Determining the Source of Methane in Arctic Ocean Waters Adjacent to Subsea Permafrost

甲烷是一种强有力的温室气体，大量存在于北极的冻土和海洋沉积物中。自然气候变化和人为气候变化都通过海底永久冻土的融化和海冰面积的减少影响着北冰洋的甲烷动态；最近的几项调查记录了海水中甲烷浓度的升高，以及甲烷向大气的释放与这两个过程有关。储存的甲烷的释放会导致进一步的变暖，然后在一个强化循环中进一步释放甲烷。海底永久冻土解冻和海冰融化都会影响几个甲烷储层，例如沉积物内部和下面储存的古代甲烷的稳定性，以及海底最近沉积的沉积物和融化的永久冻土以及与浮游植物大量繁殖有关的海底地下水中的微生物产生的甲烷。鉴于北冰洋大陆边缘的海水和大气中的甲烷来源多种多样，目前尚不清楚哪一种来源贡献了北极水域中观测到的大部分甲烷，因此也不清楚这种甲烷来源的大小可能如何变化。测量溶解在海水和沉积物中的甲烷的天然放射性碳含量可以用来确定其来源。例如，渗漏物和笼形水合物释放的甲烷不含放射性碳，最近沉积的缺氧沉积物或浮游植物大量繁殖导致的地下海洋中形成的甲烷含有更多现代水平的放射性碳，而退化的永久冻土是更新世时代的（27,000-43,000年）。这里提出的工作旨在通过对海水和沉积物中溶解的甲烷的自然放射性碳测量来阐明这些来源，这些甲烷是在波弗特海大陆边缘的横断面中收集的。测量溶解在海水中的甲烷的天然放射性碳含量的主要挑战是收集足够的甲烷碳进行定量加速器质谱（AMS）分析。以前公布的程序最近进行了修改，可以在不到两小时的时间内从大约1万升海水中提取和收集甲烷。这些新程序将在为期一周的海洋考察中收集大约100个样本。测量溶解在海水中的甲烷的天然放射性碳含量的水平和垂直分布将决定甲烷的来源如何从海岸到公海以及从海底到海面的变化。这些甲烷同位素分布也将为甲烷的有氧氧化将提供一种平衡反馈的假设提供可靠的测试，在甲烷到达大气之前，它将消除离开海底的增加数量。研究生和本科生将参与整个科学过程，不仅有机会在他们的家庭实验室，也有机会参观其他设施。调查人员将利用与罗彻斯特大学、罗彻斯特博物馆和科学中心建立的牢固途径，促进罗彻斯特市学区的教育项目，为代表性不足的少数族裔K-12学生提供服务。

Methane is a potent greenhouse gas that is present in large amounts in frozen soils and marine sediments in the Arctic. Both natural and anthropogenic climate change are influencing the methane dynamics in the Arctic Ocean, through thawing of subsea permafrost as well as the decreasing sea-ice extent; several recent investigations have documented seemingly elevated methane concentrations in seawater and releases of methane to the atmosphere associated with both of these processes. Release of stored methane can lead to further warming and then to further methane releases in a reinforcing cycle. Both subsea permafrost thaw and sea-ice melt can influence several methane reservoirs such as the stability of ancient methane stored within and below sediments as well as the production of methane by microorganisms both in the seafloor in recently deposited sediments and thawed permafrost and in subsurface ocean waters associated with phytoplankton blooms. Given the diverse sources of methane to ocean waters and the atmosphere in the continental margins of the Arctic Ocean, it is currently unknown which source is contributing most of the observed methane in Arctic waters and thus how the magnitude of this methane source may change. Measurements of the natural radiocarbon content of methane dissolved in ocean waters and sediments can be used to identify its source. For example, methane emitted from seeps and clathrate hydrates is radiocarbon-free, methane formed in recently deposited anoxic sediments or in the subsurface ocean as a result of phytoplankton blooms contains more modern levels of radiocarbon, and degrading permafrost is of Pleistocene age (27,000-43,000 years old). The work proposed here will aim to elucidate these sources through natural radiocarbon measurements of methane dissolved in ocean waters and sediments collected during transects across the continental margin of the Beaufort Sea. The major challenge associated with measuring the natural radiocarbon content of methane dissolved in seawater is collecting enough methane carbon for a quantitative Accelerator Mass Spectrometry (AMS) analysis. Previously published procedures have recently been modified enabling the extraction and collection of methane from approximately 10,000 liters of seawater in less than two hours. These new procedures will enable roughly 100 samples to be collected during a one week oceanographic expedition. Measured horizontal and vertical distributions of the natural radiocarbon content of methane dissolved in ocean waters will determine how the source of methane changes from the coast to the open ocean and from the seafloor to the sea surface. These methane isotopic distributions will also provide a solid test of the hypothesis that the aerobic oxidation of methane will provide a counterbalancing feedback removing the increased quantities of methane leaving the seafloor before it reaches the atmosphere. Graduate and undergraduate students will be involved in the entire scientific process with opportunities not only in their home lab, but also with visits to other facilities. The investigators will leverage firmly established pathways with the University of Rochester and the Rochester Museum and Science Center to promote educational programs within the Rochester City School District to serve underrepresented minority K-12 students.