Collaborative Research: Biogeochemical and Physical Conditioning of Sub-Antarctic Mode Water in the Southern Ocean

南印度洋表面的冷水下沉到约500米处，并在黑暗中流动数千英里，大约40年后，它在其他大洋盆地的赤道附近重新浮出水面。这个主要的水团被命名为亚南极模式水(SAMW)。当它变暖并上升到阳光充足的亚热带和热带水域时，它含有的营养物质估计为那里75%的微小植物生长提供了燃料。在它下沉之前，SAMW的化学性质被两种常见浮游植物的生长和独特的生理作用所改变，这两种浮游植物分别是由二氧化硅制成外壳的硅藻和具有碳酸盐外壳的球藻。当地的物理动力学影响着这两类浮游植物的生长地点和生长速度。因此，在SAMW形成点建立了不同的营养和痕量化学指纹。这个项目是一个异常详细的领域和建模工作，将记录和量化在化学上连接相隔半个地球的两个重要海洋生态系统的非凡的、相互关联的过程。领导该项目的科学家将研究SAMW生物和化学调节的复杂性，从而提供有关生物碳泵的大规模海洋控制的关键数据，该生物碳泵在千年时间尺度上将大气中的二氧化碳移至深海。该项目的科学影响将来自重要的同行评审出版物和改进的预测模型。培训一系列学者，包括高中、本科生和研究生，以及技术和博士后参与者，将产生社会效益。一名高中教师和科学传播专家将与该项目一起出海，并通过社交媒体和预定的网络互动与岸上的学生分享船上的经验。为了研究SAMW形成和俯冲如何很好地控制北部全球水域的生产力，1月份对东南印度洋的两次探险将识别、跟踪和研究独特的中尺度涡旋，这些涡旋作为离散的水块，支持丰富的球藻或硅藻及其相关微生物群落的种群。这些旋涡将用拉格朗日ARGO漂移仪进行跟踪，并将在几个月的时间尺度上准确地观察SAMW的化学条件(即Si、N、P、Fe和碳酸盐化学)。利用在这些漩涡和相关样带巡航中生态过程与营养、微量金属和碳酸盐化学之间的反馈所获得的数据，该项目将有三个主要目标：(1)确定SAMW球藻和硅藻调节碳酸盐化学以及营养和微量金属浓度的速率，以及用传统方法和下一代序列DNA/RNA剖析评估研究区域的分类和生理多样性；(2)探索铁、硅酸盐和/或硝酸盐在控制藻类组合和遗传多样性方面的生长限制，以及(3)将这些发现与埃克曼和涡流驱动的SAMW俯冲结合起来，利用观测和全球数值模型来研究盆地尺度上的生物地球化学影响。从南纬30度到60度的经向调查将被用来描述随着SAMW俯冲并向北推进，不同浮游生物组的碳酸盐化学、营养分布、生产力、遗传学和生物量的更大尺度的变异性。

Cold surface water in the southern Indian Ocean sinks to about 500 meters and travels in the dark for thousands of miles before it resurfaces some 40 years later near the equator in the other ocean basins. This major water mass is named the Sub-Antarctic Mode Water (SAMW). Nutrients it contains when it warms and rises into the sunlit subtropical and tropical waters are estimated to fuel up to 75% of the microscopic plant growth there. Before it sinks, the chemical properties of the SAMW are modified by the growth and distinct physiology of two common phytoplankton; diatoms with shells made of silica, and coccolithophores with carbonate shells. Local physical dynamics influence where and how fast these two phytoplankton classes grow. Consequently, differing nutrient and trace chemical fingerprints are established at the point of SAMW formation. This project is an exceptionally detailed field and modeling effort that will document and quantify the remarkable, interconnected processes that chemically connect two important oceanic ecosystems half a world apart. The scientists leading the project will study the complexity of the biological and chemical conditioning of the SAMW and thus provide critical data about the large-scale oceanic controls of the biological carbon pump that removes atmospheric carbon dioxide to the deep ocean over millennial timescales. Scientific impact from this project will stem from significant peer-reviewed publications and improved predictive models. Societal benefits will develop from training of a range of scholars, including high school, undergraduate, and graduate students, as well as technical and post-doctoral participants. A high school teacher and science communication specialist will go to sea with the project and share experiences from the ship with students on shore via social media and scheduled web interactions. To examine how SAMW formation and subduction controls the productivity of global waters well to the north, two January expeditions to the SE Indian Ocean will identify, track, and study the unique mesoscale eddies that serve as discrete water parcels supporting rich populations of either coccolithophores or diatoms plus their associated microbial communities. The eddies will be tracked with Lagrangian Argo drifters and observations will be made of exactly how SAMW is chemically conditioned (i.e. Si, N, P, Fe, and carbonate chemistry) over time scales of months. Using data obtained on the feedback between ecological processes and nutrient, trace metal, and carbonate chemistry in these eddies and on related transect cruises, the project will have three main goals: (1) determine the rates at which SAMW coccolithophores and diatoms condition the carbonate chemistry plus nutrient and trace metal concentrations, as well as assess taxonomic and physiological diversity in the study area with traditional methods plus next-generation sequence DNA/RNA profiling, (2) explore growth limitations by iron, silicate and/or nitrate in controlling algal assemblages and genetic diversity, and (3) combine these findings with the Ekman- and eddy-driven subduction of SAMW to examine biogeochemical impact on a basin scale, using both observations and global numerical models. A meridional survey from 30 to 60 degrees south latitude will be used to characterize the larger-scale variability of carbonate chemistry, nutrient distributions, productivity, genetics and biomass of various plankton groups as SAMW is subducted and proceeds northward.