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: 2100537
• 负责人: Ann Pearson
• 金额: $ 38.43万
• 依托单位: Harvard University
• 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=2100537&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.

该奖项全部或部分由《2021年美国救援计划法案》（公法117-2）资助。这项研究将为广泛使用的大气二氧化碳地质代用物奠定基础。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，了解温室气体（尤其是二氧化碳）与地球温度之间的关系至关重要。研究这种关系的一种方法是研究过去地球的温度和二氧化碳水平是如何一起变化的。对大气二氧化碳的直接测量只能追溯到1958年，而对冰芯中大气气泡的测量则可以追溯到大约100万年前。对超过1ma的二氧化碳的任何估计都需要使用二氧化碳浓度的代用物。一个这样的代理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，它们大多吸收同位素碳-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，它们吸收的碳-13相对较多。然而，最近的研究结果表明，除了二氧化碳水平之外，其他因素也可能改变海藻的“挑剔”程度。该项目的主要目标是通过控制实验室实验和现代海洋沉积物分析来研究这些其他因素如何影响藻类二氧化碳代理。然后，这些发现可以应用于古代海洋沉积物，以改进对过去重建的二氧化碳水平的解释。本项目将培养博士后研究员1名，博士生1名，本科生暑期实习生2名。通过提供不同于短期观测记录的地球条件下的地质测试，对过去大气二氧化碳分压水平的精确重建可以改善对未来变暖的预测。过去5500万年CO2浓度的一个重要代表是来自Noelaerhabdaceae藻类的长链不饱和酮（烯酮）的碳同位素组成。现有代理的关键假设是：(i)这些藻类的碳同位素分异（Ep）是由二氧化碳的扩散供应速率相对于碳利用速率决定的，（ii）分异主要由碳固定酶RuBisCO控制，其同位素效应假定约为每mil 25。最近的研究挑战了这些假设，表明二氧化碳的非扩散供应是普遍存在的。这些藻类中RuBisCO的动力学分馏可能小到每毫升11个，并且辐照度独立地影响碳同位素分馏。为了将这些新因素纳入二氧化碳分压的定量重建，需要更多的信息，包括重新解释新生代烯酮Ep值。项目目标包括：1。测量烯酮Ep对实验室化学调节和稀释批培养中辐照度、细胞大小和二氧化碳分压变化的响应。修订烯烃古气压测定法的定量框架，开发用于古二氧化碳分压重建的实用方程；用来自现代和更新世海洋的新的沉积物分析来测试这个修改后的基于实验室的框架。该项目的目标是提供一个基于机制而非经验证据的框架，以了解新生代烯烃Ep的变化。这将有助于建立一个Ep与CO2相关的方程，该方程使用的变量可以测量或容易估计，并且可以很容易地被古气候学界采用。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。