Isotopic composition and emission characteristics of CO2 and CH4 in glacial lakes of the Tibetan Plateau

Carbon dioxide (CO _2 ) and methane (CH _4 ) emissions from freshwater ecosystems are predicted to increase under climate warming. However, freshwater ecosystems in glacierized regions differ critically from those in non-glacierized regions. The potential emissions of CO _2 and CH _4 from glacierized environments in the Tibetan Plateau (TP) were only recently recognized. Here, the first direct measurement of CO _2 and CH _4 emission fluxes and isotopic composition during the spring of 2022 in 13 glacial lakes of the TP revealed that glacial lakes were the previously overlooked CO _2 sinks due to chemical weathering in glacierized regions. The daily average CO _2 flux was −5.1 ± 4.4 mmol m ^−2 d ^−1 , and the CO _2 consumption could reach 38.9 Gg C-CO _2 yr ^−1 by all glacial lakes in the TP. This consumption might be larger during summer when glaciers experience intensive melting, highlighting the importance of CO _2 uptake by glacial lakes on the global carbon cycle. However, the studied glacial lakes were CH _4 sources with total emission flux ranging from 4.4 ± 3.3 to 4082.5 ± 795.6 μ mol m ^−2 d ^−1 . The large CH _4 range was attributed to ebullition found in three of the glacial lakes. Low dissolved organic carbon concentrations and CH _4 oxidation might be responsible for the low CH _4 diffusive fluxes of glacial lakes without ebullition. In addition, groundwater input could alter CO _2 and CH _4 emissions from glacial lakes. CH _4 in glacial lakes probably had a thermogenic source; whereas CO _2 was influenced mainly by atmospheric input, as well as organic matter remineralization and CH _4 oxidation. Overall, glacial lakes in the TP play an important role in the global carbon cycle and budget, and more detailed isotopic and microbial studies are needed to constrain the contributions of different pathways to CO _2 and CH _4 production, consumption and emissions.