地球科学进展, 2020, 35(10): 1073-1086 DOI: 10.11867/j.issn.1001-8166.2020.086

水生关键带有机碳循环过程:从分子水平到全球尺度

北极东西伯利亚陆架沉积有机碳的源汇过程研究进展

胡利民,1,2,3, 石学法2,3, 叶君2,3, 张钰莹2

1.中国海洋大学海洋地球科学学院 海底科学与探测技术教育部重点实验室,山东 青岛 266100

2.自然资源部第一海洋研究所 海洋沉积与成矿作用重点实验室,山东 青岛 266061

3.青岛 海洋科学与技术试点国家实验室 海洋地质过程与环境功能实验室,山东 青岛 266237

Advances in the Sources and Sink of Sedimentary Organic Carbon in the East Siberian Arctic Shelf

Hu Limin,1,2,3, Shi Xuefa2,3, Ye Jun2,3, Zhang Yuying2

1.College of Marine Geosciences,Key Laboratory of Submarine Geosciences and Prospecting Technology,Ocean University of China,Qingdao 266100,China

2.Key Laboratory of Marine Geology and Metallogeny,First Institute of Oceanography,Ministry of Natural Resources,Qingdao 266061,China

3.Laboratory for Marine Geology,Qingdao National Laboratory for Marine Science and Technology,Qingdao 266237,China

收稿日期: 2020-09-01   修回日期: 2020-10-04   网络出版日期: 2020-11-30

基金资助: 国家自然科学基金优秀青年科学基金项目“海洋沉积地球化学:沉积有机质的源汇过程及其环境响应”.  41722603
国家自然科学基金面上项目“近百年亚欧北极陆架沉积有机碳源汇差异性演化:海冰变化与冻土碳输入的制约”.  42076074

Received: 2020-09-01   Revised: 2020-10-04   Online: 2020-11-30

作者简介 About authors

胡利民(1983-),男,山东聊城人,教授,主要从事海洋沉积地球化学与有机地球化学研究.E-mail:hulimin@ouc.edu.cn

HuLimin(1983-),male,LiaochengCity,ShandongProvince,Professor.Researchareasincludemarinegeochemistryandorganicgeochemistry.E-mail:hulimin@ouc.edu.cn

摘要

东西伯利亚陆架作为全球最为宽浅的陆架之一,在全球变暖和北极快速变化背景下,受海冰减少、冻土退化、径流增加和海岸侵蚀加剧等因素的影响,该区沉积有机碳的来源、输运和埋藏发生着显著变化,且不同地区之间差异显著。东西伯利亚海西部和拉普捷夫海沉积有机碳以陆源贡献为主,海岸侵蚀作用提高了冻土碳的入海通量,对气候变化具正反馈效应;楚科奇海具有较高的有机碳埋藏效率,季节性海冰变化对有机碳的源汇有直接的影响。受沉积水动力作用影响,陆源沉积有机碳从勒拿河河口输运到陆架边缘需3 000~4 000年,不同类型有机碳存在显著的分异和降解。陆架有机碳埋藏具有显著的时空差异,大量高活性的冻土碳由陆向海的快速沉积对于北极土壤碳库的稳定性、水生环境有机碳的矿化及CO2的排放等方面具有重要的意义。今后该区的研究应加强综合地球化学指标和典型有机分子碳同位素等手段的应用,开展区域对比研究,重视海冰过程与有机碳源汇的联系,结合区域碳循环多模型集成,从现代过程与地质记录、替代指标与数值模拟相结合的角度去认识不同时间尺度快速气候变化下的有机碳源汇格局及其气候环境效应。

关键词: 沉积有机碳源汇 ; 海冰 ; 冻土碳 ; 北极东西伯利亚陆架 ; 气候变化

Abstract

The East Siberian Arctic Shelf (ESAS) is one of the widest and shallowest continental shelves in the world. In the context of the global warming and rapid Arctic changes, the sources, transport and burial of sedimentary Organic Carbon (OC) in this area have experienced significant changes with spatial heterogeneity, which could be related to the sea-ice reduction, permafrost degradation, increased runoff and intensified coastal erosion. The sedimentary OC is mainly contributed by Terrestrial Organic Carbon (TerrOC) in the western East Siberian Sea and the Laptev Sea, and the coastal erosion increases the flux of Permafrost Carbon (PF/C) with a positive climate feedback effect. The Chukchi Sea has high organic carbon burial efficiency, where the seasonal variation of sea ice has direct effect on the source and sink of OC. Under the influence of hydrodynamic sorting, the cross-shelf transport times of TerrOC from the Lena estuary to the shelf edge requires approximately 3 000~4 000 years, by coupling with a significant geochemical differentiation and degradation. There existed spatio-temporal variation for the OC burial on the ESAS, and the large amount and rapid deposition of highly-reactive PF/C from the land to the sea could have important significance for the Arctic soil carbon, the OC mineralization in the aquatic environment, and CO2 outgassing. The following research should strengthen the application of the comprehensive geochemical indices and the compound-specific isotope method, emphasizing the relation between the sea-ice and the sources and sink of the OC. By coupling with the models of regional carbon cycle, we should emphasize the integration of the modern process and geological records, proxy records with the numerical simulation, which is necessary to better understand the sources and sink of sedimentary OC and the climate and environmental effect from the varied timescales.

Keywords: Source and sink of sediment organic carbon ; Sea ice ; Permafrost carbon ; The East Siberian Arctic Shelf ; Climate change

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本文引用格式

胡利民, 石学法, 叶君, 张钰莹. 北极东西伯利亚陆架沉积有机碳的源汇过程研究进展. 地球科学进展[J], 2020, 35(10): 1073-1086 DOI:10.11867/j.issn.1001-8166.2020.086

Hu Limin, Shi Xuefa, Ye Jun, Zhang Yuying. Advances in the Sources and Sink of Sedimentary Organic Carbon in the East Siberian Arctic Shelf. Advances in Earth Science[J], 2020, 35(10): 1073-1086 DOI:10.11867/j.issn.1001-8166.2020.086

1 引 言

作为有机碳的主要碳库之一,海洋沉积有机碳在全球碳循环和气候变化中扮演着重要角色1~3。占全球海洋面积8%的陆架—边缘海埋藏了绝大部分有机碳,其中仅河口陆架地区就贡献了80%以上4~6。因此,研究陆架—边缘海地区特别是大河河口—陆架沉积有机碳的源汇过程对认识全球碳循环、气候变化和陆海相互作用具有重要意义78

北极陆架的有机碳埋藏量占全球海洋的11%9,该区在碳的生物地球化学循环和全球变化中的作用一直受到密切关注10~17图1)。北极东西伯利亚陆架是全球最为宽浅的陆架,主要包括楚科奇海、东西伯利亚海和拉普捷夫海(图2);该区大部分属于俄罗斯北极陆架,地域广阔,区域环境差异显著。与中低纬大河三角洲及其毗邻陆架不同,这里不仅有世界级大河的输入(如勒拿河),还发育着广袤的冻土和季节性海冰,使得该区沉积有机碳的源汇过程独具特色(图1318,主要体现在:海源有机碳输出与季节性海冰过程关系密切19北极冻土碳(Permafrost Carbon, PF/C)占全球土壤有机碳的50%20;不同类型陆源有机碳的输入不仅受径流影响,且流域/海岸侵蚀排放(冻土老碳为主)也有重要贡献2122此外,该区沉积有机碳的迁移和沉积还直接受控于海冰搬运作用2324

图1

图1   北极陆架沉积有机碳输入示意图(据参考文献[15,17]修改)

Fig.1   Schematic illustration showing the input of sedimentary organic carbon on the Arctic shelf (modified from references [15,17])


图2

图2   北极东西伯利亚陆架区域地理位置

Fig.2   The geographic map of the location of the East Siberian Arctic Shelf (ESAS)


在全球变暖背景下,近几十年北极快速气候变化主要表现为夏季海冰覆盖面积减退、海水温度升高、陆地径流增大,以及冻土退化和初级生产力提高等25~28。东西伯利亚陆架是目前北极海冰消融、冻土退化及周边植被变化最显著的地区29,具有不同的流域—气候背景、大河输入、较高的初级生产力以及冻土沉积广泛发育等特点,因而是开展北极快速变化与沉积有机碳源汇过程及其环境响应研究的天然实验室。目前,国际上围绕该区碳的生物地球化学过程与气候环境效应已开展了较多研究30~35。本文通过对东西伯利亚陆架沉积有机碳的来源、输运和埋藏记录等国内外已有研究成果进行梳理和总结,旨在对北极快速变化下该区沉积有机碳源汇过程及其环境响应等方面有更为系统的了解,提高对气候变暖下高纬度与极地近海碳的生物地球化学过程的影响及其演化规律的认识和理解。

2 沉积有机碳的来源及其影响因素

2.1 有机碳来源

北极陆架沉积有机碳往往是多种物源信号的复杂“集合体”18。为了解和评估北冰洋边缘海沉积有机碳库的源汇格局,需对海源/陆源不同端元的有机碳贡献加以区分(如浮游植物及冰藻、土壤、冻土老碳/化石源碳等)。北冰洋边缘海的陆源有机碳主要来源于河流输入和海岸侵蚀9,随着气候变暖,冻土和海冰对有机碳输入的贡献也不可忽视;海源有机碳主要来源于海冰生物及“生物泵”作用1421图1)。北极陆架不同地区环境差异明显,大河输入、营养盐分布、冻土退化及海冰覆盖的不同都影响着沉积有机碳的来源和分布1314。东西伯利亚陆架不同地区沉积有机碳的来源具有显著的区域性差异。研究表明,在大河输入和沿岸侵蚀作用的影响下,拉普捷夫海和东西伯利亚海西部地区沉积有机碳以陆源输入为主3637;而受太平洋入流水的影响,东西伯利亚海的东部地区和楚科奇海则以海源为主3738。利用端元模型进一步解析(图3),发现在近岸以及勒拿河三角洲北部具有相对较高的陆源有机碳输入,随着离岸距离加大,陆源有机碳贡献变低,外海的沉积有机碳受海源影响较大28。楚科奇海的沉积有机碳来源主要包括苔藓、草本裸子植被、浮游植物和冰藻等输入贡献,有机碳载荷从陆架到陆坡有明显降低趋势39

图3

图3   北极东西伯利亚陆架沉积有机碳的来源及空间分布(据参考文献[21]修改,端元值来源于参考文献[21],数据来源于参考文献[21,31,34,35])

(a)沉积有机碳来源的端元分析;(b)不同来源沉积有机碳的空间分布

Fig.3   Sources and spatial distribution of sedimentary organic carbon in the ESAS modified after reference21],end-member values cited from reference21],data derived from references21313435])

(a)End-member analysis of sedimentary organic carbon sources; (b)The spatial distribution of different sedimentary organic carbon


为进一步评估北极陆架沉积有机碳的物源特征,前人运用碳同位素手段(13C和14C),将陆源有机碳的来源分为土壤源、yedoma冻土(指更新世以来形成的富冰和富含有机质的黄土沉积物)和化石源等不同端元贡献(图3),并从分子同位素水平追溯不同来源有机碳的地球化学特征224041。通过对东西伯利亚陆架周边河口地区(如鄂毕河、叶尼塞河、勒拿河和因迪吉尔卡河)有机碳的来源进行对比研究,发现河岸/海岸侵蚀、冻土融化过程中释放出的难降解老碳影响着有机碳的来源及新鲜程度;自西到东有机质的年龄逐渐变小42。近年来,也有学者利用总有机碳和单体碳同位素等手段对不同类型的陆源沉积有机质进行了详细分析,通过区分土壤有机碳和冻土老碳(主要来自沿岸侵蚀)的贡献,发现侵蚀输入的有机碳还可受水动力影响发生选择性降解和保存212240

2.2 海冰对有机碳输入的影响

北冰洋的显著特征是海冰的季节性和年际变化,例如东西伯利亚海全年的海冰变化过程呈现出5个阶段,海冰融化时间持续约3个月,冻结时间仅为1个半月;海冰的季节性变化对有机碳源汇格局有重要的影响43~47。在全球变暖背景下,海源有机碳的输入与海冰相关的“生物泵”过程密切相关4849;同时海冰对陆源有机碳的输入也有影响24。因此,评估北极快速变化下的沉积有机碳源汇过程,首先需确定海冰对沉积有机碳来源的影响,主要涉及:海源有机质的产生受海冰、光照等环境因素制约50海冰的减少影响浮游植物及微生物的种群群落,进而影响着海源有机质的组成1451沉积物通过海冰可输运到夏季海冰边缘,影响沉积有机质的组成分布52

已有研究指出,在北极快速变化背景下,气候变暖引起的海冰消退可导致光照时间变长、营养盐释放以及陆源输入的增加,显著促进“生物泵”运转,并影响浮游植物生产力和群落结构485153。研究显示,受海冰季节性变化的影响,与夏季海冰消融相关的初级生产量可高达60%以上2754;海冰较早融化带入的营养物质可引起包括冰藻在内的浮游植物勃发,改变颗粒物和有机碳的输出模式和通量2749;同时,海冰消退还会引起北极冰藻等浮游植物的消失和亚北极浮游植物入侵,改变初级生产力结构,进而影响有机碳来源51。作为全球高生产力的海区,楚科奇海具有较高的有机碳埋藏保存效率,同时也是北极海冰变化最为显著的地区之一2955。研究表明,该区的沉积物质通量和有机碳来源与海冰的季节性变化具有密切的关系,在海冰消退期,沉积颗粒物质的通量可增加1个数量级49。因此,海冰的季节性和年际变化使得该区的有机碳循环正发生着显著变化5657

由于缺乏长时间序列的海冰观测资料,限制了从较长时间尺度建立海冰与有机碳埋藏的联系,因此需利用一些能反演海冰过程的特定指标进行研究。近年来,一种新的海冰硅藻指示物——C25支链烯烃(IP25)被提出可用于北极海冰的重建58。IP25主要产生于春季北极及周边地区海冰硅藻勃发期,并能在海冰融化后保存在沉积物中59。前人利用该指标示踪了东西伯利亚陆架地区沉积有机碳的来源及近年来的海冰状况与生产力变异4960。受淡水输入影响,在河口和沿海地区陆源硅藻贡献较高;而在拉普捷夫海和喀拉海的季节性无冰区,由于海冰融化,光限制减弱,冰藻等浮游植物大量繁殖,引起初级生产力和海源有机碳发生变化60。另一方面,海冰变化也影响着陆源有机碳的输入。近岸陆源物质可随海冰被搬运到陆架和北冰洋深水区9,海冰覆盖面积的减小也会影响沉积有机碳的长距离输运2324

2.3 冻土碳输入

全球约50%的土壤有机碳埋藏在北极冻土中,其中约407 Pg的有机碳都储存于冻土与海冰混合物中,这类富冰冻土碳与一般的矿化土壤有机碳不同,它比土壤有机碳含量高10~30倍,且更容易被微生物利用61。研究发现,北极沿海多年冻土平均侵蚀速率约为0.5 m/a,最高可达10 m/a62~64,除了这种不稳定的海岸冻土输入外,海底冻土也广泛发育,平均每年约有8 Tg的CH4气体从东西伯利亚海的海底释放到大气中65。随着北极变暖,冻土退化日益加剧,不仅可引起海底冻土CH4等温室气体直接排放,还导致冰期封存的陆源有机碳的加速释放,因容易被微生物利用而快速进入现代碳循环过程,使其成为目前北极碳循环与气候变化关注的焦点116266。此外,北极陆架周边流域环境差异明显,不同陆源有机碳的输出类型多样,例如河流主要影响表层土壤有机碳的输出,而下层冻土老碳的释放则主要跟冻土发育状态、水文条件和热喀斯特侵蚀作用有关22。在此背景下,通过河流或海岸侵蚀释放的陆源有机碳通量将不断升高,组成也可能发生显著变化22406768。研究表明,自20世纪以来,泛北极地区平均每年通过径流输送约32 Tg的溶解有机碳(Dissolved Organic Carbon,DOC)到水体并最终进入北冰洋69;而俄罗斯北极陆架每年向北冰洋外海输运约12 Tg的颗粒有机碳(Particulate Organic Carbon,POC)(其中含大量的冻土老碳),且呈逐年增长的趋势,这主要跟区域水文循环过程有关2570

在北极快速变化的背景下,东西伯利亚陆架目前接收了来自河流径流和海岸侵蚀的大量陆源物质,尤其是海岸侵蚀贡献了大量冰期时形成的冻土老碳;随着海冰面积的持续减少,加剧的海岸侵蚀可增加近海有机碳的沉积通量21。为综合评估气候变化对多年冻土及有机碳输入的影响,需要考虑涉及冻土冻融过程等多方面因素的碳循环评估模型(如IBIS、STM⁃TEM和LPJ等)71;结合不同的评估研究结果,研究显示环北极海岸侵蚀引起的有机碳释放为5.84~46.54 Tg/a,而东西伯利亚陆架陆源有机碳的埋藏通量为2~44 Tg/a213371。已有研究指出,通过物理—生物地球化学模型(Baltic Sea Mode,BALTSEM),并结合野外实测数据,估算出北极四大河流向东西伯利亚陆架输入的溶解无机碳(Dissolved Inorganic Carbon,DIC)约为8.5 Tg/a,DOC约为8 Tg/a,POC约为1.1 Tg/a,另有来自侵蚀作用的POC约为14 Tg/a,其中约有0.96 Tg/a的陆源有机碳被埋藏在陆架,9.7 Tg/a的陆源有机碳矿化为DIC,剩余的碳以DIC、DOC或POC的形式被输运至更远的地方72。而据碳同位素平衡模型估算,每年因侵蚀/融化而排放输入的冻土有机碳约为44 Tg,约占总沉积有机碳的57%,其中15%~66%或可直接矿化为CO2,剩余部分(约20 Tg)则继续在陆架输运或埋藏2173。另一方面,在百年—千年尺度上,也有研究指出东西伯利亚海约55%的陆源沉积有机碳不易被降解,这可能与化石老碳的输入以及有机碳—矿物的结合保护等因素有关28。结合遥感和现场观测资料,研究显示拉普捷夫海海岸侵蚀输入的有机碳通量约为0.66 Tg/a74,大约15%的陆源冻土碳可被活性铁吸附而保存,在一定程度上或可减缓对气候的正反馈7576。然而,在气候变暖背景下,海岸侵蚀速率的加快将导致更多的冻土碳输入,同时海底多年冻土中CH4气体的排放也将加剧,这些过程对于评估全球碳循环和气候变化具有重要的影响。

3 沉积有机碳的输运与降解

东西伯利亚陆架水动力条件复杂,太平洋入流水、穿极流和沿岸流构成该区主要的环流系统,并受大气、径流和海冰过程的共同影响9。楚科奇海是北冰洋众多边缘海中海洋初级生产力最高的海区55,由于没有北极大河输入,该区的沉积物以海岸侵蚀和洋流的输入为主,并跟季节性海冰过程有关,这与周边海域的沉积物供应明显不同77。东西伯利亚海是世界上面积最大、最宽浅的陆架海,该区80%以上的海底区域发育有海底冻土70;以160°E为界,该区东西两侧的沉积环境和物质来源有明显区别,东侧与楚科奇海相连,初级生产力较高;西侧海岸类型以富冰冻土沉积为主,海岸侵蚀作用强烈,受勒拿河的输入影响显著78。拉普捷夫海受北极大河勒拿河输入的直接影响,是北极的 “海冰工厂”,陆海相互作用非常强烈,流域植被和连续永久冻土的发育对该区入海物质的组成和性质有直接的影响7479。研究显示,北极陆架沉积物的输运受波浪、海流和海冰等水动力作用,其沉积再分配过程导致有机碳的组成发生改变3580。楚科奇海的沉积物输运主要受控于区域环流系统,但冰筏作用对本区沉积物输运的影响也不可忽视81。东西伯利亚海河口区沉积物类型以粉砂和砂质粉砂为主,受波浪和海流作用,近岸沉积物容易发生再悬浮,并可随海冰的破碎传输而被搬运至外陆架区81。海冰一方面可阻隔近岸物质向外海的传输,一方面也对沉积物的分选和跨陆架搬运有影响,并伴随有明显的沉积地球化学分异81~83

国外学者对不同粒级沉积有机碳的组成和输运进行了研究,发现粗颗粒组分优先在近海岸沉降,较细的组分受分选作用影响可进行离岸输运,从而使得富含木质素、降解程度高的陆源有机碳在近岸富集,而脂类陆源有机质则易于被细颗粒物携带向外传输3580。研究显示,海源有机碳在离岸向外海输运的过程中受沉积水动力分选和降解作用的影响更显著;而随着颗粒物的逐渐变细,典型陆源有机质(木质素、角质酸和正构烷烃等)的含量呈指数性下降3479。基于典型单体生物标志物的碳同位素组成(13C和14C)和比表面积等参数模型,研究发现陆源沉积有机碳从勒拿河河口输运至陆架边缘需3 000~4 000年(图432

图4

图4   东西伯利亚陆架陆源沉积有机碳的跨陆架输运时间(据参考文献[32]修改)

Fig.4   Bounding cross-shelf transport time of terrestrial sedimentary organic carbon in the ESAS (modified from reference [32])


已有研究表明,沉积物中有机碳的降解受多种因素影响(如分子结构、吸附基质和生物酶等),在不同区域,沉积物粒度、有机碳来源以及沉积氧化还原环境等因素都会进一步影响有机碳的降解313684~86。通过区域对比,发现拉普捷夫海的陆源沉积有机碳的降解速率常数[(2.4±0.6) ka-1]高于东西伯利亚海[(1.5±0.2) ka-1],这可能是由于前者的有机碳主要来源于勒拿河的输入,而海岸侵蚀作用输入的老碳对东西伯利亚海贡献更大2832。对东西伯利亚陆架不同密度、粒级沉积有机碳的输运降解过程进行研究,发现低密度/富蜡质的颗粒碳相对于高密度/富木质素颗粒碳的降解程度更低,上层土壤碳的降解损失(90%±3.5%)较下层的yedoma(60%±11%)也更明显,这说明不同类型的陆源沉积有机碳存在不同的分配/输运降解过程35;与海源有机碳和土壤有机碳相比,冻土碳相对不易降解,更易在近岸沉积34。利用微生物培养方法和生物标志物指标,对拉普捷夫海和东西伯利亚海的沉积有机碳的降解机制进行了对比研究,结果显示前者主要受陆源有机碳控制,而后者主要由海源有机碳驱动36。受水动力分选以及原位降解等因素的影响,楚科奇海陆架和陆坡不同区域的木质素降解程度存在显著性差异39

4 沉积有机碳的埋藏保存与地质记录

4.1 有机碳的埋藏保存

北极陆架接收了大量的陆源有机碳输入,具有较高的海洋初级生产力,是有机碳主要的埋藏场所;同时受多种因素的共同影响,如物理作用(海岸侵蚀、冰川冲刷、再悬浮、冰筏输运、风和洋流)、光合作用、成岩作用(如氧化还原过程)、微生物降解等(图5),导致陆架有机碳的埋藏保存机制较为复杂,存在显著的区域性差异87

图5

图5   北极陆架沉积有机碳埋藏保存概念模式图(据参考文献[87]修改)

Fig.5   The schematic illustration showing the burial and preservation of sedimentary organic carbon in the Arctic shelf (modified after reference [87])


楚科奇海是世界上海洋初级生产力最高的海区之一,29%~90%来自初级生产力贡献的有机碳被埋藏,“生物泵”输出效率和有机碳埋藏效率较高938,埋藏速率约为1.56 mg/(cm2⋅a)88。而基于210Pb测年和沉积物干密度计算结果,从百年尺度上估算出该区有机碳埋藏速率为0~0.41 mg/(cm2⋅a)。另一方面,如前所述,受径流输入和海岸侵蚀作用的影响,每年约有20 Tg的陆源冻土有机碳沉积埋藏在东西伯利亚陆架,其中相当一部分的有机碳或可直接矿化为CO22168图6)。随着冻土退化的加剧89,一方面使土壤中温室气体CH4/CO2释放,同时也将大量河流或海岸侵蚀的陆源有机碳释放至陆架近海,导致陆源有机碳埋藏通量显著提高2267。在百年—千年尺度上,受老碳输入以及有机碳—矿物结合保护等因素影响,东西伯利亚陆架约一半的陆源有机碳可埋藏保存在海底,这可能对大气CO2起负反馈作用28

图6

图6   东西伯利亚陆架不同来源有机碳通量模式图(据参考文献[21]修改)

Fig.6   The burial fluxes patterns of different organic carbon in the ESAS (modified after reference [21])


一般来说,沉积物中有机碳的埋藏保存受多种因素的共同作用90~92,例如与矿物基质结合就是其中非常重要的一种机制9091。随着研究的深入,铁在有机碳保存过程中起到的作用受到较多的关注9394。通过对全球范围内沉积物(包括北极陆架边缘区)的研究发现,约20%的沉积有机碳会通过与活性铁结合而被保存下来,因此认为活性铁是有机碳长期储存的关键因素之一94。最新研究则表明,海冰覆盖、洋流、活性铁含量和颗粒物粒径并不是影响与铁结合有机碳(Fe-OC)性质的关键因素,更主要受有机碳自身组成的影响95。基于东西伯利亚陆架的区域性对比研究,发现Fe-OC随着有机碳的来源而变化,在海岸侵蚀和陆源输入显著的区域,活性铁主要与年龄较老的陆源有机碳相结合;而在初级生产力较强的海域,活性铁则优先与年轻的海源有机碳相结合75。据碳同位素平衡模型估算,在拉普捷夫海约15%的有机碳通过与活性铁结合而被保存,但在东西伯利亚海这方面仅有约6.4%;由于前者Fe-OC主要来源于冻土老碳,这在一定程度上或可减弱对气候的正反馈75。总体而言,目前对于东西伯利亚陆架乃至北极周边海域沉积有机碳的埋藏保存机制的研究并不多,有待于进一步研究,以期能更全面地认识北极快速变化下的沉积有机碳源汇过程。

4.2 末次冰消期以来有机碳埋藏记录

随着北极变暖,夏季海冰减少、冻土退化和径流输入增加,不同区域有机碳的埋藏格局正发生着改变,尤其是封存于冻土中的陆源有机碳正在加速迁移和释放,这对北冰洋碳循环的源汇格局及其演变有重要影响,并可能对未来的气候变化有重要意义101196。事实上,从更长时间尺度来看,末次冰消期以来北极/亚北极地区也存在着一系列快速变暖的情况9798,同样发生了类似目前的大量陆源冻土有机碳的向海迁移释放和快速的沉积埋藏过程99~103,并可能对冰后期大气CO2浓度的迅速升高和气候的快速变化具有重要的反馈作用104~106。已有研究表明,在末次冰盛期,海平面降低,东西伯利亚陆架周边没有大冰盖覆盖且形成了大范围的冻土沉积,埋藏保存了巨量的陆源有机碳107108;冰消期以来随着气温升高和海平面变化,使得相当一部分的冻土碳变得不稳定,并通过流域冲蚀和沿岸侵蚀等方式快速地向海释放和埋藏109,这些历史时期快速变暖背景下的脉冲突变式的冻土碳迁移释放与全新世以来相对稳定的陆架有机碳埋藏模式明显不同2135110,并具有显著的区域性差异35111。例如,通过重建冰后期(尤其新仙女木事件以来)拉普捷夫海的有机碳埋藏记录,发现在新仙女木事件后气温迅速升高的转换时期(11 690~11 140 cal a BP),该区陆源冻土有机碳的年均埋藏通量达(4.5±1.4) Tg C/a,是正常径流入海通量的7倍;这些巨量的冻土碳可能跟勒拿河流域上层的活动冻土不断融蚀释放有关101。而东西伯利亚海全新世以来的有机碳埋藏记录显示,受海平面快速变化影响,虽然早全新世暖期有较高的陆源有机碳输入,不过这些冻土碳的来源与拉普捷夫海有显著差异,主要来源于沿岸侵蚀作用排放的富冰冻土老碳(ICD/PF-C)111。对于楚科奇海,则发现在末次冰消期晚期的变暖阶段和海平面快速上升期间112,该区陆源冻土碳的埋藏通量比晚全新世高2倍多,且主要来自近岸侵蚀释放的富冰冻土碳(约66%),流域土壤冻土碳仅占约16%,这与受大河输入影响下的拉普捷夫海陆架冻土碳的埋藏记录明显不同109。综上,尽管东西伯利亚陆架不同时期、不同海区冻土碳的埋藏记录和来源有显著地差异,但相对于当前的河流输入,这种大量的具有高活性的陆源有机碳由陆向海的快速沉积埋藏对于北极土壤碳库的稳定性、水生环境有机碳的矿化及CO2的排放具有重要的驱动作用和气候环境效应113114

5 结语与展望

北极东西伯利亚陆架海陆相互作用强烈,接受了大量陆源沉积有机碳输入,且不同地区的沉积环境、冻土发育状况、河流/海岸侵蚀作用、海冰覆盖以及“生物泵”强度等差别显著,导致沉积有机碳的源汇过程具有显著的区域性差异。国际上对于该区沉积有机碳的源汇过程及其环境效应等方面已开展了较多的研究,尤其侧重于陆源有机碳的输入和归宿;国内学者前期的研究则主要围绕在楚科奇海有机碳的来源和埋藏等方面。此外,围绕上述科学背景,2016—2019年自然资源部第一海洋研究所、中国海洋大学和俄罗斯科学院太平洋海洋研究所等单位对俄罗斯所属的北极东西伯利亚陆架组织开展了3次多学科的联合科考,围绕陆架地质过程与环境演化、北极快速变化的气候效应以及生物多样性等方面开展了系统调查研究,这将加强我国对北极的科学认识和探索,为理解和预测北极的快速气候变化提供依据,也可为“冰上丝绸之路”建设提供科学支撑。鉴于上述,下一步围绕东西伯利亚陆架沉积有机碳源汇过程的研究工作应着力于以下几个方面:

(1)综合利用多元地球化学指标及碳循环模型从流域环境、海冰和冻土等方面揭示不同陆架地区有机碳的源汇过程及其沉积响应机制,通过区域对比,深入认识不同流域背景和外部环境因子影响下的有机碳源汇过程及其环境效应。

(2) 加强单体有机分子碳同位素技术与传统海洋沉积研究手段的结合,这有利于定量评估陆源有机碳的跨陆架输运及控制机制、降解程度和归宿;尤其是关注历史不同时期海平面和物质输入变化下陆源沉积有机碳的迁移转化过程及埋藏记录。

(3)注重海冰变化与有机碳源汇过程之间的联系。在气候变暖背景下,不同来源有机碳的输入和沉积埋藏可能随之发生变化,尤其是海源有机碳的供应与上层海洋过程(如海冰)密切相关,并可能会进一步改变沉积氧化还原环境,进而对有机质的代谢模式、微生物活动及营养盐再循环等造成影响。

(4) 重视沉积边界过程对有机碳生物地球化学行为的约束。沉积物的组成和分选作用可能是陆源沉积有机碳分布、扩散和降解保存的首要控制因素。径流输入、海冰过程、海岸侵蚀等边界条件对不同区域沉积物的来源和输运有直接的影响,进而制约着沉积有机碳的地球化学行为和归宿。

(5)加强利用海洋沉积记录和数值模式相结合的方法研究不同时间尺度沉积有机碳的源汇过程。冰消期海平面上升以来北极的陆架沉积环境、海洋过程、海冰及相关碳的生物地球化学过程可能存在不同时间尺度(百年—千年)的变化特征,对其演变规律和机制的理解需要从“指标记录与数值模拟”和“现代过程与地质记录”相结合的角度去系统认识北极快速变化下的有机碳源汇格局及其气候环境效应。

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