雅浦海溝沉積有機碳垂向分布及其指示意義

吳 彬,李 棟*,趙 軍,劉誠剛,孫承君,陳建芳,潘建明,韓正兵,胡 佶

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雅浦海溝沉積有機碳垂向分布及其指示意義

吳 彬1,2,李 棟1,2*,趙 軍1,2,劉誠剛1,2,孫承君3,陳建芳1,2,潘建明1,2,韓正兵1,2,胡 佶1,2

(1.國家海洋局第二海洋研究所海洋生態(tài)與環(huán)境實驗室,浙江 杭州 310012；2.國家海洋局海洋生態(tài)系統(tǒng)與生物地球化學(xué)重點實驗室,浙江 杭州 310012；3.國家海洋局第一海洋研究所海洋生態(tài)中心,山東 青島 266237)

利用蛟龍?zhí)栞d人深潛器采集了西太平洋雅浦海溝沉積柱樣,分析了總有機碳(TOC)、δ13C、粒度和比表面積(SSA)等參數(shù),結(jié)合端元混合模型、降解模型及主成分分析(PCA),探討不同來源有機碳(OC)垂向分布及其影響因素,估算了TOC降解速率、累積速率及通量.結(jié)果表明,雅浦海溝TOC%均值為(0.31%±0.10%),與淺海環(huán)境相當(dāng).由于外源OC主要賦存于粉砂等大粒徑顆粒物以及微生物的分解作用,TOC%與粉砂%正相關(guān),與SSA值及粘土比例負(fù)相關(guān).微生物來源OC是TOC的主要貢獻者(52%±21%),其次為海源(37%±19%)和陸源(11%±4%)OC,3種來源OC含量均隨深度增加而降低.PCA結(jié)果指出,微生物來源與外源OC相對貢獻間不耦合,表明海底地下水輸送的OC可能是海溝微生物的重要營養(yǎng)源.主導(dǎo)沉積物中TOC垂向分布的難降解OC降解速率約為0.0012a-1,略高于普通大洋環(huán)境,但較淺海環(huán)境更低.雅浦海溝TOC累積速率為1.8×10-5gC/(cm2·a),沉降通量為2.2×109gC/a.

雅浦海溝；沉積有機碳；垂向分布；來源；降解

大洋中水深超過6000m的海溝等超深淵帶,面積僅占海洋總面積的1%,但其深度變化范圍覆蓋海洋總深度的45%,在地質(zhì)、化學(xué)和生物等方面具有獨特特征,在海洋碳循環(huán)中扮演重要角色[1-5].海溝內(nèi)部的V形構(gòu)造及其引起的漏斗效應(yīng),使海溝易于收集周圍海底平原經(jīng)海底濁流和海洋生物泵等途徑輸運來的有機碳(OC),成為大洋OC的捕獲器[6-8].海溝沉積物中積累的有機質(zhì)又為該環(huán)境中底棲生物提供了理想棲息環(huán)境和可觀食物,使得海溝成為活躍的微生物反應(yīng)器[7,9-12].同時,由于兩側(cè)板塊的相向聚合運動使得海溝內(nèi)部沉積OC可以在地質(zhì)時間尺度上被裹挾進入熔融地幔,從而將地表OC輸入地球深處,使得海溝成為地球巖石圈潛在終極碳匯[13-14].然而,由于超深淵帶中極端高壓和低溫環(huán)境及其所帶來的儀器操作和樣品采集等技術(shù)挑戰(zhàn),已有的研究多限于海溝沉積OC含量空間分布及與底棲生物相互影響等方面[1,7-8,15-18],針對海溝中沉積OC生物地球化學(xué)循環(huán)的研究鮮見報道[7-8,17,19-20].

雅浦海溝緊鄰雅浦群島和帕勞群島,北起馬里亞納海溝南端,南到帕勞海溝北端,發(fā)育有典型俯沖構(gòu)造侵蝕[21].相較馬里亞納等海溝,同為世界級海溝的雅浦海溝在有機生物地球化學(xué)方面的研究未見報道.本研究利用蛟龍?zhí)栞d人深潛器在雅浦海溝內(nèi)采集無擾動沉積柱狀樣,通過對沉積物中總有機碳(TOC)、總氮(TN)、碳穩(wěn)定同位素(δ13C)、粒度組成和比表面積(SSA)等地球化學(xué)參數(shù)的測定,利用基于蒙特卡洛(Monte-Carlo,MC)模擬的三端元混合模型和主成分分析(PCA),探討沉積物中海源、陸源和微生物來源OC含量的垂向變化及其可能影響因素.使用多G降解模型和質(zhì)量累積模型估算海溝中沉積OC的降解速率、累積速率及通量,以獲取對雅浦海溝等超深淵環(huán)境中碳的生物地球化學(xué)循環(huán)的深入認(rèn)識.

1 材料與方法

于2017年6月9日,利用向陽紅10號海洋科研調(diào)查船及蛟龍?zhí)栞d人深潛器,在雅浦海溝6501m水深處使用PushCore取樣器采集了DIVE150站位的無擾動柱狀沉積物(長23cm,137.597°E,8.051°N)(采樣站位如圖1所示).沉積物樣品在船上按照1~2cm間隔進行切割,裝入封口袋并在-20℃條件下保存至岸上實驗室分析.

將沉積物裝入具有固定體積和質(zhì)量的立方體容器中,測定沉積物濕密度,隨后凍干測定干密度和含水率.取未研磨凍干沉積物,加入過量雙氧水除去顆粒物中OC,使用六偏磷酸鈉分散劑浸泡分散,在上機測試前超聲分散30s后使用馬爾文Mastersizer3000激光粒度儀進行粒度測定.取未研磨凍干沉積物,于350℃條件下加熱除去有機質(zhì),隨后使用貝士德3H-2000PS1型比表面自動分析儀,采用靜態(tài)容量法來測定SSA.取適量研磨均勻凍干沉積物,使用鹽酸熏蒸除去無機碳酸鹽,60℃條件下加熱烘干除去水分和鹽酸,使用Thermo Flash EA 1112HT氮/碳元素分析儀以及DELTA V Advantage穩(wěn)定同位素比質(zhì)譜儀對樣品中TOC%(標(biāo)準(zhǔn)偏差小于0.08%)、TN%(標(biāo)準(zhǔn)偏差小于0.07%)和δ13C(精度優(yōu)于±0.2‰)進行測定.

圖1 柱狀沉積物采樣站位

本研究使用基于δ13C和TOC/TN摩爾比(C/N比)的三端元混合模型,分析海溝沉積物中陸源、海洋浮游植物和微生物(包括微生物活體細(xì)胞、死亡后碎屑以及改造后的外源OC)3種來源OC對沉積物TOC的相對貢獻,具體方法參照文獻[22].模型公式如下:

13C海源×海源+13C陸源×陸源+13C微生物×微生物=13C樣品(1)

C/N海源×海源+C/N陸源×陸源+C/N微生物×微生物=C/N樣品(2)

海源+陸源+微生物=1 (3)

式中:13C樣品和C/N樣品分別代表所測樣品的13C值和C/N比,海源、陸源和微生物分別代表海洋、陸地和微生物來源OC的百分比貢獻(%),其他符號代表相應(yīng)來源的端元值.同時,為了盡量減少固定端元值對模型計算帶來的誤差,使用MC模擬方法來校正端元值的變化對不同來源OC比例計算的可能影響[23-26].該方法假定某種來源OC的13C或者C/N比端元值在給定范圍內(nèi)的變化符合正態(tài)分布,在此區(qū)間內(nèi)按正態(tài)分布隨機取值,然后選取部分或全部進行計算,計算過程所選端元值要同時滿足公式(1)~(3),計算結(jié)果(海源、陸源和微生物)同樣符合正態(tài)分布[27-28].使用該方法可以得到3種組分對沉積TOC相對貢獻的平均值、標(biāo)準(zhǔn)偏差、中值、最大值和最小值[29-30].多次重復(fù)運行同一樣品所得平均值相對標(biāo)準(zhǔn)偏差小于0.03%,表明該方法具有較高統(tǒng)計學(xué)穩(wěn)定性.端元模型所使用端元值δ13C海源、δ13C陸源、和δ13C微生物分別設(shè)定為-20.0‰±1.0‰[29,31]、-25.6‰± 1.0‰[32]和-20.0‰±1.0‰[33-36],C/N海源、C/N陸源和C/ N微生物設(shè)定為6.5±0.3[29]、14.6±0.8[29]和3.7±0.2[37-38].其中,本研究假設(shè)沉積物柱樣微生物群落以異養(yǎng)微生物為主,其碳源為易降解海源OC,根據(jù)微生物δ13C值取決于其碳源的原理[33-36],本研究未對微生物來源和海源OC的δ13C值進行區(qū)分,并主要依據(jù)C/N比值對海源和微生物來源OC進行區(qū)分.采樣視頻資料顯示目標(biāo)研究區(qū)域沒有冷泉或者熱液口發(fā)育,沉積物δ13C值高于-22.7‰,且熒光定量PCR結(jié)果表明化能合成微生物(例如氨氧化細(xì)菌和古菌)僅占總微生物豐度的極少部分(未發(fā)表數(shù)據(jù)).同時,在富氧、沉積速率較低以及有機質(zhì)充足的海底表層沉積物中,異養(yǎng)代謝微生物在群落結(jié)構(gòu)中的貢獻較高[39-41].因而該方法可在一定程度上用來計算微生物來源OC的相對貢獻.

本研究中使用多G降解模型對沉積物中TOC降解過程進行擬合[42],模型方程:

C=+0·(1-)·e(-k1·t)+0··e(-k2·t)(4)

式中:0表征柱狀沉積物表層TOC含量,表征沉積物剖面無窮深處TOC含量的恒定值,C表征沉積物沉降時間(a)后對應(yīng)深度沉積物中TOC含量,1(a-1)和2(a-1)分別代表易降解和難降解兩部分TOC組分的降解速率,(0<<1)表示難降解組分所占百分比,(cm)表示沉積物深度,s(cm/a)代表沉積速率,=/s.

2 結(jié)果

2.1 TOC、TN、δ13C、SSA和粒度組成

如圖2所示,TOC和TN的含量變化范圍分別為0.17%~0.46%(平均值0.31%±0.10%)和0.04%~ 0.08%(平均值0.06%±0.01%).δ13C為-22.70‰~ -20.13‰,平均-21.07‰±0.87‰.C/N比為4.4~8.2(平均值5.9±0.9).SSA平均值為(50.8±4.3)m2/g,在44.8~58.2m2/g范圍內(nèi)變化.單位SSA的TOC載荷(TOC/SSA)變化范圍為0.03~0.10mgC/m2(平均值(0.06±0.03)mgC/m2).沉積物粒度組成以粉砂(4~63μm)為主(43.1%~55.2%,均值49.4%±4.0%),粘土(<4μm)次之(48.8%~28.7%,均值38.6%±5.9%),砂(>63μm)僅占6.3%~22.5%(11.9%±4.1%).各參數(shù)均呈現(xiàn)出由上向下的降低趨勢,并最終趨于穩(wěn)定.

2.2 三端元混合模型和MC模擬

利用基于MC模擬的三端元混合模型對海溝不同來源OC百分比和含量的定量計算結(jié)果如圖3所示.DIVE150站位沉積物中海源、陸源和微生物來源OC在TOC中相對貢獻分別為37%±19%(變化范圍8%~59%)、11%±4%(變化范圍6%~24%)和52%± 21%(變化范圍16%~86%).沉積物中海源、陸源和微生物來源OC含量分別為0.13%±0.09% (0.01%~ 0.27%)、0.04%±0.02% (0.01%~0.11%)和0.14%± 0.03%(0.07%~0.19%).陸源垂向變化不明顯,海源在7.5cm以淺波動變化,7.5cm以深顯著降低至穩(wěn)定值,而微生物在6.5cm以淺無明顯變化,以深則顯著升高到80%以上.沉積物中海源和陸源OC含量分別與其百分比例具有相似垂向分布,微生物來源OC含量呈逐漸降低的變化趨勢.整體而言,8.5cm以淺沉積物以海源OC貢獻為主,而8.5cm以深以微生物來源OC為主.

圖3 基于MC模擬計算所得柱狀沉積物中海源、陸源和微生物來源OC的百分比(a)以及含量(b)的垂向分布

2.3 PCA分析

對DIVE150站位沉積物中19個參數(shù)進行PCA分析,結(jié)果表明前2個主成分的累積貢獻率為87%.其中,主成分1和2分別解釋了數(shù)據(jù)集總變異的75%和12%(圖4).主成分1在顆粒物粒級及其相關(guān)參數(shù)上具有顯著區(qū)分度,而主成分2則主要在OC來源上存在不同負(fù)荷,正負(fù)荷表征陸源和海源等外源物質(zhì)輸入,而負(fù)值則表征沉積物中微生物來源OC貢獻.

圖4　基于主成分分析所得沉積柱狀樣中不同參數(shù)的負(fù)荷分布

2.4 降解模型

表1 世界不同海域沉積柱狀樣中TOC的降解參數(shù)比較

注:為難降解OC所占比例,1(a-1)為易降解OC的降解速率,2(a-1)為難降解OC降解速率,-表示未計算.

由于雅浦海溝緊鄰馬里亞納海溝南端,兩者均位于西太平洋暖池和熱帶環(huán)流海區(qū),具有相似的初級生產(chǎn)力和沉降通量,因此,本研究使用與馬里亞納海溝南部區(qū)域相似水深處(6037m)的沉積速率(約為0.02cm/a)[7,17],對雅浦海溝沉積TOC降解速率進行估算.計算結(jié)果如表1所示.沉積物中TOC剖面符合指數(shù)型降解分布,與多G降解模型方程擬合度較高(2=0.93),沉積物中易降解OC降解速率1為5.4a-1,難降解OC降解速率2為0.0012a-1,且難降解OC的降解過程對TOC的垂向分布影響顯著.

3 討論

3.1 雅浦海溝沉積OC的組成及其影響因素

海溝等超深淵環(huán)境是深海沉積OC的潛在捕獲器.DIVE150站位柱樣0~4cm沉積物中TOC含量穩(wěn)定在0.44%以上,與鄰近的馬里亞納海溝相當(dāng)(~0.45%)[7],但明顯低于日本海溝(~1.0%)[9]、中美洲海溝(2.0%~4.0%)[51]和卡里亞科海溝(~3.0%)[52].與陸地距離的遠近和真光層初級生產(chǎn)力的差異是造成該趨勢的主要原因.雅浦海溝緊鄰馬里亞納海溝,同處于西太平洋暖池和熱帶環(huán)流海區(qū),具有相似的初級生產(chǎn)力(分別為82gC/(m2·a)和59gC/(m2·a))和顆粒OC沉降通量(分別為(0.56±0.29)gC/(m2·a)和(0.55±0.20)gC/ (m2·a))[53-54],且均位于西太平洋深水環(huán)流通道中[40],因而具有相似水動力環(huán)境并導(dǎo)致了相似TOC含量.而日本海溝、中美洲海溝以及卡里亞科海溝由于鄰近陸地并處于上升流區(qū)域,真光層水體初級生產(chǎn)力較高(例如日本海溝193gC/(m2·a)和卡里亞科海溝190gC/(m2·a)),導(dǎo)致較高的顆粒OC沉降通量(日本海溝(3.05±0.91)gC/(m2·a)、卡里亞科海溝(0.77± 0.46)gC/(m2·a))和沉積物TOC含量[40,51-52].此外,與具有較高初級生產(chǎn)力的我國東海內(nèi)陸架海域(200m以淺,0.2%~0.6%)[30]、黃海(140m以淺,0.1%~1.1%)[55]和南極半島東北海域(2000~3000m,0.4%~0.6%)[56]等淺海海洋環(huán)境同樣具有可比性,并顯著高于普通開闊大洋海底沉積物(5000m,0.1%~0.3%)[11,57],表明海溝V型構(gòu)造對海底沉積OC的物理匯聚作用顯著.

沉積物粒度組成和微生物活動對海溝沉積物中TOC垂向分布具有顯著影響.由PCA分析結(jié)果可知(圖4),沉積物粒徑大小及相關(guān)參數(shù)是影響沉積有機質(zhì)垂向分布的主要因素.沉積物TOC含量與中值粒徑大小,特別是粉砂粒級%(4~63μm)正相關(guān),而與SSA大小以及粘土粒級%(<4μm)負(fù)相關(guān)(圖5).這與在陸架邊緣海等海洋環(huán)境中所得中值粒徑越低、粘土粒級越多、SSA越高會導(dǎo)致TOC含量越高的規(guī)律相反[30,58].該現(xiàn)象可能是由海溝沉積物含水率、水動力環(huán)境、外源OC輸入以及微生物活動等多因素共同造成的.如圖4所示,含水率、外源OC百分比例和含量、TOC和TN含量、中值粒徑以及粉砂比例均分布在負(fù)荷分布圖的第一象限,表明受沉積物壓實效應(yīng)影響,沉積物含水率自上而下逐漸降低(從80%降低至60%),雖然海溝沉積物上覆水水體流速整體較低(cm/s量級),但依然會對表層沉積物造成一定程度的沖刷,導(dǎo)致沉積物粒徑自上向下的降低(圖2f).相比粒徑更小的微生物來源顆粒物,富含OC的新鮮外源有機質(zhì)可能更多的賦存于粒徑更大的粉砂粒級顆粒物上(4~63μm).同時,海溝沉積物上覆水來源于南極繞極深層水[40],溶解氧濃度較高(~5.1mg/L)且溶氧滲透深度較大[7],使得海溝等深海沉積物中好氧礦化成為有機質(zhì)分解的唯一重要路徑[7,40,59-60],因而在活躍微生物新陳代謝作用影響下,外源OC含量迅速降低,而微生物則顯著升高.同時,微生物和SSA、粘土比例等參數(shù)均位于負(fù)荷分布圖的第3象限,表明微生物在SSA高的粘土粒級顆粒物表面更活躍,這與已有的研究結(jié)果相同,即SSA越大越利于OC的附著以及被微生物吸收利用[61-63].因而形成了海溝環(huán)境中粒徑與TOC含量等參數(shù)間獨特的關(guān)系和分布特征.

原位微生物來源OC相對貢獻與外源OC輸入相對貢獻間的去耦合(圖4),以及8.5cm以下微生物來源OC的顯著貢獻(>50%),在一定程度上表明除了經(jīng)顆粒物自然沉降或海底濁流等過程輸運來的陸地和海洋初級生產(chǎn)來源OC外,還有經(jīng)其他途徑輸運而來的OC供給來維持海溝沉積物小生境中微生物的生命活動.作為連通地球內(nèi)部和表層的窗口和通道,超深淵帶中碳庫可能受到深部生物圈和海底地下水物質(zhì)輸送的調(diào)控[64-73].例如,洋殼上層沉積物及巖層是地球上容量最大的連續(xù)含水層系統(tǒng),其中存在著受潮汐驅(qū)動的可貫穿幾百米厚沉積物層的海底地下水流動(流速達到10cm/d),這些發(fā)生于沉積物間隙中的水體流動,能夠促進大洋底層水體與海底地下水之間利用沉積物內(nèi)部的裂隙和其他可滲透性的通道,進行溶解態(tài)和顆粒態(tài)的物質(zhì)交換(如O2、NO3-、溶解態(tài)和顆粒態(tài)OC等),從而更新海溝沉積物中碳等營養(yǎng)物質(zhì),并對沉積物中微生物生物量以及有機質(zhì)的降解產(chǎn)生影響[39,73-76].圖4中微生物與粘土比例的顯著正相關(guān)(=0.82)即表明小粒徑顆粒物及其附著的OC對微生物生命活動的重要作用.雅浦海溝沉積物較高的含水率和松軟質(zhì)地,更為其中物質(zhì)交換提供了基礎(chǔ).另外,作為地球上最大的微生物碳儲庫,地球總生物量的50%~ 66%存在于深部生物圈中,其中海底沉積物又是地球上最為廣袤的微生物孵化器和棲息地[39].本研究沉積物中顯著微生物來源OC相對貢獻與該結(jié)論相一致.

圖5　沉積柱狀樣中值粒徑、粉砂、粘土比例與TOC間的相關(guān)性

3.2 雅浦海溝沉積TOC的降解與埋藏

沉積OC的組成特征會決定沉積物中再礦化過程損失掉的OC的來源.本研究對沉積物中TOC/ SSA比值與δ13C·TOC/SSA比值進行線性擬合(圖6),根據(jù)擬合方程的斜率判斷再礦化過程損失的OC的來源特征[77-78].從圖6可以看出,隨著沉積OC再礦化過程(TOC/SSA比值由表層到底層逐漸降低)的進行,沉積物中被微生物降解和利用的OC的δ13C值約為-19.5‰,表明海溝沉積物中海源OC被優(yōu)先降解.可能原因是沉積物總碳庫中易降解的小分子海源OC貢獻(37%±19%)較高,而難降解大分子陸源OC百分比例相對較低(陸源=11%±4%),具有低C/N比的微生物會更傾向于分解利用含氮量高的海源有機質(zhì).雖然有研究表明,在距離陸地較近的海溝中,陸源有機質(zhì)能夠作為一種特殊食物來源促進海溝中高效、專性分解陸源高等植物來源有機質(zhì)生物的發(fā)育,例如馬里亞納海溝中專門攝食陸源木質(zhì)碎屑的可可螺科腹足類和片腳類等動物[17,79-83].同時,發(fā)生于海洋環(huán)境中的激發(fā)效應(yīng)同樣也會促進海底沉積物中陸源有機質(zhì)的快速降解[84-86].但本研究中,可能由于沉積物中陸源OC貢獻以及微生物多樣性較低(未發(fā)表數(shù)據(jù)),且缺少能夠?qū)Ｐ越到怅懺碠C的微生物,因而導(dǎo)致陸源OC被較好的保存(圖3).

圖6　沉積柱狀樣中δ13C·TOC/SSA與TOC/SSA之間的相關(guān)性

沉積速率以及沉降顆粒物的物質(zhì)組成對海溝沉積OC的降解和埋藏具有重要影響.多G降解模型結(jié)果表明,雅浦海溝與馬里亞納海溝同等深度沉積物中難降解OC所占比例相似,降解速率較為一致(表1).雅浦海溝和馬里亞納海溝沉積OC的降解速率顯著高于太平洋-南大洋洋脊、東北太平洋、西赤道太平洋等6000m以淺的普通開闊大洋的降解速率,同時,又較陸架邊緣海以及近岸海灣等淺海環(huán)境更低(表1).沉積速率可以通過對沉積物中孔隙度、溶解氧滲透深度、微生物群落組成和沉積OC礦化路徑等方式的調(diào)控,影響沉積OC降解速率[41,49].雖然一般情況下海底沉積物沉積速率與水深成反比[41,87],但由于海溝地形的匯聚作用,使得海溝中顆粒物沉積速率顯著的高于周邊海底平原(表1).與海底平原相比,較高的沉積速率和隨水深降低的水體流速導(dǎo)致了海溝沉積物中高含水率(~72%)和高溶解氧滲透深度(馬里亞納海溝6037m水深處沉積物5cm深度溶解氧濃度依然高達~170μmol/L)[7].同時,海溝中較海底平原更高的TOC含量和微生物豐度[7],更為沉積物中以好氧呼吸為主要代謝途徑的微生物活動提供了有力保證[39,60,88],因而導(dǎo)致了海溝沉積OC具有較普通開闊大洋更高的降解速率.與陸架邊緣海等淺海環(huán)境相比,雖然兩者沉積物中海源OC均有重要貢獻,但邊緣海環(huán)境中更高的初級生產(chǎn)力(例如東海161gC/(m2·a))導(dǎo)致其沉積物中易降解海源OC絕對含量顯著高于海溝(東海內(nèi)陸架約為0.4%,而雅浦海溝約為0.13%±0.09%)[30,89-91],因而可降解的潛力更高.同時,陸架淺水區(qū)中強烈的水動力條件和沉積物再懸浮[78,92],以及更高的微生物豐度(比海溝高3個數(shù)量級)[7,93-95],更使得陸架邊緣海成為沉積OC的焚化爐[78,96],因而其沉積OC降解速率顯著高于海溝.

利用雅浦海溝沉積物干、濕密度、TOC含量以及沉積物沉積速率等參數(shù),并結(jié)合質(zhì)量累積模型[17,41],估算了雅浦海溝中沉積OC的累積速率、埋藏效率和埋藏通量.結(jié)果表明,雅浦海溝表層沉積物中OC的累積速率約為1.8×10-5gC/(cm2·a),與Jamieson[40]、Lutz等[97]模型計算的雅浦海溝顆粒OC沉降速率處于同一量級(約為5.6×10-5gC/ (cm2·a)),并與鄰近馬里亞納海溝南部區(qū)域沉積OC累積速率(0.35×10-5gC/(cm2·a)~1.5×10-5gC/(cm2·a))基本相當(dāng)[17],約為全球陸架邊緣海顆粒OC累積速率平均值的4.3%(4.2×10-4gC/(cm2·a))[98]以及大洋(>1000m)顆粒OC累積速率平均值的18%(1.0× 10-4gC/(cm2·a))[99].結(jié)合雅浦海溝海域面積(1.21× 104km2)[40]計算可得雅浦海溝顆粒OC沉降通量約為2.2×109gC/a.同時,由于雅浦海溝初級生產(chǎn)力和面積均顯著低于全球其他海溝[40],因而其他海溝將具有更可觀的顆粒OC沉降速率和通量,表明超深淵帶對深海大洋顆粒OC具有顯著的匯聚作用.

4 結(jié)論

4.1 雅浦海溝TOC含量與鄰近馬里亞納海溝相當(dāng),但顯著低于日本海溝和中美洲海溝等海溝.漏斗效應(yīng)導(dǎo)致雅浦海溝表層沉積OC含量顯著高于開闊大洋,并與陸架邊緣海等淺海環(huán)境具有可比性.

4.2 沉積物粒度組成和微生物活動對海溝沉積OC垂向分布影響顯著.海溝沉積物TOC%與中值粒徑大小,特別是粉砂%具有較強正相關(guān)關(guān)系,而與SSA大小以及粘土粒級%呈現(xiàn)顯著負(fù)相關(guān).

4.3 微生物來源OC是海溝沉積OC主要來源,其次為海洋浮游植物來源和陸地來源OC.原位微生物來源OC相對貢獻與外源OC輸入相對貢獻間的去耦合,以及8.5cm以下微生物來源OC的顯著貢獻(>50%),在一定程度上表明海底地下水的流動及其輸運而來的OC對維持海溝沉積物小生境中微生物的生命活動具有重要影響.

4.4 海源OC是雅浦海溝沉積物中微生物優(yōu)先利用的營養(yǎng)來源.雅浦海溝沉積TOC降解速率高于普通開闊大洋,又較陸架邊緣海以及近岸海灣等淺海環(huán)境更低.

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Vertical distribution of sedimentary organic carbon in the Yap Trench and its implications.

WU Bin1,2, LI Dong1,2*, ZHAO Jun1,2, LIU Cheng-gang1,2, SUN Cheng-jun3, CHEN Jian-fang1,2, PAN Jian-ming1,2, HAN Zheng-Bing1,2, HU Ji1,2

(1.Laboratory of Marine Ecosystem and Environment, Second Institute of Oceanography, State Oceanic Administration, Hangzhou 310012,China；2.Key laboratory of Marine Ecosystem and Biogeochemistry, State Oceanic Administration, Hangzhou 310012, China；3.Marine Ecology Center, First Institute of Oceanography, State Oceanic Administration, Qingdao 266237, China)., 2018,38(9)：3502~3511

Sediment core from the Yap Trench located in the west Pacific Ocean was collected by manned deep-sea research submersible. Geochemical parameter analyses (e.g. total organic carbon (TOC), δ13C, grain size composition and specific surface area (SSA)), combined with an end-member mixing model, a degradation model and principal component analysis (PCA) were conducted in order to 1) study the vertical distribution of the organic carbon (OC) and corresponding affecting factors, 2) assess the degradation and accumulation rate and the flux of the sedimentary OC in the Yap Trench. The mean TOC content of the Yap Trench was (0.31%±0.10%) and was comparable with those of the neritic sediments. Probably due to enrichment of the allochthonous organic matter in the large grain-sized particles (e.g. silt) and intense decomposition of OC by microorganisms, the TOC content was positively related with silt% and negatively related with SSA and clay%. The microorganism-derived OC was the dominant proportion of the sedimentary TOC (52%±21%), followed by the marine (37%±19%) and terrestrial (11%±4%) OC. The OC contents of those three sources all decreased downward. Based on PCA, decoupling between the microorganism-derived and the allochthonous OC indicated that the OC delivered by the submarine groundwater was probably an important nutritional supply for the microorganisms in the trench. The degradation rate of the refractory OC which dominated the vertical profile of TOC was around 0.0012a-1, slightly higher than that of the normal deep ocean but lower than the neritic environment. The accumulation rate and settling flux of TOC in the Yap Trench were around 1.8×10-5gC/(cm2·a) and2.2×109gC/a.

Yap Trench；sedimentary organic carbon；vertical distribution；source；degradation

X55

A

1000-6923(2018)09-3502-10

吳 彬(1986-),女,河北省秦皇島人,博士后,主要從事海洋沉積物中生源要素的生物地球化學(xué)過程研究.發(fā)表論文4篇.

2018-02-07

國家自然科學(xué)基金青年科學(xué)基金項目(41606090);國家重點基礎(chǔ)研究發(fā)展計劃(973計劃)項目(2015CB755904);國家海洋局第二海洋研究所基本科研業(yè)務(wù)費專項(JG1516)

* 責(zé)任作者, 助理研究員, lidong@sio.org.cn