Collaborative Research: Reconstructing Temperatures during the Mid-Pliocene Warm Period in the McMurdo Dry Valleys with Cosmogenic Noble Gases

。______________________________________________________________________________________________________________ 第一部分:非技术DescriptionScientists研究地球气候的过去为了了解气候在未来将如何应对持续的全球变化。对于未来的气候，一个最好的类比可能发生在大约300万年前，在一个被称为上新世中期暖期的间隔期间。在这一时期，大气中二氧化碳的浓度与今天相似，海平面比现在高出15米或更多，这主要是由于气候变暖以及随之而来的极地冰盖融化。然而，上新世中期暖期极地地区的温度并不能很好地确定，部分原因是我们没有像冰芯这样的记录，可以追溯到这么远的时间。该项目将使用一种新的气候替代方法，即宇宙成因稀有气体古测温法，提供上新世中期暖期南极洲地表温度的约束条件。这个项目的重点是南极洲一个叫做麦克默多干谷的地区。在这个地区，气候模型显示，上新世中期的温度比现在高出10摄氏度以上，但间接的地质观测表明，温度可能与今天相似。麦克默多干谷也是一个岩石暴露在地球表面条件下数百万年的地方，在那里这种新的气候替代品可以很容易地应用。该小组将重建上新世中期暖期麦克默多干谷的温度，以解决模型和间接地质观测之间的差异，并为南极冰盖对变暖温度的敏感性提供急需的约束。该项目产生的温度重建将对气候科学、冰川学、地貌学和行星科学等多个学科产生科学影响。此外，该项目将(1)通过支持两名早期职业女性首席研究员来扩大代表性不足的群体的参与，(2)通过教育和培训一名研究生来培养STEM人才，(3)通过出版可公开访问的开源代码库来增强研究基础设施，以及(4)通过社交媒体、博客文章、出版物和会议演讲进行广泛传播。上新世中期暖期（3- 330万年前）是地质历史上最近的一个时期，当时大气二氧化碳浓度超过400ppm，被广泛认为是地球气候系统对当前全球变化的反应。气候模式预测在上新世中期暖期和当前气候变暖期间，极地放大——由于辐射强迫，高纬度地区的温度变化比全球平均温度变化更大。然而，在这两种情况下，预测的极性放大幅度是高度不确定的。极地放大的幅度对冰盖对变暖的敏感性和冰盖融化对海平面变化的贡献具有重要意义。在上新世中期暖期，基于代用物的极地地表气温约束很少甚至不存在。在南极洲，只有间接的证据表明在这段时间里变暖的程度。该项目将使用一种新开发的称为宇宙稀有气体（CNG）古测温技术，提供上新世中期暖期南极洲麦克默多干谷地表温度的约束条件。CNG古测温学利用石英中宇宙生成3He的扩散行为来量化岩石在暴露于距离地球表面几米内的宇宙射线粒子时所经历的温度。麦克默多干谷极低的侵蚀速率和零度以下的温度特征使得该地区非常适合应用压缩天然气古测温仪来解决以下问题：在上新世中期暖期，麦克默多干谷的温度特征是什么？为了解决这个问题，研究小组将在麦克默多干谷的几个地点收集基岩样本，那里的侵蚀率已知很低，宇宙射线暴露的时间可以延伸到上新世中期或更早。他们将配对宇宙起源的3He测量，记录我们样品的热历史，与宇宙起源的10Be， 26Al和21Ne测量，记录样品的暴露和侵蚀历史。我们还将对样品地点的岩石和空气温度进行现场测量，以量化辐射加热的影响，并建立岩石和空气温度之间的统计关系，以及进行扩散实验，以量化每个样品特定的3He扩散动力学。这组观测结果将用于模拟允许的热历史，并对上新世中期宇宙射线暴露的暖期区间的温度进行限制。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。

.______________________________________________________________________________________________________________Part I: Nontechnical DescriptionScientists study the Earth's past climate in order to understand how the climate will respond to ongoing global change in the future. One of the best analogs for future climate might the period that occurred approximately 3 million years ago, during an interval known as the mid-Pliocene Warm Period. During this period, the concentration of carbon dioxide in the atmosphere was similar to today's and sea level was 15 or more meters higher, due primarily to warming and consequent ice sheet melting in polar regions. However, the temperatures in polar regions during the mid-Pliocene Warm Period are not well determined, in part because we do not have records like ice cores that extend this far back in time. This project will provide constraints on surface temperatures in Antarctica during the mid-Pliocene Warm Period using a new type of climate substitute, known as cosmogenic noble gas paleothermometry. This project focuses on an area of Antarctica called the McMurdo Dry Valleys. In this area, climate models suggest that temperatures were more than 10 C warmer during the mid-Pliocene than they are today, but indirect geologic observations suggest that temperatures may have been similar to today. The McMurdo Dry Valleys are also a place where rocks have been exposed to Earth surface conditions for several million years, and where this new climate substitute can be readily applied. The team will reconstruct temperatures in the McMurdo Dry Valleys during the mid-Pliocene Warm Period in order to resolve the discrepancy between models and indirect geologic observations and provide much-needed constraints on the sensitivity of Antarctic ice sheets to warming temperatures. The temperature reconstructions generated in this project will have scientific impact in multiple disciplines, including climate science, glaciology, geomorphology, and planetary science. In addition, the project will (1) broaden the participation of underrepresented groups by supporting two early-career female principal investigators, (2) build STEM talent through the education and training of a graduate student, (3) enhance infrastructure for research via publication of a publicly-accessible, open-source code library, and (4) be broadly disseminated via social media, blog posts, publications, and conference presentations.Part II: Technical DescriptionThe mid-Pliocene Warm Period (3-3.3 million years ago) is the most recent interval of the geologic past when atmospheric CO2 concentrations exceeded 400 ppm and is widely considered an analog for how Earth’s climate system will respond to current global change. Climate models predict polar amplification - the occurrence of larger changes in temperatures at high latitudes than the global average due to a radiative forcing - both during the mid-Pliocene Warm Period and due to current climate warming. However, the predicted magnitude of polar amplification is highly uncertain in both cases. The magnitude of polar amplification has important implications for the sensitivity of ice sheets to warming and the contribution of ice sheet melting to sea level change. Proxy-based constraints on polar surface air temperatures during the mid-Pliocene Warm Period are sparse to non-existent. In Antarctica, there is only indirect evidence for the magnitude of warming during this time. This project will provide constraints on surface temperatures in the McMurdo Dry Valleys of Antarctica during the mid-Pliocene Warm Period using a newly developed technique called cosmogenic noble gas (CNG) paleothermometry. CNG paleothermometry utilizes the diffusive behavior of cosmogenic 3He in quartz to quantify the temperatures rocks experience while exposed to cosmic-ray particles within a few meters of the Earth’s surface. The very low erosion rates and subzero temperatures characterizing the McMurdo Dry Valleys make this region uniquely suited for the application of CNG paleothermometry for addressing the question: what temperatures characterized the McMurdo Dry Valleys during the mid-Pliocene Warm Period? To address this question, the team will collect bedrock samples at several locations in the McMurdo Dry Valleys where erosion rates are known to be low enough that cosmic ray exposure extends into the mid-Pliocene or earlier. They will pair cosmogenic 3He measurements, which will record the thermal histories of our samples, with measurements of cosmogenic 10Be, 26Al, and 21Ne, which record samples exposure and erosion histories. We will also make in situ measurements of rock and air temperatures at sample sites in order to quantify the effect of radiative heating and develop a statistical relationship between rock and air temperatures, as well as conduct diffusion experiments to quantify the kinetics of 3He diffusion specific to each sample. This suite of observations will be used to model permissible thermal histories and place constraints on temperatures during the mid-Pliocene Warm Period interval of cosmic-ray exposure.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.