南黃海沉積物中活性鐵氧化物對有機碳的保存作用

陶婧，馬偉偉，李文君，李鐵，朱茂旭*

(1．中國海洋大學 化學化工學院，山東 青島 266100)

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南黃海沉積物中活性鐵氧化物對有機碳的保存作用

陶婧1，馬偉偉1，李文君1，李鐵1，朱茂旭1*

(1．中國海洋大學 化學化工學院，山東 青島 266100)

土壤和沉積物中活性鐵對有機質的吸附對有機質具有長期穩定和保存作用，從而在地質時間尺度上緩沖大氣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)及與之結合的有機碳(Fe-OC)，定量研究了南黃海沉積物中FeR與OC之間的結合方式以及FeR對OC的保存作用，討論了深度增加對二者相互作用的影響。結果表明，南黃海沉積物中Fe-OC占沉積物總有機碳的份數(fFe-OC)為(13.2±7.47)%，即活性鐵對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，表明共沉淀作用對有機質的保存起重要作用，且其比值隨海源有機質含量增加而增加。Fe-OC穩定碳同位素(δ13CFe-OC)結果表明，FeR優先保存活性有機質，但這種選擇性隨OC∶Fe增大而減弱。隨深度增加，fFe-OC和δ13CFe-OC均未表現出顯著變化，這與該海域沉積物中有機質活性較低、鐵還原作用較弱有關。

活性鐵氧化物；有機碳保護；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fFe-OC范圍為2.4%～16.7%，平均值(11.6±5.2)%，上部5 cm范圍內其值波動明顯，但深部較穩定；A07站點fFe-OC范圍為4.0%～19.4%，平均值(11.9±4.9)%，其波動較其他兩站位小。盡管A03和A07兩站點的Fe-OC含量差異明顯，但其對應的fFe-OC卻無明顯差異(P< 0.05)。

A01、A03、A07三站點的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站點的OC∶Fe明顯小于其他兩站點(P<0.05)?？傮w而言，A03和A07兩站點深部OC∶Fe高于淺部，而A01站點無明顯深度變化(P< 0.1)。與文獻數據相比，三站點OC∶Fe變化幅度較小[10,13]。

A01、A03、A07三站點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δ13CFe-OC平均值高于對應的非鐵結合態OC的δ13C平均值(P< 0.1)，表明前者13C 相對“富集”，而A01站點則相反，表明13CFe-OC相對虧損。三站點多數樣品(65%)的δ13CFe-OC高于對應的非鐵結合態OC的δ13C，總體上表明13CFe-OC相對“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FeR對OC的穩定作用處于較低水平。根據本研究得到的fFe-OC平均值(13.2±7.47)%以及近100年來南黃海TOC埋藏通量(5.48 Mt/a)[27]，可估算出該海域沉積物FeR對OC的年吸附量為0.72 Mt。南黃海占全球邊緣海總面積的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OC的保存作用較低。

圖4 鐵結合態和非鐵結合態有機質的碳同位素組成(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、海洋環流以及氣旋渦流共同作用下，沉積物經歷長距離輸送以及長時間氧氣暴露有關[28,30]。在沿岸流作用下，從渤海進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沉積，另一部分繞過成山角并入南黃海循環[31]。古黃河口侵蝕沉積物通過黃海海岸流向西南方向傳輸、經黃海暖流向北傳輸，通過再懸浮并入南黃海沉積系統[32]。以上復雜的水動力學條件導致表層沉積物經歷長距離的反復懸浮-再沉積傳輸，可導致活性鐵氧化物的逐漸老化和結晶，降低鐵氧化物表面吸附容量；該過程也導致鐵氧化物表面的有機質逐步氧化[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制

OC與Fe的相互作用有表面吸附和共沉淀兩種機制，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]。該比值遠超過FeR對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質，在氧化還原震蕩條件下易形成“洋蔥”結構的Fe-OC共沉淀復合體[35—36]。南黃海OC與FeR的共沉淀與該海域強烈的再懸浮作用密切相關[30]。該海域水深較淺，強烈的潮汐流、低能量沉積區和高能量沉積區之間的物質交換以及冬季季風誘發的強烈底部剪應力等使沉積物易發生反復的再懸浮[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引起的生物擾動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Fe-OC組成的限定

多數樣品(65%)的Fe-OC比對應的非鐵結合態OC相對富集13C(即δ13C更大)(圖6a)，前者δ13C平均值比后者“重”(0.62±3.5)‰。該沉積物中鐵結合態有機質(Fe-OM)的C/N摩爾比總體上小于非鐵結合態有機質的C/N(平均小2.90±1.61)(P< 0.05)(圖6b)，這表明前者相對富集N。富δ13C及富N的天然有機質包括氨基酸、蛋白質、碳水化合物等活性組分[39]。本研究區δ13CFe-OC和C/N均表明FeR傾向于吸附活性有機質，從而有利于這些活性組分的長期保存。這與其他海洋沉積物以及土壤中FeR對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質含量[27—28]以及FeR對活性有機質的吸附性保存可能是該沉積物中有機質總體降解活性較低(即隨深度變化小)的重要原因。

圖6 鐵結合態與非鐵結合態有機質之間的碳同位素組成(δ13C)及C/N摩爾比的對比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 鐵結合態有機質的C/N與OC:Fe摩爾比關系Fig.7 Molar ratio relationship of C/N versus OC∶Fe for Fe-bound organic matter

A01、A03和A07三站點鐵結合態有機質的C/N與OC∶Fe之間有良好正相關性(圖7)。這表明隨共沉淀比例的增大(即OC∶Fe增大)，N的相對含量減小，即對富N活性有機質吸附傾向性減弱。最近的實驗也發現，共沉淀對OC的吸附選擇性明顯弱于表面吸附過程[11,36]。因此，FeR與OC結合方式的改變是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，從氧化環境變為還原環境。在還原環境下FeR的還原溶解將導致Fe-OC釋放，削弱FeR對OC的穩定作用[16]。南黃海三站點fFe-OC隨深度的增加未出現顯著減小(A01表層除外)(圖3b)，這可能與沉積物中有機質活性低、鐵還原作用較弱有關。研究表明，陸源惰性有機質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]；盡管海源有機質占沉積物TOC的比例超過50%[26]，但總體上較低的TOC含量(0.39%～1.06%)可能無法導致大規模的鐵還原，因此深度的增加未導致fFe-OC明顯減小。三站點TOC隨深度增加變化不明顯也反映了有機質的低活性(圖2a)。此外，研究區三站點OC-Fe共沉淀是OC的重要結合方式，這也可能是fFe-OC隨深度無明顯減小的另一原因，因為與表面吸附相比，Fe-OC共沉淀復合體的穩定性受還原環境影響較小[17]。

盡管三站點的fFe-OC變化范圍較大，就其平均值而言，fFe-OC最高的A01站點，其沉積速率和FeR含量也最高。與A01站點相比，A03和A07兩站點的沉積速率和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OC的吸附性保存。

δ13CFe-OC主要受氧化還原環境和物源輸入的影響。一方面，鐵還原溶解及有機質選擇性釋放可導致13CFe-OC相對“富集”[15]；另一方面，以陸源輸入為主的沉積物中FeR優先結合δ13C相對虧損的木質素組分，而在海洋有機質較多的沉積物中δ13CFe-OC則更具有海洋特征(即13C相對富集)[10—11]。南黃海3個站點δ13CFe-OC隨深度變化幅度很小(圖4)，表明較弱的氧化還原過程對δ13CFe-OC無明顯影響。A01、A03、A07三站點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中海洋有機質逐漸增多。這與該研究區OC物源空間分布趨勢一致，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質含量增加[40]。

上述有機質來源及活性的差異也可能是A01站點OC∶Fe平均值小于A03和A07兩站點的原因之一(圖3a)。A01 站點較高的陸源惰性有機質導致鐵的氧化還原循環較弱，除了OC與FeR共沉淀，OC在FeR的表面吸附也起較重要作用；也正因為較弱的鐵還原，使得該站點OC∶Fe隨深度無明顯變化。A03和A07兩站點海洋活性有機質相對較高，有利于鐵的氧化還原循環以及OC-Fe共沉淀，從而導致OC∶Fe較高。由于鐵還原主要釋放表面吸附的OC，對共沉淀OC的影響較小[17]，這一因素可能導致了這兩站點深部OC∶Fe較高。

5 結論

本研究表明，南黃海沉積物中的OC(13.2±7.47)%直接與FeR結合，即FeR對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結合的重要方式。相對于非鐵結合態有機質，FeR結合的有機質相對富集13C和N，表明FeR選擇性結合活性有機質，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質含量較高、鐵還原較弱，導致fFe-OC和δ13CFe-OC隨深度的增加未呈現顯著減小。在海源有機質含量較高的站點(A03，A07)，因鐵氧化還原循環較活躍，有利于OC∶Fe(6.0±2.9，5.1±1.8)較高的共沉淀復合體形成，也促進了更多海洋活性有機質的保存；在陸源惰性有機質較多的站點(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，且無明顯深度變化。沉積速率和FeR含量的提高有利于FeR對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。

國家自然科學基金(41576078)；山東省自然科學基金(ZR2015DM006)；國家重點研發計劃項目(2016YFA0601301)。

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

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

P736.21

A

0253-4193(2017)08-0016-09

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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