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

南印度洋的冰冷的地表水下沉到500米左右，在黑暗中漂流数千英里，大约40年后才在赤道附近的其他海洋盆地重新浮出水面。这一主要水团被命名为亚南极模态水（SAMW）。据估计，当海水变暖并上升到有阳光照射的亚热带和热带水域时，它所含的营养物质为那里75%的微观植物生长提供了燃料。在下沉之前，SAMW的化学性质被两种常见浮游植物的生长和不同的生理特性所改变；有硅壳的硅藻和有碳酸盐壳的球石藻。当地的物理动态影响着这两类浮游植物生长的地点和速度。因此，在SAMW形成点建立了不同的营养和微量化学指纹图谱。这个项目是一个非常详细的领域和建模工作，将记录和量化显著的，相互关联的过程，化学连接两个重要的海洋生态系统相隔半个地球。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源于重要的同行评审出版物和改进的预测模型。社会效益将从一系列学者的培训中得到发展，包括高中生、本科生和研究生，以及技术和博士后参与者。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的形成和俯冲如何控制全球北部水域的生产力，一月份对印度洋东南部的两次考察将识别、跟踪和研究独特的中尺度漩涡，这些漩涡作为离散的水体包裹，支持丰富的粒石藻或硅藻种群及其相关的微生物群落。这些漩涡将被拉格朗日阿尔戈漂移器跟踪，并将在几个月的时间尺度上精确地观察SAMW的化学条件（即Si， N， P， Fe和碳酸盐化学）。利用这些漩涡中生态过程与营养物质、微量金属和碳酸盐化学之间的反馈数据以及相关的样带巡航数据，该项目将有三个主要目标：(1)利用传统方法和下一代序列DNA/RNA谱分析，确定SAMW球石藻和硅藻对碳酸盐化学、营养物质和痕量金属浓度的调节速率，并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；(3)利用观测和全球数值模型，将这些发现与SAMW的Ekman和涡旋驱动俯冲结合起来，研究盆地尺度上的生物地球化学影响。在南纬30至60度的经向调查将用于表征随着SAMW俯冲并向北推进，碳酸盐化学、营养分布、生产力、遗传和各种浮游生物群生物量的更大规模变化。

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.