Basal Signatures of Subglacial Impact Structures

The search for impact structures beneath Earth's ice sheets has been long been hindered by our relatively sparse knowledge of subglacial topography and the expected significant subglacial erosion of ancient impact structure morphology. New satellite datasets and increasingly extensive aerogeophysical surveys of these ice sheets warrant a renewed search. Here we discuss two large impact structures beneath the Greenland Ice Sheet, focusing on the unique features of the basal ice and subglacial interface of one of the structures that is situated directly adjacent to the ice-sheet margin. This structure was mapped in detail using a new ultra-wideband airborne ice-penetrating radar system, which observed bed-originating reflectors emanating from the central uplift, innumerable point scatterers within basal ice, and a remarkably flat reflection approximately 15 meters below the ice-bed interface. We interpret these reflections as evidence of ongoing and vigorous subglacial erosion of the impact structure, including quarrying of boulder-sized clasts, and the first detection of a subglacial groundwater table below presumably well-drained impact breccia. Observed radiostratigraphy reflects changing ice flow over this structure since the Last Glacial Period. The remaining compelling questions regarding these impact structures suggests significant future opportunities for remote and direct investigations to better understand the ice sheet's interactions with these unusual bed features and - more generally - time-varying basal processes beneath ice-sheet margins.