Methane emissions from subglacial meltwater of three alpine glaciers in Yukon, Canada

Subglacial meltwater of land-terminating glaciers in Greenland and Iceland are sources of methane (CH4) to the atmosphere, but sparse empirical data exist about the spatial distribution of subglacial CH4 production and emission from glaciers in other regions of the world. This study presents the first measurements of CH4 emissions from the subglacial meltwater of three outlet glaciers of the St. Elias Mountains in Yukon, Canada. Dissolved CH4 concentrations were highly elevated at 45, 135, and 250 times compared to the atmospheric equilibrium concentration in the meltwater of Dusty, Kluane, and Donjek glaciers, respectively. Dissolved CO2 concentrations were depleted relative to the atmospheric equilibrium. This points to the meltwater being a source of CH4 and a sink of CO2. Stable carbon (13C) and hydrogen (2H) isotopic signatures of the subglacial CH4 were depleted compared to atmospheric CH4 at all sites, indicating both biotic and abiotic sources and possible alteration from bacterial CH4 oxidation in the meltwater. No relation was found between CH4 concentrations in the meltwater and the meltwater chemistry or the size of the glaciers in this study. These findings suggest that CH4 emissions from subglacial environments under alpine glaciers may be a more common phenomenon than previously thought.

格陵兰岛与冰岛的陆地终止型冰川（land-terminating glaciers）下的冰下融水是大气甲烷（CH₄）的排放源，但全球其他区域冰川的冰下甲烷生成与排放的空间分布特征目前仍缺乏充足的实测数据。本研究首次针对加拿大育空地区圣伊莱亚斯山脉（St. Elias Mountains）的3条出口冰川（outlet glaciers）的冰下融水开展甲烷排放测量。达斯蒂、克卢安尼与冬杰克冰川的融水中，溶解态甲烷浓度分别较大气平衡浓度升高45倍、135倍与250倍，处于极高水平；而溶解态二氧化碳浓度则低于大气平衡浓度。这表明该冰下融水既是甲烷的排放源，同时也是二氧化碳的汇。所有采样点的冰下甲烷的稳定碳同位素（¹³C）与氢同位素（²H）特征值均低于大气甲烷，表明其同时存在生物与非生物成因来源，且融水中可能发生了细菌介导的甲烷氧化作用并改变了同位素组成。本研究未发现融水中的甲烷浓度与融水化学性质或冰川规模之间存在关联。上述研究结果表明，山地冰川（alpine glaciers）下冰环境的甲烷排放现象或许比此前认知中更为普遍。