Correlating Permafrost Organic Matter Composition and Characteristics with Methane Production Potentials in a First Generation Thermokarst Lake and Its Underlying Permafrost Near Fairbanks, Alaska, USA

Abstract:

Thermokarst lakes, formed in permafrost-thaw depressions, are known sources of atmospheric methane (CH₄) and carbon dioxide (CO₂). The organic carbon (OC) utilized in the production of these greenhouse gases originates from microbial decomposition of aquatic and terrestrial organic matter (OM) sources, including soils of the lakes' watersheds and permafrost thaw beneath the lakes. OM derived from permafrost thaw is particularly important given the thickness of permafrost soils underlying some lakes (typically 10-30 m in yedoma permafrost); however, OM heterogeneity remains a significant uncertainty in estimating how microbial decomposition responds to permafrost thaw. This study correlates OM and water-extractable OC (WEOC) composition with CH₄ production potentials determined from anaerobic laboratory incubations. Samples were collected from 21 depths along a 5.9-m deep thermokarst-lake sediment core and 17 depths along an adjacent 40-m deep undisturbed yedoma permafrost profile near Vault Creek, Alaska. The Vault Lake core, collected in the center of a 3230 m² first generation thermokarst lake, includes surface lake sediments, the talik (thaw bulb), and permafrost actively thawing beneath the lake. Soil OM composition was characterized using pyrolysis-gas chromatography/mass spectrometry (py-GC/MS) and the most prevalent compounds were grouped into six indices based on their likely origin. WEOC was characterized using fluorescence spectrometry. Using stepwise multiple linear regression analyses, we found that CH₄ production was negatively correlated with WEOC aromaticity (p = 0.018) and fulvic acids (p = 0.027). CH₄ production was positively correlated with lipids and carboxylic acids (p < 0.001), polysaccharides (p = 0.036) and the degree of WEOC humification (p = 0.013). Results suggest OM and WEOC composition can be correlated with CH₄ production, indicating potential for model building to better predict greenhouse gas release from permafrost thaw.