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

全球气温如何对大气二氧化碳的变化做出反应是地球科学中一个具有巨大社会意义的中心问题。地球大气中二氧化碳的深层历史可能有助于我们量化这种温室气体在地质过去突然气候变化事件中的作用，为未来大气二氧化碳增加的情景下的气候预测提供更好的视角。该项目将改进对大气二氧化碳的替代估计，这些估计基于对银杏叶片气孔特性如何对二氧化碳浓度升高做出反应的理解。通过对种植的银杏叶进行受控实验建立的这种代理关系，随后将应用于使用化石银杏叶记录大气二氧化碳的古代记录，这些化石银杏叶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准确估计将有助于完善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人为二氧化碳排放将导致的气候、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有关这项研究的科学方法和见解，让他们参与收集样本、进行测量和分析数据。全球温度对大气二氧化碳的敏感度很难通过直接大气测量来确定，因为地球系统的反馈作用要持续数千年或更长时间。这项研究将利用银杏化石叶片的气孔特性，开发出古新世晚期和始新世早期温暖气候(~60-53 Ma)的可靠、时间密集的二氧化碳替代记录。研究人员将首先细化银杏叶片气孔特性之间的代理关系，然后将该代理关系应用于密集且时间限制良好的银杏化石记录。为了确定现代银杏对二氧化碳浓度升高的响应，将在实验条件下在开顶式气室中培养三种二氧化碳浓度升高的树木，同时监测气孔密度、大小和指数，以及碳同位素组成、光合作用同化和蒸腾速率、温度、相对湿度和光合作用有效辐射。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和化石气孔之间的模型关系。一旦对现代银杏的这种反应进行了校准，结果将应用于从落基山脉古近纪陆地沉积物中收集的大量银杏化石。这一记录与来自海洋和陆地的古温度记录相结合，将被用来评估大气二氧化碳在地质上快速升温(高温)时期的作用。对古二氧化碳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控制的理解。这一奖项反映了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，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.