Spatial distribution and influencing factors of organic carbon in surface sediments along the Bohai and Yellow Sea
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摘要: 了解海洋沉积物有机碳的分布特征及其影响因素对深入探究全球碳循环机制并应对气候变化意义重大。本研究通过渤、黄海表层沉积物碳-氮-硫元素及其同位素（TOC、TN、TS、S2‒、δ13C、δ15N、δ34S）测定，结合比表面积、色素生物标志物、热重分析等，阐明了沉积物有机碳的来源组成、分布特征及其主要约束机制。结果表明，受水动力条件控制，TOC和TN含量呈现近岸低、远岸高的分布特征，且受沉积物比表面积显著影响。同位素端元分析结果表明沉积物中混合了陆地植物、土壤和海洋藻类有机碳。其中黄河口及辽东半岛近岸海域土壤输入影响较大，有机碳碳反应指数较小；而南黄海海洋藻类影响较大，有机碳碳反应指数较高。渤海及北黄海沉积物TOC与TS的显著相关性表明有机碳降解与硫酸盐还原过程密切相关，较负的δ34S表明沉积物-水界面的硫酸盐还原-硫化物氧化-海水硫酸盐补充的循环过程，暗示了该区域有机质有氧到无氧的降解过程，其中色素降解潜在导致其与表层沉积物TOC之间相关性较弱。随着日益频发的藻华和水体缺氧效应，未来研究需进一步关注海水水体氧化还原条件与表层沉积物元素循环的耦合关系，这对理解大陆架沉积物有机碳埋藏具有重要意义。Abstract: Understanding the distribution, migration, and transformation of organic carbon in the ocean is of great significance to study the global carbon cycle and tackle climate change. Here, surface sediment samples were collected from the Bohai and the Yellow Sea. For illustrating the source composition, distribution pattern, and their constraints, material characterization such as specific surface area, total organic carbon, nitrogen, sulfur contents and their isotopes (TOC, TN, TS, S2‒, δ13C, δ15N, and δ34S), as well as pigment biomarkers and thermos-gravimetric analysis were conducted. The results showed that the TOC and TN contents were lower in the nearshore but higher in the offshore region due to stronger hydrodynamics nearshore. The specific surface area had significant impacts on the distribution of organic carbon. Three-endmember mixing model suggested that sediments near the Yellow River mouth and the shallower region along the Liaodong Peninsula in the Bohai Sea were dominated by soil-derived organic carbon with a smaller carbon reactivity index, while marine phytoplankton contributed mostly to the southern Yellow Sea with increasing carbon reactivity index. Significant correlation between TOC and TS in the Bohai and northern Yellow Sea indicated significant linkages between organic carbon degradation and sulfate reduction. The negative δ34S indicated the process of sulfate reduction-sulfide oxidation-seawater sulfate diffusion at the sediment-water interface, which hinted the oxidation of organic carbon from oxic to anoxic condition. Pigment degradation potentially resulted in its weak correlation with the TOC content. As the risk of harmful algal bloom and hypoxia is becoming more severe, it is essential to monitor the water chemistry and elemental cycling in the sediment to comprehensively understand the role of the continental shelf on organic carbon burial.
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Key words:
- Bohai and Yellow Sea /
- Surface sediments /
- Organic carbon /
- Sulfur isotope /
- Pigment /
- Thermo-gravimetric analysis
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图 1 渤、黄海表层沉积物采样站位示意图
Fig. 1 Sampling sites of the surface sediments in the Bohai and Yellow Sea
图 2 渤、黄海表层沉积物的比表面积（a）；TOC（b）；TN（c）；摩尔C/N（d）；δ13C（e）；δ15N（f）；TS（g）；S2‒（h）；和δ34S（i）的空间分布
Fig. 2 Spatial distributions of specific surface area (a), TOC (b), TN (c), molar C/N ratio (d), δ13C (e), δ15N (f), TS (g), S2‒ (h), and δ34S (i) in the marine sediments from the Bohai and Yellow Sea investigated in this study
图 3 渤、黄海表层沉积物色素生物标志物、热重分析参数及来源贡献的空间分布。（a）色素（nmol/g）；（b）归一化色素（nmol/g OC）；（c）OMTotal（%）；（d）OML（%）；（e）OMR（%）；（f）CRI；（g）土壤贡献；（h）陆源植物贡献；（i）海洋藻类贡献
Fig. 3 Spatial distributions of pigment, thermo-gravimetric analysis parameters, and source contributions of the surface sediments in the Bohai and Yellow Sea. (a) Pigment content (nmol/g); (b) Normalized pigment content (nmol/g OC); (c) OMTotal (%); (d) OML (%); (e) OMR (%); (f) CRI; (g) Soil fraction; (h) Terrestrial vegetation; (i) Marine fraction
图 4 表层沉积物色素生物标志物组成及相对比例。（a）渤海；（b）北黄海；（c）南黄海
Fig. 4 Pigment composition and relative proportion in surface sediments of (a) Bohai, (b) Northern Yellow Sea, and (c) Southern Yellow Sea
图 5 （a）渤、黄海表层沉积物中TOC和TN含量关系；（b）C/N与δ13C关系；（c）TOC和比表面积（SSA）间关系。两条黑色实线分别代表有机碳载荷为1.0 mg OC/m2和0.4 mg OC/m2
Fig. 5 (a) Correlation between TOC and TN contents; (b) Relationship between molar C/N and δ13C; (c) Correlation between TOC and specific surface area (SSA): Solid lines represent organic carbon (OC) load of 1.0 mg OC/m2 and 0.4 mg OC/m2, respectively.
图 6 渤、黄海表层沉积物中（a）TOC含量与比表面积的相关关系：两条黑色实线分别代表有机碳载荷为1.0 mg OC/m2和0.4 mg OC/m2，以及（b）有机碳的δ13C和δ15N的相关关系
Fig. 6 (a) Relationship between TOC content and specific surface area and (b) between δ13C and δ15N of organic matter in the surface sediments of the Bohai and Yellow Sea. Solid lines in panel a represent organic carbon (OC) load of 1.0 mg OC/m2 and 0.4 mg OC/m2, respectively
图 7 渤、黄海表层沉积物各参数的主成分分析
Fig. 7 Principal component analysis of the bulk organic carbon parameters in surface sediments of the Bohai and Yellow Sea
表 1 渤、黄海表层沉积物有机碳端元分析结果
Tab. 1 Endmember mixing model results of the Bohai and Yellow Sea
海区 陆源植物（%） 土壤有机碳（%） 海洋藻类（%） 渤海 20 ± 8 54 ± 16 28 ± 11 北黄海 21 ± 7 37 ± 16 42 ± 10 南黄海 18 ± 11 39 ± 36 53 ± 25 
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