Collaborative Research: A revised Plio-Pleistocene view of the effect of climate on North Pacific oxygenation from foraminifera-bound nitrogen isotopes

像大多数陆地生物一样，海洋中的生物，如浮游动物、鱼类和海洋哺乳动物，需要氧气来分解它们的食物并产生能量。然而，在海洋的三个不同区域，有一些深度(从大约200米到500米)由于细菌而耗尽了氧气。这些区域被称为缺氧区(ODZ)，其中最大的缺氧区位于热带北太平洋东部。臭氧管制区限制海洋生物的栖息地和活动，包括人类赖以生存的渔业。它们还影响海洋中重要营养物质的可获得性和循环。对这些营养物的影响可能会影响海洋的生物生产力和不同类型浮游植物的流行。最后，臭氧管制区可能会影响海洋与大气的温室气体(二氧化碳和一氧化二氮)的交换。这些气体会改变地球的气候。研究表明，ODZ在过去一个世纪里一直在扩大，可能是由于持续的全球变暖。然而，气候模型对未来ODZ范围的预测产生了一幅不确定的图景。重建ODZ过去的变化将澄清对ODZ范围的主要控制，提高预测未来变化的能力。在这个项目中，将在深海沉积物岩芯中测量捕获在浮游生物化石壁上的氮的化学成分，以重建热带北太平洋东部ODZ过去的变化。这些数据将与过去的气候和海洋属性进行比较，表明这些属性中哪些对ODZ的控制最强。在普林斯顿，从事这项研究的团队将领导一个关于气候和海洋的研讨会，供初中和高中STEM教师使用。这将是向公众公布结果的多个机会之一。马萨诸塞大学波士顿分校的研究团队将为本科生提供有偿研究机会。马萨诸塞大学波士顿分校是一所为少数族裔服务的机构，第一代学生和佩尔奖学金获得者的比例都很高。这些学生将与普林斯顿大学的研究团队密切合作，学习和应用尖端的地球化学分析技术。海洋的ODZ及其随时间的变化值得我们从多方面关注：作为海洋和大气环流的监测；作为生物地球化学通量的热点；作为主要的海洋次环境和对海洋物种的限制；以及作为海洋的生物碳泵的反映。沉积氮(N)同位素被用来通过ODZ对水柱反硝化的敏感性来重建过去的变化，水柱反硝化增加了供应给地表水的硝酸盐的15N/14N(δ15N)。块状沉积N的δ15N(δ15N Bulk)已被广泛应用于这项工作，从而得出了更新世冰期与ODZ收缩有关的普遍结论。然而，δ15NBulk可能会因海底成岩作用和外源N输入的变化而叠加。为了避免这些潜在的偏差，现在可以测量浮游有孔虫贝壳中结合和保护的少量有机物质的δ15N(结合有孔虫的δ15N，或FB-δ15N)。最近来自赤道东太平洋的FB-δ15N数据和来自热带北太平洋东部的新的试点数据与以前基于δ15N的大范围冰期/间冰期强烈对比的推论相矛盾。为了测试先前的推论，并对ETNP反硝化和ODZ范围的过去变化提供新的见解，将在ETNP的海洋钻探计划站点1241生成FB-δ15N记录。将分析过去3.3 Ma期间300-500KYR的四个时间窗，每个窗的分辨率为亚轨道(~3KYR)。ETNP ODZ历史的重建将为这一关键区域的古气候数据集增加一个关键的生物地球化学成分，从而深入了解(1)对热带太平洋上层海洋环流的控制，包括风场和温度场，(2)对气候驱动的这些物理参数变化的生物地球化学反应，以及(3)对海洋的生物条件和海洋内部生物储存的过量二氧化碳分布的影响。这项裁决反映了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，认为值得支持。

Like most life on land, life in the ocean such as zooplankton, fish, and marine mammals require oxygen to break down their food and generate energy. However, in three different regions of the ocean, there are depths (from roughly 200 to 500 m) where the oxygen has been exhausted due to bacteria. These are known as the oxygen-deficient zones (ODZs), with the largest of these occurring in the eastern tropical North Pacific. The ODZs restrict the habitats and movements of ocean life, including fisheries on which humans rely. They also affect the availability and cycling of important nutrients in the ocean. The impact on these nutrients can affect the ocean’s biological productivity and the prevalence of different types of phytoplankton. Finally, the ODZs may affect the ocean’s exchange of greenhouse gases (both carbon dioxide and nitrous oxide) with the atmosphere. These gases can alter Earth’s climate. Studies have suggested that the ODZs have expanded over the last century, possibly due to ongoing global warming. However, climate model predictions of the ODZ extent in the future yield an uncertain picture. Reconstructing ODZ changes in the past will clarify the main controls on ODZ extent, improving the ability to predict future changes. In this project, the chemical composition of nitrogen trapped in the wall of fossil plankton will be measured in a deep-sea sediment core to reconstruct past changes in the ODZ of the eastern tropical North Pacific. The data will be compared with climate and ocean properties in the past, indicating which of these properties most strongly controls the ODZs. At Princeton, the team pursuing this research will lead a workshop on climate and the ocean for middle and high school STEM teachers. This will be one of multiple opportunities to communicate the results to the public. The research team at University of Massachusetts Boston will support paid research opportunities for undergraduate students. University of Massachusetts Boston is a minority-serving institution with a high proportion of both first-generation and Pell grant recipient students. These students will learn and apply cutting-edge geochemical analysis techniques in close collaboration with the research team at Princeton University. The ocean’s ODZs and their changes over time deserve our attention in multiple capacities: as monitors of ocean and atmospheric circulation; as hotspots of biogeochemical fluxes; as major ocean sub-environments and constraints on ocean species; and as reflections of the ocean’s biological carbon pump. Sedimentary nitrogen (N) isotopes have been used to reconstruct past changes in the ODZs through their sensitivity to water column denitrification, which increases the 15N/14N (δ15N) of the nitrate supplied to surface waters. The δ15N of bulk sedimentary N (δ15Nbulk) has been applied extensively in this effort, leading to the general conclusion that the Pleistocene ice ages have been associated with ODZ contraction. However, δ15Nbulk may be overprinted by variations in seafloor diagenesis and exogenous N inputs. To avoid these potential biases, it is now possible to measure the δ15N of the small quantity of organic matter bound within and protected by planktonic foraminifera shells (foraminifera-bound δ15N, or FB-δ15N). Recent FB-δ15N data from the eastern equatorial Pacific and new pilot data from the eastern tropical North Pacific (ETNP) argue against previous δ15Nbulk-based inferences of a strong ice age/interglacial contrast in ODZ extent. To test prior inferences and provide new insight into past variations in ETNP denitrification and ODZ extent, a FB-δ15N record will be generated at Ocean Drilling Program Site 1241 in the ETNP. Four time-windows of 300-500 kyr will be analyzed over the last 3.3 Ma, each at suborbital (~3 kyr) resolution. The reconstruction of the history of the ETNP ODZ will add a key biogeochemical component to the paleoclimate data sets from this key region, yielding insights into (1) the controls on tropical Pacific upper ocean circulation, which include the wind and temperature fields, (2) the biogeochemical responses to climate-driven changes in these physical parameters, and (3) the consequences for the biological conditions of the ocean and the distribution of biologically-stored excess CO2 in the ocean interior.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.