南海海盆陆源颗粒大气沉降通量的长寿命Th同位素示踪

Terrestrial particle deposition from the atmosphere has been demonstrated to play a key role in regulating the ecosystem and carbon cycles in the low nutrient low chlorophyll South China Sea (SCS) Basin. However, the published atmospheric deposition fluxes of terrestrial particles to the SCS differ from each other to an extent of two magnitudes with an ambiguous spatiotemporal variability. Consequently, the quantitative assessment of biogeochemical impact of atmospheric deposition of terrestrial particles on the SCS still appears unlikely and further hinders the comprehensive investigation of biogeochemical cycles in the SCS. In this proposed research, a newly developed geochemical approach involving the tracer of long-lived thorium isotopes (230Th and 232Th) will be applied to better quantify the atmospheric deposition flux of terrestrial particles to the SCS Basin. In this approach, the residence time of thorium calculated from the radioactive disequilibrium between 230Th and 234U can be used to establish the supply rate of dissolved 232Th in the upper water column of the SCS combining with the 232Th inventory. The supply rate of dissolved 232Th, which is mainly derived from the partial dissolution of terrestrial particles supplied by atmospheric deposition, can be further converted into the atmospheric deposition flux of terrestrial particles in the SCS Basin with the field measured aerosol thorium content and solubility in seawater. We propose to first validate the long-lived thorium isotopic tracer for the estimation of atmospheric deposition of terrestrial particles in the SCS with a synthetic intercomparison between our newly produced atmospheric deposition data and historic datasets obtained with traditional approaches, including island- and ship-based aerosol observations, satellite remote sensing and atmospheric deposition model. Then the atmospheric deposition flux of terrestrial particles in the SCS Basin will be accurately estimated and its spatiotemporal variability will be clearly presented afterwards. The impact of anthropogenic activities in the continent and islands around the SCS, seasonal and interannual monsoon variability, and aerosol source variability on the atmospheric deposition flux of terrestrial particles in the SCS Basin will be further investigated for a better atmospheric deposition modelling and projection practice in this region. The research outcome will provide key geochemical data input for the development of regional ecosystem and biogeochemical model in the SCS and also provide scientific information and support for the Chinese marine ecosystem and climate policies.

南海海盆是典型的寡营养低生产力海区，陆源颗粒的大气沉降对该海区生态系统及碳循环起着重要的调控作用。迄今为止，已有的南海陆源颗粒的大气沉降通量数据差别甚大，时空分辨率低，阻碍了对其调控作用的量化，以及对南海生物地球化学循环的深入研究。针对这一研究缺陷，本项目将首次采用新兴的长寿命Th同位素示踪法来估算南海海盆陆源颗粒的大气沉降通量。依据上层水柱中230Th与其母体234U的放射性不平衡得到Th的停留时间，结合陆源232Th在上层水柱中的贮量以及实测的气溶胶中Th的含量与海水溶解度数据计算南海海盆中陆源物质的大气沉降通量。通过与传统研究手段的综合比对分析，论证长寿命Th同位素示踪法在南海应用的可行性，得到精确的南海陆源颗粒的大气沉降通量及其时空变化图像，揭示人为活动、东亚季风、气溶胶来源的影响，为构建南海生物地球化学与生态模型提供准确的基础参数，为相关海洋生态及气候政策的制定提供科学支撑。

南海及毗连的西太平洋是典型的寡营养低生产力海区，大气沉降是东亚大陆陆源营养要素向南海海盆及西太平洋输送的主要途径，并成为维持南海及西太平洋浮游植物初级生产力、驱动南海及西太平洋碳循环及碳汇效应的关键过程。本项目采用长寿命Th同位素示踪技术，结合实测的气溶胶Fe、Th溶解度，准确估算了南海及西太平洋中陆源颗粒及Fe的大气沉降通量，为构建全球大气沉降通量数据库、标定全球海洋大气沉降模型以及研究大气沉降的海洋生态效应提供了第一手数据。.本项目基于东亚气溶胶源区土壤的实验结果建立了气溶胶中Fe、Th溶解度的化学浸取程序。应用该程序成功获得了厦门城市气溶胶、南海及西太平洋海洋气溶胶中Fe、Th的溶解度。根据获得的南海及西太平洋海水中长寿命Th同位素含量的剖面分布图像，结合实测的气溶胶Fe、Th溶解度数据估算了南海及西太平洋陆源颗粒及Fe的大气沉降通量，并剖析其时空变化特征及气溶胶来源与东亚季风的影响机制。实测的气溶胶溶解度数据证实基于大气沉降模型反推的气溶胶Th溶解度存在明显偏差，导致此前研究对大气沉降通量的低估。这一技术路线的应用也使本项目大大提高了所获得的大气沉降通量数据的准确性与可靠性，解决了长寿命Th同位素示踪技术的固有缺陷。.本项目系统阐述了南海东北部210Po-210Pb不平衡的形成机制，阐述了陆架沉积物再悬浮及侧向输运对南海颗粒动力学及物质循环的潜在影响，探讨了南海海盆Pa/Th分馏及边界清除效应，构建了南海东北部颗粒物质量平衡模型，定量了南海东北部中层水颗粒物侧向运移通量。本项目还搭建了国内首套符合国际GEOTRACES计划准入标准的大体积、全水深海洋痕量元素洁净采水系统，同时建立了大洋海水痕量金属Fe的流动注射分析及同位素稀释质谱分析技术，并在西太平洋4个航次中成功应用，为提升中国痕量元素海洋生物地球化学研究水平提供了扎实的技术支撑。