Investigating controls over methane production and bubbling from Interior Alaskan lakes using stable isotopes and radiocarbon ages

Abstract

"Large uncertainties in first-order estimates of the magnitude of CH₄ emissions from lakes (global lakes: 8-48 Tg CH₄ yr⁻¹ Bastviken et al. 2004) result from variation in ebullition (bubbling) rates between and within lakes. Based on a comparison of two interior Alaska thermokarst lakes, I suggest that variation in CH₄ ebullition observed within and between lakes can be explained by a few key differences in substrate quality and sediment density. Killarney Lake, which has a 130 cm-thick modern sediment package, emitted 120 mg CH₄ M⁻² day⁻¹ produced from a mixture of modern C and permafrost C sources, while Goldstream Lake, a younger lake with only 2-5 cm of modern lake sediment, emitted more CH₄ (183 mg CH₄ M⁻² day⁻¹) produced mostly from thawed permafrost. Incubated thawed permafrost supported production of substantially more CH₄ (0.25 ± 0.04 mg CH₄ g TC⁻¹ d⁻¹) than did taberal lake sediments (0.08 ± 0.02 mg CH₄ g TC⁻¹ d⁻¹). Together, these lines of evidence support the importance of permafrost C availability as control on CH₄ production and bubbling in thermokarst lakes. Stable isotope and radiocarbon values of contemporary interior Alaska thermokarst lake CH₄ emissions reported in this study could help constrain contributions of thermokarst lakes to the global atmospheric CH₄ budget. I show here that methanogens in close proximity to thermokarst utilized pore water derived from melted permafrost ice as a hydrogen source, and that [delta]DCH₄ values reflected ancient [delta]D of precipitation. [Delta]DCH₄ values from Alaskan thermokarst lakes were less-depleted than [delta]DCH₄ values from Siberian lakes. Thus, thermokarst lake contributions to early Holocene atmospheric CH₄ concentrations were likely higher than originally thought"--Leaf iii