Permafrost organic matter quality and biolability in the Vault Lake thermokarst environment, Interior Alaska, USA

Abstract

Warming and thawing of permafrost soils removes a major barrier to soil organic carbon (SOC) mineralization, leading to the mobilization and microbial degradation of previously frozen, inactive permafrost organic carbon (OC) into the greenhouse gases carbon dioxide (CO₂) and methane (CH₄). Many thermokarst (thaw) lakes formed in permafrost-dominated landscapes have high rates of CO₂ and CH₄ emission; however, the composition and biodegradability of the thawed permafrost OC as they relate to the relative magnitudes of anaerobic OC mineralization at different depths throughout the vertical profile of a thermokarst-lake talik system have, to my knowledge, never been measured. My research examined OC composition and mineralization potentials at the Vault Creek (VC) permafrost tunnel and Vault Lake, located 20 km north of Fairbanks, Alaska, USA, to better constrain these uncertainties. I found that, in a 590-cm long sediment core collected from the center of Vault Lake, whole-column CH₄ production is dominated by methanogenesis in the organic-rich mud facies, which occurred in the surface 0 to 152 cm. CH₄ production potential rates positively associated with substrate availability (carbon and nitrogen concentrations) and the relative abundances of terrestrially-derived organic matter compounds (alkanes, alkenes, lignin products, and phenols and phenolic precursors), measured using pyrolysis-gas chromatography-mass spectrometry. Temperature sensitivity analyses conducted on a subset of samples from the Vault Lake sediment core suggest century-scale time since permafrost thaw affects temperature sensitivities of CH₄ production. Freshly-thawed taberite sediments at the base of the talik (thaw bulb) were most sensitive to warming at lower incubation temperatures (0 °C to 3 °C), while the overlying taberite sediments thawed in situ longer periods of time (up to 400 years based on radiocarbon dating) did not experience statistically significant increases in CH₄ production until higher incubation temperatures (10 °C to 25 °C). Finally, using anaerobic incubations and ultrahigh-resolution mass spectrometry of water-extractable organic matter along a 12-m yedoma profile in the VC permafrost tunnel, I show that yedoma OC biolability increases with depth as indicated by increasing proportions of aliphatics and peptides (reduced, high H/C compounds). These compounds also positively correlated with anaerobic CO₂ and CH₄ production, and corresponded to high proportions (5.6% to 118 %) of OC mineralization rates in incubations. This suggests that as yedoma permafrost thaws beneath a thermokarst lake greenhouse gas production potentials may increase with thaw depth.