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UBC Theses and Dissertations

Refining estimates of the stable carbon isotope signature attributed to methane emissions from boreal wetlands Hosseini Alhashemi, Azamalsadat

Abstract

Northern wetlands are a major global source of methane (CH₄) emissions to the atmosphere. How carbon cycling in these peat-rich ecosystems will respond to ongoing climate change remains uncertain. Stable isotopes provide a means to trace CH₄ from different emission sources, including different wetland types; however, upscaling of emissions relies upon accurate knowledge of CH₄ source strengths and isotope composition, and delineation of wetland types and areas. This study developed a comprehensive probabilistic inventory of wetland occurrence in Manitoba, Canada using Earth observation satellite data and ancillary data characterizing vegetation structure and environmental conditions. Google Earth Engine was used to acquire and process synthetic aperture radar (SAR) and multi-spectral data from Sentinel-1 and Sentinel-2 for 2018. Satellite imagery was combined with vegetation structure information, pH and topographic derivatives in a random forest model to predict the occurrence and type of wetland. The approach yielded a high overall predictive accuracy (0.86) and strong discrimination between different wetland types. The results demonstrated a significant role for use of optical variables to model wetland distribution at regional scales when coupled with LiDAR-derived metrics and soil pH data. A literature review identified limited new stable isotope data for wetlands globally but the available data agreed well with previous assessments of δ¹³C(CH₄) values for bogs (-74.9 ± 9.8‰) and fens (-64.8 ± 4.0‰). The δ¹³C(CH₄) values were upscaled using published CH₄ flux rates and the areas of bogs and fens in Manitoba determined in this study. Manitoban bogs and fens are estimated to emit 0.8 ± 0.1 Mt CH₄ yr-¹ collectively, which is a large proportion of total flux from northern Canadian wetlands (4.1 - 6.9 Mt). Integrated δ¹³C(CH₄) values are more negative than compositions typically attributed to wetlands (-62 to -58‰) in global budgets because of the endmember compositions assigned to bogs and fens. However, the integrated values are more consistent with δ¹³C(CH₄) values reported from diel inversion and aircraft studies. Refinement of the approach will rely on new observational δ¹³C(CH₄) and source strength data for all types of boreal wetland in combination with measured δ²H(CH₄) values, which remain unavailable globally.

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Attribution-NonCommercial-NoDerivatives 4.0 International