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。生物可利用氮(或“固定氮”)是海洋中微型植物(浮游植物)的基本营养物质。固定氮的供应会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。随着固定氮的流动和转化，是了解物理、生物、化学如何相互作用来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途径。有了这样的理解，我们或许能够预测海洋将如何随着人类活动的变化而变化，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，它的氮(N)和氧(O)原子都存在于一个以上的质量中，或者说“同位素”：N为14N和15N，O为16O和18O。这些同位素的比率受到生物过程的影响，因此对硝酸盐的同位素测量揭示了海洋中发挥作用的过程，否则很难分离。此外，随着时间的推移，海洋中N的同位素比率记录在海底积累的沉积物的有机质中。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封存在过去的气候变化中发生了怎样的变化，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，也是被研究最少的一个，由于其沉积物中含有丰富的微化石，它可能被证明是恢复过去表层海洋状况的最佳盆地。该项目中产生的硝酸盐同位素的测量将有多种用途。作为一个例子，它将允许确定硝酸盐从深水向阳光充足的表面水域向上转移的循环路径，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，以恢复过去的海洋状况，该方法利用了捕获在有孔虫化石外壳中的有机物，有孔虫是一种浮游动物。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，在该项目的两年中，每年都会为普林斯顿大学教师培训计划的教师学者计划举办为期一天的研讨会，该计划将把初中和高中教师带到普林斯顿与教职员工互动，学习重要的科学问题和研究。讲习班将侧重于获取海洋数据集和其他海洋学内容并将其可视化，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/或高级论文参与这项研究。该项目将对即将到来的2019-2020年从南非到澳大利亚的南印度洋I05纬向断面30-33度的样本进行硝酸盐15N/14N和18O/16O的分析。硝酸盐15N/14N和18O/16O对海洋固定N的周期和投入/产出预算提供关键限制。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，特别是在远离北部低[O2]地区的主要盆地，该项目将填补一个重大的地理空白。此外，它将以目前可用的最高精度提供密集的空间和深度覆盖范围，从而能够以新的水文学为基础计算N通量。硝酸盐同位素样本将从进行Go-Ship水文和生物地球化学测量的相同瓶子中提取，从而最大限度地发挥硝酸盐同位素数据集的作用。该数据集将用于包括生物地球化学在内的越来越多的海洋数值模型模拟，用于识别区域海洋过程，以及用于地面真实古海洋学工具。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.