Temporal and spatial variability of the Greenland firn aquifer revealed by ground and airborne radar data

Abstract:

During the last two decades, the Greenland ice sheet has been losing mass, significantly contributing to sea level rise (0.33±0.08 mm yr^-1). In the meantime, summer surface melt has been increasing in both duration and extent, and subsequent runoff represents about half of the total mass lost. However, small-scale heterogeneous physical processes and residence times associated with meltwater formation, infiltration in the firn, refreezing and/or runoff remain unconstrained in coarser resolution numerical models, leading to significant error bars while estimating total runoff. In Southeast and South Greenland, widespread aquifers have been observed in relative high accumulation and melt regions, persisting throughout the year, storing a significant mass of water within the firn. The presence of a persistent water table within the firn aquifer is observed using a 400 MHz ground-penetrating radar and the 750 MHz airborne Accumulation Radar over the same location. In both radar echograms, a strong reflection is present, illustrating the important dielectric contrast between dry firn and water-saturated firn. Since 2011, NASA’s Operation IceBridge mission allows us to produce an ice-sheet-wide map of the location and depth of the firn aquifer using the Accumulation Radar echograms. Over the last four years, from one spring to the next, repeated flight lines demonstrate a relatively steady short-term behavior of water in the aquifer with constant lateral boundaries (with a few exceptions) and water table surface. An earlier radar survey (1993) implies the aquifer presence by lack of bed return, but the study area was limited to the Helheim Glacier region. Within the aquifer, a relatively slow flow of water is inferred from 2-D hydrological flow modeling, while assuming a constant hydraulic conductivity in the aquifer. On the aquifer low-elevation lateral boundary, connection with crevasses are observed in the airborne radar echograms and documented in this study. More work is required to quantify the water volume potentially draining from the aquifer into the crevasse system. The aquifer presence will need to be taken into consideration during mass balance estimations as it could impact the elevation change to mass change conversion for certain regions of the Greenland ice sheet.