Large Variability in the Radiocarbon Signature of Greenhouse Gases From Incubations of Thermokarst Lake Sediments Linked to Methane Production Rates and CH4:CO2 Ratios

Thermokarst lakes have the potential to contribute to permafrost climate feedbacks through large methane (CH₄) and carbon dioxide (CO₂) emissions. However, it is not fully understood how the mobilization of aged carbon from permafrost contributes to greenhouse gas (GHG) production in lake sediments. We carried out anaerobic incubations of thermokarst lake sediments to better understand the factors influencing CH₄ and CO₂ radiocarbon (¹⁴C) values. We observed an unexpectedly large variability of CO₂ and CH₄ radiocarbon values (Δ¹⁴C-GHG; −790 to +70‰) across incubation experiments. This variation is much greater than the ¹⁴C variation in bulk sediment organic matter (OM) (Δ¹⁴C-OM, −400 to −40‰) while Δ¹⁴CH₄ and Δ¹⁴CO₂ were close to each other and were strongly positively correlated. We observed much lower Δ¹⁴C-GHG when sediments were stored for a longer period (11 vs. 3 months) prior to incubation, likely because of a loss of young and labile dissolved organic carbon during storage. Δ¹⁴C values of both GHGs were strongly positively correlated with net CH₄ production rates and CH₄:CO₂ ratios, implying that when younger OM is decomposed, CH₄ production is faster, although net CO₂ production rates were not correlated with Δ¹⁴C-GHG. Δ¹⁴CH₄ was also negatively correlated with sediment OC:N ratios, suggesting that greater contributions of carbon-rich peat OM to sediments is associated with the respiration of older carbon. Our results indicate that Δ¹⁴C-GHG in thermokarst lake sediments is not strongly controlled by overall sediment Δ¹⁴C-OM, but is linked to the availability of labile younger C pools.