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

The lakes that form via ice-rich permafrost thaw emit CH4 and CO2 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 (C-14) and stable carbon (C-13) isotopes in the summer and winter dissolved CH4 and CO2 pools, ebullition-CH4, 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 C-14 age of discrete ebullition-CH4 seeps ranged from 39515 to 28,240150 YBP across all study lakes; however, dissolved (CH4)-C-14 was younger than 4,730 YBP. In the primary study lake, Goldstream L., the integrated whole-lake C-14 age of ebullition-CH4, as determined by three different approaches, ranged from 3,290 to 6,740 YBP. A new dissolved-C-14-CH4-based approach to estimating ebullition C-14 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 C-14 and C-13 isotopes revealed the influence of winter ice trapping and forcing ebullition-CH4 into the underlying water column, where it comprised 50% of the total dissolved CH4 pool by the end of winter. Across the study lakes, we found a relationship between the whole-lake C-14 age of dissolved CH4 and CO2 and the extent of active thermokarst, representing a positive feedback system that is sensitive to climate warming. Plain Language Summary Lakes that form as a result of thawing permafrost (perennially frozen ground) can release new greenhouse gases from ancient carbon reservoirs, which further warm the atmosphere and promote more permafrost thaw. Existing observations of this phenomenon are insufficient to fully understand the impact that thaw lakes have on the current atmosphere, let alone the atmosphere of a future warmer world. In this study, we used a novel approach and made open water and ice-covered measurements of rare carbon isotopes in methane and carbon dioxide dissolved in lake water and in bubbles emitted from sediments to determine the whole-lake-scale environmental drivers that regulate gas emissions from thawing permafrost. We learned that despite highly variable carbon source ages within single lakes, the presence of winter-ice traps and mixes all lake sources proportionally into the dissolved gas pool during winter and allows the stronger greenhouse gas, methane, to be oxidized to carbon dioxide before emission to the atmosphere in spring. This study also confirmed that higher levels of permafrost thaw within a lake are related to older carbon sources fueling whole-lake gas emissions, which, if consistent with lakes across northern permafrost regions, is evidence of a potential positive feedback to further climate warming.