High-resolution study of layering within the percolation and soaked facies of the Greenland ice sheet

Within the percolation and soaked facies of the Greenland ice sheet, the relationship between radar-derived internal reflection horizons and the layered structure of the firn column is unclear. We conducted two small-scale ground-penetrating radar (GPR) surveys in conjunction with 10m firn cores that we collected within the percolation and soaked facies of the Greenland ice sheet. The two surveys were separated by a distance of ~50 km and ~340m of elevation leading to ~40 days of difference in the duration of average annual melt. At the higher site (~1997ma.s.l.), which receives less melt, we found that internal reflection horizons identified in GPR data were largely laterally continuous over the grid; however, stratigraphic layers identified in cores could not be traced between cores over any distance from 1.5 to 14.0 m. Thus, we found no correlation between firn core stratigraphy observed directly and radar-derived internal reflection horizons. At the lower site (~1660ma.s.l.), which receives more melt, we found massive ice layers >0.5m thick and stratigraphic boundaries that span >15m horizontally. Some ice layers and stratigraphic boundaries correlate well with internal reflection horizons that are laterally continuous over the area of the radar grid. Internal reflection horizons identified at ~1997ma.s.l. are likely annual isochrones, but the reflection horizons identified at ~1660ma.s.l. are likely multi-annual features. We find that mapping accumulation rates over long distances by tying core stratigraphy to radar horizons may lead to ambiguous results because: (1) there is no stratigraphic correlation between firn cores at the 1997m location; and (2) the reflection horizons at the 1660m location are multi-annual features.