| Summary: | Anthropogenic warming is amplified in the northern high latitudes, and disturbance processes such as wildfire and thermo-erosion (i.e., ground subsidence transport of ice-rich sediments) have increased in frequency and magnitude in tundra ecoregions in recent decades. These novel disturbance regimes could increase the role of the Arctic in exacerbating climate warming through the release of large carbon stocks stored in frozen soils. Fires can directly release soil carbon through combustion, and catastrophic thermo-erosion can rapidly increase the carbon pool available for microbial decomposition. On longer timescales, fires can enhance permafrost thaw by reducing surface albedo, enhancing surface roughness, and facilitating shrub expansion and snow drift, which can alter the soil thermal regime. Thus, these novel disturbance regimes may result in more rapid ecosystem changes in the Arctic compared to warming alone. However, the paucity of observational data from remote tundra ecoregions, spatial heterogeneity of ground-ice deposits, and rare burning in many tundra ecoregions at present limits our understanding of the drivers of and potential interactions between fire and permafrost disturbance. I overcome these challenges with paleoecological techniques to examine these disturbance processes over a broad range of scenarios, which is vital given the novel climate and vegetation settings predicted for the future. The main questions of my dissertation research are: Chapter 2 - How do modern tundra fire regimes compare to long-term natural variability? Chapter 3 - How does catastrophic permafrost thaw vary through time and what are the dominant drivers? Chapter 4 -What is the relationship between wildfire and thermo-erosion? Paleorecords of fire and thermo-erosional activity provide critical information regarding natural variability, which is difficult to assess give the rarity of modern tundra burning and the spatial variability of catastrophic permafrost thaw. To address my first question (Chapter 2), I used macroscopic charcoal stored in lake sediments to reconstruct past fire regimes from sites located in several Alaskan tundra ecoregions, which span a variety of modern vegetation and climate combinations. These paleofire records, spanning the past ~10,000 to 35,000 years, are among the first from these remote tundra ecoregions, and show that the spatial pattern of tundra burning on the Alaskan landscape has been in place for millennia, suggesting that ongoing changes to the fire regime will have profound impacts to ecosystem dynamics and carbon storage in historically undisturbed regions. For Chapter 3, I used sophisticated geochemical techniques, including X-ray fluorescence, X-ray diffraction, and strontium isotopes to identify past episodes of watershed thermo-erosion and examine the drivers of these events over the past 6000 years. This record of thermo-erosion from the Alaskan North Slope provided one of the first reconstructions of thermo-erosional activity from the Arctic. Intervals of shoreline thaw over the past 6000 years broadly corresponded to periods of high summer temperature, illustrating strong climatic controls on thermo-erosion in tundra regions that rarely burn. In addition, the lack of thermo-erosional episodes at a nearby site over the same time period suggests that positive feedbacks facilitate catastrophic thaw in ice-rich areas where it has previously occurred. These multi-millennial records of fire and permafrost activity provided critical information on the long-term dynamics of Arctic disturbance regimes, as well as a template to assess future change. However, interactions between these processes may further enhance the impacts of Arctic warming. To assess the relationship between fire and thermo-erosion, and gain a deeper understanding of the factors that facilitate catastrophic permafrost thaw, I combined paleoecological and soil analyses from sites in the Noatak River Watershed (NRW), a region that is a potential analogue for Arctic tundra ecoregions in the future (Chapter 4). Using lake sediments from the NRW, I reconstructed past episodes of shoreline thermo-erosion and watershed fires to directly test the relationship between these disturbance regimes using superposed epoch analysis (SEA). The SEA shows a significant relationship between watershed fires and thermo-erosional episodes over the past 3000 years, illustrating a key feedback mechanism that exacerbates catastrophic thaw. Moreover, thermo-erosion occurred several decades after fire events, suggesting long-term feedbacks between fires, landscape-scale vegetation structure, ground-heat flux, and permafrost dynamics. This record provides new insight into how these novel disturbances interact, suggesting that climate-driven changes to Arctic fire regimes can facilitate catastrophic permafrost thaw.
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