叶绿素单体氮同位素（δ15N）重建长江口及毗邻海域低氧区氮丢失强度记录

Nitrogen cycling in the oxygen minimum zone (OMZ) in coastal waters is closely connected with the development of marine ecosystem. The long-term variations of nitrogen loss in association with OMZ signal the changes in the processes of nitrogen source, sink and internal cycling. A high-resolution reconstruction of nitrogen loss intensity over the last 100 years is able to reveal the variations of nitrogen cycling in the OMZ that is located in the vicinity of Yangtze River Estuary and adjacent East China Sea. Differing from the δ15N of total nitrogen in sediment, the δ15N of chlorophyll pigment keeps its isotopic integrity during various diagenetic processes that incur isotopic fractionation, thus it could accurately record the signal of nitrogen loss in the OMZ in shelf waters. The recently-developed analytical method of compound specific δ15N of chlorophyll pigment at a concentration of nano-molar level could facilitate the achievement of this research objective. .This study aims at reconstructing annual/decadal variations of nitrogen loss intensity over the last 100 years in the OMZ in shelf waters of Yangtze River Estuary and adjacent East China Sea. The nitrogen loss variations are screened from the comparisons of the core records of δ15N of chlorophyll pigment in two sediment cores, which are retrieved separately from OMZ and Kuroshio upwelling zones. Analyses of other geochemical proxies, like opal content and alkenone-derived sea surface temperature etc., further constrain the results of nitrogen loss variations. Historical dataset of Yangtze River input and Kuroshio current flux are assimilated with the reconstructed nitrogen loss variations, for a projection of the nitrogen loss intensity in association with OMZ development in the studied area.

近海生态系统低氧区中的氮循环过程已成为近年来海洋科学研究的热点问题之一，其长期变化与近海生态系统的演化和发展密切相关。因此，重建长江口毗邻海域低氧区中氮丢失强度近百年来的高精度变化，对于我们认知低氧区中氮这一重要生源要素的源、汇和转化过程，揭示低氧区的发展趋势，具有十分重要的基础科学意义。与沉积物总氮同位素相比，叶绿素单体氮同位素不受氮同位素分馏过程影响，可灵敏表征水体中氮丢失强度信号，使以上研究成为可能。.本课题针对长江口及毗邻海域低氧区中氮丢失强度近百年来的年代际变化和演化趋势这一科学问题，采用叶绿素单体氮同位素分析方法，通过分别分析低氧核心区和黑潮涌升区沉积物柱状样品中叶绿素单体的氮同位素值，结合其它生物地球化学参数的沉积地质记录（海表温度、生物硅含量等），重建低氧水体中氮丢失强度的历史变化。同时结合长江径流入海通量和黑潮观测等历史数据，初步分析低氧区发生、发展的驱动机制和演化趋势。

该项目研究海域是长江口及其毗邻东海，利用叶绿素单体氮同位素示踪东海主要氮循环过程，通过沉积物柱中叶绿素单体氮同位素反演重建东海长江口低氧区氮循环过程。在该项目的支持下，系统建立了基于细菌反硝化法的海水硝酸盐、溶解有机氮、叶绿素单体同位素测定方法，分析技术在国内处于领先。首次完成了黄海表层沉积物和长江口表层沉积物的叶绿素单体氮同位素测定，刻画了水体颗粒氮循环转化路径；完成了黄海水体、东海水体硝酸盐氮氧同位素的测定，刻画了东海硝酸盐源汇过程及转化过程；评估了大型海藻爆发对于水体氮库结构的影响和机制。项目先后参加长江口基金委共享航次2018-2019年共6个航次，搭载其它东海航次5个，取得了极为丰富的水体、颗粒物和沉积物样品。此外，项目选取了30个夏季航次代表性样品与国际著名同位素测定实验室美国UC-Davis分校的SIF开展测试比较。项目已经发表论文3篇（SCI 1篇，EI 1篇，中文核心1篇），投稿SCI论文3篇；会议论文2篇。项目培养硕士研究生1名，培养8位本科毕业生，在读博士研究生1名，美籍学生交换生1名。