Mapping bedrock topography and ice thickness distribution of Patagonian outlet glaciers using ground-penetrating radar

The Southern Patagonian Icefield, the largest temperate ice body in the Southern Hemisphere, is among the regions with the highest mass-loss rates globally. Most of this mass loss is driven by large, water-terminating outlet glaciers. Their response to climate change, however, is heterogeneous in time and space. For water-terminating glaciers, subglacial topography plays a key role in modulating their response to climate change, yet it remains largely unknown, especially near glacier termini.We present results from two campaigns in 2022 and 2024, during which we conducted helicopter-borne GPR measurements of three of the largest Argentine outlet glaciers: Glaciar Upsala, Glaciar Viedma, and Glaciar Perito Moreno. Our measurements, covering 232 km of flight tracks, reveal the complex subglacial topography in the lower regions of these glaciers and show bed reflections of up to 800 m depth at Glaciar Upsala. Our data also shows Glaciar Perito Moreno lies on a pronounced subglacial bedrock ridge, which has largely contributed to its past stability.In addition, we incorporated our measurements into an established ice-thickness reconstruction to derive the basin-wide ice-thickness distribution and, thus, the subglacial topography. Our ice-thickness maps indicate that the three glaciers had a combined ice volume of 831 km³ in the year 2000, which is more than six times the total ice volume of the European Alps combined.While the rerteat of Glaciar Upsala and Viedma has slowed down, Glaciar Perito Moreno shows increased surface-lowering rates, from 0.34 m a⁻¹ (2000-2019) to 5.5 m a⁻¹ (2019-2024), accompanied by glacier acceleration and frontal retreat. After almost 100 years, the glacier has started to detach from its pinning point. Using a simple numerical model, we show that buoyancy-driven retreat of several kilometres could occur in the near future if lowering rates persist.