DOI: 10.1594/pangaea.893097
title: Greenland basal water distribution from airborne radar sounding (2003-2014)

description: There is widespread, but often indirect, evidence that a significant fraction of the bed beneath the Greenland Ice Sheet is thawed (at or above the pressure melting point for ice). This includes the beds of major outlet glaciers and their tributaries and a large area around the NorthGRIP borehole in the ice-sheet interior. The ice-sheet scale distribution of basal water is, however, poorly constrained by existing observations. In principle, airborne radio-echo sounding (RES) enables the detection of basal water from bed-echo reflectivity, but unambiguous mapping is limited by uncertainty in signal attenuation within the ice. Here we introduce a new, RES diagnostic for basal water that is associated with wet-dry transitions in bed material: bed-echo reflectivity variability. This technique acts as a form of edge detector and is a sufficient, but not necessary, criteria for basal water. However, the technique has the advantage of being attenuation-insensitive and suited to data combination enabling combined analysis of over a decade of Operation IceBridge survey data.The basal water predictions are compared with existing analyses of the basal thermal state (frozen and thawed beds) and geothermal heat flux. In addition to the outlet glaciers, we demonstrate widespread water storage in the northern and eastern interior. Notably, we observe a quasi-linear 'corridor' of basal water extending from NorthGRIP to Petermann glacier that spatially correlates with elevated heat flux predicted by a recent magnetic model. Finally, with a general aim to stimulate regional- and process-specific investigations, the basal water predictions are compared with bed topography, subglacial flow paths, and ice-sheet motion. The basal water distribution, and its relationship with the thermal state, provides a new constraint for numerical models.
description: The basal water distribution is derived from the ACORDS, MCRDS and MCoRDS airborne radar sensors over 13 field seasons from 2003-2014. The flight-track data is provided as csv files on a season-by-season basis and for the full (13 season) data set. The data columns correspond to: (A) latitude, (B) longitude, (C) water binary value, at a 1 km along-track posting. The water binary value corresponds to: 1 == a 5 km bin with water detected, 0 == a 5 km bin with no water detected, NaN == a 5 km bin with no coverage.The basal water predictions are shown in Figures 7-10 of Jordan et al. 2018 and correspond to a subset of flight-track data where basal water is present: specifically, where there are rapid horizontal gradients in the bed dielectric (wet/dry transitions). The predictions therefore act as a sufficient constraint upon the basal water distribution rather than being a fully comprehensive flight-track map for basal water extent.
description: Supplement to: Jordan, Thomas M; Williams, Christopher N; Schroeder, Dustin M; Martos, Yasmina M; Cooper, Michael A; Siegert, Martin J; Paden, John D; Huybrechts, Philippe; Bamber, Jonathan L (2018): A constraint upon the basal water distribution and thermal state of the Greenland Ice Sheet from radar bed echoes. The Cryosphere, 12(9), 2831-2854