US GEOTRACES GP17-OCE: Nitrate isotopic signals of the Southern Ocean's circulation and biogeochemistry

US GEOTRACES GP17-OCE：南大洋环流和生物地球化学的硝酸盐同位素信号

• 批准号: 2049416
• 负责人: Daniel Sigman
• 金额: $ 48.65万
• 依托单位: Princeton University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2021
• 资助国家: 美国
• 起止时间: 2021-10-01 至 2024-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2049416&HistoricalAwards=false
• 关键词: US; GEOTRACES; GP17; OCE; Nitrate

The U.S. GEOTRACES GP17-OCE section is an upcoming water sampling and measurement campaign that will focus on the Southern Ocean, the ocean around Antarctica. The Southern Ocean appears to play an outsized role in Earth’s climate and environmental conditions. For example, it appears to strongly affect: (1) the biological productivity of the surface ocean across the globe, (2) the levels of the greenhouse gas carbon dioxide (CO2) in the atmosphere, and (3) the concentration of oxygen (O2) in deep ocean waters. In GEOTRACES GP17-OCE, water samples and water-borne particles will be collected from the surface to the ocean bottom. On these samples, a suite of new, technically advanced chemical measurements will be made to better understand the physical, chemical, and biological processes of the Southern Ocean. The project described here will contribute to GP17-OCE by measuring the stable isotopes of nitrate in the seawater samples. Nitrate is the primary form of nitrogen in the sea that is available to photosynthetic life. This nitrogen is an essential nutrient for the ocean’s phytoplankton (floating, single-celled algae). Phytoplankton are the base of the marine food web, and the sinking of dead phytoplankton causes carbon dioxide to be stored in deep waters, away from the atmosphere. There are multiple forms (“isotopes”) of both the nitrogen and the oxygen atoms that make up nitrate. The heavier isotopes are nitrogen-15 and oxygen-18, and the lighter isotopes are nitrogen-14 and oxygen-16. When phytoplankton use nitrate to build their tissue, the lighter isotopes are converted slightly faster. As a result, the isotope ratios of nitrate show the fingerprints of biological nitrate use and of the degradation of the nitrogen in phytoplankton tissue back to nitrate. The nitrate isotope measurements made in this project will provide a three-dimensional picture of the physical transport and biochemical conversions of nitrate through the ocean regions from which the samples are collected. This will address how nitrate is supplied to Southern Ocean surface waters, what proportion of the supply is consumed by Southern Ocean phytoplankton, and what controls the amount of nitrate that flows from the Southern Ocean into the subtropical, tropical, and equatorial ocean to fuel phytoplankton productivity in those regions. Because phytoplankton consume carbon dioxide and nitrate in a given ratio, the information on nitrate will also indicates how the Southern Ocean impacts the carbon dioxide concentration in the air. The key lessons from this work will be distilled for middle and high school science teachers. This will be done in the context of a workshop for teachers lead by the project’s principal investigator. The workshop will focus on ocean monitoring and the powerful tools that are available to the general public to visualize, present, and interpret environmental data.The U.S. GEOTRACES GP17-OCE section will provide the opportunity to generate complementary data sets of trace elements and isotopes (TEI) across the South Pacific and the Southern Ocean from Tahiti to the Amundsen Sea, and to the Chilean shelf. For all stations of GP17-OCE, the investigators will measure the delta15N and delta18O of nitrate in full depth profiles and the delta15N and concentration of dissolved organic nitrogen and the concentration of dissolved organic phosphorus in a subset of samples from low-nitrate shallow waters. These measurements will address two sets of topics, with implications both for modern ocean processes and for the interpretation of paleoceanographic nitrogen (N) isotope data from the Southern Ocean and other ocean regions. First, a mechanistic understanding of Southern Ocean physical, biogeochemical, and carbon cycle processes requires seasonal models that simulate conditions in both the Antarctic and the Sub-Antarctic zones (AZ and SAZ, respectively). To yield robust information, these models must simultaneously simulate a comprehensive suite of geochemical measurements that trace nutrient transport (e.g., upwelling and mixing), nutrient consumption, export production, and remineralization. GP17-OCE will be unique among nitrate isotope data sets in the complementary trace elements and isotopes that can be applied to their interpretation, providing the opportunity to apply such seasonal models in a multifaceted, data-rich context. This will help to address longstanding questions regarding (1) in situ nutrient and carbon cycling and (2) physical and biogeochemical exchanges between the summer surface layer, the underlying remnant winter mixed layer, and the deeper ocean. For example, what proportion of the nitrate in the SAZ surface derives from the AZ vs. wintertime mixing with the underlying thermocline, and given other constraints on the wind-driven circulation, what does this imply for eddy-driven transport rates across the Polar Frontal Zone? Second, mid-depth waters formed in the Southern Ocean – Antarctic Intermediate Water and Subantarctic Mode Water – sustain life in the low latitude surface ocean by injecting nutrients into the global ocean’s thermocline. A central question in global-scale chemical oceanography is how this Southern Ocean nutrient source compares with the input of nitrate to the low latitude thermocline by vertical mixing and the widespread upwelling of deep water. Nitrate deriving from the Southern Ocean surface is elevated in delta15N whereas deep nitrate is not. As a result, the delta15N of the nitrate in the global pycnocline reflects the fraction of pycnocline nitrate that derives from the Southern Ocean surface vs. the low latitude deep ocean. The data from GP17-OCE will help to better constrain this fraction by providing new data for both the Southern Ocean sources and the isotopic characteristics of the South Pacific interior, within and below the pycnocline.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

美国GEOTRACES GP17-OCE部分是即将到来的水采样和测量活动，将重点关注南极周围的南大洋。南大洋似乎在地球的气候和环境条件中扮演着巨大的角色。例如，它似乎强烈影响：(1)全球海洋表面的生物生产力，(2)大气中温室气体二氧化碳（CO2）的水平，以及(3)深海中氧气（O2）的浓度。在GEOTRACES GP17-OCE中，将从海洋表面到海底收集水样和水性颗粒。为了更好地了解南大洋的物理、化学和生物过程，将对这些样本进行一套新的、技术先进的化学测量。这里描述的项目将通过测量海水样本中硝酸盐的稳定同位素来促进GP17-OCE。硝酸盐是海洋中氮的主要形式，可供光合生物使用。这些氮是海洋浮游植物（漂浮的单细胞藻类）必需的营养物质。浮游植物是海洋食物网的基础，死亡浮游植物的下沉导致二氧化碳被储存在远离大气的深水中。构成硝酸盐的氮原子和氧原子有多种形式（“同位素”）。重的同位素是氮-15和氧-18，轻的同位素是氮-14和氧-16。当浮游植物利用硝酸盐来构建它们的组织时，较轻的同位素转化得稍微快一些。因此，硝酸盐的同位素比值显示了生物硝酸盐利用和浮游植物组织中氮降解回硝酸盐的指纹。在这个项目中进行的硝酸盐同位素测量将提供硝酸盐在收集样本的海洋区域的物理运输和生化转化的三维图像。这将探讨硝酸盐是如何供应到南大洋表层水域的，南大洋浮游植物消耗了多少比例的供应，以及是什么控制了从南大洋流入亚热带、热带和赤道海洋的硝酸盐的数量，从而促进了这些地区浮游植物的生产力。因为浮游植物以一定的比例消耗二氧化碳和硝酸盐，所以硝酸盐的信息也将表明南大洋如何影响空气中的二氧化碳浓度。从这项工作中提炼出的关键经验教训将供初中和高中科学教师使用。这将在一个由项目首席研究员领导的教师研讨会的背景下完成。研讨会将集中讨论海洋监测和公众可用的强大工具，以便将环境数据可视化、呈现和解释。美国GEOTRACES GP17-OCE部分将提供机会，生成从塔希提岛到阿蒙森海以及智利大陆架的南太平洋和南大洋的微量元素和同位素（TEI）的补充数据集。对于GP17-OCE的所有站点，研究人员将在全深度剖面中测量硝酸盐的三角洲15n和三角洲18o，并在低硝酸盐浅水的一小部分样品中测量三角洲15n和溶解有机氮浓度和溶解有机磷浓度。这些测量将涉及两组主题，对现代海洋过程和对来自南大洋和其他海洋区域的古海洋学氮同位素数据的解释都有影响。首先，对南大洋物理、生物地球化学和碳循环过程的机制理解需要模拟南极区和亚南极区（分别为AZ和SAZ）条件的季节性模型。为了获得可靠的信息，这些模型必须同时模拟一套全面的地球化学测量，以追踪营养物质的运输（例如，上升流和混合）、营养物质的消耗、出口生产和再矿化。GP17-OCE将是硝酸盐同位素数据集中唯一的补充微量元素和同位素，可以应用于它们的解释，为在多方面、数据丰富的背景下应用这种季节性模型提供机会。这将有助于解决长期存在的问题，包括：(1)原位营养和碳循环；(2)夏季表层、下层残余冬季混合层和更深海洋之间的物理和生物地球化学交换。例如，南区表面的硝酸盐有多大比例来自于南区与下层温跃层的混合，以及考虑到风驱动环流的其他限制条件，这对涡旋驱动的极地锋区运输率意味着什么？其次，在南大洋形成的中深水——南极中间水和亚南极模态水——通过向全球海洋的温跃层注入营养物质来维持低纬度海洋表面的生命。全球尺度化学海洋学的一个中心问题是，南大洋的营养来源如何与通过垂直混合和深水的广泛上涌向低纬度温跃层的硝酸盐输入相比较。来自南大洋表面的硝酸盐在三角洲15n中升高，而深层硝酸盐则没有升高。因此，全球斜斜中硝酸盐的delta15N反映了来自南大洋表面与低纬度深海的斜斜硝酸盐的比例。GP17-OCE的数据将有助于更好地限制这一比例，为南大洋的来源和南太平洋内部、背斜内部和下方的同位素特征提供新的数据。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。