From the Plateau to the Bog: How Environmental Variables Control Permafrost Progression and Methane Emissions

Abstract

Permafrost, or ground at or below 0 °C for two or more years, covers approximately 24% of the northern hemisphere and contains around two times the amount of carbon currently in Earth’s atmosphere. Atmospheric temperatures in the Northern Hemisphere have warmed faster than the global average, causing widespread warming and thawing of permafrost. As permafrost thaws, carbon that was once relatively inaccessible becomes available for microbes to process and release as greenhouse gases. When this process occurs in an oxygen-poor wetland, methane, a greenhouse gas with 28-35 times the global warming potential of carbon dioxide, is released.Permafrost thaw and methane emissions vary significantly across the permafrost zone. Environmental factors other than air temperature, such as ecologic, hydrologic, and topographic conditions, also regulate permafrost temperature, progression, and stability. Methane emissions have spatial and temporal variability caused by complex interactions of environmental factors, including soil moisture, soil temperature, vegetation, and nutrient access. To better understand the environmental factors that cause this variability, we instrumented Alaskan permafrost sites that experience varying climate conditions. In Chapter 2, we investigate environmental controls on permafrost thaw at a warm and wet site at the southern fringe of the permafrost zone. Our study (2020-2022) captured three of the snowiest years and three of the four wettest years since site monitoring began in 2015. Average thaw rates along an across-site transect increased nine-fold from 6 ± 5 cm/year (2015-2020) to 56 ±12 cm/year (2020-2022). This thaw was not uniform. Hummock locations, residing on topographic high points with relatively dense canopy, experienced only 8 ± 9 cm/year of thaw, on average. Hollows, topographic low points with low canopy cover, and transition locations, which had canopy cover and elevation between hummocks and hollows, thawed 44 ± 6 cm/year and 39 ± 13 cm/year, respectively. Mechanisms of thaw differed between these locations. Hollows had high warm-season soil moisture, which increased thermal conductivity, and deep cold-season snow coverage, which insulated the soil. Transition locations thawed primarily due to thermal energy transported through subsurface taliks during individual rain events. Most increases in depth to permafrost occurred below the ~45 cm thick seasonally frozen layer and expanded site taliks. This chapter highlights the importance of canopy cover and microtopography in controlling soil thermal inputs, the ability of subsurface runoff from individual rain events to trigger warming and thaw, and the acceleration of thaw caused by consecutive wet and snowy years. As northern high-latitudes become warmer and wetter and precipitation events become more extreme, the importance of these controls on soil warming and thaw is likely to increase. In Chapter 3, we investigate how environmental variables affected the spatial and temporal variability of depth to permafrost at our colder, drier, and more stable permafrost site. To better understand the impacts of these variables, we installed 25 high-resolution soil temperatures along a transect into permafrost at a discontinuous permafrost site in Interior, Alaska, with broad variation in environmental variables. Local slope and wetland proximity of transect locations caused variations in thermal regime and depth to permafrost. Locations on mild slopes had high near-surface soil moisture entering the cold season, increasing thermal conductivity and making them colder than drier locations. The bog complex, which remains unfrozen all year, acted as a thermal buffer for nearby permafrost locations, keeping them warmer than comparable locations. Warm season meteorological forcing controlled year-to-year variation in depth to permafrost at locations with consistent amounts of seasonal ground ice from year to year. Cold-season meteorological forcing was the primary control on locations with variable year-to-year ground ice content. Two locations that only overcame the zero curtain in 2023 experienced 15 cm and 58 cm shallowing of the permafrost table, respectively, while all other locations experienced permafrost thaw. This result shows the relative sensitivity of permafrost locations at a thermal tipping point compared to locations that are either warmer or colder. Chapter 4 investigates the impact of advectively transported nutrient inputs from the permafrost plateau on methane emissions from a downgradient bog complex. We found that the advection of nitrogen from the surrounding permafrost plateau and warm soil temperatures significantly enhanced methane emissions. Root damage sustained from rapid soil freezing inhibited nutrient use from plateau vegetation during the early growing season, doubling nitrogen availability in the plateau. Early season June rain flowed through the top ~40 cm of plateau soils, picking up excess nitrogen and transporting it into the wetland complex. This flux of nutrients fueled a 65% increase in methane emissions and a 30% increase in gross primary productivity. Our study identifies rain's important and unconsidered role in governing the nutrient balance of thawing permafrost landscapes.