Multiyear Isotopic Observations Reveal that Northern Lake Methane Emissions are Hidden in Wetland Estimates

Abstract:

Lakes are a substantial source of atmospheric methane (CH₄) at high latitudes. In spite of this, regional and global budget estimates leave little or no room for large emissions from these abundant landscape features, assuming that wetlands account for all biogenic CH₄ released from northern boreal and arctic regions (Bruhwiler et al., 2014). Sources are partitioned into general categories using inverse (top-down) mass balance modeling of measured overall CH₄ emissions and stable isotopic ratios of ¹³C/¹²C in the CH₄ (δ¹³C-CH₄). Source specific δ¹³C-CH₄ and emission are also used in bottom-up models for comparison with atmospheric mixing and isotopic ratios (Bousquet et al., 2006; Ghosh et al., 2015). Here, we present a large, multiyear dataset on both δ¹³C-CH₄ and δD-CH₄ emitted via ebullition from three interconnected, subarctic post-glacial lakes. The isotopic signatures vary largely (-78.4 to -53.1‰ and -383.3 to -232.3, n = 171 and 251) and overlap those reported from the nearby wetland. Adding our data to the signatures of other studied lakes, we show that, overall, the δ¹³C-CH₄ emitted from lakes range across all wetland signatures in the published literature. Based on this, we raise the question whether lake emissions are hidden isotopically and, thus, being double-counted within the estimates of other natural CH₄ sources, such as wetlands.