Date of Award

Winter 2020

Project Type

Thesis

Program or Major

Earth Sciences

Degree Name

Master of Science

First Advisor

Ruth K Varner

Second Advisor

Jessica Ernakovich

Third Advisor

Julie G Bryce

Abstract

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‰) did not differ between sampling years or months. Pond types that are partially thawed and have a lower daily CH4 ebullitive flux appear to have a higher relative contribution of hydrogenotrophic methanogenesis while types that are partially or fully thawed and have a higher daily CH4 ebullitive flux appear to have a higher contribution of acetoclastic methanogenesis relative to other pond types. Differences in CH4 isotopic composition between pond types indicates that shifts in isotopic emissions could occur as thaw progresses in northern permafrost ecosystems. While we did observe expansion of ponds and landscape slumping at Stordalen Mire over our 7-year study, a strong trend in isotope signal was not observed likely due to the high interannual variability. This unique multi-year study characterized δ13C-CH4 and δD-CH4 values for ebullition and porewater from open-water thaw ponds providing valuable data to constrain the global CH4 budget and improve modeling of the contribution of these systems to emissions now and in the future.

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