Collaborative Research: Illuminating the Cenozoic Alkenone pCO2 Record

合作研究：阐明新生代烯酮 pCO2 记录

• 批准号: 2100509
• 负责人: Pratigya Polissar
• 金额: $ 39.87万
• 依托单位: University of California-Santa Cruz
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2021
• 资助国家: 美国
• 起止时间: 2021-08-01 至 2024-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2100509&HistoricalAwards=false
• 关键词: Collaborative; Research; Illuminating; Cenozoic; Alkenone

This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117-2). This study will improve the foundation of a widely used geologic proxy for atmospheric carbon dioxide. To correctly predict the magnitude of future global warming from rising greenhouse gas (GHG) concentrations, it is essential to understand the relationship between GHGs – particularly carbon dioxide (CO2) – and Earth’s temperature. One way to investigate this relationship is to examine how Earth’s temperature and CO2 levels have varied together in the past. Direct measurements of atmospheric CO2 extend back only to 1958, while measurements from bubbles of atmosphere trapped in ice cores extend back to about 1 million years (Ma) ago. Any estimate of CO2 older than about 1 Ma requires the use of a proxy for CO2 concentrations. One such proxy is based upon how ‘picky’ marine algae are about the two different isotopes of carbon. When algae grow with high CO2 levels they mostly take up the isotope carbon-12, and use less of the isotope carbon-13. When they grow with low CO2 levels they take up relatively more carbon-13. However, recent findings show that marine algae may also change how ‘picky’ they are due to other factors besides CO2 levels. The primary goal of this project is to investigate how these other factors affect the algal CO2 proxy through controlled laboratory experiments and analysis of modern ocean sediments. These findings can then be applied to ancient ocean sediments, in order to improve interpretations of reconstructed CO2 levels in the past. This project will train a post-doctoral investigator, a Ph.D. student, and two summer undergraduate interns.Accurate reconstructions of past atmospheric pCO2 levels can improve predictions of future warming by providing geologic tests during Earth conditions that were different from the short observational record. An important proxy for CO2 concentrations during the past 55 million years is the carbon isotopic composition of long-chain unsaturated ketones (alkenones) from the Noelaerhabdaceae family of algae. The crucial assumptions of the existing proxy are that: (i) carbon isotope fractionation (Ep) by these algae is set by the rate of diffusive supply of CO2 relative to the rate of carbon use, and (ii) the fractionation is primarily governed by the carbon-fixing enzyme RuBisCO, with an assumed isotope effect of roughly 25 per mil. Recent work challenges these assumptions, suggesting that non-diffusive supply of CO2 is ubiquitous, that kinetic RuBisCO fractionation in these algae may be as small as about 11 per mil and that irradiance independently influences carbon isotope fractionation. Additional information is needed to incorporate these new factors into quantitative reconstructions of pCO2, including re-interpreting Cenozoic alkenone Ep values. Project objectives include:i. Measure the response of alkenone Ep to variations in irradiance, cell size, and pCO2 in laboratory chemostat and dilute batch cultures,ii. Revise the quantitative framework for alkenone paleobarometry, developing practical equations to use for paleo-pCO2 reconstruction, andiii. Test this revised laboratory-based framework with new sediment analyses from the modern and Pleistocene ocean.The project goal is to provide a framework, based in mechanistic rather than empirical evidence, for understanding alkenone Ep variations during the Cenozoic. This will help develop an equation relating Ep to CO2 that uses variables that can be measured or easily estimated, and that could be adopted readily by the paleoclimate community.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.

该奖项全部或部分根据2021年美国救援计划法案（公法117-2）资助。这项研究将改善广泛使用的大气二氧化碳地质代用指标的基础。 为了正确预测温室气体（GHG）浓度上升导致的未来全球变暖的程度，必须了解GHG--特别是二氧化碳（CO2）--与地球温度之间的关系。 研究这种关系的一种方法是研究地球的温度和二氧化碳水平在过去是如何变化的。 对大气中二氧化碳的直接测量只能追溯到1958年，而对冰芯中捕获的大气气泡的测量可以追溯到大约100万年前。 任何对年龄超过1 Ma的CO2的估计都需要使用CO2浓度的替代物。 其中一个替代指标是基于海藻对两种不同的碳同位素的“挑剔”程度。 当藻类在高二氧化碳水平下生长时，它们主要吸收同位素碳12，而使用较少的同位素碳13。 当它们在低二氧化碳水平下生长时，它们会吸收相对更多的碳13。然而，最近的研究结果表明，除了二氧化碳水平外，海藻也可能会因为其他因素而改变它们的“挑剔”程度。 该项目的主要目标是通过受控实验室实验和现代海洋沉积物分析，研究这些其他因素如何影响藻类CO2代用指标。 这些发现可以应用于古代海洋沉积物，以改善对过去重建的二氧化碳水平的解释。 该项目将培养一名博士后研究员，一名博士生。过去大气pCO 2水平的准确重建可以通过在地球条件下提供地质测试来改善对未来变暖的预测，这些地质测试与短期观测记录不同。 在过去的5500万年中，二氧化碳浓度的一个重要指标是来自Noelaerhabdaceae藻类家族的长链不饱和酮（烯酮）的碳同位素组成。 现有委托书的关键假设是：（i）这些藻类的碳同位素分馏（Ep）由CO2的扩散供应速率相对于碳使用速率设定，以及（ii）分馏主要由碳固定酶RuBisCO控制，假设同位素效应约为25/mil。最近的工作挑战了这些假设，表明CO2的非扩散供应是普遍存在的，在这些藻类中的动力学RuBisCO分馏可能小到约11/mil，并且辐照度独立地影响碳同位素分馏。 需要更多的信息，将这些新的因素纳入定量重建pCO 2，包括重新解释新生代烯酮Ep值。 项目目标包括：i.测量实验室恒化器和稀释分批培养物中烯酮Ep对辐照度、细胞大小和pCO 2变化的响应，ii.修订定量框架烯酮古压力，开发实用的方程用于古二氧化碳分压重建，andiii。利用现代和更新世海洋的新沉积物分析来测试这个基于实验室的修订框架。该项目的目标是提供一个基于机制而非经验证据的框架，以了解新生代期间烯酮Ep的变化。 这将有助于开发一个方程Ep到CO2，使用的变量，可以测量或容易估计，并可以随时通过古气候社区。这个奖项反映了NSF的法定使命，并已被认为是值得的支持，通过评估使用基金会的知识价值和更广泛的影响审查标准。