中尺度气旋式涡旋作用下有机碳与生源硅输出通量的动态变化与耦合：涡旋演化与亚中尺度过程的重要性

Mesoscale eddies induce extensive spatial variability in biological production and are critical to the biologically mediated downward transport known as biological pump and sequestration of carbon in the ocean interior. Limited field observations that typically only sample the eddy center at a specific stage of the eddy lifespan have however shown contradictory results on whether eddy enhances export productivity. Questions also remain on whether export follows the biomass stimulation with time lags during the eddy lifecycle. It is also unclear how organic carbon and opal (biogenic silica) are coupled or decoupled during eddy processes. Our prior researches based on a month long field observation of two cyclonic eddies in Western South China Sea have shown significant variability in 234Th-based export fluxes due to submesoscale interactions between eddy edges and cores. We also show significantly enhanced export in one eddy during its early intensification-mature stage but much less export in another eddy during its decaying period, and such timely but transient export fluxes are ubiquitous in oligotrophic subtropical ocean by revisiting the literature eddy export flux data. Furthermore, our data have demonstrated that carbon and opal exports are coupled during the eddy intensifying stage but decoupled during its decaying stage. We suggest therefore a better understanding to submesoscale variability and the stage-dependence of export productivity critical in revisiting prior hypothesis and framing eddy biogeochemistry in the ocean. This proposal chose a prominent cyclonic eddy in the Northwest Pacific Ocean off Taiwan in spring (Eddy SILICON) to examine the eddy evolution and its submesoscale variability and how these spatial-temporal variability modulate the exports of carbon and opal and their coupling. Through intensive and high spatial-temporal resolved field observations and numerical modeling, we are to answer the following critical questions: (1) How do submesoscale processes of a cyclonic eddy modulate the spatial variability of carbon and opal exports? (2) how does eddy evolution affect carbon and opal exports? (3) Are the exports of carbon and opal coupled or decoupled on the spatial scale during the submesoscale variability and on the temporal scale during eddy evolution? We content that the proposed research is crucial in better understanding the biological carbon pump and its role in the global carbon cycle.

海洋生物泵是影响海洋-大气碳交换的关键过程之一，调控着海洋碳储量和大气CO2浓度，影响全球气候系统。过去30年，纵然国际学界长期投入，对生物泵的认识仍然十分有限。本项目基于申请团队扎实的研究基础，以中尺度冷涡为案例解剖“麻雀”，围绕表征生物泵效率的有机物输出通量的时空格局这一主线，探讨冷涡的时空演变，特别是冷涡的演化以及冷涡系统在亚中尺度水平上的变异及其与有机碳、生源硅输出通量之间的动态变化关系，揭示其中的生物泵的关键过程与机制；项目拟解决如下关键科学问题：1）亚中尺度过程如何调节中尺度涡中颗粒有机碳与生源硅的输出？2）冷涡动力场随时间的演变如何影响颗粒有机碳和生源硅的输出？3）中尺度涡的时空变异如何影响颗粒有机碳与生源硅输出的耦合或非耦合？这些过程如何调控海-气界面CO2的源汇格局？项目的实施可为全面认识、定量解析海洋生物泵及其在碳循环中的作用奠定基础。

海洋生物泵是影响海洋-大气碳交换的关键过程之一，调控着海洋碳储量和大气CO2浓度，影响全球气候系统。过去30年，纵然国际学界长期投入，对生物泵的认识仍然十分有限。本项目以中尺度冷涡为案例解剖“麻雀”，围绕表征生物泵效率的有机物输出通量的时空格局这一主线，探讨冷涡的时空演变，特别是冷涡的演化以及冷涡系统在亚中尺度水平上的变异及其与有机碳、生源硅输出通量之间的动态变化关系，揭示其中的生物泵的关键过程与机制。项目开展了大量的历史数据集成工作，并于2019年3- 4月期间针对位于西北太平洋的特征冷涡开展了多手段(船基观测与采样，生物光学漂流剖面浮标，载带生物地球化学传感器的水下滑翔机以及遥感观测等)多学科高时空分辨率观测。通过本项目的实施，首次揭示了涡旋中心与边缘均存在亚中尺度变异，厘清了亚中尺度过程对生物泵特别是输出生产力的影响，并发现在系统水平上(同时考虑时间与空间)，冷涡增强了硅输出却并未引起碳输出增强。冷涡还改变了浮游植物利用营养盐的过程，引发硝化作用两步氧化过程的空间解耦，并可强化固氮作用。基于系统观测，本项目基于运动控制体内的质量守恒方程，在等密度面坐标上建立物质守恒方程，解析了该涡旋的营养盐收支，计算了中尺度涡旋内净群落生产力，并进一步估算了中尺度涡旋对北太平洋流涡区新生产力的贡献和中尺度冷涡的“泵吸”效率。结合固氮速率，在营养盐匮乏层，固氮是新生产力/输出生产力主要的新氮来源；而在营养盐充足层，下层输入的无机氮才是支持新生产力营养盐的主要来源。整体上，下层输入的新氮营养盐是固氮的2.6倍。对比整个北太平洋流涡区（NPSG）平均的新生产力状况，中尺度涡旋内的新生产力分别以氮和磷计算比周围大洋高167±45% 和247±67%。中尺度涡旋所贡献的新氮（磷）占比整个NPSG的39±11%（51±14%）。以上一系列的创新性发现，进一步加强了对涡旋生物地球化学的认识，为最终回答中尺度涡旋对全球海洋生物泵以及输出生产力的贡献奠定了基础。

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