Isotopic composition and emission characteristics of CO ₂ and CH ₄ in glacial lakes of the Tibetan Plateau

Abstract

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 μmol 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.