石學(xué)法，胡利民， 喬淑卿，白亞之

(1.國家海洋局 第一海洋研究所，山東 青島 266061；2. 海洋沉積與環(huán)境地質(zhì)國家海洋局重點(diǎn)實(shí)驗(yàn)室，山東 青島 266061；3. 青島海洋科學(xué)與技術(shù)國家實(shí)驗(yàn)室 海洋地質(zhì)過程與環(huán)境功能實(shí)驗(yàn)室，山東 青島 266061)

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中國東部陸架海沉積有機(jī)碳研究進(jìn)展：來源、輸運(yùn)與埋藏*

石學(xué)法1,2,3*，胡利民1,2,3， 喬淑卿1,2,3，白亞之1,2

(1.國家海洋局 第一海洋研究所，山東 青島 266061；2. 海洋沉積與環(huán)境地質(zhì)國家海洋局重點(diǎn)實(shí)驗(yàn)室，山東 青島 266061；3. 青島海洋科學(xué)與技術(shù)國家實(shí)驗(yàn)室 海洋地質(zhì)過程與環(huán)境功能實(shí)驗(yàn)室，山東 青島 266061)

大河影響下的河口陸架區(qū)陸海相互作用活躍，是有機(jī)碳的主要沉積區(qū)，在全球碳的生物地球化學(xué)循環(huán)中發(fā)揮著重要作用。中國東部陸架海包括渤海、黃海和東海陸架，黃河和長江兩條大河直接流入該區(qū)。從有機(jī)碳來源、輸運(yùn)和埋藏保存等方面總結(jié)介紹了近年來中國東部陸架海沉積有機(jī)碳源匯過程的研究進(jìn)展。研究表明，土壤有機(jī)碳和海洋初級(jí)生產(chǎn)力是本區(qū)沉積有機(jī)碳的主要來源；有機(jī)分子標(biāo)志物和多元統(tǒng)計(jì)分析模型的綜合應(yīng)用能更準(zhǔn)確地評(píng)估近海沉積有機(jī)碳的源匯特征；陸架泥質(zhì)區(qū)是長江和黃河陸源沉積有機(jī)碳的重要儲(chǔ)庫；河流輸入、沉積再懸浮和遠(yuǎn)距離物質(zhì)輸運(yùn)等沉積動(dòng)力過程顯著影響著本區(qū)不同來源、不同類型陸源沉積有機(jī)碳的輸運(yùn)和歸宿；大河物質(zhì)輸入、海洋初級(jí)生產(chǎn)力以及陸架沉積作用共同支撐著該區(qū)較高的有機(jī)碳埋藏能力。本區(qū)的研究今后需要從海陸結(jié)合的地球系統(tǒng)科學(xué)的角度，從整體上研究沉積作用過程與有機(jī)碳遷移轉(zhuǎn)化的影響機(jī)制和響應(yīng)關(guān)系；結(jié)合我國東部陸架海特有的沉積物源匯體系特征，揭示從流域到近海物質(zhì)輸運(yùn)體系中不同來源有機(jī)碳的年齡分布和歸宿，并在氣候變化和人類活動(dòng)影響日益顯著的背景下，探究不同時(shí)空尺度沉積有機(jī)碳的輸運(yùn)和埋藏保存及其對大氣CO2和全球變化的影響和意義。

中國東部陸架海；沉積有機(jī)碳；來源；輸運(yùn)；埋藏

大河控制影響下的河口陸架地區(qū)是沉積有機(jī)碳的主要堆積區(qū)，超過80%的有機(jī)碳沉積發(fā)生在這一區(qū)域[1-2]。這一地區(qū)是沉積有機(jī)碳的重要儲(chǔ)庫，沉積通量高、動(dòng)力環(huán)境多樣、生物地球化學(xué)過程復(fù)雜、人類活動(dòng)影響顯著[3-6]，同時(shí)對碳的生物地球化學(xué)循環(huán)過程、海洋生態(tài)系統(tǒng)以及全球氣候變化也有重要影響[4,7-8]。作為陸源入海物質(zhì)的主要堆積區(qū)，河口陸架有機(jī)碳的堆積速率往往比大洋高出一個(gè)數(shù)量級(jí)[9]，其較高的海洋固碳能力和碳匯效應(yīng)潛力已引起人們的關(guān)注[10-14]。另一方面，在大河影響下的河口陸架，特別是三角洲地區(qū)，有機(jī)碳雖然具有較高的沉積埋藏速率，但部分有機(jī)碳并沒有真正保存下來，而是受該區(qū)較為活躍的沉積水動(dòng)力條件(如再懸浮)及生物擾動(dòng)等因素的影響[15-20]；同時(shí)由于再次暴露在相對氧化的水體環(huán)境中，加之微生物作用，沉積有機(jī)碳容易發(fā)生礦化分解[21-24](圖1)。上述過程能顯著影響區(qū)域有機(jī)碳的循環(huán)及其源匯效應(yīng)[7, 25-26]，這一過程的研究對于深入認(rèn)識(shí)不同類型沉積有機(jī)碳的物源、歸宿及其環(huán)境指示等方面具有重要的科學(xué)意義[4, 27]。

圖1　近海沉積有機(jī)碳的循環(huán)模式圖[28]Fig.1　The sketch of sedimentary organic carbon cycling in the coastal margin [28]

圖2 中國東部陸架海沉積物輸運(yùn)與泥質(zhì)沉積區(qū)分布及海流分布示意圖[29-30]Fig.2　Schematic diagram showing patterns of oceanic currents, sediment dispersal and distribution of mud deposits over the East China Sea[29-30]

中國東部邊緣海陸架寬廣，兼有長江和黃河兩條世界性大河攜帶巨量沉積物入海，在西邊界流和東亞季風(fēng)驅(qū)動(dòng)的復(fù)雜環(huán)流體系的影響下，形成了該區(qū)沉積物“夏儲(chǔ)冬輸”的宏觀輸運(yùn)格局[29, 31-35](圖2)。除了黃河、長江等河流物質(zhì)的巨量供應(yīng)及其季節(jié)性變化外，復(fù)雜的水動(dòng)力條件和陸架沉積作用也對該區(qū)沉積物的源匯特征有重要影響[36-41]。對該區(qū)陸架沉積物輸運(yùn)和沉積特征的大量研究表明，黃河作為全球懸沙濃度最高和多年平均入海沉積物通量居世界第二的大河，是影響中國東部近海陸架現(xiàn)代沉積格局的主要物質(zhì)來源，黃河沉積物經(jīng)現(xiàn)代河口入海后，受沿岸流作用經(jīng)渤海海峽南部進(jìn)入北黃海，然后沿山東半島北部陸架繼續(xù)向東輸運(yùn)和沉積，并繞過成山頭最終向南黃海輸運(yùn)和沉積[35, 42-48]；而黃、東海海域發(fā)育的中尺度氣旋型渦旋(冷渦)等陸架環(huán)流和沉積作用則控制性影響著黃海和東海陸架泥質(zhì)沉積區(qū)的形成[40, 49-50]。長江沉積物入海后主要向東南方向輸運(yùn)，受臺(tái)灣暖流阻隔作用，入海沉積物通量的70%～90%堆積于長江口及東海內(nèi)陸架(水深<50 m)，其中大部分在夏季優(yōu)先堆積在123°E以西的長江口區(qū)[33, 51-52]，冬季在沉積再懸浮作用下，懸浮泥沙在閩浙沿岸流驅(qū)動(dòng)下主要沿海岸向南輸運(yùn)，最終可達(dá)閩江口[36, 52-54]。綜上所述，該區(qū)陸海相互作用強(qiáng)烈、沉積物輸運(yùn)機(jī)制復(fù)雜，不僅是河流入海細(xì)顆粒沉積物匯聚埋藏的主要區(qū)域，也是沉積有機(jī)碳發(fā)生堆積、遷移和轉(zhuǎn)化的重要區(qū)域，記錄了豐富的環(huán)境變異和人類活動(dòng)等信息，因此是開展大河河口-近海陸海相互作用影響下沉積有機(jī)碳源匯過程研究的理想?yún)^(qū)域。本文旨在結(jié)合該領(lǐng)域有關(guān)國際研究背景和前沿科學(xué)問題，總結(jié)近年來我國東部陸架海沉積有機(jī)碳來源、輸運(yùn)和埋藏等研究的主要進(jìn)展，以期深入了解該區(qū)沉積有機(jī)碳的生物地球化學(xué)過程及其對碳匯效應(yīng)、物質(zhì)來源和環(huán)境演變的指示作用。

1 河口-陸架沉積有機(jī)碳的物源表征及影響因素

近海沉積有機(jī)碳的來源與陸源輸入和海洋自身生產(chǎn)密切相關(guān)。區(qū)分有機(jī)碳的陸源和海源貢獻(xiàn)，除利用傳統(tǒng)的總有機(jī)碳C/N比值及穩(wěn)定碳同位素(δ13CTOC)等經(jīng)典指標(biāo)外[55-57]，近年來發(fā)展起來的有機(jī)分子標(biāo)志物(如長鏈烷烴、甾醇類、木質(zhì)素等)，以其來源明確和性質(zhì)穩(wěn)定的特點(diǎn)具有更加敏感和直觀的優(yōu)勢，得到較廣泛的應(yīng)用[57-60]。研究表明，來自于陸源高等植物葉蠟的奇碳數(shù)長鏈烷烴是反映沉積物中陸源有機(jī)質(zhì)輸入的良好替代指標(biāo)[61]。低碳數(shù)(

蔡德陵等[65-66]基于總有機(jī)碳C/N比值和δ13CTOC等指標(biāo)的研究發(fā)現(xiàn)，山東半島水下三角洲沉積物主要來源于現(xiàn)代黃河物質(zhì)，南黃海中部陸源沉積物來自廢黃河和現(xiàn)代黃河物質(zhì)，來自長江的物質(zhì)比例較少，并基于有機(jī)質(zhì)δ13CTOC估算了沉積物中海源碳的貢獻(xiàn)，以此恢復(fù)重建了該區(qū)200 a來的初級(jí)生產(chǎn)力歷史。研究顯示，長江河口區(qū)和東海內(nèi)陸架δ13CTOC的信號(hào)明顯受長江陸源輸入的影響[67]；Kao等[68]對比研究了東海內(nèi)陸架和沖繩海槽區(qū)沉積有機(jī)質(zhì)的δ13CTOC的組成特征，發(fā)現(xiàn)兩者的相似性，進(jìn)而提出了內(nèi)陸架沉積物跨陸架輸運(yùn)的可能性?；讦?3CTOC等指標(biāo)，將長江口地區(qū)沉積有機(jī)碳來源分為河流、三角洲和海洋三個(gè)端元，發(fā)現(xiàn)沉積水動(dòng)力過程影響沉積有積碳來源和組成[69]。Hu等[70-72]分別對渤、黃海陸架和長江口-東海內(nèi)陸架泥質(zhì)區(qū)沉積有機(jī)碳的分布與來源進(jìn)行了系統(tǒng)研究，證實(shí)黃河口、長江口及內(nèi)陸架細(xì)顆粒沉積區(qū)是陸源有機(jī)碳的主要堆積區(qū)；研究也表明黃河口及鄰近海域δ13CTOC的分布與現(xiàn)代黃河入海沉積物的擴(kuò)散輸運(yùn)路徑基本一致，說明其可以作為指示黃河物質(zhì)在渤、黃海地區(qū)擴(kuò)散的特征指標(biāo)?；诓煌嗽摩?3CTOC組成研究，發(fā)現(xiàn)黃河口陸源有機(jī)碳貢獻(xiàn)可達(dá)50%，但向外海迅速降低，估計(jì)只有約10%～20%的黃河物質(zhì)到達(dá)渤海中部泥質(zhì)區(qū)和渤海海峽[73]。長江口沉積物δ13CTOC的變化范圍較小，海洋源約占60%，基于δ13CTOC的分布模式得出顆粒有機(jī)碳(POC)主要是在東-東南方向上輸送[74]。最近，基于蒙特卡洛模擬的三端元混合模型(陸地維管、土壤和海源有機(jī)碳)，Li 等[75]對長江口-東海內(nèi)陸架沉積有機(jī)碳來源的研究表明，海源有機(jī)碳(平均64%)是沉積有機(jī)碳的主要貢獻(xiàn)源，而在陸源有機(jī)碳中，土壤有機(jī)碳(平均27%)的相對貢獻(xiàn)是陸地高等C3維管植物的2倍。

基于傳統(tǒng)的總有機(jī)碳C/N和δ13CTOC指標(biāo)雖能大體判斷沉積有機(jī)碳的物源(圖3)，但由于海洋浮游生物生產(chǎn)的有機(jī)碳δ13CTOC變化范圍較大[76]，且陸地植被存在較大偏差，同時(shí)早期成巖降解作用能改變有機(jī)質(zhì)初始的C/N和δ13CTOC[77-78]，導(dǎo)致這類指標(biāo)對有機(jī)碳(尤其是陸源有機(jī)碳)物源識(shí)別的低敏感性[71, 79-80]。此外，近海沉積物無機(jī)氮吸附和人類活動(dòng)(如石油污染、施肥)等因素也使得這類指標(biāo)在物源識(shí)別時(shí)出現(xiàn)偏差[21, 81-84]。相比之下，生物標(biāo)志物法可更加準(zhǔn)確地區(qū)分陸源和海洋源有機(jī)碳。這在我國河口-陸架地區(qū)已有較多應(yīng)用，例如，基于對渤、黃海表層沉積物脂類標(biāo)志物(正構(gòu)烷烴、脂肪醇和甾醇等)的分析，發(fā)現(xiàn)沉積物中不同脂類標(biāo)志物隨著離岸距離的遠(yuǎn)近具有不同的降解程度，河口地區(qū)陸源貢獻(xiàn)更大，河流輸入的陸源信號(hào)主要集中在近岸區(qū)[85]。南黃海沉積物中正構(gòu)烷烴的組成多呈現(xiàn)雙峰分布，前峰群短鏈烷烴主要來自浮游藻類和細(xì)菌，而長鏈烷烴則主要跟陸地高等植物葉蠟來源有關(guān)[86]。長江口-東海內(nèi)陸架是長江入海物質(zhì)的主要堆積區(qū)，該區(qū)有機(jī)碳的生物地球化學(xué)過程得到了廣泛關(guān)注，已應(yīng)用了多種生物標(biāo)志物對沉積有機(jī)碳的來源進(jìn)行表征，主要包括正構(gòu)烷烴[80, 87-88]、甾醇和烯酮[80, 89]、木質(zhì)素[87, 90-92]及脂肪酸等[93-94]。近幾年，也有學(xué)者利用與微生物合成有關(guān)的四醚膜酯(GDGTs)[95]和放射性14C[91-92, 96]等新指標(biāo)和方法對本區(qū)沉積有機(jī)碳的來源及其生物地球化學(xué)過程進(jìn)行表征，表明陸架細(xì)顆粒沉積區(qū)是有機(jī)碳特別是陸源有機(jī)碳的重要匯，有機(jī)碳的來源、分布及歸宿與區(qū)域沉積動(dòng)力環(huán)境關(guān)系密切。Yao等[97]綜合利用總有機(jī)碳和多種生物標(biāo)志物(正構(gòu)烷烴、木質(zhì)素、四醚膜酯)及沉積物粒度和比表面積等多元地球化學(xué)分析手段，并結(jié)合多元統(tǒng)計(jì)分析模型，估算得出長江口及鄰近陸架沉積有機(jī)碳的來源主要為海洋自生源(35%)、土壤源(47%)和陸源植物貢獻(xiàn)(18%)。綜上所述，對不同來源沉積有積碳的物源識(shí)別這一重要科學(xué)問題，東部陸架?？傆袡C(jī)質(zhì)的C/N和δ13CTOC指標(biāo)的應(yīng)用受多種因素影響(如無機(jī)氮吸附、水動(dòng)力分選、微生物和早期成巖作用等)而表現(xiàn)出多解性[70,80,98]，而具有一定的來源特異性的生物標(biāo)志物，通過其不同來源的分子組成比值及單位分子的同位素組成特征，往往能更加準(zhǔn)確地量化沉積有機(jī)碳的海源與陸源相對貢獻(xiàn)[80,98]。

圖3　基于C/N和有機(jī)碳δ13CTOC組成的沉積有機(jī)碳源解析[111]Fig.3　The bio-plot of the C/N and δ13CTOC of the surface sediments in the East China Sea and its comparison with other river-dominated margins around the world[111]

2 沉積有機(jī)碳輸運(yùn)與沉積水動(dòng)力條件的控制

事實(shí)上，顆粒物在河口-陸架環(huán)境中的運(yùn)移過程具有選擇性[99]，受水動(dòng)力條件影響，其分布特征主要取決于顆粒物的粒徑、密度、礦物組成和比表面積等要素，即大部分細(xì)顆粒、低密度的顆粒物質(zhì)可能會(huì)被波浪和沿岸流等輸運(yùn)到距離河口更遠(yuǎn)的海域，而粒徑較粗、密度較高的顆粒物則會(huì)優(yōu)先在河口附近沉積[19]。在這一過程中，受波浪和海流的動(dòng)力分選作用，具有不同來源和活性的不同粒度、礦物及地球化學(xué)組成的物質(zhì)會(huì)發(fā)生分選或分異[17,19,100-101]，從而導(dǎo)致吸附于這些顆?；|(zhì)上不同類型、不同性質(zhì)的有機(jī)碳組分也發(fā)生分異，即有機(jī)碳會(huì)在不同的粒級(jí)和礦物組成的顆粒物甚至不同相之間進(jìn)行重新分配[18, 20, 102]，進(jìn)而顯著影響沉積有機(jī)碳在河口-陸架海的地球化學(xué)行為和歸宿[15-16,19,101,103]。

國外學(xué)者在這方面開展了系統(tǒng)研究，例如，基于對墨西哥灣不同粒級(jí)沉積物中的木質(zhì)素的分析，發(fā)現(xiàn)砂質(zhì)沉積物中以木本植物有機(jī)質(zhì)為主，而泥質(zhì)沉積物中則富含草本植物有機(jī)質(zhì)，砂質(zhì)沉積物對陸源沉積有機(jī)質(zhì)入海也具有重要作用[18]。Tesi等[104-105]利用典型生物標(biāo)志物(木質(zhì)素、正構(gòu)烷烴)和穩(wěn)定碳同位素組成對地中海沿岸和東西伯利亞海陸架地區(qū)有機(jī)碳的來源、傳輸和動(dòng)力分異進(jìn)行了研究，發(fā)現(xiàn)在陸源有機(jī)碳向海輸運(yùn)過程中，木質(zhì)有機(jī)組分更易停留在河口附近，而賦存于細(xì)顆粒中降解程度較高的土壤有機(jī)碳更容易被優(yōu)先搬運(yùn)到離岸區(qū)域。運(yùn)用粒度、有機(jī)碳和生物標(biāo)志物(脂類、木質(zhì)素和四醚膜酯)等多參數(shù)指標(biāo)，發(fā)現(xiàn)伊比利亞海沉積物在水動(dòng)力分選的作用下，來自土壤和維管植物的陸源有機(jī)碳主要賦存于粉砂粒級(jí)顆粒中，這類降解程度較高的有機(jī)碳能被輸運(yùn)到離岸較遠(yuǎn)的地方，并在水動(dòng)力能量較低的離岸陸架泥質(zhì)區(qū)沉積埋藏，而海洋自生的藻類等新鮮有機(jī)碳則主要沉積在內(nèi)陸架區(qū)域[20]。

在我國，沉積動(dòng)力對顆粒有機(jī)碳擴(kuò)散輸運(yùn)的影響也受到較多關(guān)注，研究主要集中在長江口-東海內(nèi)陸架。Bouloubassi等[106]對長江口烴類化合物空間分布的研究發(fā)現(xiàn)，烴類化合物的運(yùn)移與其所吸附的顆粒的大小與密度有直接的關(guān)系，且不同烴類化合物的吸附能力有差異，這對它們的沉積分異特征有重要影響?；谀举|(zhì)素等參數(shù)的分析，發(fā)現(xiàn)長江口及鄰近陸架區(qū)陸源有機(jī)質(zhì)降解程度較高，且長江口31°30′N以南木質(zhì)素降解程度比北部區(qū)域更高，草本植物貢獻(xiàn)更大[90]。對長江口海域沉積物進(jìn)行粒徑分級(jí)，并結(jié)合同位素示蹤研究，發(fā)現(xiàn)沉積物中的粗顆粒物(>250 μm)主要來源于人類活動(dòng)產(chǎn)生的有機(jī)物，細(xì)顆粒物(<25 μm)主要來自于河口懸浮物，黏土組分中有機(jī)碳的含量最高[107]?；陂L江口及鄰近海域表層沉積物中正構(gòu)烷烴、木質(zhì)素和GDGTs的分子標(biāo)志物組成特征，探討了不同沉積動(dòng)力環(huán)境中不同性質(zhì)陸源有機(jī)碳的運(yùn)移及其環(huán)境指示意義，發(fā)現(xiàn)除了細(xì)顆粒沉積物表面吸附作用外，河口區(qū)由于再懸浮過程導(dǎo)致的有機(jī)質(zhì)解吸附以及陸架殘留有機(jī)質(zhì)對有機(jī)碳的擴(kuò)散和循環(huán)也有重要作用，并以此劃分了6類有機(jī)碳的賦存沉積環(huán)境[88]。Hu等[71]基于長江口-東海內(nèi)陸架表層沉積物正構(gòu)烷烴組成特征，發(fā)現(xiàn)吸附于細(xì)顆粒物質(zhì)的貧木質(zhì)陸源有機(jī)碳組分優(yōu)先在河口-內(nèi)陸架地區(qū)向南輸運(yùn)，并從物質(zhì)匯和沉積水動(dòng)力過程耦合的角度揭示了長江入海的陸源沉積有機(jī)質(zhì)的環(huán)境歸宿。Wang等[108]通過對長江口及鄰近陸架區(qū)的沉積物不同密度和不同粒級(jí)的有機(jī)碳組成研究，則發(fā)現(xiàn)有機(jī)碳主要富集在密度為2.0～2.5 g/cm3的組分中，陸源有機(jī)碳主要富集在密度小的顆粒物上，海源有機(jī)碳優(yōu)先富集在密度大的顆粒物上，表明不同密度級(jí)上不同來源有機(jī)碳輸運(yùn)與埋藏機(jī)制的不同；富含木質(zhì)素的新鮮植物碎屑主要與粗顆粒相聯(lián)系，并優(yōu)先堆積在河口附近，而貧木質(zhì)素、降解程度較高的土壤有機(jī)碳則主要賦存在細(xì)顆粒物上，可輸運(yùn)到離河口較遠(yuǎn)的位置，水動(dòng)力分選過程在決定陸源有機(jī)碳(包括土壤、維管植物)和海洋源有機(jī)碳在河口外的輸運(yùn)和散布過程中發(fā)揮重要作用[108]。除東海外，基于渤、黃海沉積物總有機(jī)碳和生物標(biāo)志物的多參數(shù)統(tǒng)計(jì)分析，并結(jié)合沉積物粒度組成和粒徑趨勢分析的結(jié)果，Hu等[70, 72]研究了水動(dòng)力條件對沉積有機(jī)碳區(qū)域分布的控制作用，印證了陸源有機(jī)碳同位素的空間分布特征與黃河入海沉積物的擴(kuò)散路徑具有較好的一致性，黃、渤海陸架水動(dòng)力能量較低的泥質(zhì)區(qū)是黃河入海陸源沉積有機(jī)碳的重要遠(yuǎn)端匯；發(fā)現(xiàn)來源于植物葉蠟的正構(gòu)烷烴作為土壤有機(jī)質(zhì)的重要組成部分，受沉積水動(dòng)力分選的作用易被優(yōu)先傳輸并沉積在水動(dòng)力能量較低的離岸泥質(zhì)區(qū)，進(jìn)而指出大河輸入和陸架沉積水動(dòng)力環(huán)境對本區(qū)陸源沉積有機(jī)碳選擇性輸運(yùn)的控制作用，并提出現(xiàn)代黃河入海陸源沉積有機(jī)碳向渤、黃海遠(yuǎn)距離、選擇性輸運(yùn)保存的概念模式(圖4)。

圖4　黃、渤海陸源沉積有機(jī)質(zhì)的概念輸運(yùn)模式[72]Fig.4　Schematic plot showing the simplified circulation system and the transport pathways of terrigenous sedimentary organic carbon on the shelf coupled with spatial variability of terrigenous n-alkanes proportion[72]

3 陸架沉積有機(jī)碳的埋藏與保存

研究表明，全球大約90%的有機(jī)碳埋藏發(fā)生在大河影響下的邊緣海及相應(yīng)陸架區(qū)，這里是全球重要的碳儲(chǔ)庫[1-2]，其變化對全球物質(zhì)循環(huán)和氣候變化有重要影響[8]。研究河口陸架地區(qū)有機(jī)碳的埋藏保存對于深入了解沉積有機(jī)質(zhì)的生物地球化學(xué)循環(huán)及其對氣候變化和人類活動(dòng)的響應(yīng)等方面具有重要意義[1,4,109-110]。前人對河口陸架沉積有機(jī)碳埋藏能力大小的評(píng)估往往取決于區(qū)域沉積有機(jī)碳的含量和質(zhì)量年堆積速率(g·cm-3)，后者主要是由放射性核素(如210Pb，137Cs，14C等)得到的沉積速率再結(jié)合沉積物干密度計(jì)算得出[12-14,111]。因此，在大河影響下的邊緣海不同沉積環(huán)境中，由于TOC含量和質(zhì)量埋藏速率的不同，有機(jī)碳的埋藏速率往往也有很大差異(表1)。

表1　世界上不同河口-近海沉積有機(jī)碳埋藏速率的比較

注：“-”表示無數(shù)據(jù)

Deng 等[14]根據(jù)表層沉積物TOC含量和沉積速率及干密度資料，對東海陸架不同區(qū)域沉積有機(jī)碳的埋藏速率和收支進(jìn)行了系統(tǒng)研究，估算得到東海陸架有機(jī)碳的年平均埋藏速率為14.7 g·m-2，區(qū)域年埋藏通量為7.4×106t；其中，長江三角洲附近有機(jī)碳年埋藏速率最高，超過200 g·m-2，堆積在沉積物中的陸源和海源有機(jī)碳占各自總量的10%和5.5%，高于全球平均水平，并指出該區(qū)較高的有機(jī)碳埋藏特點(diǎn)可能跟本區(qū)年沉積速率較高有關(guān)，河流輸入和黑潮侵入是影響東海陸架有機(jī)碳埋藏的主要因素；通過比較還發(fā)現(xiàn)，該結(jié)果(14.7 g·m-2)高于從PN斷面利用同樣的估算方法得到的結(jié)果(4.7～10.9 g·m-2)[112]，同時(shí)也高于基于質(zhì)量收支平衡模型估算出的區(qū)域有機(jī)碳年沉積通量結(jié)果(8.03 g·m-2)[114]，這可能與不同研究的估算方法及所覆蓋區(qū)域范圍的差異有關(guān)?；诜派湫蕴纪凰啬甏蚣?，Yang等[115]對長江口和珠江口全新世以來沉積有機(jī)碳埋藏的演變過程進(jìn)行了比較研究，發(fā)現(xiàn)珠江口的沉積有機(jī)碳埋藏記錄可較好用于重建東亞季風(fēng)的演化史，而長江口沉積有機(jī)碳的埋藏記錄及季風(fēng)替代指標(biāo)的關(guān)系較復(fù)雜，指示了該區(qū)沉積有機(jī)碳埋藏保存的影響因素較多(如流域風(fēng)化和土壤類型)。通過收集近十年以來區(qū)域尺度的渤、黃海沉積有機(jī)碳和質(zhì)量埋藏速率資料，Hu等[112]估算了渤、黃海陸架區(qū)百年尺度內(nèi)沉積有機(jī)碳的年平均埋藏速率約為15.3 g·m-2，區(qū)域年埋藏量達(dá)5.6×106t，通過與世界上其他陸架地區(qū)的結(jié)果對比(表1)，提出渤、黃海陸架區(qū)域是全球近海有機(jī)碳的重要儲(chǔ)庫，并指出大河物質(zhì)輸入、較高的海洋初級(jí)生產(chǎn)力以及陸架沉積作用可能共同支撐著本區(qū)較高的碳埋藏能力。通過對水體和沉積物中有機(jī)碳含量的分析，并結(jié)合文獻(xiàn)資料，劉軍等[116]對渤、黃海有機(jī)碳的收支進(jìn)行了評(píng)估，結(jié)果表明有機(jī)碳的主要來源為初級(jí)生產(chǎn)力貢獻(xiàn)，水體中外源輸入和海洋自生有機(jī)碳約1.6%被埋藏，說明渤、黃海具有相對較高的生產(chǎn)力和埋藏效率，表現(xiàn)出潛在碳匯特性。利用淺地層剖面調(diào)查獲得的全新世地層厚度，結(jié)合南黃海西部內(nèi)陸架沉積柱樣和鉆孔巖心的沉積速率和有機(jī)碳含量資料，侯雪景[117]研究認(rèn)為老黃河三角洲沉積區(qū)和山東半島南部水下泥質(zhì)區(qū)全新世以來有機(jī)碳年沉積通量較高(>30 g·m-2)，并比較了海岸帶和內(nèi)陸架不同環(huán)境單元的碳匯能力。綜上所述，不同地區(qū)、不同沉積年代的河口陸架環(huán)境中沉積有機(jī)碳的埋藏能力差異較大，沉積有機(jī)碳的埋藏主要還是受控于物源供應(yīng)和沉積環(huán)境。

盡管河口陸架環(huán)境中沉積物被認(rèn)為是陸源輸入及海洋自生物質(zhì)的歸宿，但如前所述，受河口近海沉積水動(dòng)力條件的控制，不同來源、不同性質(zhì)的沉積有機(jī)質(zhì)往往發(fā)生選擇性輸運(yùn)和埋藏；而且，伴隨這種活躍的沉積動(dòng)力環(huán)境，沉積物中還廣泛存在各種化學(xué)氧化過程和微生物活動(dòng)，導(dǎo)致不同來源、不同類型的有機(jī)質(zhì)組分的降解速率(早期成巖礦化)或生物可利用性并不相同，從而使得不同沉積環(huán)境中的有機(jī)碳的埋藏保存效率存在較大差異(圖5)[24,118]。例如，在亞馬孫河口-近海的移動(dòng)泥(mobile mud)中，超過50%的河流輸入的有機(jī)碳被氧化而分解，再懸浮過程顯著促進(jìn)了底邊界層有機(jī)碳的再礦化降解，從而使得河口三角洲陸源有機(jī)碳的埋藏保存效率較低(約24%)，而陸架能儲(chǔ)存約30%～35%的河流輸入的有機(jī)碳[10]，表明這些大河影響下的河口陸架地區(qū)雖然是有機(jī)碳的堆積中心，但卻并不一定是長期的保存中心[22,118]。我國東部陸架海地區(qū)沉積有機(jī)質(zhì)保存及降解礦化的研究尚處在起步階段。早期Aller等[119]對長江口和東海沉積物孔隙水的氧化還原條件及其中生源要素的擴(kuò)散通量和降解成巖過程進(jìn)行了初步研究，估算了陸源有機(jī)碳的保存效率；Yao等[120]發(fā)現(xiàn)長江口-東海內(nèi)陸架沉積物單位比表面積有機(jī)碳含量普遍較低(<0.40 mg/m2)，表明有機(jī)碳的保存效率不高，上層沉積物有機(jī)碳再礦化速率與上述熱帶海域的結(jié)果具有可比性，顯示可能經(jīng)歷了比較顯著的再礦化分解過程?；诹６确旨?jí)研究，發(fā)現(xiàn)長江口最大渾濁帶附近小于32 μm的顆粒物有機(jī)碳保存效率相對較低，約71%的長江所輸入的陸源有機(jī)碳在沉積過程中發(fā)生礦化分解[121]。受黃河輸入的直接影響，沉積有機(jī)碳經(jīng)歷了較強(qiáng)的水動(dòng)力分選和再礦化作用；渤、黃海陸架泥質(zhì)區(qū)有機(jī)碳保存能力相對偏低(圖6)，可能與陸源顆粒物長距離輸運(yùn)、水柱沉降過程中的降解有關(guān)，并表現(xiàn)出泥質(zhì)區(qū)的沉積動(dòng)力機(jī)制對有機(jī)碳保存的影響[122]?？傊?，不同地區(qū)沉積有機(jī)碳的保存效率和再礦化路徑(如有氧呼吸和反硝化等)不盡相同[123]，定量開展這方面的研究對于揭示能量分配和碳循環(huán)過程具有重要的生態(tài)學(xué)和環(huán)境學(xué)意義[117,124-126]。

圖5　不同沉積環(huán)境中有機(jī)碳的埋藏保存效率[26] Fig.5　The percentage of organic carbon deposited on the seafloor that is buried as a function of net sediment accumulation rate[26]

圖6　單位比表面積不同來源沉積有機(jī)碳組成[121]Fig.6　The regional comparisons of TOC/SSA in varied origins in different depositional settings[121]

另一方面，除了海洋源自生的新鮮的有機(jī)質(zhì)容易被有效分解，同時(shí)也發(fā)現(xiàn)以往認(rèn)為難降解的陸源有機(jī)碳(如木質(zhì)素)在這一過程中也會(huì)顯著降解，這種被稱為“激發(fā)效應(yīng)”的現(xiàn)象(priming effect)可能是通過微生物引發(fā)、共氧化和共代謝作用而促進(jìn)更難降解的陸源有機(jī)質(zhì)的分解[10,24,128]，但其中具體的過程和機(jī)制還不十分清楚。沿長江陸源沉積物的擴(kuò)散輸運(yùn)路徑，Hu等[71]發(fā)現(xiàn)在東海內(nèi)陸架伴隨著新鮮海洋源有機(jī)質(zhì)的混入，陸源沉積有機(jī)質(zhì)由北向南進(jìn)一步發(fā)生降解，表現(xiàn)為陸源正構(gòu)烷烴碳優(yōu)勢指數(shù)進(jìn)一步的降低。在沉積物再懸浮-再搬運(yùn)過程中，反復(fù)的氧化還原交替、局部新鮮海洋源有機(jī)質(zhì)的供給和微生物分解作用可能共同支撐著沉積有機(jī)碳的再礦化分解[22]。

4 結(jié)　語

總體來說，近年來針對中國東部陸架海沉積有機(jī)碳來源、輸運(yùn)和埋藏等方面的研究取得了較多成果，對在大河輸入、季節(jié)性再懸浮及沉積物遠(yuǎn)距離輸運(yùn)的宏觀格局下，不同來源、不同類型沉積有機(jī)碳的遷移轉(zhuǎn)化的源-匯過程有了較深入認(rèn)識(shí)。但是，目前的研究進(jìn)展與國際前沿研究仍有一定差距，今后應(yīng)進(jìn)一步從海陸結(jié)合的地球系統(tǒng)科學(xué)的角度，重點(diǎn)加強(qiáng)從流域到河口陸架乃至更廣闊的空間尺度和地質(zhì)時(shí)間尺度上對上述復(fù)雜生物地球化學(xué)過程的影響機(jī)制和環(huán)境響應(yīng)開展研究，主要可包括如下幾方面：1)加強(qiáng)對沉積作用過程的系統(tǒng)分析與有機(jī)碳遷移轉(zhuǎn)化機(jī)制的整體研究，開展不同沉積環(huán)境下有機(jī)碳的埋藏保存及其對陸源物質(zhì)供應(yīng)通量變異、沉積過程和人類活動(dòng)的響應(yīng)研究；2)加強(qiáng)有機(jī)地球化學(xué)與礦物學(xué)、微生物學(xué)等其他多學(xué)科交叉的研究，從有機(jī)-無機(jī)相互作用的角度探討不同類型有機(jī)碳與不同礦物和地球化學(xué)組成沉積物及微生物種群的相互關(guān)系及沉積物中有機(jī)碳的保存方式和歸宿的影響機(jī)制；3)結(jié)合我國東部陸架海特有的沉積物源匯體系特征，重點(diǎn)從分子-同位素層面定量厘清不同來源沉積有機(jī)碳的年齡分布，研究不同類型陸源有機(jī)碳從流域到河口陸架這一連續(xù)體中的遷移轉(zhuǎn)化過程，并在氣候變化和人類活動(dòng)影響日益顯著的背景下，著重探究不同時(shí)空尺度下沉積有機(jī)碳的搬運(yùn)和埋藏保存及其對大氣CO2和全球變化的影響意義。

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Progress in Research of Sedimentary Organic Carbon in the East China Sea: Sources, Dispersal and Sequestration

SHI Xue-fa1,2,3, HU Li-min1,2,3, QIAO Shu-qing1,2,3, BAI Ya-zhi1,2

(1.TheFirstInstituteofOceanography,SOA, Qingdao 266061, China;2.KeyLaboratoryofMarineSedimentologyandEnvironmentalGeology, Qingdao 266061, China;3.LaboratoryforMarineGeology,QingdaoNationalLaboratoryforMarineScienceandTechnology,Qingdao 266061, China)

The coastal system, especially for the river-influenced estuary and shelf serve as the major repository of sedimentary organic carbon (OC), is an active interface between terrestrial and oceanic environments, and plays an important role in the global biogeochemical cycles of carbon. The East China Shelf Sea (ECS) includes the Bohai Sea, Yellow Sea and East China Sea, which was directly influenced by the Yellow River (Huanghe) and Yangtze River (Changjiang). This study overviewed the recent developments of the study of this field, especially focusing on the key points of the sources, dispersal and burial of the sedimentary OC. It is found that soil-derived OC is a major contributor to the sedimentary OC, and marine primary production is the main source for the OC deposited in the shelf region. The multiproxies approach (e.g., biomarkers), together with the statistics analysis model, could yield a more precise assessment for the sources of sedimentary OC. The shelf mud deposit areas in the ECS are the main repository of land-based OC from the Yellow River and Yangtze River. Some factors such as riverine input, sediment re-suspension and long-distance transportation, can influence the quality, fate and behavior of the terrigenous OC. The overall organic carbon storage capacity in this region is mainly sustained by the fluvial/land-based input, high phytoplankton primary productivity, convergent hydrodynamic and stable depositional settings. However, a large knowledge gap exists between the international frontier and the present research progress in ECS, and more efforts could be put on the systematic and multidisciplinary work on the land-sea interaction, especially for indicating its mechanism and the relationship between the sedimentary processes and transformation of carbon components as a whole system. Based on the typical sediment source to sink property in the ECS, the future work could pay attention to the residence time and age of sedimentary OC from the drainage basin to the sea, and examine the significance of the behavior and burial of OC on the atmospheric carbon dioxide and global change.

east China shelf sea; sedimentary organic carbon; sources; dispersal path; carbon sequestration

June 16,2016

2016-06-16

國家自然科學(xué)基金項(xiàng)目——亞洲大陸邊緣演化及環(huán)境效應(yīng)(U1606401)；“全球變化與海氣相互作用”專項(xiàng)——亞洲大陸邊緣“源-匯”過程與陸海相互作用(GASI-GEOGE-03);中央級(jí)公益性科研院所基本科研業(yè)務(wù)費(fèi)專項(xiàng)資助項(xiàng)目——不同能源下的有機(jī)分子標(biāo)記物海洋沉積記錄(GY0214G28)；青島海洋科學(xué)與技術(shù)國家實(shí)驗(yàn)室鰲山科技創(chuàng)新計(jì)劃項(xiàng)目——亞洲大陸邊緣地質(zhì)過程與資源環(huán)境效應(yīng)(2015ASKJ03)

石學(xué)法(1965-)，男，山東濰坊人，研究員，博士，主要從事海洋沉積學(xué)方面研究. E-mail: xfshi@fio.org.cn

(陳靖編輯)

P736

A

1671-6647(2016)03-0313-15

10.3969/j.issn.1671-6647.2016.03.001