Investigation of Past Atmospheric Methane Variability with Stable Isotopes in Antarctic Ice Cores

大气中的甲烷会导致温室效应，从而影响地球的温度。自工业革命以来，人类已经导致甲烷增加了近三倍。随着地球变暖，地球表面和大气层的变化将进一步改变甲烷水平。这些反馈将如何随着时间的推移而变化还不清楚。通过测量被困在冰中的古代空气，对甲烷过去变化的研究提供了这些变化的例子，并有助于更好地预测未来的变化。在这个项目中，研究人员将使用甲烷中碳和氢的不同化学形式的测量来更好地了解过去变化的原因。拟议的研究将开发氢同位素的实验室测量方法，使用甲烷的不同化学形式来了解过去冰河时代结束和地球变暖时大气甲烷的变化，并使用来自南极洲的独特的非常古老的冰样本（100万年）来了解当气候比今天温暖或更温暖时，是什么控制着大气中的甲烷水平。开发新方法涉及测试现有的甲烷净化系统，以通过质谱法测量氢同位素。该系统已经投入运行，并对甲烷中的碳同位素进行了校准。接下来，研究人员将测量来自南极洲泰勒冰川和艾伦山的甲烷样本中的氢同位素，这些样本涵盖了从冰河时代到间冰期温暖时期的两个过渡时期。这些数据将用于限制在这些时间发生的甲烷增加的起源，因为氢同位素比率对甲烷来源的纬度很敏感。最后，研究人员将测量来自南极洲艾伦山蓝冰区的100万至300万年样本中的碳和氢同位素。这些测量结果和现有的数据之间的比较年轻的样本可能会揭示重要的差异之间的混合源类型，弥补了甲烷预算在这些不同的时间段。例如，在100万年前，冰川期明显缺乏大的北方冰盖，导致冰川海平面上升，这将对全球湿地甲烷源产生影响。此外，温度升高可能增加了湿地的甲烷排放量，但也增加了平流层中羟基自由基的产生和甲烷的破坏。该项目将支持一名博士后研究员，他将与达特茅斯学院的冰钻项目合作，创建用于高中和大学课堂的虚拟现场实验室。该奖项反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

Atmospheric methane contributes to the greenhouse effect and therefore Earth’s temperature. Humans have caused an almost three-fold increase in methane since the industrial revolution. As the planet warms, changes on the Earth surface and in the atmosphere will further change methane levels. How these feedbacks will change over time is not well known. Studies of past variations in methane, by measuring ancient air trapped in ice, provide examples of these changes and help build better predictions of future change. In this project researchers will use measurements of different chemical forms of the carbon and hydrogen in methane to better understand reasons for past changes. The proposed research will develop laboratory measurement methods for hydrogen isotopes, use the different chemical forms of methane to understand the changes in atmospheric methane that happened when past ice ages ended and the Earth warmed, and use unique, very old ice samples ( 1 million years) from Antarctica to understand what controls methane levels in the atmosphere when climate was as warm or warmer than today. Developing new methods involves testing an existing system for purifying methane for measurements of hydrogen isotopes by mass spectrometry. The system is already operational and calibrated for carbon isotopes in methane. Next, the researchers will measure hydrogen isotopes in methane from samples from Taylor Glacier and Allan Hills, Antarctica, that cover two transitions from ice ages to interglacial warm periods. These data will be used to constrain where the methane increase that happened at these times originated, because hydrogen isotope ratios are sensitive to the latitude of the methane source. Finally, the researchers will measure both carbon and hydrogen isotopes in samples from 1 to 3 million years in age from the Allan Hills Blue Ice Area in Antarctica. Comparisons between these measurements and existing data for younger samples may reveal important differences between the mixture of source types that make up the methane budget in these different time periods. For example, prior to 1 million years ago, the apparent lack of large Northern Hemisphere ice sheets during glacial periods, and resulting higher glacial sea levels, would have had impacts on the global wetland methane source. Additionally, warmer temperatures might have increased CH4 emissions from wetlands but also increased production of the hydroxyl radical and methane destruction in the stratosphere. The project will support a post-doctoral researcher who will collaborate with the Ice Drilling Program at Dartmouth College to create virtual field labs used in high school and college classes.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.