Ground ice distribution and degradation rates in the near-shore coastal zone

Abstract. Ice-rich permafrost coasts are among the most rapidly changing shorelines on Earth, and their retreat is episodic and highly variable along-shore, which makes planning and management difficult. Massive ground ice within and beneath coastal bluffs is a primary control on that retreat, yet its three-dimensional distribution cannot be assessed from surface observations and is rarely characterised at the resolution coastal protection decisions require. We present a dense three-dimensional Ground Penetrating Radar (GPR) survey, combined with passive seismic Horizontal-to-Vertical Spectral Ratio (HVSR) measurements, photogrammetry, and active layer thickness probing with validation pits, acquired across the beach at Qikiqtarjuak  Tuktoyaktuk Island), Beaufort Sea, Northwest Territories, Canada, over three summer campaigns. The GPR resolves the upper surface of a massive ice body dipping seaward at approximately 1:5, from 0.4 to 2 m below the beach, with along-shore variability resolvable only through dense three-dimensional survey. Repeat survey over a five-year interval yields the first direct measurements of coastal massive ice degradation at this site: a mean of 6 ± 2 cm yr⁻¹ along a mid-beach transect, rising to 14 ± 2 cm yr⁻¹ in the shallowest ice sampled. The rate decays exponentially with overburden thickness, with a decay length of 40 cm. Read through the thermal damping-depth relation, that length scale implies a surface forcing period of 7-27 days, excluding the diurnal tidal constituents and the annual cycle and pointing instead to fortnightly-scale wetting of the beach by spring tides and storm surge. Rising thawing indices account for the magnitude of the mean rate; the fortnightly signal governs how it varies with depth. The depth-dependence of ground ice loss therefore appears to be set by the frequency with which the surface is wetted, rather than by overburden insulation alone. That control is not fixed: it will shift as open-water seasons lengthen and storm frequency changes, and armouring a beach necessarily alters it. The baseline established here, immediately prior to rock armour installation in March 2025, provides a direct test of that expectation.