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Journal of Tropical Oceanography >

2024 , Vol. 43 >Issue 4: 20 - 32

DOI: https://doi.org/10.11978/2023117

Review

Geochemistry of black carbon in marine extreme environments and its environmental implications*

  • LI Dai ,
  • WANG Xudong ,
  • JIA Zice ,
  • FENG Dong
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  • College of Oceanography and Ecological Science, Shanghai Ocean University, Shanghai 201306, China
WANG Xudong. email:

Copy editor: SUN Cuici

Received date: 2023-08-12

  Revised date: 2023-09-08

  Online published: 2023-11-16

Supported by

National Natural Science Foundation of China(42106059)

Shanghai Sailing Program(21YF1416800)

Shanghai Chenguang Program(22CGA58)

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Abstract

Black carbon is one of the carbonaceous materials, it exists ubiquitously in the environment and could be resistant to oxidation and decomposition. Black carbon might strongly affect the global carbon cycle as an important component of recalcitrant organic carbon. The current discrepancy of marine black carbon budgets indicates that there are unknown sources or buried pathways of black carbon in the ocean. It has been confirmed that marine extreme environments, such as abyssal trenches, hydrothermal vents and cold seeps, may be important sources or sinks of marine black carbon with the continuous deepening of the research on black carbon in these environments. In this review, the geochemical characteristics of black carbon in extreme marine environment are summarized. It is found that the unique “V”-shaped terrain of the abyssal trenches is conducive to the accumulation of materials, in which the black carbon is older than the syn-sedimentary organic carbon, and the annual buried amount of black carbon is about (1.0±0.5) Tg. The high-temperature fluid in hydrothermal vents forms in-situ authigenic black carbon by “burning” organic matter, and its annual contribution to the ocean is about 1.6~9.7Tg, which is an important source of marine black carbon. The source and sink process of black carbon in cold seeps remain unclear, but the high abundance of anaerobic methanotrophic archaea in these areas has recently been proved to directly produce black carbon, and its carbon isotope value is negatively below -60‰. As the only microbial source of black carbon found so far, it is an important supplement to the traditional understanding of black carbon types. The overall framework of marine black carbon source and sink process has been established notwithstanding, there is a lack of direct morphological observation and characterization of black carbon in marine extreme environments. It is necessary to clarify the ratio between terrestrial black carbon input and marine authigenic black carbon in extreme marine environments, to further understand the source and sink process of marine black carbon and explore the role of extreme environmental black carbon in marine black carbon budgets in the future.

Key words： black carbon; marine extreme environments; geochemical characteristics; source-sink processes

Cite this article

LI Dai , WANG Xudong , JIA Zice , FENG Dong . Geochemistry of black carbon in marine extreme environments and its environmental implications*[J]. Journal of Tropical Oceanography, 2024 , 43(4) : 20 -32 . DOI: 10.11978/2023117

黑碳(black carbon, BC)是由生物质或化石燃料不完全燃烧所产生的一种碳质颗粒。根据颗粒大小、传输距离、生成温度和反应活性的不同, 可划分为焦炭、木炭、烟炱和石墨型黑碳等类型(图1, Hedges et al, 2000; Dickens et al, 2004a; Masiello, 2004)。依照不同的BC类型以及研究目的, 发展出多种BC提取方法, 如显微镜法、热光反射法、热氧化法、化学氧化法、苯多羧酸分子标志物法等(图1, Masiello, 2004; Hammes et al, 2007)。高度浓缩的芳香结构使BC具备抗氧化、耐分解的特性, 可在环境中保存千年乃至百万年, 因此BC是惰性有机碳的重要组分。海洋沉积物中BC与总有机碳(total organic carbon, TOC)的比值(即BC/TOC)约为2.0%~38.0%(Bird et al, 2015)。BC的形成代表着将生物质活性的碳转换为稳定的惰性碳, 在碳循环中充当着碳汇的角色, 并且可能对全球变暖产生负反馈效应(Kuhlbusch, 1998; Masiello et al, 1998; 曹军骥 等, 2011; Coppola et al, 2022)。基于BC的高度稳定性和长期碳汇的角色, 已提出把科学化生产BC作为有效的储碳手段的构想, 据估计在全球范围内每年可减少约3.4~6.3Pg CO2的排放量(1Pg=1× 1015g) (Lehmann, 2007, 2021; Woolf et al, 2010)。在全球气候变化日益加剧的当下, 探究BC循环有助于我国“双碳”目标的实现(Liu et al, 2022; 夏翠梅 等, 2023)。
图1 黑碳的理化性质与提取方法

a.实线为提取方法的适用范围, 虚线代表暂未清楚其适用范围, TOR为热光反射法(thermal-optical reflectance), CTO-375(chemothermal oxidation)为在375℃下热氧化24h, BPCA(benzene polycarboxylic acid)为苯多羧酸分子标志物法; b. 透射电镜(TEM)下的烟炱颗粒; c. 扫描电镜(SEM)下的焦炭/木炭颗粒(据Masiello, 2004; Hammes et al, 2007; Buseck et al, 2012; Bird et al, 2015修改)

Fig. 1 Physicochemical properties and extraction methods of black carbon.

(a) The solid line shows the range of applicability of the extraction method and the dashed line represents the range of applicability that is not clear yet. TOR is thermal-optical reflectance. CTO-375 is thermal oxidation at 375℃ for 24h. BPCA is benzene polycarboxylic acid molecular marker method; (b) TEM image of soot; (c) SEM image of char (modified from Masiello, 2004; Hammes et al, 2007; Buseck et al, 2012; Bird et al, 2015)

陆地和海洋沉积物都是BC的储库。陆源BC主要包括生物质不完全燃烧和化石燃料不完全燃烧2种, 其中生物质不完全燃烧每年产生约114~383Tg的BC(1Tg=1×1012g), 是陆地最大的BC来源; 化石燃料不完全燃烧每年贡献约2~29Tg的BC(Coppola et al, 2018; Liu et al, 2021; Coppola et al, 2022)。陆源BC最终会通过大气沉降和河流运输等途径进入海洋并埋藏在沉积物中, 海洋沉积物因此成为最大的海洋BC储库(480~1440Pg)并被视为不同环境中BC的最终归宿, 近岸/陆架是区域最重要的BC埋藏区, 埋藏于边缘海沉积物BC约400~1200Pg, 而深海沉积物BC约80~240Pg(Bird et al, 2015)。根据海洋学中对有机碳在操作上的分类方法, 可将BC按照其颗粒大小, 以是否能通过0.45μm的滤膜为标准, 分为溶解态黑碳(dissolved black carbon, DBC)和颗粒态黑碳(particulate black carbon, PBC)(Coppola et al, 2022; Zhang et al, 2023)。大部分PBC会随河流搬运而埋藏于近岸地区的沉积物中, 深海沉积物中除了大气干湿沉降所带来的PBC, 只有少部分PBC会被河流运输到开阔大洋, DBC则常常吸附于颗粒物从而下沉埋藏到深海沉积物中(Coppola et al, 2014, 2016; Bird et al, 2015)(图2、图3)。
图2 全球海洋沉积物黑碳含量(干重)汇编

该图基于国家测绘地理信息标准地图服务网站下载的审图号为GS(2016)1665号的标准地图制作。数据源自Smith et al, 1973; Griffin et al, 1975; Lim et al, 1996; Gustafsson et al, 1998; Kang et al, 2009; Lohmann et al, 2009; Salvadó et al, 2017; Yang et al, 2018; Ren et al, 2019, 2022; Wu et al, 2019; Dan et al, 2022; Wulandari et al, 2023

Fig. 2 Global compilation of marine sediment black carbon content (data from Smith et al, 1973; Griffin et al, 1975; Lim et al, 1996; Gustafsson et al, 1998; Kang et al, 2009; Lohmann et al, 2009; Salvadó et al, 2017; Yang et al, 2018; Ren et al, 2019, 2022; Wu et al, 2019; Dan et al, 2022; Wulandari et al, 2023)

图3 海洋黑碳循环

a. 黑色数值代表黑碳的年通量(Tg·yr−1), 蓝色数值代表黑碳的总储量(Pg); b. 通过BPCA法提取出的不同类型黑碳的稳定碳同位素(δ13C)值(δ13CVPDB); 高温高压下浓硝酸氧化DBC会形成带有3~6个羧基的苯多酸(BPCA, 即B3CA-B6CA), 苯多酸单体浓度比例会受到DBC结构及来源等因素的影响, 因此苯五多酸(B5CA)和苯六多酸(B6CA)等特定DBC的δ13C值将有助于追踪黑碳的来源和过程; 另外, B6CA:B5CA是常用来表示DBC芳香性程度的指标, 该值越大, DBC芳香性越强(据方仔铭, 2018; Wagner et al, 2019修改)

Fig. 3 Marine black carbon cycle.

(a) The black value represents the annual flux of black carbon (Tg·yr−1), the blue value represents the total storage of black carbon (Pg); (b) it is the δ13C values representing different types of black carbon extracted by the BPCA method; the oxidation of DBC by concentrated nitric acid under high-temperature and high-pressure conditions results in the formation of benzene polyacid (BPCA, B3CA-B6CA) compounds, which contain 3~6 carboxyl groups; the monomer concentration ratio of benzene polyacid is influenced by the structure and source of DBC; therefore, the stable carbon isotope (δ13C) values of specific DBCs, such as benzene pentapolyacid (B5CA) and benzene hexapolyacid (B6CA), can be utilized to track the origin and formation process of BC; furthermore, the B6CA:B5CA ratio is a widely employed index for assessing the aromaticity of DBC; a higher value of this ratio indicates a greater degree of aromaticity in DBC (modified from Fang, 2018; Wagner et al, 2019)

吸附于颗粒物下沉到深海沉积物的DBC通量约为40~85Tg·yr−1, 与河流加大气DBC向海洋年输入量之和接近, 其中河流是更主要的输入源。然而, 实测的δ13CDBC值显示河流DBC比海洋DBC亏损约6‰, 表明除河流外, 海洋中可能存在未知的BC来源(Wagner et al, 2019; Coppola et al, 2022; Yamashita et al, 2022)。在深海沉积物中, BC通常比TOC老2400~13900年, 河流搬运的古老PBC大部分埋藏于近岸, 只有少部分会被运输至大洋中, 因此BC与TOC之间的年龄差异可能是古老DBC与颗粒物结合进入深海沉积物中的结果(Masiello et al, 1998; Coppola et al, 2014)。目前海洋DBC总储量约为36Pg, 河流DBC的通量约66.3Tg·yr−1, 因此河流DBC在海洋中的驻留时间应< 600年(驻留时间=总储量/年通量)(Bird et al, 2015; Fang et al, 2021; Coppola et al, 2022), 但根据目前海水放射性碳十四(Δ14CDBC)值推算的表层海水DBC的平均年龄为(4800±620)年, 而深层海水DBC平均年龄大于20000年, 这还没有考虑DBC清除过程(海洋表面的光降解, 微生物降解作用, 吸附于颗粒物等)的周转时间, 这些证据都暗示海洋中存在未知的古老DBC源(Kuzyakov et al, 2009; Stubbins et al, 2012; Coppola et al, 2014, 2016)(图3)。
综上, 目前对海洋BC循环过程的有限研究还不足以精确限定海洋BC循环对海洋碳循环的贡献。海洋极端环境(深渊海沟、热液区和冷泉区)是近年来才开始涉足的海洋黑碳研究区域, 初步的研究表明其特异性的地形地貌、温压条件和生物地球化学作用会塑造独特的黑碳循环过程, 可能是未来海洋黑碳循环研究的关键地带。本文将围绕深海极端环境BC的地球化学特征、源汇过程及其意义展开介绍, 以期为完善深海黑碳收支理论提供新的视角。

1 深海极端环境黑碳的地球化学特征

1.1 深渊区

深渊区水深超过6000m, 涵盖了遍布全球的30多条海沟(Xu et al, 2018b)。海沟是地球构造运动最活跃的地带, “V”字型的地形和靠近大陆边缘分布的特点使其如漏斗一般汇聚有机质, 成为海洋TOC和BC埋藏的热点区域(Glud et al, 2013; Zhang et al, 2022)。
近期, 根据对全球典型的6条深渊海沟沉积物(太平洋阿塔卡马、克马德克、新不列颠、布干维尔、玛索和马里亚纳海沟)TOC和BC含量及其各自碳同位素特征的分析, Zhang等(2022)揭示了深渊海沟沉积物BC的地球化学性质并估算了BC的埋藏通量, 主要认识如下(图4):
图4 海沟沉积物黑碳的地球化学特征

该图基于国家测绘地理信息标准地图服务网站下载的审图号为GS(2016)1665号的标准地图制作; a. 饼图面积大小表示各站点TOC含量, 饼图中绿色代表除黑碳外的有机碳含量, 黄色代表黑碳含量(干重), 饼图旁“字母+数字”代表站位号, “数字”代表该站位的TOC含量, 单位为mg·g−1; b. 海沟沉积物黑碳与其他海洋沉积物黑碳的δ13C和Δ14C值(据Zhang et al, 2022修改)

Fig. 4 Geochemistry of black carbon in trench sediments.

(a) The area of the pie chart indicates the TOC content of each site, green in the pie chart represents the organic carbon content except black carbon, yellow represents the black carbon content, the ‘letter+number’ next to the pie chart represents the station number, and the ‘number’ next to the pie chart represents the TOC content of the station in mg·g−1; (b) cross-plot of δ13C and Δ14C for the black carbon (BC) in trench sediments and other marine sediments (modified from Zhang et al, 2022)

(1)在深渊海沟沉积物中, BC是TOC的重要组分。6条深渊海沟的BC含量在0.1~1.8mg·g−1(干重)之间, BC/TOC的范围为1.8%~32.4%, 与其他海洋沉积物有机碳组成特征类似(Bird et al, 2015; Zhang et al, 2022);
(2)相比其他海洋沉积物(δ13CBC: -25.0‰~ -19.4‰, 均值为-21.3‰), 深渊海沟沉积物BC的δ13C值更轻(δ13CBC: -27.7‰~-22.3‰, 均值为-25.1‰), 表现出更典型C3植物的特征(Masiello et al, 2003; Dickens et al, 2004b; Ren et al, 2019; Zhang et al, 2022);
(3)与其他海洋沉积物(Δ14CBC: -989‰~-352‰)类似, 深渊海沟沉积物BC比共同沉积的TOC(-891‰~ -71‰)更为古老, Δ14CBC介于-897‰至-346‰之间(Masiello et al, 2003; Dickens et al, 2004b; Ren et al, 2019; Zhang et al, 2022);
(4)马里亚纳海沟沉积物BC的δ13CBC(-27.7‰~ -22.3‰, 均值为-24.3‰)显著低于上层水体DBC的δ13CBC(-20.9‰~-18.4‰)。海沟沉积物BC更为偏负的δ13CBC组成可能是微生物或光对水体DBC的降解所造成, 或是存在其他来源的BC(Qi et al, 2020; Zhang et al, 2022);
(5)深渊海沟沉积物每年埋藏BC量可达约(1.0± 0.5)Tg(Zhang et al, 2022)。

1.2 热液区

热液区是深海典型的高温环境, 喷发流体温度可高达464℃(赵维殳 等, 2023)。热液流体释放出大量颗粒物, 与冷海水混合并扩散, 形成独特的热液区物质能量循环(Dick, 2019)。热液区强烈的热量交换可能会产生一种分子结构与陆地生物质焦化后类似的物质, 即热液区海水或沉积物中的有机质会经过热液的“灼烧”而形成BC(Coppola et al, 2022), 因此热液区一直被认为是海洋部分热成因有机质和深海BC的来源(Dittmar et al, 2006)。这些观点已得到东太平洋海隆低—高温热液喷口区发现非生物合成纳米/亚微米级石墨碳(Estes et al, 2019)和喷射状热液羽流外部存在热液BC(Yamashita et al, 2023)的确认。热液区BC的主要特征包括(图5):
图5 热液区溶解有机碳变化示意图

SPE-DOC (solid phase extraction-dissolved organic carbon) 代表被固相萃取的惰性溶解有机碳, 与周围海水相比, 约94%的SPE-DOC在热液喷口处被清除 (据Hawkes et al, 2015; Niggemann et al, 2016; Dick, 2019修改)

Fig. 5 Schematic diagram of dissolved organic carbon changes in the hydrothermal systems. SPE-DOC (solid phase extraction-DOC) represents recalcitrant dissolved organic carbon extracted by solid phase extraction, and about 94% of SPE-DOC is removed at hydrothermal vents compared to surrounding seawater (modified from Hawkes et al, 2015; Niggemann et al, 2016; Dick, 2019)

(1)热液喷口处的DBC浓度相对于周围海水低(Hawkes et al, 2015; Niggemann et al, 2016);
(2)热液区产生的DBC在海洋中可以进行长距离的运输(Yamashita et al, 2023);
(3)热液区BC的年产量约为1.6~9.7Tg(Yamashita et al, 2023);
(4)海洋DBC的δ13C相对河流偏高6‰(海洋: -27.2‰~-21.9‰, 均值: -23.6‰, 河流: -31.6‰~ -27.7‰, 均值: -30.0‰), 可能是热液活动将海水中的颗粒/溶解有机碳改造成具有偏高δ13C特征的BC(Wagner et al, 2019; Jones et al, 2020);
(5)东太平洋溶解有机碳(DOC)年龄超过6000年, 该老化的有机碳来自于海水经过洋壳后从热液喷口涌出的可能性最高(Luther, 2021)。已确认热液区由微生物产生的具有芳香结构的DOC极度亏损Δ14C, 因此热液区BC也可能继承极度亏损的Δ14C特征(McCarthy et al, 2011; Lin et al, 2019)。

1.3 冷泉区

冷泉是指富含碳氢化合物和硫化氢的流体向上运移到浅层沉积物甚至渗漏到海水中的现象(Suess, 2020)。冷泉系统常与天然气水合物相伴生, 全球约有0.5~2.0Tg碳被封存于天然气水合物中(Boetius et al, 2013)。冷泉区以渗漏甲烷为基础演化而来的DIC(dissolved inorganic carbon, 溶解无机碳)和DOC是深海碳循环研究的重要窗口(Wang et al, 2001; Boetius et al, 2013; Feng et al, 2020; Yang et al, 2020; Li et al, 2023b)。微生物介导的甲烷厌氧氧化作用(anaerobic oxidation of methane, AOM)将大量甲烷转化为DIC, 其中一部分与Ca2+/Mg2+结合形成自生碳酸盐岩固结于沉积物中, 是重要的碳汇; 另一部分随着流体进入海洋无机碳库中, 成为海洋的碳源之一(Xu et al, 2018a; 徐翠玲 等, 2020)。此外, 最近研究表明, 冷泉区可能是深海有机碳的重要来源。冷泉沉积物中约有10%~20%的有机碳来自于渗漏甲烷, 甲烷厌氧氧化古菌(anaerobic methanotrophic archaea, ANME)通过AOM作用可将3%~25%的甲烷碳转化为有机碳(醋酸盐), 以持续支持冷泉区大量的异养生物(Joye, 2020; Yang et al, 2020; Feng et al, 2021; Li et al, 2023b)(图6)。
图6 a. 冷泉系统碳循环示意图, 硫酸盐-甲烷转换带(sulfate-methane transition zone, SMTZ); b. 甲烷厌氧氧化古菌(ANME)所产生的黑碳、活性碳和石墨的拉曼光谱特征; c、d: 黑色小球状物质为ANME所产生的黑碳(据Boetius et al, 2013; Allen et al, 2021修改)

Fig. 6 (a) Carbon cycle of cold seeps, sulfate-methane transition zone (SMTZ); (b) Raman spectroscopic characterization of the black carbon in methane anaerobic oxidizing archaea (ANME), activated carbon, and graphite. (c) and (d) the black spherical material is the black carbon produced by ANME (modified from Boetius et al, 2013; Allen et al, 2021)

传统BC研究概念认为, 作为有机碳重要组分之一的BC仅能通过非生物途径(燃烧)产生, 未见有微生物合成途径的报道。近期的一项培养实验发现, 冷泉系统发育的ANME能够利用甲烷产生BC(Allen et al, 2021)。这一研究打破了微生物仅能降解而不能产生BC的传统认知, 为海洋BC来源研究提供了新的猜想与思考(Kuzyakov et al, 2009; Zimmerman, 2010; Allen et al, 2021)。针对这一特殊BC的主要认知如下(图6):
(1)微生物所产生的BC大小在微米级范围内, 拉曼光谱与活性碳相似, 有较宽的G峰(石墨)和D峰(无定形), 与具有窄高G峰的石墨碳具有明显的差异(Allen et al, 2021);
(2)该类BC不溶于强酸强碱以及一些有机溶剂, 具有较强的稳定性, 但与传统BC不同的是, 它们在分子结构上不具有芳香结构(Allen et al, 2021);
(3)相对已有报道的极度亏损δ13C的BC(δ13C: -48‰和-45‰(Flores-Cervantes et al, 2009), 此类BC具有更加亏损的δ13C特征(δ13C: 约-60.0‰) (Allen et al, 2021);
(4)基于冷泉渗漏甲烷常常具有极度亏损的Δ14C (Pohlman et al, 2011), 可以合理推测这种生物成因BC也可能继承极度亏损的Δ14C特征;
(5)根据从干重约15.8mg的细胞团中可以获取0.5mg BC的实验结果粗略判断, 该BC约占所产生的颗粒物的3.2%(Allen et al, 2021)。
遗憾的是, 目前无法确认这一微生物过程产生的BC总量以及对海洋BC循环的可能影响。

2 深海极端环境下黑碳的源汇过程及其环境意义

海洋BC的来源、运输及清除过程及其影响因素(如运输距离、海底地形、海流搬运、微生物降解、吸附在下沉颗粒等)是研究海洋BC循环的关键问题。例如, 边缘海和开阔大洋BC的分布显然表明, 绝大多数BC仅经历短距离河流运输便会沉积在海洋中, 而通过空气进行长距离运输的BC则以气溶胶为主, 颗粒小于1μm(Masiello et al, 1998; Coppola et al, 2014)。通过对太平洋海盆尺度DBC分布的研究发现, 太平洋深处的DBC浓度随着深海经向环流而减少, DBC通过吸附在下沉颗粒上被清除到深海沉积物中(Yamashita et al, 2022)。据估计, 全球从海洋下层到深海沉积物的清除通量要大于已知的通过河流和大气沉积的全球DBC输入通量, 这在一定程度上导致了海洋DBC池的放射性碳年龄无法与现有模型中DBC的Δ14C观测值相匹配(Coppola et al, 2016, 2022)。从同位素角度来说, 海洋DBC至少有两个组成部分: 来自河流的现代成分(+58 ± 207)‰加上古老的背景DBC池(-945 ± 5)‰, 这暗示了其可能有两种截然不同的组成成分, 一种古老、丰富且稳定; 另一种则年轻、稀少且活跃(Coppola et al, 2016), 可能与海洋DOC遵循相似的混合过程(Beaupré et al, 2020)。因此, 在维持海洋DBC库方面, 来自海洋内部原生性DBC(以亏损放射性碳和富集稳定碳为特征)的输入通量很可能与河流DBC同样重要, 这需要进一步研究以确定海洋中DBC的来源(Coppola et al, 2022)。
海洋BC收支的不平衡表明海洋中可能存在未知的BC源或汇, 但目前绝大部分BC研究集中于边缘海特别是陆架区, 无法涵盖整个海洋的黑碳循环过程。针对深海极端环境BC的研究有望为解答这个问题提供全新的视角(Bird et al, 2015; Coppola et al, 2022) (图2)。
深渊海沟是被忽略的重要海洋BC汇。海沟BC主要来源于陆地C3植物和化石燃料的不完全燃烧, 表现出C3植物典型的δ13C值以及化石燃料古老的Δ14C特征。基于Δ14C值的两端元混合模型计算出生物质和化石燃料分别贡献了海沟BC的36%和64%, 表明化石燃料是海沟BC更为重要的来源(Zhang et al, 2022)。此外, 独特的地质过程也会影响海沟BC的输入量。在阿塔卡马海沟沉积物中, 公元1700年左右BC和TOC含量均有所增加, 很可能是由于当时的海沟区地震扰动了原本沉积的BC, 并通过海沟独特的地形运输至海沟轴部(Zhang et al, 2022)。就海沟BC的碳同位素特征来看, 由于光降解能够使DOC的δ13C值偏正约1.0‰~4.5‰和DBC容易受到光降解的影响, 海沟δ13CDBC值偏正很可能是陆源DBC被搬运入海时受到了光降解作用的控制; PBC可能受到其他矿物基质的保护, 使得沉积BC保留了原本偏负的δ13C特征(Opsahl et al, 2001; Osburn et al, 2001; Stubbins et al, 2012; Jones et al, 2020; Qi et al, 2020)。最后, 海沟超深的水深一定程度上会影响BC的埋藏。通过将深渊海沟区沉积物BC与更浅水深沉积物BC的绝对含量、BC/TOC值、δ13CBC值和Δ14CBC值汇总比较发现, 深渊海沟区BC的绝对含量更低, 这可能与更深的埋藏路径有关。BC/TOC值与其他水深相似, 表明TOC中的活跃组分似乎在下沉过程中会与BC经历相似的改造过程。δ13CBC值更低(C3植物的贡献)且Δ14CBC值分布范围很广, 是生物和化石燃料不同程度混合的反映(图7)。
图7 海洋沉积物不同深度剖面BC的地球化学特征

按海水深度划分为浅海区(0~200m)、半深海区(200~2000m)、深海区(2000~6000m)和深渊海沟区(> 6000m)。a.沉积物中BC的含量(干重); b. 沉积物中BC/TOC值; c.沉积物BC的δ13C值; d.沉积物BC的Δ14C值 (图中数据源自Middelburg et al, 1999; Kang et al, 2009; Lohmann et al, 2009; Li et al, 2012; Salvadó et al, 2017; Zhou et al, 2017; Shen et al, 2018; Yang et al, 2018; Ren et al, 2019, 2022; Wu et al, 2019; Pei et al, 2020; Dan et al, 2022; Liu et al, 2022; Zhang et al, 2022; Fu et al, 2023; Li et al, 2023a; Wulandari et al, 2023)

Fig. 7 Geochemical characteristics of BC in different depth profiles of marine sediments.

According to the depth of seawater: shallow sea (0~200m), semi-deep sea (200~2000m), deep sea (2000~6000m) and hadal zone (> 6000m). (a) The content of BC in the sediment; (b) the BC/TOC value in the sediment; (c) the δ13C value of BC in the sediment; (d) the Δ14C value of BC in the sediment (the data in the figure are cited from Middelburg et al, 1999; Kang et al, 2009; Lohmann et al, 2009; Li et al, 2012; Salvadó et al, 2017; Zhou et al, 2017; Shen et al, 2018; Yang et al, 2018; Ren et al, 2019, 2022; Wu et al, 2019; Pei et al, 2020; Dan et al, 2022; Liu et al, 2022; Zhang et al, 2022; Fu et al, 2023; Li et al, 2023a; Wulandari et al, 2023)

以往基于固相萃取法(solid phase extraction, SPE)所提取的SPE-DOC结果认为, 热液区可能是深海DBC的汇(Hawkes et al, 2015; Niggemann et al, 2016)。大西洋、太平洋以及南大洋热液喷口处和周围海水的SPE-DOC浓度显示, 热液喷口处约94%的SPE-DOC被清除(Hawkes et al, 2015)。DBC与SPE-DOC的含量分布类似, 热液喷口处的DBC浓度明显低于周围海水(Niggemann et al, 2016)。然而, 由于热液所产生的DBC是一种具有浓缩芳香结构的不溶胶体且可能在热液流体与海水混合过程中被吸附于无机颗粒上, 这两种形式的颗粒物均不能被SPE法收集, 因此混淆了热液区BC的源汇角色(Simoneit, 1993; Hawkes et al, 2016; Yamashita et al, 2023)。实际上, 越来越多的研究认为热液区产生的BC是海洋DBC的重要来源, 其中包括2种可能的BC形成途径: 一是热液区纳米/亚微米级石墨的非生物合成途径, 热液区BC可能是通过CO2或CO的还原作用而形成(Estes et al, 2019; Yamashita et al, 2023); 二是高温热液流体可能将深部沉积物中古老的有机质或海水有机碳“灼烧”产生BC, 即有机碳的热解作用(Dittmar et al, 2006; Yamashita et al, 2023)。先前根据全球的BC通量、储量、年龄以及源汇过程数据建立的箱式模型所推测的热液系统BC通量仅为0.12~1.2Tg·yr-1, 远远低于全球热液系统实际BC通量 1.6~9.7Tg·yr−1, 暗示热液区BC在释放后存在除下沉颗粒吸附以外的未知去除过程(Coppola et al, 2022; Yamashita et al, 2023)。
基于冷泉系统遍布于全球大陆边缘和ANME是冷泉系统主导性的微生物群落的特点, 冷泉区BC很可能是陆源BC通过大气/河流运输入海和微生物产生的特殊BC的混合, 其地球化学性质取决于两种来源BC的相对比例(Knittel et al, 2009; Suess, 2020; Allen et al, 2021)。冷泉区的陆源BC与正常海洋环境相似, 而ANME产BC的碳源主要来自于海水或CH4转化的DIC(Allen et al, 2021)。ANME产BC作为一种独特的生物来源BC, 目前还不清楚是以PBC还是DBC的形式存在(Coppola et al, 2022), 但已有研究认为冷泉BC是一个潜在的海洋DBC来源。例如, 烃类渗漏是墨西哥湾老DOC的来源, 在原油渗漏中也出现了DBC的释放(Pohlman et al, 2011; Brünjes et al, 2022)。墨西哥湾沥青渗漏所释放的DBC的δ13C值(-28.0‰)明显低于已报道的其他海洋DBC的δ13C值(-27.2‰~-21.9‰)且具有极度亏损的Δ14C特征(Wagner et al, 2019)。根据两端元混合模型计算出石油沥青渗漏DBC对深海老DBC的贡献高达94.2%, 表明烃类(甲烷/原油/沥青)渗漏显著影响了局部海域的碳循环, 并对深海老DBC以及未解的深海惰性溶解有机碳具有重要贡献(Brünjes et al, 2022)。

3 结论与展望

近年来为数不多的深渊海沟、热液区和冷泉区的BC研究显示了这些深海极端环境独特的BC地球化学特征和源汇过程。深渊海沟是海洋BC埋藏的热点区域和以往未被充分关注的海洋BC的汇; 热液区是海洋BC的重要来源, 随高温流体长距离运输的BC可能是揭秘海洋古老BC未解之谜的突破口; 冷泉区微生物产BC的发现打破了以往BC只来源于非生物过程的传统认知, 但目前还未被纳入到海洋BC循环模型中, 冷泉区BC很可能是局部环境中的潜在碳源。然而, 关于深海极端环境BC的研究数据有限, 其迁移转化过程存在诸多空白。深渊海沟中不同形式BC的地球化学过程, 热液区BC的形成途径、在长距离运输中的清除机制以及冷泉区全新类型BC的鉴别, 都需要进一步的探索以帮助解释海洋黑碳收支的不平衡问题。
未来海洋BC研究应重点关注:
(1) 深渊海沟是海陆板块俯冲的地形表现。在深渊地幔橄榄岩中已发现了纳米级的非生物成因有机质(Nan et al, 2021), 全球深渊俯冲带很可能埋藏了大量未发现的碳。海沟作为重要的海洋黑碳汇, 未来的研究应考虑多种黑碳来源;
(2) 热液区作为海洋BC源或汇的争论依然存在, 热液区对海洋BC循环的贡献并未得到很好的限定。目前对热液区DBC的研究程度高于PBC, 今后应加强对热液区PBC的研究;
(3) 冷泉系统BC是陆源BC和生物BC的混合, 冷泉微生物ANME产生的BC过程局限于实验室培养。针对自然环境, 围绕ANME产BC的效率及量级进行系统采样、分析并进行数值计算, 评估生物BC对海洋BC循环的影响是今后值得关注的一个方向;
(4) 未来的建模方法将受益于海洋中不同水团PBC和DBC浓度和放射性碳年龄测量空间覆盖率的增加, 因此需要增加海洋中BC含量及其放射性碳年龄测量的空间覆盖范围, 以在十年和更长时间尺度上约束大规模过程的周转时间。沿不同年龄水团的测量将进一步限制DBC的净生产或消耗率, 有助于我们确定海洋DBC是否存在其他来源, 以解释海洋DBC和河流DBC之间稳定同位素的差异(Coppola et al, 2022)。

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