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

.______________________________________________________________________________________________________________第一部分：科学家研究地球过去的气候，以了解气候将如何应对未来持续的全球变化。未来气候的最佳模拟之一可能是大约300万年前发生的时期，在上新世中期温暖时期。在此期间，大气中的二氧化碳浓度与今天相似，海平面高出15米或更多，主要是由于极地地区的变暖和随之而来的冰盖融化。然而，在上新世中期温暖时期，极地地区的温度并没有很好的确定，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。该项目将利用一种称为宇宙成因惰性气体古温度测定法的新型气候替代物，对上新世中期温暖期南极洲的地表温度进行限制。这个项目的重点是南极洲的一个地区称为麦克默多干谷。在这一地区，气候模型表明，上新世中期的温度比今天高出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，这种新的气候替代品可以很容易地应用。该团队将重建上新世中期温暖时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，并提供南极冰盖对变暖温度敏感性的急需限制。该项目产生的温度重建将对多个学科产生科学影响，包括气候科学，冰川学，地貌学和行星科学。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）通过研究生的教育和培训来培养STEM人才，（3）通过公开访问，开源代码库的出版来加强研究基础设施，以及（4）通过社交媒体，博客文章，出版物，第二部分：技术说明上新世中期温暖期（300 - 330万年前）是地质过去的最近间隔，大气中二氧化碳浓度超过400 ppm，并被广泛认为是地球气候系统将如何应对当前全球变化的模拟。气候模型预测极地放大--由于辐射强迫，高纬度地区的气温变化大于全球平均水平--无论是在上新世中期温暖期还是由于当前的气候变暖。然而，在这两种情况下，极性放大的预测幅度是高度不确定的。极地放大的幅度对于冰盖对变暖的敏感性以及冰盖融化对海平面变化的贡献具有重要意义。在上新世中期温暖期，基于代理的极地表面气温的限制是稀疏的，不存在。在南极洲，只有间接证据表明这段时间的变暖幅度。该项目将利用一种新开发的称为宇宙成因惰性气体古温度测定法的技术，对南极洲麦克默多干谷在上新世中期温暖期的地表温度进行限制。CNG古温度计利用宇宙成因的3 He在石英中的扩散行为来量化岩石在暴露于地球表面几米范围内的宇宙射线粒子时所经历的温度。非常低的侵蚀率和零度以下的温度特点的麦克默多干谷，使这个地区特别适合应用CNG古测温解决的问题：什么温度的特点麦克默多干谷在上新世中期温暖期？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，已知那里的侵蚀率足够低，宇宙射线暴露延伸到上新世中期或更早。他们将对宇宙成因的3 He测量，这将记录我们样品的热历史，与宇宙成因的10 Be，26 Al和21 Ne的测量，记录样品暴露和侵蚀的历史。我们还将在现场测量岩石和空气的温度在采样点，以量化辐射加热的影响，并制定岩石和空气温度之间的统计关系，以及进行扩散实验，以量化的动力学3 He扩散具体到每个样品。这套观测结果将用于模拟允许的热历史，并在上新世中期宇宙射线暴露的温暖期间隔期间对温度进行限制。该奖项反映了NSF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.