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，它们共同限制了大部分热带、亚热带和温带海洋的生产力。氮的循环及其投入/产出预算都被认为可以控制海洋的肥力和海洋在大气二氧化碳排放中的作用。有孔虫的CaCO3测试可以代表相当一部分海洋沉积物，在古海洋学中得到了广泛的应用；它们是微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开发了一种用于有孔虫壳层结合N的同位素分析的协议。目前的协议比典型的在线燃烧至少敏感100倍，从而使N同位素档案的快速进展得以实现，而这在以前是无法测量的。对表层沉积物的测量和来自加勒比的岩心下部记录表明，受有孔虫限制的del15N(FB-del15N)有望为古海洋学提供一个强大的N同位素档案，而且鉴于存在具有不同深度生境和行为的多个有孔虫物种，该档案具有独特的丰富潜力。此外，加勒比海记录中的这一发现--冰河时代大西洋的氮固定减少--改变了关于海洋固定氮的输入/输出预算的性质及其改变海洋肥力和大气二氧化碳的潜力的科学对话。然而，对FB-del15N的控制还没有得到充分的研究。在这个项目中，作为为FB-del15N的古海洋学应用奠定基础的第一个重要步骤，同一个普林斯顿大学团队将研究其在水柱中的成因、向海底的输送和早期成岩作用。他们将在百慕大以南的马尾藻海进行这项研究。总体而言，这是海洋中研究得最好的区域之一，就有孔虫而言，这一区域在过去十年和以前一直是PI的N同位素研究的重点。此外，其显著的季节性--在物理海洋学、生物地球化学和有孔虫物种丰度方面--将有助于在机械水平上理解FB-del15N的控制。该研究小组将在两年内参与六次百慕大大西洋时间序列研究(BATS)，收集有孔虫和其他N表，可能有助于深入了解FB-del15N的控制。他们将从附近的大洋流计划(OFP)系泊沉积物圈闭和该地区收集的浅层沉积物中挑选有孔虫贝壳，并再次进行相关的辅助测量。这项工作将建立不同物种的有孔虫生物量与壳层结合的del15N的关系，有孔虫同位素数据与上层海洋N池的比较将产生经验同位素关系，并努力从机理上深入了解FB-del15N(例如，不同N池对不同有孔虫饲料的重要性；藻类共生体的作用)。沉积物圈闭和表层沉积物数据将通过整合较长时间尺度的浮游生物TOW数据来支持浮游生物TOW数据，还将解决关于后期(例如配子形成)钙化和FB-del15N和FB-N含量的早期成岩作用的问题。更广泛的影响：这项研究将更好地理解有孔虫的营养动态，有孔虫是一类其贝壳是微古生物学和古气候学的核心工具的生物。该项目还将以首席调查员对百慕大海洋科学研究所的参与为基础，将其作为整合本科生和研究生与海洋有关的教育和研究的资产。

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.