Entrained debris records regrowth of the Greenland Ice Sheet after the last interglacial

Radio-echo sounding data capture intensely deformed structures deep within the ice sheet in Northern Greenland. Using technology developed for radar swath imaging, we measure the 3D scattering field of englacial structures and infer their physical properties. High amplitude, off-nadir backscattered energy shows these structures are not simply deformed meteoric layering as previously hypothesized, but contain distinct horizons of subglacially sourced debris (described here as "debris trains"). In many places, debris is transported through the ice column more than 1000 m off the ice sheet bed, modifying the strength of the ice and localizing deformation in ways not captured by current ice sheet models. These structures are found across Northern Greenland despite being absent in similar glaciological environments in Greenland and Antarctica. Based on their geometry, distribution, and composition, we argue they are a product of unique ice flow conditions experienced in Greenland during regrowth of a substantially reduced ice sheet during the last interglacial period. These results indicating the possibility for old ice recovery from within these structures.