Using Stable Isotopes to Determine Dominant Methane Production Pathways of Thaw Ponds in a Subarctic Peatland

• Main Author: Bennett, Kathryn Ann
• Format: Text
• Language: unknown
• Published: University of New Hampshire Scholars' Repository 2020
• Subjects: methane; permafrost; stable isotopes; thaw; Arctic; Ice; Northern Sweden; Subarctic
• Online Access: https://scholars.unh.edu/thesis/1447; https://scholars.unh.edu/cgi/viewcontent.cgi?article=2486&context=thesis

Arctic and subarctic ecosystems are currently warming faster than any other region of the globe, accelerating seasonal permafrost thaw. As thaw progresses, small water bodies can form due to slumping of the peatland surface. These ponds emit methane (CH4), a strong, radiatively important trace gas, predominantly through ebullition (bubbling). Two different types of methanogenic Archaea present in these systems produce CH4 through their respective production pathways: acetoclastic and hydrogenotrophic methanogenesis. The acetoclastic pathway forms CH4 using CH3COOH, an organic carbon (C) source while hydrogenotrophic methanogenesis uses CO2, an inorganic C source. Stable isotopes can be used to characterize the relative contribution of these two pathways in overall CH4 production and to better constrain the global CH4 budget and improve modeling of future emission scenarios. We used stable isotopes, carbon-13 (13C) of CH4 and CO2, deuterium (D) of CH4, and calculated apparent fractionation factors to determine the relative contribution of acetoclastic versus hydrogenotrophic pathways of methanogenesis in thaw ponds in a subarctic peatland located in the discontinuous permafrost region of northern Sweden. Isotopic analysis was performed on porewater samples (n = 310) and gas captured from ebullition (n = 177). Samples were collected from nine ponds over seven years (2012 to 2019) during the ice-free months (June to September). We tested important physical attributes of the ponds that were related to their formation and CH4 production pathways. Results indicated that δ13C-CH4 of ebullition (-86.3‰ to -49.2‰) and porewater (98.2‰ to -42.9‰) and the inferred contribution of hydrogenotrophic vs. acetoclastic methanogenesis differed significantly between certain ponds and pond types. Over the course of this study dissolved and ebullitive δ13C-CH4 remained relatively constant between years but varied significantly between months. Alternatively, δD-CH4 of ebullition (-397.0‰ to -199.4‰) and porewater (-383.4‰ to -184.8‰) ...