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

海洋覆盖了地球的大部分表面，它的物理、生物和化学过程决定了地球气候和大气化学的许多方面。生物可利用氮（或“固定氮”）是海洋中微小植物（浮游植物）必需的营养物质。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。跟踪固定氮的流动和转化是了解物理、生物、化学如何相互作用以决定海洋的生物肥力及其对大气中二氧化碳的储存的有力方法。有了这样的了解，我们也许能够预测海洋将如何响应人类活动而变化，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”的形式出现：N为14N和15N， O为16O和18O。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在海洋中起作用的过程，否则这些过程很难解开。此外，海洋中N的同位素比值随时间的变化记录在海底沉积物的有机质中。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吸收如何随着过去的气候变化而变化，这可能表明这种吸收在未来将如何变化。这个项目将产生迄今为止最大的印度洋硝酸盐同位素组成数据集。印度洋是全球海洋的三个主要盆地之一，也是被研究最少的一个，它可能被证明是重建过去海洋表面状况的最佳盆地，因为它的沉积物中有丰富的微化石。在这个项目中产生的硝酸盐同位素的测量将有许多用途。例如，它可以确定硝酸盐从深水转移到阳光照射的表层水域的循环路径，而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来重建过去的海洋条件，这些方法利用了有孔虫（一种浮游动物）化石壳中捕获的有机物。为了解释这些目标，并将我们对海洋的理解传播给教育工作者，在项目的两年里，每年都会为普林斯顿大学教师准备项目的教师学者项目举办为期一天的研讨会，该项目将把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，以便在课堂上主动学习。本科生将通过暑期实习、初级项目和/或高级论文参与研究。该项目将对即将到来的2019-2020年GO-SHIP I05在南印度洋从南非到澳大利亚30-33°s的区域剖面收集的样品中的硝酸盐15N/14N和18O/16O比率进行分析。硝酸盐15N/14N和18O/16O为海洋固定氮的循环和输入/输出预算提供了关键约束。I05硝酸盐同位素数据将广泛有用，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，特别是在远离低[O2]区域的主要盆地向北，该项目将填补一个主要的地理空白。此外，它将以目前最高的精度提供密集的空间和深度覆盖，允许基于水文的新型N通量计算。硝酸盐同位素样本将从与GO-SHIP水文和生物地球化学测量相同的瓶子中提取，最大限度地提高硝酸盐同位素数据集的能力。该数据集将用于越来越多的数值海洋模式模拟，包括生物地球化学，用于识别区域海洋学过程，以及用于古海洋学工具的地面真实性。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。

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.