NOVEL DISTURBANCE REGIMES IN THE ARCTIC: PALEOECOLOGICAL RECORDS OF FIRE AND PERMAFROST THAW FROM ALASKAN TUNDRA

Anthropogenic warming has resulted in profound environmental changes in the Arctic that may increase the vulnerability of the large carbon stocks in permafrost soils. Disturbance processes such as wildfire and thermal degradation of permafrost (i.e., thermo-erosion) have increased in both frequency and magnitude in recent decades. These disturbance processes may interact with one another, exacerbating ecosystem response beyond the direct impacts of warming alone. Paleoecological reconstructions from lake sediments can provide information on the long-term dynamics of tundra disturbance regimes.

We present several reconstructions of wildfire and thermo-erosion from tundra ecoregions of Alaska, based on the analyses of charcoal, isotopes, lithology, X-Ray fluorescence, and X-Ray diffraction on lake-sediment cores. Paleofire reconstructions from tundra ecoregions that span a broad range of modern vegetation and climate show spatial heterogeneity in fire regimes, with mean fire-return intervals ranging from 140 to 6050 years. In some ecoregions, these past fire-return intervals are longer than fire cycles estimated from modern observations, suggesting higher rates of burning over the recent past. On the Alaskan North Slope, where fires are rare, we identified ten episodes of shoreline thermo-erosion over the past 6000 years that coincided with periods of warm summer temperatures. In contrast, records of fire and thermo-erosion from the Noatak Watershed, a high-fire ecoregion, suggest that thermo-erosion (14 episodes over the past 3000 years) was facilitated by watershed fires. Furthermore, these records show that shoreline thermo-erosional features formed ~20 years after fire events, suggesting a lagged response of permafrost thawing to climate-driven fire activity. These records provide valuable new information for understanding the natural variability, drivers, and interactions of tundra disturbances, which is critical given the rapidly changing state of the Arctic and the potential for novel disturbance regimes in tundra ecosystems.