A high-density, high-precision zonal section of nitrate isotopes across the South Indian Ocean

海洋覆盖了地球表面的大部分，其物理、生物和化学过程决定了地球气候和大气化学的许多方面。生物有效氮（或“固定氮”）是海洋中微生物（浮游植物）的必需营养素。 固定氮的供应可以限制浮游植物的生长和海洋吸收二氧化碳的能力，二氧化碳是最重要的温室气体。遵循固定氮的流动和转化是理解物理学，生物学，化学如何相互作用以确定海洋的生物肥力及其远离大气的二氧化碳储存的有力方法。有了这样的理解，我们也许能够预测海洋将如何响应人类活动而发生变化，从而影响未来的生命条件。硝酸盐（NO3-）是海洋中固定氮的主要形式，其氮（N）和氧（O）原子都以不止一种质量或“同位素”存在：14 N和15 N为N，16 O和18 O为O。这些同位素的比例受到生物过程的影响，因此硝酸盐的同位素测量揭示了海洋中的工作过程，否则很难解开。此外，随着时间的推移，海洋中氮的同位素比率记录在海底沉积物的有机物中。因此，同位素使我们能够重建过去海洋的重要特征，例如海洋生物对二氧化碳的封存在过去的气候变化中如何变化，这可能表明这种封存在未来将如何变化。该项目将产生迄今为止关于印度洋硝酸盐同位素组成的最大数据集。印度洋是全球海洋的三大盆地之一，也是研究最少的一个，它可能被证明是重建过去表层海洋条件的最佳盆地，因为它的沉积物中含有丰富的微体化石。在这个项目中产生的硝酸盐同位素的测量将服务于许多目的。作为一个例子，它将允许确定硝酸盐从深水沃茨向上转移到阳光照射的表面沃茨的循环路径，在那里生长的浮游植物需要这种营养。作为第二个例子，这些测量将有助于校准新的氮同位素方法，以重建过去的海洋条件，该方法利用了浮游动物有孔虫化石外壳中捕获的有机物质。为了解释这些目标，并向教育工作者传播我们对海洋的理解，该项目的两年中，每年将为普林斯顿大学教师准备计划的教师学者计划举办为期一天的研讨会，该计划将初中和高中教师带到普林斯顿与教师互动，了解重要的科学问题和研究。讲习班将侧重于获取和可视化海洋数据集和其他海洋学内容，以便在课堂上积极学习。本科生将通过暑期实习，初级项目和/或高级论文参与研究。该项目将分析在即将到来的2019-2020年GO-SHIP I 05横跨南印度洋从南非到澳大利亚的30-33 ° S的带状断面中收集的样品中硝酸盐15 N/14 N和18 O/16 O的比例。硝酸盐15 N/14 N和18 O/16 O是海洋固定氮循环和输入输出平衡的重要制约因子。I 05硝酸盐同位素数据将具有广泛的用途，有可能成为全球海洋硝酸盐同位素数据集的基石。鉴于印度洋缺乏硝酸盐同位素测量，特别是在远离北部低[O2]区域的主要盆地，该项目将填补一个重大的地理空白。此外，它将提供密集的空间和深度覆盖面，具有目前最高的精度水平，允许新的水文为基础的计算氮通量。硝酸盐同位素样品将取自GO-SHIP水文和地球化学测量所用的同一个瓶子，以最大限度地利用硝酸盐同位素数据集。该数据集将用于越来越多的数值海洋模型模拟，包括海洋地球化学，用于识别区域海洋学过程，并为地面实况古海洋学工具。该奖项反映了NSF的法定使命，并已被认为是值得通过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的支持。

The ocean covers most of the Earth's surface, and its physical, biological, and chemical processes determine many aspects of Earth's climate and the chemistry of the atmosphere. Biologically available nitrogen (or "fixed N") is an essential nutrient for the microscopic plants (phytoplankton) in the ocean. The supply of fixed N can limit the growth of phytoplankton and the capacity of the ocean to absorb carbon dioxide, the most important greenhouse gas. Following the flow and transformation of fixed N is a powerful approach for understanding how the physics, biology, chemistry interact to determine the biological fertility of the ocean and its storage of carbon dioxide away from the atmosphere. With such an understanding, we may be able to predict how the ocean will change in response to human activities and thus affect the conditions for life in the future. Nitrate (NO3-) is the primary form of fixed N in the ocean, and both its nitrogen (N) and oxygen (O) atoms occur in more than one mass, or "isotope": 14N and 15N for N and 16O and 18O for O. The ratios of these isotopes are affected by biological processes, and so isotopic measurements of nitrate reveal processes at work in the ocean that are otherwise hard to disentangle. Moreover, the isotope ratio of N in the ocean through time is recorded in the organic matter of the sediments that accumulate on the seafloor. Thus, the isotopes allow us to reconstruct important features of the past ocean, such as how the sequestration of carbon dioxide by ocean life has changed over past climate changes, which may indicate how this sequestration will change in the future. This project will produce the largest data set so far on the isotopic composition of nitrate in the Indian Ocean. The Indian Ocean is one of the three major basins of the global ocean, the one that has been studied the least, and it may prove to be the best basin for reconstructing past surface ocean conditions because of the abundance of microfossils in its sediments. The measurements of nitrate isotopes generated in this project will serve many purposes. As one example, it will allow for a determination of the circulation path by which nitrate is transferred up from the deep waters to the sunlit surface waters where growing phytoplankton require this nutrient. As a second example, the measurements will help to calibrate new nitrogen isotopic methods for reconstructing past ocean conditions, which utilizes the organic matter trapped in the fossil shells of foraminifera, a type of zooplankton. In order to explain these goals and to spread our understanding of the ocean to educators, each of the two years of the project, a 1-day workshop will be held for the Teachers as Scholars program of the Princeton University Program in Teacher Preparation, which brings middle and high school teachers to Princeton to interact with faculty and learn about important scientific questions and research. The workshop will focus on accessing and visualizing ocean data sets and other oceanographic content for active learning in the classroom. Undergraduates will be involved in the research through summer internships, junior projects, and/or senior theses.This project will produce analyses of the nitrate 15N/14N and 18O/16O ratios in samples collected during the upcoming 2019-2020 GO-SHIP I05 zonal section across the South Indian Ocean from South Africa to Australia at 30-33 deg S. Nitrate 15N/14N and 18O/16O provide critical constraints on the cycle and input/output budget of marine fixed N. The I05 nitrate isotope data will be broadly useful, potentially serving as a cornerstone in the global ocean's nitrate isotope data set. Given the lack of nitrate isotope measurements in the Indian Ocean, especially in the main basin away from the low-[O2] regions to the North, this project will fill a major geographic gap. Moreover, it will provide dense spatial and depth coverage with the highest level of precision currently available, allowing for novel hydrography-based calculations of N fluxes. The nitrate isotope samples will be taken from the same bottles from which the GO-SHIP hydrographic and biogeochemical measurements will be made, maximizing the power of the nitrate isotope data set. The data set will be of use to the growing number of numerical ocean model simulations including biogeochemistry, for identifying regional oceanographic processes, and for ground-truthing paleoceanographic tools.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.