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Methane (CH₄) and carbon dioxide (CO₂) emissions from small peatland lakes may be highly sensitive to climate warming and thermokarst expansion caused by permafrost thaw. We studied effects of thermokarst expansion on ebullitive CH₄ and CO₂ fluxes and diffusive CH₄ fluxes from a peatland thaw lake in boreal western Canada. Ebullitive CH₄ fluxes from the thaw edge (236 ± 61 mg CH₄ m⁻² d⁻¹) were double and quadruple that of the stable lake edge and center, respectively. Modeled diffusive CH₄ fluxes did not differ between the thawing and stable edges (~ 50 mg CH₄ m⁻² d⁻¹) but were double that of the center. Radiocarbon (¹⁴C) analysis of CH₄ and CO₂ bubbles from the thaw edge was older (~ 1211 and 1420 ¹⁴C yr BP) than from the stable edge and the center (modern to ~ 102 and 50 ¹⁴C yr BP, respectively). Incubations indicated that deep, old peat sediment was more labile along the thaw edge than in the center. While our study suggested increase CH₄ emissions partly derived from millennial-aged carbon along the thaw edge, accounting for these emissions only increased the estimated total lake CH₄ emissions by ~ 10%, which is a much smaller contribution than measured from thermokarst lakes in yedoma regions. Our study suggests that it is important to account for landscape history and lake types when studying the processes that govern the sensitivity of lake greenhouse gas emissions to climate change.