南黃海沉積物中活性鐵氧化物對(duì)有機(jī)碳的保存作用

陶婧，馬偉偉，李文君，李鐵，朱茂旭*

(1．中國(guó)海洋大學(xué) 化學(xué)化工學(xué)院，山東 青島 266100)

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南黃海沉積物中活性鐵氧化物對(duì)有機(jī)碳的保存作用

陶婧1，馬偉偉1，李文君1，李鐵1，朱茂旭1*

(1．中國(guó)海洋大學(xué) 化學(xué)化工學(xué)院，山東 青島 266100)

土壤和沉積物中活性鐵對(duì)有機(jī)質(zhì)的吸附對(duì)有機(jī)質(zhì)具有長(zhǎng)期穩(wěn)定和保存作用，從而在地質(zhì)時(shí)間尺度上緩沖大氣CO2濃度。本文利用連二亞硫酸鈉還原性溶解提取活性鐵氧化物(FeR)及與之結(jié)合的有機(jī)碳(Fe-OC)，定量研究了南黃海沉積物中FeR與OC之間的結(jié)合方式以及FeR對(duì)OC的保存作用，討論了深度增加對(duì)二者相互作用的影響。結(jié)果表明，南黃海沉積物中Fe-OC占沉積物總有機(jī)碳的份數(shù)(fFe-OC)為(13.2±7.47)%，即活性鐵對(duì)OC的年吸附量為0.72 Mt，占全球邊緣海沉積物TOC年埋藏通量的0.44%。Fe-OC的平均OC∶Fe為4.50±2.61，表明共沉淀作用對(duì)有機(jī)質(zhì)的保存起重要作用，且其比值隨海源有機(jī)質(zhì)含量增加而增加。Fe-OC穩(wěn)定碳同位素(δ13CFe-OC)結(jié)果表明，F(xiàn)eR優(yōu)先保存活性有機(jī)質(zhì)，但這種選擇性隨OC∶Fe增大而減弱。隨深度增加，fFe-OC和δ13CFe-OC均未表現(xiàn)出顯著變化，這與該海域沉積物中有機(jī)質(zhì)活性較低、鐵還原作用較弱有關(guān)。

活性鐵氧化物；有機(jī)碳保護(hù)；海洋沉積物；吸附；南黃海

圖2 TOC、FeR以及Fe-OC含量隨深度的變化(FeR的誤差棒為1倍方差)

Fig.2 Depth profile of TOC, FeR and Fe-OC contents(the error bar for FeR is one standard deviation)

A01站點(diǎn)fFe-OC范圍為3.3%～38.9%，平均值(16.1±10.8)%(圖3a)，表層fFe-OC值明顯高于深部的值(P<0.05)；A03站點(diǎn)fFe-OC范圍為2.4%～16.7%，平均值(11.6±5.2)%，上部5 cm范圍內(nèi)其值波動(dòng)明顯，但深部較穩(wěn)定；A07站點(diǎn)fFe-OC范圍為4.0%～19.4%，平均值(11.9±4.9)%，其波動(dòng)較其他兩站位小。盡管A03和A07兩站點(diǎn)的Fe-OC含量差異明顯，但其對(duì)應(yīng)的fFe-OC卻無(wú)明顯差異(P< 0.05)。

A01、A03、A07三站點(diǎn)的OC∶Fe摩爾比(圖3b)分別為0.44～4.0，平均值2.4±1.3、1.0～9.5，平均值6.0±2.9以及2.0～6.8，平均值5.1±1.8，A01站點(diǎn)的OC∶Fe明顯小于其他兩站點(diǎn)(P<0.05)。總體而言，A03和A07兩站點(diǎn)深部OC∶Fe高于淺部，而A01站點(diǎn)無(wú)明顯深度變化(P< 0.1)。與文獻(xiàn)數(shù)據(jù)相比，三站點(diǎn)OC∶Fe變化幅度較小[10,13]。

A01、A03、A07三站點(diǎn)δ13CFe-OC范圍(圖4)分別為-30.9‰～-22.0‰，平均值(-24.1±3.1)‰、-25.5～-16.4‰，平均值(-21.8±2.8)‰及-22.5‰～-15.4‰，平均值(-20.1±2.7)‰。A03和A07站點(diǎn)δ13CFe-OC平均值高于對(duì)應(yīng)的非鐵結(jié)合態(tài)OC的δ13C平均值(P< 0.1)，表明前者13C 相對(duì)“富集”，而A01站點(diǎn)則相反，表明13CFe-OC相對(duì)虧損。三站點(diǎn)多數(shù)樣品(65%)的δ13CFe-OC高于對(duì)應(yīng)的非鐵結(jié)合態(tài)OC的δ13C，總體上表明13CFe-OC相對(duì)“富集”。

圖3 OC∶Fe摩爾比和fFe-OC隨深度的變化Fig.3 Depth profile of OC∶Fe molar ratios and fFe-OC for iron-bound organic matter

4 討論

4.1 FeR對(duì)OC的保存作用

南黃海沉積物中fFe-OC平均值(13.2±7.47)%低于全球海洋沉積物平均值[9](圖5)，也低于青藏高原(QTP)永凍土平均值[12]，更低于森林土壤平均值[13]，但與Wax Lake三角洲沉積物[11]以及北極陸架海沉積物[10]平均值接近?？傮w而言，本研究區(qū)FeR對(duì)OC的穩(wěn)定作用處于較低水平。根據(jù)本研究得到的fFe-OC平均值(13.2±7.47)%以及近100年來(lái)南黃海TOC埋藏通量(5.48 Mt/a)[27]，可估算出該海域沉積物FeR對(duì)OC的年吸附量為0.72 Mt。南黃海占全球邊緣?？偯娣e的1.55%，其占全球邊緣海沉積物TOC年埋藏通量(164 Mt)的0.44%，再次表明該海域沉積物中FeR對(duì)OC的保存作用較低。

圖4 鐵結(jié)合態(tài)和非鐵結(jié)合態(tài)有機(jī)質(zhì)的碳同位素組成(δ13C)隨深度的變化Fig.4 Depth profile of carbon isotopic compositions (δ13CFe-OC) for iron-bound and non-iron-bound organic matter

該海域fFe-OC較低的原因可能與在沿岸流、海洋環(huán)流以及氣旋渦流共同作用下，沉積物經(jīng)歷長(zhǎng)距離輸送以及長(zhǎng)時(shí)間氧氣暴露有關(guān)[28,30]。在沿岸流作用下，從渤海進(jìn)入黃海的沉積物一部分在山東半島北岸近海和北黃海中部氣旋型渦流區(qū)沉積，另一部分繞過(guò)成山角并入南黃海循環(huán)[31]。古黃河口侵蝕沉積物通過(guò)黃海海岸流向西南方向傳輸、經(jīng)黃海暖流向北傳輸，通過(guò)再懸浮并入南黃海沉積系統(tǒng)[32]。以上復(fù)雜的水動(dòng)力學(xué)條件導(dǎo)致表層沉積物經(jīng)歷長(zhǎng)距離的反復(fù)懸浮-再沉積傳輸，可導(dǎo)致活性鐵氧化物的逐漸老化和結(jié)晶，降低鐵氧化物表面吸附容量；該過(guò)程也導(dǎo)致鐵氧化物表面的有機(jī)質(zhì)逐步氧化[9]，以上因素可能是fFe-OC較小的原因。此外，南黃海沉積物中較低的活性鐵含量可能也是fFe-OC較小的原因之一。有研究表明，作為南黃海沉積物的重要物源，黃河懸浮顆粒物的高活性鐵與總鐵比值(FeHR/FeT= 0.27)明顯小于長(zhǎng)江懸浮物比值(0.38)以及全球河流懸浮物平均值(0.43±0.03)[33]。

圖5 不同區(qū)域沉積物和土壤中fFe-OC的比較(誤差棒為1倍方差)Fig.5 Comparison of fFe-OC for sediments and soils of vari-ous regions(the error bar is one standard deviation)FS：森林土壤；MS：全球海洋沉積物；WLD：Wax Lake三角洲沉積物；QTP: 青藏高原永凍土；SYS：南黃海沉積物；EAS：歐亞北極陸架沉積物FS: forest soils; MS: global marine sediments; WLD: Wax Lake Delta sediments; QTP: Qinghai-Tibetan Plateau permafrost; SYS: South Yellow Sea sediments; EAS: Eurasian Arctic Shelf sediments

4.2 Fe-OC的表面吸附與共沉淀機(jī)制

OC與Fe的相互作用有表面吸附和共沉淀兩種機(jī)制，OC∶Fe摩爾比小于或等于1表明OC主要吸附于鐵氧化物表面，而OC∶Fe在6～10之間則表明OC與Fe主要以共沉淀形式存在[9,34]。本研究中OC∶Fe平均值為4.50±2.61，與全球水體富氧陸架海沉積物平均值4.0±2.8相近[9]。該比值遠(yuǎn)超過(guò)FeR對(duì)OC的最大表面吸附容量[33]，這表明，除了表面吸附，OC與FeR共沉淀也是重要結(jié)合方式。在氧化還原界面上Fe(Ⅱ)被氧化沉淀為Fe(Ⅲ)氧化物后可吸附溶解有機(jī)質(zhì)，在氧化還原震蕩條件下易形成“洋蔥”結(jié)構(gòu)的Fe-OC共沉淀復(fù)合體[35—36]。南黃海OC與FeR的共沉淀與該海域強(qiáng)烈的再懸浮作用密切相關(guān)[30]。該海域水深較淺，強(qiáng)烈的潮汐流、低能量沉積區(qū)和高能量沉積區(qū)之間的物質(zhì)交換以及冬季季風(fēng)誘發(fā)的強(qiáng)烈底部剪應(yīng)力等使沉積物易發(fā)生反復(fù)的再懸浮[30,32]，有利于Fe(Ⅱ)的氧化以及Fe(Ⅲ)-OC共沉淀[35—36]。另外，南黃海較高豐度大型底棲生物活動(dòng)引起的生物擾動(dòng)和生物灌溉可使沉積物-水接觸面增加50%～400%[37—38]，也有利于OC與FeR共沉淀。

4.3 δ13CFe-OC及C/N對(duì)Fe-OC組成的限定

多數(shù)樣品(65%)的Fe-OC比對(duì)應(yīng)的非鐵結(jié)合態(tài)OC相對(duì)富集13C(即δ13C更大)(圖6a)，前者δ13C平均值比后者“重”(0.62±3.5)‰。該沉積物中鐵結(jié)合態(tài)有機(jī)質(zhì)(Fe-OM)的C/N摩爾比總體上小于非鐵結(jié)合態(tài)有機(jī)質(zhì)的C/N(平均小2.90±1.61)(P< 0.05)(圖6b)，這表明前者相對(duì)富集N。富δ13C及富N的天然有機(jī)質(zhì)包括氨基酸、蛋白質(zhì)、碳水化合物等活性組分[39]。本研究區(qū)δ13CFe-OC和C/N均表明FeR傾向于吸附活性有機(jī)質(zhì)，從而有利于這些活性組分的長(zhǎng)期保存。這與其他海洋沉積物以及土壤中FeR對(duì)OC的吸附傾向性一致[9,13]。南黃海沉積物中較高的陸源惰性有機(jī)質(zhì)含量[27—28]以及FeR對(duì)活性有機(jī)質(zhì)的吸附性保存可能是該沉積物中有機(jī)質(zhì)總體降解活性較低(即隨深度變化小)的重要原因。

圖6 鐵結(jié)合態(tài)與非鐵結(jié)合態(tài)有機(jī)質(zhì)之間的碳同位素組成(δ13C)及C/N摩爾比的對(duì)比Fig.6 Comparison of carbon isotopic compositions (δ13C) and molar ratios of C/N between iron-bound and non-iron-bound organic matter

圖7 鐵結(jié)合態(tài)有機(jī)質(zhì)的C/N與OC:Fe摩爾比關(guān)系Fig.7 Molar ratio relationship of C/N versus OC∶Fe for Fe-bound organic matter

A01、A03和A07三站點(diǎn)鐵結(jié)合態(tài)有機(jī)質(zhì)的C/N與OC∶Fe之間有良好正相關(guān)性(圖7)。這表明隨共沉淀比例的增大(即OC∶Fe增大)，N的相對(duì)含量減小，即對(duì)富N活性有機(jī)質(zhì)吸附傾向性減弱。最近的實(shí)驗(yàn)也發(fā)現(xiàn)，共沉淀對(duì)OC的吸附選擇性明顯弱于表面吸附過(guò)程[11,36]。因此，F(xiàn)eR與OC結(jié)合方式的改變是OC吸附傾向性的重要影響因素。

4.4fFe-OC、OC∶Fe及δ13CFe-OC的影響因素

理想條件下，隨海洋沉積物深度增加，從氧化環(huán)境變?yōu)檫€原環(huán)境。在還原環(huán)境下FeR的還原溶解將導(dǎo)致Fe-OC釋放，削弱FeR對(duì)OC的穩(wěn)定作用[16]。南黃海三站點(diǎn)fFe-OC隨深度的增加未出現(xiàn)顯著減小(A01表層除外)(圖3b)，這可能與沉積物中有機(jī)質(zhì)活性低、鐵還原作用較弱有關(guān)。研究表明，陸源惰性有機(jī)質(zhì)廣泛分布于南黃海沉積物中，其中，陸源老化土壤OC及化石OC占TOC的37%～44%[27—28]；盡管海源有機(jī)質(zhì)占沉積物TOC的比例超過(guò)50%[26]，但總體上較低的TOC含量(0.39%～1.06%)可能無(wú)法導(dǎo)致大規(guī)模的鐵還原，因此深度的增加未導(dǎo)致fFe-OC明顯減小。三站點(diǎn)TOC隨深度增加變化不明顯也反映了有機(jī)質(zhì)的低活性(圖2a)。此外，研究區(qū)三站點(diǎn)OC-Fe共沉淀是OC的重要結(jié)合方式，這也可能是fFe-OC隨深度無(wú)明顯減小的另一原因，因?yàn)榕c表面吸附相比，F(xiàn)e-OC共沉淀復(fù)合體的穩(wěn)定性受還原環(huán)境影響較小[17]。

盡管三站點(diǎn)的fFe-OC變化范圍較大，就其平均值而言，fFe-OC最高的A01站點(diǎn)，其沉積速率和FeR含量也最高。與A01站點(diǎn)相比，A03和A07兩站點(diǎn)的沉積速率和FeR含量都較低，但兩者間的差異較小，其fFe-OC差異也不明顯。這表明沉積速率和FeR含量的提高有利于FeR對(duì)OC的吸附性保存。

δ13CFe-OC主要受氧化還原環(huán)境和物源輸入的影響。一方面，鐵還原溶解及有機(jī)質(zhì)選擇性釋放可導(dǎo)致13CFe-OC相對(duì)“富集”[15]；另一方面，以陸源輸入為主的沉積物中FeR優(yōu)先結(jié)合δ13C相對(duì)虧損的木質(zhì)素組分，而在海洋有機(jī)質(zhì)較多的沉積物中δ13CFe-OC則更具有海洋特征(即13C相對(duì)富集)[10—11]。南黃海3個(gè)站點(diǎn)δ13CFe-OC隨深度變化幅度很小(圖4)，表明較弱的氧化還原過(guò)程對(duì)δ13CFe-OC無(wú)明顯影響。A01、A03、A07三站點(diǎn)δ13CFe-OC平均值依次增大，分別為(-24.1±3.1)‰、(-21.8±2.8)‰、(-20.1±2.7)‰，表明Fe-OC中海洋有機(jī)質(zhì)逐漸增多。這與該研究區(qū)OC物源空間分布趨勢(shì)一致，即南黃海西北和東北部陸源輸入較多，而東南方向海洋有機(jī)質(zhì)含量增加[40]。

上述有機(jī)質(zhì)來(lái)源及活性的差異也可能是A01站點(diǎn)OC∶Fe平均值小于A03和A07兩站點(diǎn)的原因之一(圖3a)。A01 站點(diǎn)較高的陸源惰性有機(jī)質(zhì)導(dǎo)致鐵的氧化還原循環(huán)較弱，除了OC與FeR共沉淀，OC在FeR的表面吸附也起較重要作用；也正因?yàn)檩^弱的鐵還原，使得該站點(diǎn)OC∶Fe隨深度無(wú)明顯變化。A03和A07兩站點(diǎn)海洋活性有機(jī)質(zhì)相對(duì)較高，有利于鐵的氧化還原循環(huán)以及OC-Fe共沉淀，從而導(dǎo)致OC∶Fe較高。由于鐵還原主要釋放表面吸附的OC，對(duì)共沉淀OC的影響較小[17]，這一因素可能導(dǎo)致了這兩站點(diǎn)深部OC∶Fe較高。

5 結(jié)論

本研究表明，南黃海沉積物中的OC(13.2±7.47)%直接與FeR結(jié)合，即FeR對(duì)OC的年吸附量為0.72 Mt，占全球陸架海OC年埋藏通量的0.44%。與已有研究相比，該海域沉積物中FeR對(duì)OC的保存作用較低。南黃海沉積物中OC∶Fe平均值為4.50±2.61，表明共沉淀作用是OC與FeR結(jié)合的重要方式。相對(duì)于非鐵結(jié)合態(tài)有機(jī)質(zhì)，F(xiàn)eR結(jié)合的有機(jī)質(zhì)相對(duì)富集13C和N，表明FeR選擇性結(jié)合活性有機(jī)質(zhì)，但這種選擇性隨OC∶Fe增大而減弱。

總體而言，南黃海海域沉積物惰性有機(jī)質(zhì)含量較高、鐵還原較弱，導(dǎo)致fFe-OC和δ13CFe-OC隨深度的增加未呈現(xiàn)顯著減小。在海源有機(jī)質(zhì)含量較高的站點(diǎn)(A03，A07)，因鐵氧化還原循環(huán)較活躍，有利于OC∶Fe(6.0±2.9，5.1±1.8)較高的共沉淀復(fù)合體形成，也促進(jìn)了更多海洋活性有機(jī)質(zhì)的保存；在陸源惰性有機(jī)質(zhì)較多的站點(diǎn)(A01)，較弱的鐵氧化還原作用不利于FeR與OC共沉淀，OC∶Fe(2.4±1.3)較小，且無(wú)明顯深度變化。沉積速率和FeR含量的提高有利于FeR對(duì)OC的吸附性保存。

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Organic carbon preservation by reactive iron oxides in South Yellow Sea sediments

Tao Jing1, Ma Weiwei1, Li Wenjun1, Li Tie1, Zhu Maoxu1

(1.CollegeofChemistryandChemicalEngineering,OceanUniversityofChina,Qingdao266100,China)

Sorption of organic carbon (OC) on reactive iron (FeR) plays an important role in OC stabilization and preserving in sediments and soils, and thus can buffer the concentration of atmospheric CO2on geological timescales. Based on the amount of OC associated with FeR (Fe-OC) in three cores from the South Yellow Sea determined by the dithionite reduction extraction, we quantitatively investigated the role of FeR in OC stabilization, mechanisms of OC and FeR association, and variation of Fe-OC with depth. Our results showed that Fe-OC accounted for (13.2±7.47)% of sedimentary total OC in the South Yellow Sea. This means that annually 0.72 Mt of OC buried in the sediments is sequestered by FeR, which is 0.44% of the global OC buried in the continental shelf sea annually. Molar ratios of OC to FeR (average 4.50±2.61) indicate that coprecipitation of OC with FeR plays an important role in OC stabilization, and the ratios increased with an increase in fractions of marine OC in the sediments. Stable isotopic compositions of Fe-OC (δ13CFe-OC) suggested that more labile OC is preferentially trapped by FeR, but this preferential trend decreases with an increase in OC/Fe ratio. No obvious changes infFe-OCand δ13CFe-OCwith depth were observed, which can be ascribed to low degradability of organic matter and consequently weak iron reduction.

reactive iron oxides; organic carbon protection; marine sediments; sorption; South Yellow Sea

10.3969/j.issn.0253-4193.2017.08.002

2016-12-07；

2017-02-22。

國(guó)家自然科學(xué)基金(41576078)；山東省自然科學(xué)基金(ZR2015DM006)；國(guó)家重點(diǎn)研發(fā)計(jì)劃項(xiàng)目(2016YFA0601301)。

陶婧(1992—)，女，河南省駐馬店市人，從事海洋化學(xué)和環(huán)境分析化學(xué)研究。E-mail: m15954098032@163.com

*通信作者：朱茂旭(1967—)，男，教授，博士生導(dǎo)師，從事海洋化學(xué)研究。E-mail: zhumaoxu@ouc.edu.cn

P736.21

A

0253-4193(2017)08-0016-09

陶婧，馬偉偉，李文君，等. 南黃海沉積物中活性鐵氧化物對(duì)有機(jī)碳的保存作用[J].海洋學(xué)報(bào)，2017，39(8)：16—24,

Tao Jing, Ma Weiwei, Li Wenjun，et al. Organic carbon preservation by reactive iron oxides in South Yellow Sea sediments[J]. Haiyang Xuebao，2017，39(8)：16—24, doi:10.3969/j.issn.0253-4193.2017.08.002