南海北部陸坡冷泉區(qū)和非冷泉區(qū)地球化學(xué)特征對甲烷滲漏活動的響應(yīng)

南海北部陸坡冷泉區(qū)和非冷泉區(qū)地球化學(xué)特征對甲烷滲漏活動的響應(yīng)目錄一 引言 引言（一）選題依據(jù)與研究目的在冷泉進入人類視野的30年中，全世界范圍內(nèi)發(fā)現(xiàn)了各種油氣滲漏點這些地點已經(jīng)成為了解甲烷滲透和相關(guān)生物地球化學(xué)過程變化的模型系統(tǒng)。目前，中國南海的冷泉研究，多集中于單個區(qū)域，或者單一地研究冷泉碳酸鹽巖、沉積物或沉積物孔隙水的地球化學(xué)特征，少有對南海多個冷泉活動區(qū)、各種沉積特征綜合的對比研究。因此針對以上存在的問題，結(jié)合已有的研究條件，本文測試了南海北部陸坡F站位冷泉區(qū)、海馬冷泉區(qū)和九龍峽谷非冷泉區(qū)的沉積物和沉積物孔隙水地球化學(xué)特征，對比研究冷泉區(qū)和非冷泉區(qū)、冷泉活動在不同時間尺度上的沉積特征。（二）全球冷泉分布大陸邊緣斜坡海底之下有著豐富的甲烷貯藏，以固態(tài)天然氣水合物、溶解于海水或氣體的形式存在ADDINCSL_CITATION{"citationItems":[{"id":"ITEM-1","itemData":{"ISSN":"0029-8182","author":[{"dropping-particle":"","family":"Gross","given":"MG","non-dropping-particle":"","parse-names":false,"suffix":""}],"container-title":"Oceanus","id":"ITEM-1","issue":"3","issued":{"date-parts":[["1984"]]},"page":"2-6","title":"Introduction:Deep-seahotspringsandcoldseeps","type":"article-journal","volume":"27"},"uris":["/documents/?uuid=82685057-5d9e-3bfd-bc92-3d9f10af69e5"]}],"mendeley":{"formattedCitation":"(Gross,1984)","plainTextFormattedCitation":"(Gross,1984)","previouslyFormattedCitation":"(Gross,1984)"},"properties":{"noteIndex":0},"schema":"/citation-style-language/schema/raw/master/csl-citation.json"}(Gross,1984)。在重力和構(gòu)造應(yīng)力的作用下產(chǎn)生甲烷通道，海底之下的以甲烷等有機質(zhì)為主要成分的流體從通道中泄漏進海水，被稱為冷泉ADDINCSL_CITATION{"citationItems":[{"id":"ITEM-1","itemData":{"DOI":"10.1038/ngeo1926","ISSN":"17520894","abstract":"Theleakageofcold,methane-richfluidsfromsubsurfacereservoirstotheseaflooratspecificsitesoncontinentalslopes,termedcoldseeps,sustainssomeoftherichestecosystemsontheseabed.Theseseep-fuelledcommunitiesutilizearoundtwoordersofmagnitudemoreoxygenperunitareathannon-seepseafloorcommunities.Muchoftheoxygenisconsumedbymicrobesandanimal–microbesymbiosesthatusemethaneasanenergysource.Theproportionofmethaneconsumedvarieswithfluidflowrate,rangingfrom80%inseepswithslowfluidflowtolessthan20%inseepswherefluidflowishigh.Assumingthepresenceofafewtensofthousandsofactivecoldseepsystemsoncontinentalslopesworldwide,weestimatethatthetotaleffluxofmethanetotheoverlyingoceancouldreach0.02Gtofcarbonannually.Asmuchmoremethaneislostfromcontinentalslopes,beitthroughemissiontothehydrosphereorconsumptionbymicrobes,thancanbeproduced,wesuggestthatasubstantialfractionofthemethanethatfuelsseepecosystemsissourcedfromdeepcarbonburiedkilometresundertheseafloor.","author":[{"dropping-particle":"","family":"Boetius","given":"Antje","non-dropping-particle":"","parse-names":false,"suffix":""},{"dropping-particle":"","family":"Wenzh?fer","given":"Frank","non-dropping-particle":"","parse-names":false,"suffix":""}],"container-title":"NatureGeoscience","id":"ITEM-1","issue":"9","issued":{"date-parts":[["2013"]]},"page":"725-734","publisher":"NaturePublishingGroup","title":"Seaflooroxygenconsumptionfuelledbymethanefromcoldseeps","type":"article-journal","volume":"6"},"uris":["/documents/?uuid=1f4f7131-4375-40f7-bc05-09b03b57d0da"]}],"mendeley":{"formattedCitation":"(BoetiusandWenzh?fer,2013)","plainTextFormattedCitation":"(BoetiusandWenzh?fer,2013)","previouslyFormattedCitation":"(BoetiusandWenzh?fer,2013)"},"properties":{"noteIndex":0},"schema":"/citation-style-language/schema/raw/master/csl-citation.json"}(BoetiusandWenzh?fer,2013)。冷泉流體的溫度通常低于周圍海水溫度不知道出處。冷泉滲漏會在海底產(chǎn)生一系列物理、化學(xué)、生物地質(zhì)作用，同時促進不需要光合作用、通過化能合成作用自養(yǎng)生存的生物群落發(fā)育，形成獨特的冷泉系統(tǒng)ADDINCSL_CITATION{"citationItems":[{"id":"ITEM-1","itemData":{"DOI":"10.1126/science.226.4677.965","ISSN":"00368075","abstract":"DensebiologicalcommunitiesoflargeepifaunaltaxasimilartothosefoundalongridgecrestventsattheEastPacificRisewerediscoveredintheabyssalGulfofMexico.TheseassemblagesoccuronapassivecontinentalmarginatthebaseoftheFloridaEscarpment,theinterfacebetweentherelativelyimpermeablehemipelagicclaysofthedistalMississippiFanandthejointedCretaceouslimestoneoftheFloridaPlatform.Thefaunaapparentlyisnourishedbysulfiderichhypersalinewatersseepingoutatnearambienttemperaturesontotheseafloor.","author":[{"dropping-particle":"","family":"Paull","given":"C.K.","non-dropping-particle":"","parse-names":false,"suffix":""},{"dropping-particle":"","family":"Hecker","given":"B.","non-dropping-particle":"","parse-names":false,"suffix":""},{"dropping-particle":"","family":"Commeau","given":"R.","non-dropping-particle":"","parse-names":false,"suffix":""},{"dropping-particle":"","family":"Freeman-Lynde","given":"R.P.","non-dropping-particle":"","parse-names":false,"suffix":""},{"dropping-particle":"","family":"Neumann","given":"C.","non-dropping-particle":"","parse-names":false,"suffix":""},{"dropping-particle":"","family":"Corso","given":"W.P.","non-dropping-particle":"","parse-names":false,"suffix":""},{"dropping-particle":"","family":"Golubic","given":"S.","non-dropping-particle":"","parse-names":false,"suffix":""},{"dropping-particle":"","family":"Hook","given":"J.E.","non-dropping-particle":"","parse-names":false,"suffix":""},{"dropping-particle":"","family":"Sikes","given":"E.","non-dropping-particle":"","parse-names":false,"suffix":""},{"dropping-particle":"","family":"Curray","given":"J.","non-dropping-particle":"","p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圖1.1全球冷泉。具有水合物-變質(zhì)帶-微生物-碳酸鹽特征的滲漏部位；主動大陸邊緣（藍色），被動大陸邊緣包括地下水滲漏（橙色）；轉(zhuǎn)換斷層邊緣（綠色）。ADDINCSL_CITATION{"citationItems":[{"id":"ITEM-1","itemData":{"DOI":"10.1007/s00531-014-1010-0","ISBN":"0053101410100","author":[{"dropping-particle":"","family":"Suess","given":"Erwin","non-dropping-particle":"","parse-names":false,"suffix":""}],"id":"ITEM-1","issued":{"date-parts":[["2014"]]},"page":"1889-1916","title":"Marinecoldseepsandtheirmanifestations:geologicalcontrol,biogeochemicalcriteriaandenvironmentalconditions","type":"article-journal"},"uris":["/documents/?uuid=22ffe03f-41df-470b-8645-3e3fcb6a61bc"]}],"mendeley":{"formattedCitation":"(Suess,2014)","plainTextFormattedCitation":"(Suess,2014)","previouslyFormattedCitation":"(Suess,2014)"},"properties":{"noteIndex":0},"schema":"/citation-style-language/schema/raw/master/csl-citation.json"}(Suess,2014)（二）冷泉中甲烷的來源冷泉流體的最主要成分為甲烷，甲烷的來源可分為有機成因和無機成因。有機成因的甲烷主要為生物有機質(zhì)降解形成，無機成因的甲烷主要為蛇紋石化形成或地球形成過程中形成保存在地球內(nèi)部ADDINCSL_CITATION{"citationItems":[{"id":"ITEM-1","itemData":{"DOI":"10.1007/978-1-4684-2757-8_8","abstract":"Methanehasbeendetectedinseveralcoresofrapidlydeposited({>}50m/my)deepseasediments.Othergases,suchascarbondioxideandethane,arecommonlypresentbutonlyinminorandtraceamounts,respectively.ThemethaneoriginatespredominantlyfrombacterialreductionofC02,asindicatedbycomplimentarychangeswithdepthintheamountandisotopiccompositionofredox-linkedporewaterconstituents:sulfate-bicarbonateandbicarbonate-methane.Presently,noprecisedeterminationexistsoftheamountofgaspresentunderinsituconditionsindeepseasediments.UsingC13/C12isotoperatiosofthedissolvedbicarbonateandmethane,andemployingkineticcalculationsbasedonRayleighdistillationequa-tions,theamountsofmethanegeneratedbyreductionofcarbondi-oxidebyhydrogenhasbeenestimated.Theamountscalculatedsuggestthataminimumof20mmolCH4/kginterstitialwaterisformed.Amethaneconcentrationof20mmol/kgapproachestheamountre-quiredfortheformationofgashydratesunderpressure-temperatureconditionscorrespondingtoawatercolumnofabout500meters,withatemperatureof5°Catthesediment-waterinterface.Depthofsta-bilityofthegashydratewithinthesedimentisdirectlypropor-tionalto:hydrostaticpressure,orheightofthewatercolumnabovethesediment,temperatureatthesedimentsurface,thegeothermalgradient,andconcentrationofmethane.Underaverageoceaniccon-ditions,gashydratescouldbestableinsedimentundera3kmwatercolumntodepthsofapproximately600meters,ifsufficientmethaneispresent.","author":[{"dropping-particle":"","family":"Claypool","given":"GeorgeE.","non-dropping-particle":"","parse-names":false,"suffix":""},{"dropping-particle":"","family":"Kaplan","given":"I.R.","non-dropping-particle":"","parse-names":false,"suffix":""}],"container-title":"NaturalGasesinMarineSediments","id":"ITEM-1","issued":{"date-parts":[["2012"]]},"title":"TheOriginandDistributionofMethaneinMarineSediments","type":"chapter"},"uris":["/documents/?uuid=aaefd384-06b3-3ac4-b644-45a6792bde12"]},{"id":"ITEM-2","itemData":{"author":[{"dropping-particle":"","family":"王先彬","given":"","non-dropping-particle":"","parse-names":false,"suffix":""},{"dropping-particle":"","family":"歐陽自遠","given":"","non-dropping-particle":"","parse-names":false,"suffix":""},{"dropping-particle":"","family":"卓勝廣","given":"","non-dropping-particle":"","parse-names":false,"suffix":""},{"dropping-particle":"","family":"張明峰","given":"","non-dropping-particle":"","parse-names":false,"suffix":""},{"dropping-particle":"","family":"鄭國東","given":"","non-dropping-particle":"","parse-names":false,"suffix":""},{"dropping-particle":"","family":"王永莉","given":"","non-dropping-particle":"","parse-names":false,"suffix":""}],"id":"ITEM-2","issued":{"date-parts":[["2014"]]},"title":"蛇紋石化作用、非生物成因有機化合物與深部生命.pdf","type":"article"},"uris":["/documents/?uuid=080e9c3b-91bc-4fbe-b8ab-2af2b8fa1854"]}],"mendeley":{"formattedCitation":"(ClaypoolandKaplan,2012;王先彬<i>etal.</i>,2014)","plainTextFormattedCitation":"(ClaypoolandKaplan,2012;王先彬etal.,2014)","previouslyFormattedCitation":"(ClaypoolandKaplan,2012;王先彬<i>etal.</i>,2014)"},"properties":{"noteIndex":0},"schema":"/citation-style-language/schema/raw/master/csl-citation.json"}(ClaypoolandKaplan,2012;王先彬etal.,2014)。（三）冷泉系統(tǒng)主要的生物地球化學(xué)過程在正常海洋沉積環(huán)境中，海底沉積物中的有機質(zhì)在埋藏過程中被O2、NO3-、Mn4+、Fe3+和SO42-等電子受體氧化，在沉積物-底層水界面以下形成氧還原帶、硝酸鹽還原帶、錳還原帶、鐵還原帶、硫酸鹽還原帶和產(chǎn)甲烷帶（圖2）由于海水中的硫酸根離子濃度遠高于其他電子受體的濃度，因此硫酸鹽還原作用是早期成巖作用中的主要過程，硫酸鹽還原作用消耗的有機碳占沉積物中總有機碳的50%以上（Canfieldetal.，1993;Jorgensenetal.，1982）還沒引。在濱海沉積環(huán)境中，沉積物沉積速率較高，陸源物質(zhì)充足，鐵錳氧化物和氫氧化物含量高，各反應(yīng)帶都較為發(fā)育，硫酸鹽還原帶深度較淺（Daleetal.，2006）沒引；；在上升流發(fā)育區(qū)，有機質(zhì)含量很高，為相對缺氧環(huán)境，各種受體濃度增加，硫酸鹽還原反應(yīng)速率大，硫酸鹽還原帶深度較淺；在陸坡沉積環(huán)境種，初級生產(chǎn)力和沉積速率較低，缺少陸源物質(zhì)，鐵錳氧化物和氫氧化物含量較低，硫酸鹽還原反應(yīng)速率較低，硫

還沒引沒引圖1.2海洋沉積物中各種氧化還原反應(yīng)及各種物質(zhì)的礦化帶分布示意圖（胡鈺，2015）。酸鹽還原帶深度較深（表1）。硫酸鹽還原作用主要通過有機質(zhì)還原硫酸鹽和缺氧條件下甲烷還原硫酸鹽，即甲烷缺氧氧化作用（Anaerobicoxidationofmethane，簡稱AOM）完成，兩種作用的反應(yīng)方程式分別為：2CH2O+SO42-2HCO3-+H2S（1.1）CH4+SO42-HCO3-+HS-+H2O（1.2）硫酸鹽驅(qū)動的甲烷缺氧氧化作用是由甲烷氧化古菌和硫酸鹽還原菌共同完成的。甲烷氧化古菌簡稱為ANME。ANME與產(chǎn)甲烷菌的關(guān)系非常密切，最近的研究結(jié)果表明，AOM作用可能是產(chǎn)甲烷過程中酶的逆轉(zhuǎn)錄（Nauhausetal.，2005）沒引。目前人們對于甲烷氧化古菌和硫酸鹽還原菌在AOM反應(yīng)中直接的相互作用以及其中的中間體仍然知之甚少。海洋沉積物是生產(chǎn)甲烷非常重要的來源，但大氣中只有2%的甲烷來自海洋，絕大多數(shù)來自陸地，如濕地或水稻田發(fā)生的產(chǎn)甲烷作用。這主要是因為來自深海的甲烷在向上運移到海表的過程中，有85%以上的甲烷被硫酸鹽還原甲烷缺氧氧化作用所消耗，因此AOM作用能有效的減少具有溫室效應(yīng)的甲烷排放到大氣中，對調(diào)節(jié)大氣甲烷濃度和遏制全球氣候的快速變暖具有非常重要的意義（Nauhausetal.，2005）沒引。沒引沒引正常海洋沉積環(huán)境，沉積物深部埋藏的有機質(zhì)含量較低時，如開放的大洋盆地環(huán)境中，產(chǎn)甲烷作用較弱，向上擴散的甲烷濃度很低，因此硫酸鹽主要被有機質(zhì)還原，沉積物孔隙水中的硫酸鹽濃度在沉積物—水界面以下隨深度緩慢降低最后趨于穩(wěn)定；而有機質(zhì)含量較高時，如海岸帶和陸坡等環(huán)境，地層中產(chǎn)甲烷菌產(chǎn)生的甲烷在向上擴散時會與海水中向下擴散的硫酸鹽發(fā)生甲烷缺氧氧化作用，也就是說硫酸鹽主要通過甲烷厭氧氧化作用消耗，沉積物孔隙水中的硫酸鹽濃度下降速度與深度呈正比關(guān)系（圖3）（Borowskietal.，1996，1999;Michaelisetal.，2002）沒引。在水合物發(fā)育區(qū)，由于水合物分解產(chǎn)生的甲烷氣體在向上運移時會與海水中向下擴散的硫酸鹽發(fā)生缺氧氧化作用，沉積物孔隙水中的硫酸鹽濃度隨深度下降趨勢在剖面上也呈直線下降；甲烷濃度在產(chǎn)甲烷帶達到飽和保持不變，甲烷向上擴散，濃度隨深度變淺呈直線減少；在硫酸鹽—甲烷轉(zhuǎn)換帶（sulfatemethanetransitionzone，簡稱SMTZ）附近，硫酸鹽和甲烷含量最低，甲烷缺氧氧化作用和硫酸鹽還原作用最強（Borowskietal.，1996;1999）。沒引圖1.3不同沉積環(huán)境下：1正常深海沉積物；2高有機質(zhì)含量沉積物，如海岸帶；3天然氣水合物發(fā)育區(qū)沉積物，甲烷氣體的來源和甲烷厭氧氧化作用，POC代表顆粒有機碳，SAT代表甲烷飽和濃度，修改自Haeckel，2015

圖1.4冷泉沉積物孔隙水中發(fā)生的主要生物地球化學(xué)過程及相關(guān)的早期成巖作用過程示意圖(參考Rodriguezetal.(2000)).在SMTZ的下邊界，硫酸鹽濃度幾乎為0，而甲烷濃度則迅速增加。在水合物發(fā)育區(qū)，甲烷擴散到SMTZ附近，發(fā)生強烈的甲烷缺氧氧化作用，導(dǎo)致SMTZ較淺，SMTZ界面之上的孔隙水中硫酸根大量被消耗。SMTZ在一定程度上能指示深部的甲烷通量，較淺的SMTZ通常指示高通量的甲烷，因此能指示下伏的天然氣水合物藏（圖4）（Borowskietal.，1996;Hensenetal.，2003）沒有引。根據(jù)前任研究，在全球范圍內(nèi)，一般水合物發(fā)育區(qū)的SMTZ深度都小于50米，平均深度在20米左右；而在沒有水合物藏的大洋和大陸邊緣SMTZ深度一般大于50米，部分大于100米（Borowskietal.，1996）沒有引。在我國南海已探明的水合物發(fā)育區(qū)，SMTZ深度一般小于15米，有的甚至小于5米。通過對南海東沙海域兩個水合物有利區(qū)域：九龍甲烷礁和―海洋四號‖高速沉積體的表層沉積物孔隙水的地球化學(xué)研究發(fā)現(xiàn)，東沙海域水合物有利區(qū)域的SMTZ一般小于8米，通過硫酸鹽濃度梯度計算得出的甲烷通量為3.8×10-3~5.9×10-3mmol/（cm2.a），類似于國際上已發(fā)現(xiàn)水合物區(qū)域的甲烷通量水平，暗示該海域深部可能存在天然氣水合物藏（陸紅峰等，2005，2006；鄔黛黛等，2009a，2013；吳時國等，2004）。

沒有引沒有引圖1.6甲烷通量與SMTZ界面深度的關(guān)系（Borowskietal.，1996）海洋沉積根據(jù)來源和組成的不同一般可以分為三種類型：（1）成巖沉積物（lithogenoussediments），主要由陸源碎屑及火山物質(zhì)組成；（2）生物成因沉積物（biogenoussediments）；（3）自生沉積物（authigenicsediments）。冷泉沉積物基本也是由這三類物質(zhì)組成，但與正常海洋沉積物相比生物成因和自生沉積所占的比例很高，因此也是研究的重點。冷泉活動強烈時，冷泉生物及自生沉積物發(fā)育，研究冷泉相關(guān)的科學(xué)問題完全可以依賴冷泉生物（如管狀蠕蟲類、貽貝類及蛤類等）及自生沉積巖（如冷泉碳酸鹽巖及重晶石）進行。但當冷泉活動相對較弱或微弱時，冷泉生物并不發(fā)育，形成的自生沉積物含量低并散布在沉積物中，從而增加了研究的難度。盡管如此，冷泉沉積物中的地球化學(xué)研究則可以在某種程度上克服了上述困難。目前，冷泉沉積物中總的無機碳含量（TIC）、鉻還原性硫的含量（chromiumreduciblesulfur;CRS）及它們各自的同位素值δ13CTIC和δ34SCRS經(jīng)常被用來識別古代的硫酸鹽-甲烷轉(zhuǎn)換帶（SMTZ），古代SMTZ界面附近由于AOM和SR作用導(dǎo)致TIC和CRS含量異常高、δ13CTIC偏負、δ34SCRS大幅偏正，代表了一次古代的甲烷滲漏事件（Limetal.,2011;Peketietal.,2012;Borowskietal.,2013）。此外，古代SMTZ界面有時可以發(fā)育Ba的異常富集（即“Ba峰”）及氧化還原敏感元素Mo的富集現(xiàn)象（Torresetal.,1996;Dickens,2001;Kastenetal.,2012;Peketietal.,2012;Satoetal.,2012）。上述指標均可以有效地被用于研究古代的甲烷滲漏事件及活動特征。在記錄古代甲烷滲漏事件方面，沉積物中所含的底棲有孔蟲殼體的13C值可以用指示古代甲烷滲漏事件，同時結(jié)合各種定年手段（如浮游有孔蟲14C定年），來探討古代甲烷滲漏發(fā)生的時間及觸發(fā)機制等（Weferetal.,1994;Rathburnetal.,2003;Martin,etal.,2007,2010;Fontanier,etal.,2014;Consolaroetal.,2015）。正常情況下，沉積物一般具有較好的沉積序列，在定年的基礎(chǔ)上獲得時間框架，結(jié)合沉積物中記錄的古海洋學(xué)信息，有望能夠更好地用于研究古冷泉形成時的環(huán)境、探索地質(zhì)歷史時期中甲烷滲漏事件的觸發(fā)機制、過程及影響。綜上所述，冷泉系統(tǒng)中的冷泉生物、流體、自生礦物及沉積物均能夠記錄下AOＭ信號，是了解和研究冷泉環(huán)境中的生物地球化學(xué)過程、探索冷泉形成的機制和演過等的重要載體。研究冷泉環(huán)境中正在進行的生物地球化學(xué)過程及相關(guān)的機制，冷泉流體（孔隙水）及部分活的冷泉生物是理想的載體。然而，探索過去的冷泉活動及特征，則只能依靠冷泉自生沉積巖（主要是碳酸鹽巖和重晶石）、沉積物及少部分可以保存的生物殼體。作為冷泉流體強烈滲漏作用的產(chǎn)物，冷泉碳酸鹽巖和重晶石能夠很好地記錄古代冷泉流體的特征，結(jié)合定年數(shù)據(jù)能夠揭示古代冷泉流體的演化和控制因素。但與冷泉沉積物相比，還是缺乏連續(xù)的時間記錄。同時，冷泉沉積物記錄的古海洋學(xué)信息也能提供冷泉的形成和演化的諸多信息。因此，為了更全面、深入地了解冷泉系統(tǒng)的形成、演化及所涉及到的生物地球化學(xué)過程，有必要對冷泉系統(tǒng)中的各種產(chǎn)物（冷泉生物、流體、自生礦物及沉積物）進行綜合的研究和對比分析。胡玉-沒改過胡玉-沒改過（四）冷泉系統(tǒng)生物群落除了硫酸鹽還原細菌和甲烷缺氧氧化古菌等微生物外，在海底冷泉區(qū)常發(fā)育肉眼可見的白色的或橙色的細菌席（Fischeretal.,2012；圖1-3）。同時，冷泉系統(tǒng)還孕育有大量化能自養(yǎng)型的生物群落，常見管狀蠕蟲類、貽貝類及蛤（Macdonaldetal.,1990；圖1-3）。某些種屬的管狀蠕蟲生長長度可達3m、壽命可長達幾百年(Fisheretal.,1997;Bergquistetal.,2000)。最近研究表明某些管狀蠕蟲的根部能改造周圍沉積物的地球化學(xué)特征（Cordesetal.,2003；Dattaguptaet.,2008），并且相應(yīng)的地球化學(xué)特征能記錄在冷泉碳酸鹽巖中（Fengetal.,2010c,2013a）。冷泉環(huán)境中貽貝主要依靠嗜甲烷的或者嗜硫化氫的細菌共生體獲取營養(yǎng)而生存(e.g.Paulletal.,1985;Childressetal.,1986;Cordesetal.,2009;Duperron,2010)。不同的細菌共生體所需要的含碳和含硫的種類和同位素特征又各不相同(Fisher,1990;Conwayetal.,1994;VetterandFry,1998;Yamanakaetal.,2000)，因此，自養(yǎng)型生物中的碳和硫的穩(wěn)定同位素組成能夠用來揭示冷泉生物的生活方式(e.g.Childressetal.,1986;Levin,2005;Macavoyetal.,2008;Beckeretal.,2010,2013,2014;Rodriguesetal.,2013).。最近的研究表明利用貽貝的軟組織中碳、氮和硫穩(wěn)定同位素能夠有效地示蹤冷泉環(huán)境中的生物地球化學(xué)過程（Fengetal.,2015）。因此，冷泉環(huán)境中對冷泉生物進行地球化學(xué)研究不僅有助于探索冷泉生物的生活習性和方式，而且有希望用于示蹤冷泉環(huán)境中的生物地球化學(xué)過程插圖。沒改過插圖沒改過地質(zhì)背景（一）地理位置南海位于太平洋板塊、歐亞板塊和印度洋板塊交匯處，是西太平洋上最大的邊緣海之一（圖2-1）。南海的水團主要通過巴士海峽和馬六甲海峽分別與太平洋和印度洋相連。南海是我國最大的邊緣海，面積約350×104km2，平均水深1212米，馬尼拉海溝南端為最深處5567米。研究區(qū)主要位于南海北部陸坡水合物發(fā)育區(qū)。由于各種海山、陡崖和海槽的廣泛發(fā)育，南海北部大陸坡地形非常復(fù)雜。大陸坡水深150~3500米之間，很多新生代的含油氣盆地發(fā)育，如鶯歌海盆地、瓊東南盆地、臺西南盆地和北部灣盆地等，被認為是洋盆二次擴張作用的結(jié)果（張光學(xué)等，2002；王宏斌等，2003）。

圖2.1南海地理位置圖（李牛，2015）（二）構(gòu)造背景南海兼具主動大陸邊緣和被動大陸邊緣的特征，可能由三次大規(guī)模的構(gòu)造運動塑造了如今的構(gòu)造格局。南海第一次大規(guī)模的海底擴張運動主要發(fā)生在中生代末期-新生代早期。太平洋版塊對華南微板塊的俯沖結(jié)束，華南微板塊開始向北部運動，區(qū)域應(yīng)力場從原先的擠壓變?yōu)樗沙冢⑶以谌A南微板塊前緣形成了NE向的地塹式斷陷盆地，由此拉開了南海陸源擴張的序幕。華南微板塊在古新世-始新世向南漂移并伴隨順時針的轉(zhuǎn)動。中-晚始新世，華南微板塊向南運動，在其東南邊緣出現(xiàn)北東-南西向的擠壓應(yīng)力場，使早期的地塹式斷陷盆地開始沿東南方向擴張。同時，大陸地殼持續(xù)拉張減薄，盆地不斷切割加深，基性巖漿冒出，形成南海海洋地殼和慢速擴張洋中脊，形成了原始南海。南海的第二次擴張活動的開始在大約32~17百萬年，即中漸新世-早中新世，此時沿NWW方向擠壓的太平洋板塊和由南向北運動的華南微板塊使南海形成東兩向的擴張軸，西沙海槽在此時期形成。南海的第三次擴張活動在中中新世-上新世，構(gòu)造運動主要以垂向為主，并伴隨著大范圍的沉降活動。在太平洋板塊的擠壓作用之下，南海洋殼消減于呂宋島弧的仰沖之下?，F(xiàn)代南海的構(gòu)造格局在上新世之后基本形成（劉昭蜀等，2002）。（三）沉積環(huán)境特征南海北部陸坡的沉積環(huán)境隨著ODP184航次的實施得到了非常廣泛的研究。南海北部陸坡的物源主要是來自珠江、臺灣海峽、巴士海峽和呂宋的大陸陸源碎屑。南海的三次擴張運動，從大陸輸入了大量的陸源碎屑，沉積速率較高。而且青藏高原的快速隆升也為南海北部陸坡帶來了豐富的陸源物質(zhì)。鶯歌海盆地在上新世的沉積速率，最高可達40cm/ka，而瓊東南盆地最高為60cm/ka；ODP1144站位，位于東沙群島陸坡區(qū)的沉積速率為48cm/ka（Bühringetal.，2004），晚中新世的1143站位，位于南部陸坡區(qū)，沉積速率為11.4cm/ka（吳必豪等，2003）。一般認為水合物發(fā)育需要的沉積速率為3~30cm/ka（Kvenvold，1985）。而且ODP深海鉆探的結(jié)果也表明，0.5~100cm/ka的沉積速率利于水合物的發(fā)育。由此可見，南海的沉積速率具有發(fā)育水合物的條件。而且南海發(fā)育巨厚的新生代沉積盆地，厚度達到2~11km，有機質(zhì)豐富，包含大量的甲烷等烴類資源。南海北部陸坡沉積物中的有機碳含量為0.46%~1.95%，超過了水合物發(fā)育所需要的沉積物中有機碳含量0.5%的下限（Kvenvold，1985），而且深海鉆探表明南海北部陸坡多個站位深部富含生物成因甲烷和熱解成因甲烷。因此南海北部陸坡這些高沉積速率、高有機質(zhì)含量的地層以及各種等深流、重力流沉積和斷裂構(gòu)造的發(fā)育為天然氣水合物的發(fā)育和冷泉流體的運移提供了良好的條件。南海北部陸坡是我國開展水合物調(diào)查研究的重點海域，已通過地球物理、地球化學(xué)等方法圈定數(shù)個水合物遠景區(qū)域，并于2007年和2013年在神狐海域和珠江口盆地鉆獲水合物實物樣品（Zhangetal.,2007；Zhangetal.,2014）。此外，通過近20年的調(diào)查研究結(jié)果還發(fā)現(xiàn)了大量與海底冷泉滲漏有關(guān)的冷泉碳酸鹽巖和冷泉生物，如蛤和貽貝。在瓊東南盆地、西沙海槽、神狐海域、東沙海域及臺西南盆地等40多個站位采集到冷泉碳酸鹽巖樣品和/或冷泉生物樣品（圖2-2）。通過德國“太陽號”在東沙東北海域海底發(fā)現(xiàn)目前已知最大面積的冷泉碳酸鹽巖礁——九龍甲烷礁，引起國內(nèi)及國際同行的廣泛關(guān)注。因此，南海北部陸坡是我國水合物勘探重點區(qū)域和研究冷泉生物地球化學(xué)過程和冷泉生物圖2-2南海北部陸坡目前已知的冷泉碳酸鹽巖發(fā)現(xiàn)點（馮東，內(nèi)部資料）的天然實驗室。圖2.2南海北部陸坡目前已知的冷泉碳酸鹽巖發(fā)現(xiàn)點（馮東，內(nèi)部資料）

采樣和測試方法(一)研究樣品（二）樣品的前處理首先，將采集得到的樣品切割為50-100cm的長度，中間切開，進行拍照和樣品描述，并用密封袋包好以后放入4℃的冷柜中保存。然后在中科院地化所邊緣海地質(zhì)重點實驗室對樣品進行切割和拍照描述。每隔2cm一個樣品，樣品經(jīng)冷凍干燥后用瑪瑙碾缽手工碎樣，進一步的粒度、化學(xué)和同位素分析在中科院地化所、中科院廣州地化所和中科院南海海洋研究所等地進行。（三）分析方法1.TOC、TIC、N和總S分析沉積物中的TOC、TIC、N和總S采用ThermoScientificFlashSmart元素分析儀進行測試。選取120~150mg樣品，加入過量2mol/L的HCl除去碳酸鈣，再用超純水進行多次稀釋，直到樣品為中性，然后冷凍干燥24小時以上。采用未經(jīng)HCl處理的樣品測試總硫、總氮和總碳的含量，經(jīng)HCl處理后的樣品測試TOC的含量，TIC為總碳和TOC者之間的差值，儀器的精度和準確度均優(yōu)于1%。2.孔隙水陰、陽離子濃度分析孔隙水中陰離子（SO42?）和陽離子（Mg2+和Ca2+）濃度用戴安公司DionexICS-900型離子色譜儀在中科院廣州地球化學(xué)研究所完成。SO42?濃度用超純水稀釋500倍后上機測試，Mg2+和Ca2+濃度則用超純水稀釋200倍后上機測試。AS23型色譜分離柱（用于陰離子）和CS12A型色譜分離柱（用于陽離子）用來進行各種離子的分離。測試陰離子濃度用4.5mMNa2CO3/0.8mMNaHCO3混合液作為為淋洗液，25mMH2SO4為再生液，進樣體積為50μL，流速設(shè)定為1ml/min。測試陽離子濃度時用11mMH2SO4作為淋洗液，自動電解水為再生液，進樣體積為50μL，流速設(shè)定為1ml/min。通過用標準水體進行檢測和質(zhì)量控制，陰陽離子的標準偏差均小于2%。

C、N、S元素含量和陰陽離子濃度特征（一）C、N、S結(jié)果分析1.F站位沉積物SiteFPC1總硫含量總體在0.15%~0.45%之間，略高于正常氧化環(huán)境海相沉積物的值（Berneretal.,1980）。含量在0.44%~0.66%之間，總體在0.5%左右，有隨深度增加TOC含量減小的趨勢。TIC總體含量偏高，在0.79%~1.67%之間，大致與TOC含量變化相反。C/N原子比值在3~5之間，顯示沉積物中陸源和海源有機質(zhì)輸入通量的變化(Meyers,1994)放到討論說。放到討論說SiteFPC2總硫含量在0.4%~0.7%之間，高于正常氧化環(huán)境海相沉積物的值（Berneretal.,1980）。TOC含量在0.3%~0.7%之間，并隨深度逐漸減小。TIC含量在0.5%~0.7%之間，基本保持在0.6%左右變化很小。C/N原子比值在1~4之間，與TOC含量變化趨勢一致。SiteFPC3總硫含量在0.60%~0.80%之間，遠高于正常氧化環(huán)境下的海洋沉積物中總硫含量。TOC含量在0.44%~0.68%之間，并隨深度緩慢增加。TIC含量在0.33%~0.77%之間，并隨深度緩慢增加。C/N含量在3~5之間。SiteFPC4總硫含量在0.11%~0.61%之間，上段含量較低，為正常水平，下段含量突然增加至0.3%以上。TOC含量在0.48%~0.66%之間。TIC含量在0.53%~0.90%之間，與TOC含量變化呈反相關(guān)。C/N含量在3~5之間。

圖4.1F站位沉積物TS、TOC、TIC含量和C/N比值剖面上隨深度變化圖樣品點深度(cm)TN(%)TS(%)TOC(%)TIC(%)C/NSiteFPC11.000.160.150.641.213.953.000.150.170.551.283.665.000.140.240.660.794.567.000.140.290.501.083.4511.000.160.450.601.443.7413.000.140.380.591.674.2815.000.140.350.571.334.1117.000.140.320.441.463.07SiteFPC21.000.150.480.450.533.053.000.170.540.630.633.715.000.130.580.520.543.857.000.150.600.440.652.999.000.140.530.490.643.4711.000.160.690.310.611.89SiteFPC31.000.140.650.580.374.103.000.150.750.630.334.155.000.120.780.500.524.027.000.110.780.460.544.039.000.120.740.440.573.5811.000.120.700.570.414.9113.000.130.660.560.444.1815.000.140.650.610.344.3017.000.150.670.550.413.7619.000.140.720.680.354.7521.000.140.680.680.394.7523.000.150.680.560.773.8325.000.140.700.630.524.3527.000.140.680.610.624.3229.000.150.600.670.604.63SiteFPC41.000.140.160.510.903.693.000.150.110.660.564.325.000.140.110.480.703.387.000.140.110.490.773.509.000.140.150.580.534.2511.000.140.370.500.703.5713.000.130.590.630.504.8615.000.140.610.430.733.1317.000.130.500.500.663.9919.000.120.470.580.564.83表4.1F站位沉積物TN、TS、TOC、TIC含量和C/N比值2海馬冷泉區(qū)Haiyang4PC1沉積物總硫含量在0.32%~0.72%之間，含量隨深度增加明顯。TOC含量在0.6%~0.8%之間，含量隨深度緩慢減小。TIC含量在0.31%~0.41%之間，變化不明顯。C/N含量在4~6之間。Haiyang4PC2沉積物總硫含量在0.08%~0.31%之間，含量變化不明顯。TOC含量在0.09%~0.14%之間，含量隨