Seasonal Sources of Whole‐Lake CH₄ and CO₂ Emissions From Interior Alaskan Thermokarst Lakes

Abstract

Abstract The lakes that form via ice‐rich permafrost thaw emit CH 4 and CO 2 to the atmosphere from previously frozen ancient permafrost sources. Despite this potential to positively feedback to climate change, lake carbon emission sources are not well understood on whole‐lake scales, complicating upscaling. In this study, we used observations of radiocarbon ( 14 C) and stable carbon ( 13 C) isotopes in the summer and winter dissolved CH 4 and CO 2 pools, ebullition‐CH 4 , and multiple independent mass balance approaches to characterize whole‐lake emission sources and apportion annual emission pathways. Observations focused on five lakes with variable thermokarst in interior Alaska. The 14 C age of discrete ebullition‐CH 4 seeps ranged from 395 ± 15 to 28,240 ± 150 YBP across all study lakes; however, dissolved 14 CH 4 was younger than 4,730 YBP. In the primary study lake, Goldstream L., the integrated whole‐lake 14 C age of ebullition‐CH 4 , as determined by three different approaches, ranged from 3,290 to 6,740 YBP. A new dissolved‐ 14 C‐CH 4 ‐based approach to estimating ebullition 14 C age and flux showed close agreement to previous ice‐bubble surveys and bubble‐trap flux estimates. Differences in open water versus ice‐covered dissolved gas concentrations and their 14 C and 13 C isotopes revealed the influence of winter ice trapping and forcing ebullition‐CH 4 into the underlying water column, where it comprised 50% of the total dissolved CH 4 pool by the end of winter. Across the study lakes, we found a relationship between the whole‐lake 14 C age of dissolved CH 4 and CO 2 and the extent of active thermokarst, representing a positive feedback system that is sensitive to climate warming.