P2C2: Collaborative Research: New Estimates of Atmospheric pCO2 for the Paleocene-Eocene

全球气温如何对大气中二氧化碳的变化作出反应是地球科学的一个核心问题，具有巨大的社会意义。地球大气中二氧化碳的深时间历史可以帮助我们量化这种温室气体在过去地质时期气候突变事件中的作用，从而更好地预测未来大气二氧化碳增加情景下的气候变化。该项目将改进大气二氧化碳的代理估计，该估计是基于对银杏叶片气孔特性如何响应二氧化碳浓度升高的理解。这种代理关系是通过对栽培银杏叶的对照实验建立起来的，然后将利用银杏叶化石应用于大气二氧化碳的古代记录。银杏叶化石在地球大气二氧化碳浓度和气候发生重大变化的时期沉积的沉积物中很常见。因此，这些对古代大气二氧化碳的更精确估计将有助于改进我们对地球气候系统对温室气体浓度的敏感性的理解，并有助于更准确地预测由人为二氧化碳排放引起的气候、冰量和海平面的变化。对这些未来情景的更准确预测将对基础设施和粮食生产产生重大的社会和经济影响。该项目还将教育学生和公民科学家，让他们参与收集标本、测量和分析数据，了解这项研究的科学方法和见解。全球温度对大气二氧化碳分压的敏感性很难通过直接的大气测量来确定，因为地球系统中的反馈作用要持续数千年或更长时间。本研究将利用银杏叶化石的气孔特性，为古新世晚期和始新世早期的温暖气候（~60-53 Ma）建立一个可靠的、时间密集的pCO2代用记录。研究人员将首先完善银杏叶片气孔特性之间的代理关系，然后将此代理应用于银杏密集且时间约束良好的化石记录。为了确定现代银杏对pCO2浓度升高的响应，在3个CO2浓度升高的实验条件下，在开顶箱中栽培银杏，同时监测其气孔密度、大小和指数、碳同位素组成、光合同化和蒸腾速率、温度、相对湿度和光合有效辐射。这些来自实验栽培的数据将与现代银杏叶的调查数据一起使用，使研究人员能够评估古二氧化碳分压与化石气孔之间的模型关系。一旦这种反应被校准为现代银杏，结果将应用于从落基山脉古近系陆相沉积物中广泛收集的wyomingensis银杏化石。这一记录，结合来自海洋和陆地领域的古温度记录，将被用来评估大气二氧化碳在地质上温度快速上升（热液）时期的作用。对古二氧化碳分压的更好估计将大大有助于完善我们对地球气候对二氧化碳的敏感性的理解，并增加我们对地质记录中过去温暖气候的控制的理解。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。

How global temperature responds to changes in atmospheric carbon dioxide is a central question in Earth science with enormous societal significance. The deep-time history of carbon dioxide in Earth's atmosphere may help us quantify the role of this greenhouse gas in abrupt climate change events in the geological past, giving a better perspective towards climate projections under future scenarios of increasing atmospheric carbon dioxide. This project will refine proxy estimates of atmospheric carbon dioxide that are based on an understanding of how the properties of stomatal pores in the leaves of Ginkgo trees respond to elevated carbon dioxide concentration. This proxy relationship, established using controlled experiments on cultivated Ginkgo leaves, will then be applied to the ancient record of atmospheric carbon dioxide using fossil Ginkgo leaves that are common in sediments deposited during periods when Earth's atmospheric carbon dioxide concentration and climate varied substantially. These more precise estimates of ancient atmospheric carbon dioxide will thus help to refine our understanding of the sensitivity of Earth's climate system to greenhouse gas concentrations and facilitate more accurate projections of changes in climate, ice volume and sea level that will result from anthropogenic carbon dioxide emissions. More accurate projections of these future scenarios will have significant societal and economic implications for infrastructure and food production. The project will also educate students and citizen scientists about the scientific method and insights from this research by involving them in collecting specimens, making measurements and analyzing data.Sensitivity of global temperature to atmospheric pCO2 is difficult to determine by direct atmospheric measurements because feedbacks in the Earth system play out over millennia or longer. This research will develop a reliable, temporally dense proxy record of pCO2 for the late Paleocene and Early Eocene period of warm climates (~60-53 Ma) using stomatal properties of fossil Ginkgo leaves. The researchers will first refine a proxy relationship between Ginkgo leaf stomatal properties, and then apply this proxy to dense and temporally well-constrained fossil records of Ginkgo. To establish the response of modern Ginkgo to elevated pCO2 concentrations, trees will be cultivated under experimental conditions at three elevated CO2 concentrations in open-top chambers, while monitoring stomatal density, size and index, as well as carbon isotopic composition and rates of photosynthetic assimilation and transpiration, temperature, relative humidity, and photosynthetically active radiation. These data from experimental cultivation will be used along with survey data of modern Ginkgo leaves, to allow the researchers to evaluate modeled relationships between paleo-pCO2 and fossil stomata. Once this response is calibrated for modern Ginkgo biloba, the results will be applied to extensive collections of fossil Ginkgo wyomingensis from the terrestrial sediments of the Paleogene in the Rocky Mountains. This record, combined with records of paleotemperature from the marine and terrestrial realms will be used to evaluate the role of atmospheric CO2 during episodes of geologically rapid temperature increases (hyperthermals). Better estimates of paleo-pCO2 will contribute substantially to refining our understanding of Earth's climate sensitivity to carbon dioxide and increase our understanding of controls on past warm climates in the geological record.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.