Characteristics of methane and carbon dioxide in ice caves at a high-mountain glacier of China

The exploration of the spatiotemporal distribution of greenhouse gas (GHG) exchange in the cryosphere (including ice sheet, glaciers, and permafrost) is important for understanding its future feedback to the atmosphere. Mountain glaciers and ice sheets may be potential sources of GHG emissions, but the magnitude and distribution of GHG emissions from glaciers and ice sheets remain unclear because observation data are lacking. In this study, in situ CH4 and CO2 and the mixing ratios of their carbon isotope signatures in the air inside an ice cave were measured, and CH4 and CO2 exchange in the meltwater of Laohugou glacier No. 12, a high-mountain glacier in an arid region of western China, was also analyzed and compared with the exchange in downstream rivers and a reservoir. The results indicated elevated CH4 mixing ratios (up to 5.7 ppm) and depleted CO2 (down to 168 ppm) in the ice cave, compared to ambient levels during field observations. The CH4 and CO2 fluxes in surface meltwater of the glacier were extremely low compared with their fluxes in rivers from the Tibetan Plateau (TP). CH4 and CO2 mixing ratios in the air inside the ice cave were mainly controlled by local meteorological conditions (air temperature, wind speed and direction) and meltwater runoff. The carbon isotopic compositions of CH4 and CO2 in the ice cave and terminus meltwater indicated δ13C-CH4 depletion compared to ambient air, suggesting an acetate fermentation pathway. The abundances of key genes for methanogenic archaea/genes encoding methyl coenzyme M reductase further indicated the production of CH4 by methanogenic archaea from the subglacial meltwater of high-mountain glaciers. The discovery of CH4 emissions from even small high-mountain glaciers indicates a more prevalent characteristic of glaciers to produce and release CH4 from the subglacial environment than previously believed. Nevertheless, further research is required to understand the relationship between this phenomenon and glacial dynamics in the third pole.