Controls on Stable Methane Isotope Values in Northern Peatlands and Potential Shifts in Values Under Permafrost Thaw Scenarios

Abstract

Northern peatlands are a globally significant source of methane (CH₄), and emissions are projected to increase due to warming and permafrost loss. Understanding the microbial mechanisms behind patterns in CH₄ production in peatlands will be key to predicting annual emissions changes, with stable carbon isotopes (δ¹³C-CH₄) being a powerful tool for characterizing these drivers. Given that δ¹³C-CH₄ is used in top-down atmospheric inversion models to partition sources, our ability to model CH₄ production pathways and associated δ¹³C-CH₄ values is critical. We sought to characterize the role of environmental conditions, including hydrologic and vegetation patterns associated with permafrost thaw, on δ¹³C-CH₄ values from high-latitude peatlands. We measured porewater and emitted CH₄ stable isotopes, pH, and vegetation composition from five boreal-Arctic peatlands. Porewater δ¹³C-CH₄ was strongly associated with peatland type, with δ¹³C enriched values obtained from more minerotrophic fens (−61.2 ± 9.1‰) compared to permafrost-free bogs (−74.1 ± 9.4‰) and raised permafrost bogs (−81.6 ± 11.5‰). Variation in porewater δ¹³C-CH₄ was best explained by sedge cover, CH₄ concentration, and the interactive effect of peatland type and pH (r² = 0.50, p < 0.001). Emitted δ¹³C-CH₄ varied greatly but was positively correlated with porewater δ¹³C-CH₄. We calculated a mixed atmospheric δ¹³C-CH₄ value for northern peatlands of −65.3 ± 7‰ and show that this value is more sensitive to landscape drying than wetting under permafrost thaw scenarios. Our results suggest northern peatland δ¹³C-CH₄ values are likely to shift in the future which has important implications for source partitioning in atmospheric inversion models.