河口近海环境中陆源有机质的激发效应研究

As the most active zone of land-ocean interaction, estuaries and coastal seas, despite representing only 7% of global ocean surface area, contribute about 80% of organic carbon burial, and thus play a key role in marine carbon cycle. The question about “fate of terrestrial organic carbon in the ocean” remains one of the biggest unresolved mysteries for marine organic geochemists for long time. Each year, global rivers transport enormous amount of terrestrial organic carbon from continent to sea, but most previous studies have demonstrated that only very small fraction of organic carbon in seawater and marine sediments is of a terrestrial origin. “Priming effect” intrigued by the mixing of land and ocean materials in estuaries and coastal seas is likely an important driver for rapid degradation of terrestrial organic matter in seas. However, so far, no study has reported priming effect in the estuary and coastal sea. In this project, the applicants will use the biomarker, carbon stable isotope, optical spectral and high-resolution mass spectrometry approaches to trace terrestrial organic carbon in the river-mouths of the Yangtze River and Yellow River, two world large rivers, and their adjacent seas. We attempt to assess the contribution of priming effect to terrestrial organic carbon degradation as well as its mechanism based on laboratory incubation experiments and in situ analyses. This project will shed light on “the fate of terrestrial organic carbon in the ocean” and improve our understanding about marine carbon cycle and regional responses to global change.

河口近海作为海陆相互作用最为活跃的区域，尽管面积只占全球海洋的7%，但有机碳埋藏量却达80%，是研究海洋碳循环的关键区域。长期以来关于“陆源有机碳在海洋中的归宿”一直是困扰海洋有机地球化学界的未解之谜。世界河流每年输送巨量的陆源有机碳入海，但海洋水体和沉积物中却只有很少一部分有机碳具有陆源性质。海洋和陆源物质在河口近海混合产生的“激发效应”很可能是导致陆源有机质在海洋中快速降解的重要原因，然而目前还未见这方面的深入报道。本项目中，申请人拟以长江和黄河这两条世界级大河河口及其邻近海域为研究对象，应用生物标志物、同位素、光谱和高分辨率质谱等多种方法示踪陆源有机质的迁移和转化，并结合实验室模拟和现场分析，评估激发效应对海洋中陆源有机质降解的贡献和机制。本项目将尝试揭示“海洋中陆源有机碳的归宿”这一重大科学问题，这对于理解海洋有机碳循环和全球变化下的区域响应具有重要的意义。

河流每年输入大量的陆源有机碳入海，然而海洋中的陆源有机碳却非常少。这种陆源有机碳在海洋中的“碳失汇”现象是困扰海洋学界多年的学术难题。本项目推测在河口近海区域，海陆强烈混合引起的“激发效应”可能是造成陆源有机碳快速降解的原因。为了验证该假说，课题组以长江下游-河口-东海为研究区域，开展了大量的野外调查和实验室模拟，通过改变河水-海水的混合比例、微生物丰度、水体浊度、营养盐浓度、培养时间等条件，运用总有机碳分析仪、紫外和荧光光谱仪、质谱仪等分析手段，监测陆源有机碳随培养时间的变化，以及调查断面水体和沉积物有机碳含量、来源和组成的时空变化。结果显示，尽管部分培养实验显示海陆混合增加了陆源有机碳的微生物降解作用，但单纯的微生物作用无法解释河口近海区域水体中陆源有机碳浓度的大幅度下降，而浊度的降低引起的光降解增强可能是富含腐殖酸类陆源有机碳去除的重要原因。此外，河口沉积物中大量的陆源有机碳信号显示在陆源有机碳进入河口后，浊度、盐度、水动力的剧烈变化引起了显著的物理絮凝作用，导致大量的陆源有机碳进入沉积物中，进而发生一系列的生物地球化学过程。因此光降解和絮凝作用可能是引起近海水体陆源有机碳浓度大幅度下降的主要原因。此外，河口区的水体和沉积物有机碳含量和组成均显示出明显的季节性变化，进一步证实了陆源有机碳在河口活跃的降解和埋藏过程。总之，基于课题组四年的研究，陆源有机碳在河口近海区域出现的快速变化是一个物理、化学和生物作用的综合结果，单纯的激发效应不能解释海洋中陆源有机碳的失汇现象。课题组将在后期继续通过野外调查和模拟实验开展这种综合效应的评估工作。