authigenic pyrite content the Fepy/jeHR value is about 0.45,indicating that the pyrite formation is mainly limited by the sulfate reduction.In the LSMTZ,the authigenic pyrite formation conditions are gradually unsuitable,the authigenic siderite content raised from 0.23%to 0.64%in the sediment.(6)The pyrite framboidal cluster has a very different distribution characteristics in upper and lower SMTZ.The average diameter of pyrite framboidal cluster in the LSMTZ(51.59?m)is twice bigger than the average diameter in the USMTZ(25.43?m)and has a more broadly distribution range.The lower pyrite content and the larger mean diameter in LSMTZ indicate that the pyrite in LSMTZ is not easy to nucleate,but the nucleated pyrite cluster has been growing longer.The formation of irregular pyrite aggregates has been through four stages as follow:conglutination,cementation,compaction and formation.(7)The organic sulfur content in the SMTZ(0.35%)of this area is significant higher than the other layer(0.15%).The large abound of intermediate sulfide in SMTZ(element sulfur and acid volatile sulfide)are the key factor to the diagenetic organic sulfur formation.The geochemical conditions in LSMTZ is most favorable for diagenetic organic sulfur formation.The organic sulfur content accounts for about 40%of the total sulfur.The large abundant of organic sulfur and intermediate sulfur in sediment shows that their sulfur pool effect is 'underestimated during the early diagenesis in the sediment of methane hydrate area.(8)973-4 core is located at methane hydrate area and have been through three times of methane leakage.There are two pathway,including the dominated AOM-SR,of sulfate reduction in this area due to the upward-moving methane.The sulfate reduction transfers the sulfate to hydrogen sulfide and then to sulfur-containing products to complete the sulfur migration in the upper sediment of methane hydrate area.The direct factors affecting the sulfur reduction products in 973-4 area including sulfate reduction rate,hydrogen sulfide content and sulfate content(SO42-supplemental environment).The indirect influencing factors include the content and activity of organic matter,the content of active iron in sediments and the concentration of upward-moving methane.All these factors joint influence the sulfur-containing reduction products. |
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