Investigating permafrost degradation below beaver ponds using ground-penetrating radar in northwestern Alaska

The Arctic is warming, resulting in permafrost degradation. Beavers are rapidly colonizing the Arctic tundra and altering the landscape, which can further increase permafrost thaw. While surface changes caused by beaver ponds have been shown using remote sensing imagery, the subsurface changes caused by beaver ponds remain theoretical and undocumented. Here, we used ground-penetrating radar (GPR) to non-invasively measure permafrost table depth under beaver pond complexes in northwestern Alaska and quantify the rate of permafrost thaw based on pond age in beaver and non-beaver-affected waterbodies. GPR measurements were collected in August 2024 along transects at 11 sites associated with beaver ponds. These included six sites near Nome on the Seward Peninsula, characterized by discontinuous permafrost with low to moderate ground-ice content, and five sites near Kotzebue on the Baldwin Peninsula, characterized by continuous permafrost with high ground-ice content. An antenna frequency of 160 MHz was used inside a packraft, and measurements were recorded at 0.3 s intervals. We also measured thaw depth (average of 0.5 m +/- 0.2 m) using a probe around the water bodies to calibrate the radar velocity (0.05 m/ns) within the thawed layer. Our results show deeper thaw beneath old (>20 years) beaver ponds and shallower thaw beneath young (~1 year) beaver ponds. Permafrost tables are more clearly identified in radargrams in sites with continuous permafrost and high ground-ice content compared to sites with discontinuous permafrost and low ground-ice content. This study quantifies the effects of beaver engineering on permafrost degradation, enhancing our understanding of the Arctic beaver pond environment and the future of the Arctic tundra ecosystems in a changing climate.