Understanding the nitrogen isotopes of planktonic foraminifera: A modern Sargasso Sea study

氮（N）和磷是海洋中浮游植物大量需要的两种营养物质，它们一起限制了大多数热带，亚热带和温带海洋的生产力。N的循环及其输入/输出预算都被认为控制了海洋的肥力和海洋在设定大气CO2中的作用。 有孔虫的碳酸钙测试可以代表海洋沉积物的很大一部分，并已被广泛用于古海洋学;它们是微体化石结合有机质同位素分析的明显目标。近年来，普林斯顿大学的研究人员已经开发出一种用于有孔虫壳结合氮同位素分析的方法。 目前的协议是至少100倍以上的敏感性比典型的在线燃烧，允许快速进展的N同位素档案，以前是不可行的测量。从加勒比海的表层沉积物和downcore记录的测量显示有孔虫约束的del 15 N（FB-del 15 N）的承诺，既提供了一个强大的N同位素档案古海洋学，一个具有独特的潜力的丰富性，鉴于存在多个有孔虫物种与不同的深度栖息地和行为。 此外，加勒比海记录的发现-冰河时期大西洋的氮固定减少-改变了关于海洋固定氮的投入/产出预算性质及其改变海洋肥力和大气CO2的潜力的科学讨论。 然而，对fb-del 15 N的控制还没有得到充分的研究。在这个项目中，作为为fb-del 15 N的古海洋学应用奠定基础的第一个重要步骤，普林斯顿大学的同一个团队将研究其在水柱中的成因，向海底的迁移和早期成岩作用。他们将在百慕大以南的马尾藻海进行这项研究。这是海洋中研究得最好的区域之一，一般来说，就有孔虫而言，这是一个在过去十年和以前一直是PI N同位素研究重点的区域。此外，其显着的季节性-在物理海洋学，地球化学和有孔虫物种丰度-将有助于努力了解的控制fb-del 15 N在机械水平。 该研究小组将在两年内参加六次百慕大大西洋时间序列研究（BATS）巡航，收集有孔虫和其他N形式，可能提供对fb-del 15 N控制的见解。从附近的海洋通量计划（OFP）系泊沉积物陷阱和该地区收集的浅层沉积物中，他们将挑选有孔虫壳并再次进行相关的辅助测量。这项工作将建立有孔虫生物量与不同物种的壳结合del 15 N的关系，有孔虫同位素数据与上层海洋N库的比较将产生经验同位素关系，并致力于fb-del 15 N的机理洞察（例如，不同的N库对不同有孔虫食物的重要性;藻类共生体的作用）。沉积物收集器和表层沉积物数据将通过在较长时间尺度上进行整合来支持浮游生物拖曳数据，并将解决有关后期阶段的问题（例如，配子形成）钙化和早期成岩作用的fb-del 15 N和fb-N含量。更广泛的影响： 这项研究将有助于更好地了解有孔虫的营养动态，有孔虫是一类生物，其外壳是微体古生物学和古气候学的核心工具。 该项目还将利用首席研究员参与百慕大海洋科学研究所的工作，将其作为本科生和研究生两级海洋教育和研究一体化的一项资产。

Nitrogen (N) and phosphorus are the two nutrients required in large quantity by phytoplankton in the ocean, and together they limit productivity throughout most of the tropical, subtropical, and temperate ocean. Both the cycling of N and its input/output budget have been argued to control the fertility of the ocean and the ocean's role in setting atmospheric CO2. The CaCO3 tests of foraminifera can represent a substantial fraction of marine sediments and have been used extensively in paleoceanography; they are an obvious target for isotopic analysis of microfossil-bound organic matter. In recent years, researchers at Princeton have developed a protocol for the isotopic analysis of foraminiferal shell-bound N. The current protocol is at least 100 times more sensitive than typical on-line combustion, allowing for rapid progress with a N isotope archive that was previously not feasible to measure. Measurements on surface sediments and a downcore record from the Caribbean show the promise of foraminifera-bound del15N (fb-del15N) to provide both a robust N isotope archive for paleoceanography, and one with a unique potential of richness, given the existence of multiple foraminiferal species with different depth habitats and behaviors. Moreover, the finding from the Caribbean Sea record -- reduced N fixation in ice age Atlantic -- has changed the scientific conversation about the nature of the input/output budget of oceanic fixed N and its potential to change ocean fertility and atmospheric CO2. However, the controls on fb-del15N have not yet been adequately studied.In this project, as a first major step in developing a foundation for the paleoceanographic application of fb-del15N, the same Princeton University team will study its genesis in the water column, transport to the seafloor, and early diagenesis. They will undertake this study in the Sargasso Sea south of Bermuda. This is one of the best studied regions of the ocean, in general and with respect to foraminifera, and a region that has been has been a focus of the N isotope research of the PI for the last decade and others previously. Moreover, its significant seasonality -- in physical oceanography, biogeochemistry, and foraminiferal species abundance -- will facilitate the effort to understand the controls on fb-del15N at a mechanistic level. The research team will participate in six Bermuda Atlantic Time-series Study (BATS) cruises over two years, collecting foraminifera and other N forms likely to provide insight into the controls on fb-del15N. From the nearby Oceanic Flux Program (OFP) moored sediment traps and from shallow sediments collected in the region, they will pick foraminifera shells and again make relevant ancillary measurements. This work will establish the relationship of foraminiferal biomass to shell-bound del15N for different species, and comparison of the foraminiferal isotope data with the upper ocean N pools will yield empirical isotopic relationships and work toward a mechanistic insight of fb-del15N (e.g., the importance of different N pools to the diets of different foraminifera; the role of algal symbionts). The sediment trap and surface sediment data will support the plankton tow data by integrating over longer time scales and will also address questions regarding late stage (e.g., gametogenic) calcification and the early diagenesis of fb-del15N and fb-N content. Broader Impacts: This study will yield an improved understanding of the nutrient dynamics of foraminifera, a class of organisms whose shells are a central tool in micropaleontology and paleoclimatology. The project will also build on the principal investigator's involvement in the Bermuda Institute of Ocean Sciences as an asset for integrating ocean-related education and research at both the undergraduate and graduate levels.