现生浮游有孔虫壳体键结有机氮同位素特征与真光层氮循环

The changing nitrogen cycle caused by anthropogenic excess nitrogen is among the urgent key issues in studying the global change. Understanding the natural process of marine nitrogen cycle is essential for an objective assessment about the impact of human activities on modern marine ecosystem, and also providing fundamental knowledge to predict future marine ecosystem change. Planktonic foraminifer is single cell phytoplankton inhabiting in the euphotic zone. Foraminifera shell-bound nitrogen isotope (δ15Nfb) has been reported as a potential tool to decipher the history of marine nitrogen processes. Currently interpretation of δ15Nfb relies on several assumptions that have not been properly validated. It is not yet known where the shell-bound nitrogen was sourced from? What is and what controls the δ15Nfb differences among foraminifera species? Moreover, selective feeding may complicate δ15Nfb record. In this proposal, I select the western North Pacific and South China Sea as research areas. This is the first attempt to trace the source origin of δ15Nfb by combining analyze the nitrogen isotopes in foraminifera shell, foraminifera biomass, phytoplankton, zooplankton, suspended particulate organic matter and water nitrate, also considering the in situ chemical, biological, and physical parameters. The isotopic signal transmission from inorganic nutrient to δ15Nfb signal and its controlling factors will be quantified. The species-specific δ15Nfb will be identified. Results may bridge the knowledge gap and hopefully benefit the applicability of δ15Nfb as a proxy to reconstruct the marine nitrogen cycle history.

人类活动过剩氮对海洋天然氮循环的干扰是全球变化研究中的关键科学问题之一。认识海洋氮循环的自然变化规律，能为客观评估人类活动对现代海洋生态系统的影响，以及对未来趋势预测提供基础参考。浮游有孔虫壳体键结有机氮同位素（δ15Nfb）记录了上层海洋氮循环的信息，具有应用于重建海洋氮循环的潜力。但目前对浮游有孔虫壳体δ15Nfb所记录信息的解读极度缺乏现代调查资料的支撑。例如，尚不清楚浮游有孔虫壳体δ15Nfb信号的来源及影响因素，壳体δ15Nfb的种间差异及受控因素等。通过收集并分析西太平洋典型站位现生浮游有孔虫壳体和有机体，以及浮游动物、浮游植物、分粒级颗粒物、和水体硝酸盐样本的氮同位素信息，同时结合船测水文参数，本项目将首次系统性探索无机活性氮同位素信息向浮游有孔虫壳体传递的过程及影响因素，以期客观评估浮游有孔虫壳体δ15Nfb指标被用于研究地质历史时期真光层氮循过程的潜力与可靠性。

本项目计划采集西太平洋不同种类的现生浮游有孔虫，开展浮游有孔虫壳体键结氮同位素（δ15Nfb）信号来源的研究。通过三年的工作开展，项目组基本完成了计划书的拟定任务，回答了项目书所列科学问题。利用采集自南海和孟加拉湾的拖网样品，发现浮游有孔虫Globigerinoides sacculifer种的配子生殖钙化（gametogenic calcification）过程没有向成年壳体中引入温跃层水体环境的氮同位素信号，证实了该浮游种壳体的δ15Nfb可以用来指示真光层水体氮同位素信息。进一步的，项目组利用南海北部沉积捕获拖网样品，分别对上真光层（Globigerinoides sacculifer 和 Orbulina universa）和下真光层（Globigerinella siphonifera）浮游有孔虫壳体进行键结氮同位素分析，研究发现随着栖息深度的增加，δ15Nfb逐渐增加并且与水体悬浮颗粒有机物（sPOM）的δ15N分布相似。项目组提出sPOM应该是浮游有孔虫的主要食物来源，而南海真光层δ15Nfb的种间差值的变化可能反应上层水体的混合强度。上述成果为利用浮游有孔虫δ15Nfb指标重建上层海洋氮循环过程提供了现代依据。此外，借助本项目资助，项目组对冲绳海槽生物泵演化和有机碳埋藏等方面开了部分工作，获得了创新性成果。项目执行期间，项目负责人以第一作者/通讯作者发表标注SCI论文4篇（项目预期结果为发表论文1-3篇），其中包括《Geology》、《Marine Geology》、《Deep Sea Research Part I: Oceanographic Research Papers》等。