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

大气中的甲烷导致了温室效应，从而导致了地球温度。自工业革命以来，人类导致甲烷的排放量几乎增加了三倍。随着地球变暖，地球表面和大气中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并不为人所知。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，并有助于建立对未来变化的更好预测。在这个项目中，研究人员将使用对甲烷中不同化学形式的碳和氢的测量，以更好地了解过去变化的原因。这项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(100万年)来了解当气候像今天一样温暖或更温暖时，是什么控制了大气中的甲烷水平。开发新的方法包括测试现有的甲烷净化系统，用于通过质谱学测量氢同位素。该系统已经投入使用，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品中的氢同位素，这些样品涵盖了从冰期到间冰期暖期的两次转变。这些数据将被用来限制在这些时间发生的甲烷增加的来源，因为氢同位素比率对甲烷来源的纬度很敏感。最后，研究人员将测量南极洲Allan Hills蓝色冰区100万到300万年前的样品中的碳和氢同位素。将这些测量结果与更年轻样本的现有数据进行比较，可能会发现构成这些不同时间段甲烷收支的来源类型混合之间的重要差异。例如，在100万年前，冰川时期北半球明显缺乏大冰盖，导致冰川海平面上升，这将对全球湿地甲烷来源产生影响。此外，较暖的温度可能增加了湿地的甲烷排放量，但也增加了羟基自由基的产生和平流层中甲烷的破坏。该项目将支持一名博士后研究人员，他将与达特茅斯学院的冰钻计划合作，创建用于高中和大学课堂的虚拟野外实验室。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.