Collaborative Research: Using New Ice Cores from Dome C to Test the Assumption of a Constant Galactic Cosmic Ray Flux and Improve Understanding of the Holocene Methane Budget

该项目将与法国极地研究所合作，在南极洲获取冰芯。对这些冰芯的分析将提高我们对起源于太阳系以外的宇宙射线历史的理解。宇宙射线到达太阳系的历史速率对于研究过去的太阳活动和过去的冰川范围很重要。太阳活动是地球气候的重要驱动力，而冰川范围是地球气候系统的重要组成部分。该项目还将提高我们对大气甲烷循环的理解。甲烷既有自然来源，也有人为来源，是一种重要的温室气体，也是全球大气化学的关键参与者。大气层中产生的宇宙核素的记录已被用来重建过去的太阳活动和太阳辐射。地表岩石中产生的宇宙源核素被广泛应用于过去的冰动力学和冰区研究。所有这些研究通常都假设地球上的星系宇宙射线流量在时间上是恒定的，但这一点的不确定性为30%或更多。该项目将利用对南极冰穹C新冰芯中的一氧化碳(14CO)的原位宇宙成因碳-14的测量来测试恒定的银河系宇宙射线流量假设。来自穹顶C的冰中几乎所有的14CO都来自高能、深穿透的介子，14CO的产生率只对星系宇宙射线通量敏感。该项目还将研究甲烷的碳14(14CH4)，它可用于明确区分同时代的甲烷来源(例如湿地、动物、生物质燃烧)和古代的碳源(地质甲烷渗漏、甲烷水合物、永久冻土)。特别是自然地质来源是全球甲烷预算中最不确定的术语之一。这项拟议的工作将有助于通过对过去7000年≈大气14CH4的新测量来改进对自然地质源星等的估计。该项目将帮助培训三名早期职业研究人员和两名研究生。该奖项反映了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。

This project will obtain ice cores in Antarctica in collaboration with the French Polar Institute. The analysis of these ice cores will improve our understanding of the history of cosmic rays originating from outside of our solar system. The historical rate at which cosmic rays reach the solar system is important to understand for studies of past solar activity and past glacier extent. Solar activity is an important driver of Earth’s climate, and glacier extent is an important part of the Earth’s climate system. This project will also improve our understanding of the cycle of atmospheric methane. Methane has both natural and human-caused sources and is an important greenhouse gas and a key player in global atmospheric chemistry. Records of atmospherically produced cosmogenic nuclides have been used to reconstruct past solar activity and solar irradiance. Cosmogenic nuclides produced in surface rock are widely used in studies of past ice dynamics and extent. All these studies generally assume that the galactic cosmic ray flux at Earth is constant in time, but this is uncertain by 30% or more. This project will use measurements of in situ cosmogenic carbon-14 of carbon monoxide (14CO) in new ice cores from Dome C, Antarctica to test the assumption of constant galactic cosmic ray flux. Almost all 14CO in ice from Dome C is from high-energy, deep-penetrating muons, and the 14CO production rate is only sensitive to the galactic cosmic ray flux. This project will also look at carbon-14 of methane (14CH4), which can be used to unambiguously distinguish contemporaneous sources of methane (e.g., wetlands, animals, biomass burning) from ancient carbon sources (geologic methane seeps, methane hydrates, permafrost). The natural geologic source in particular is one of the most uncertain terms in the global methane budget. The proposed work would help to improve estimates of the natural geologic source magnitude via new measurements of atmospheric 14CH4 over the last ≈7000 years. This project will help to train three early-career researchers and two graduate students.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.