Seasonal Sources of Whole-Lake CH_4 and CO_2 Emissions From Interior Alaskan Thermokarst Lakes

摘要

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.