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 部分是一项即将开展的水采样和测量活动，重点关注南极洲周围的海洋。 例如，南大洋似乎在地球气候和环境条件中发挥着巨大作用。强烈影响：(1) 全球表层海洋的生物生产力，(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，将从表面到海底收集水样和水载颗粒，并对这些样本进行一系列新的、技术先进的化学测量，以更好地了解水的物理、化学、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为 GP17-OCE 做出贡献，硝酸盐是海洋中可进行光合作用的氮的主要形式。氮是海洋浮游植物（漂浮的单细胞藻类）的必需营养素，浮游植物是海洋食物网的基础，死亡浮游植物的下沉会导致二氧化碳储存在远离大气的深水中。是构成硝酸盐的氮原子和氧原子的多种形式（“同位素”），较重的同位素是氮 15 和氧 18，以及较轻的同位素是氮 14 和氧 16 当浮游植物使用硝酸盐构建其组织时，较轻的同位素转化得稍快，因此，硝酸盐的同位素比率显示了生物硝酸盐使用和降解的指纹。该项目中进行的硝酸盐同位素测量将提供物理运输和生化转化的三维图像。这将解决硝酸盐如何供应到南大洋表层水域、南大洋浮游植物消耗了多少供应量以及控制从南大洋流出的硝酸盐量的问题。海洋进入亚热带、热带和赤道海洋，以提高这些地区浮游植物的生产力，因为浮游植物以给定的比例消耗二氧化碳和硝酸盐，有关硝酸盐的信息也将表明这些区域的浮游植物的生产力。南大洋对空气中二氧化碳浓度的影响将在由该项目首席研究员主持的教师研讨会上为初中和高中科学教师总结。专注于海洋监测以及公众可用于可视化、呈现和解释环境数据的强大工具。美国 GEOTRACES GP17-OCE 部分将提供生成痕量元素和同位素 (TEI) 补充数据集的机会南方太平洋和南大洋，从塔希提岛到阿蒙森海，再到智利大陆架 对于 GP17-OCE 的所有站，研究人员将测量全深度剖面的硝酸盐 delta15N 和 delta18O，以及溶解有机氮和浓度。这些浓度测量将解决两组主题，对现代海洋过程和古海洋学的解释都有影响。首先，对南大洋物理、生物地球化学和碳循环过程的机械理解需要模拟南极和亚南极地区（AZ和亚南极地区）条件的季节模型。为了产生可靠的信息，这些模型必须同时模拟一套全面的地球化学测量，用于追踪养分输送（例如上升流和混合）、养分消耗、出口生产和再矿化。 GP17-OCE 在可用于解释的互补微量元素和同位素方面在硝酸盐同位素数据集中是独一无二的，为在多方面、数据丰富的背景下应用此类季节性模型提供了机会，这将有助于解决长期存在的问题。关于（1）原位养分和碳循环以及（2）夏季表层、下面的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，硝酸盐在海洋中的比例是多少。 SAZ 表面源自 AZ 与冬季与底层温跃层的混合，并且考虑到风驱动环流的其他限制，这对于极地锋区的涡流驱动传输速率意味着什么？南大洋——南极中层水和亚南极模式水——通过向全球海洋温跃层注入营养物质来维持低纬度海洋表层的生命。全球范围化学海洋学的一个核心问题是南大洋如何维持生命。海洋营养物来源与通过垂直混合输入低纬温跃层的硝酸盐相比，来自南大洋表面的硝酸盐的广泛上升在三角洲15N中升高，而深层硝酸盐则没有。全球比重跃层中的硝酸盐反映了来自南大洋表面与低纬度深海的比重比重硝酸盐的比例。 GP17-OCE 将通过提供南大洋来源和南太平洋内陆、重斜斜线内部和下方的同位素特征的新数据，帮助更好地限制这一部分。该奖项反映了 NSF 的法定使命，并被认为值得通过以下方式获得支持：使用基金会的智力价值和更广泛的影响审查标准进行评估。