The response of permafrost to inundation below a rapidly eroding Arctic island

Abstract. Tuktoyaktuk Island acts as a natural breakwater, protecting the harbour and townsite of Tuktoyaktuk — an Arctic community that has faced coastal retreat and its consequences for decades. Increasing storm activity, coupled with a longer open-water season, is rapidly eroding the island's shoreline and inundating the underlying permafrost. Once inundated, permafrost warms and degrades, further undermining coastal stability. This study investigates both short and long-term permafrost changes during the transition from terrestrial to subsea. We used Electrical Resistivity Tomography (ERT) to estimate the depth of the ice-bearing subsea permafrost table (IBPT), capturing the short-term response. By integrating subsurface resistivity data with historical shoreline positions and thermal modelling, we also gain insights into long-term degradation patterns. Our results reveal a distinct contrast in IBPT shape between the ocean-facing and harbour-facing nearshore zones, indicating the influence of coastal erosion rates and corresponding inundation times. Additionally, small-scale variations appear linked to local geological differences. In the long term, changes in subsurface composition point to more rapid ice loss within the permafrost than can be explained by the temperature gradient caused by inundation alone. We suggest that subsea permafrost north of the island is more degraded than previously thought, potentially accelerating the projected breach, which was last estimated to occur by 2044. These findings enhance our understanding of subsurface processes driven by coastal retreat and offer valuable insights that can inform engineering strategies to fortify the island.