A constraint upon the basal water distribution and basal thermal state of the Greenland Ice Sheet from radar bed-echoes

Abstract

Abstract. 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 North-GRIP 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) en-5 ables the detection of basal water from bed-echo reflectivity, but unambiguous mapping is limited by uncertainty in signal attenuation. Here we introduce a new RES diagnostic for basal water that is associated with wet to dry transitions in bed material: bed-echo reflectivity variability. Importantly, this diagnostic is demonstrated to be attenuation-insensitive and the technique enables combined analysis of over a decade of Operation IceBridge survey data. 10The basal water predictions are compared with existing analyses for 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 stim-15 ulate 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 basal thermal state, provides a new constraint for numerical models. 1The Cryosphere Discuss., https://doi

Surface Expression of Basal and Englacial Features, Properties, and Processes of the Greenland Ice Sheet

“…The latter of these mechanisms, initiated either by velocity changes within the ice column or in basal thermal state, would give rise to large‐scale folding and deformation like that seen here (Weertman, ; Wolovick et al, ). While it has been noted that some UDRs in Greenland do not have a clear relationship to known basal water networks (Wolovick et al, ), the distribution of observed features is consistent with radar basal water predictions (Chu et al, ; Jordan et al, ). Additionally, these are within a region of elevated geothermal heat flux (>58 mW/m 2 ; Martos et al, ; Figure b), and transitional basal thermal state (Chu et al, ; MacGregor et al, ).…”

“…Various geometric features, including lineations, troughs, and peaks, are visible in the RSE (Movie S1). Interpretation of these features is facilitated by the comparison with BM3, (Morlighem et al, ) and along‐track radargrams from National Aeronautics and Space Administration's Operation IceBridge (Rodriguez‐Morales et al, ; Figure a), as well as with complementary data sets (Figure ), including interferometric synthetic aperture radar‐derived surface ice velocity (Joughin et al, ), radar‐derived basal water (Jordan et al, ), and magnetic‐derived geothermal heat flux (Martos et al, ). On initial comparison to BM3, the RSE (Figure b) replicates fundamental landscape features.…”

“…Ice‐penetrating radar coverage as per Figure a and Greenland Ice Sheet drainage basins are delineated (Zwally et al, ), surface velocity contours at 10‐m/a intervals (Joughin et al, ), and profiles A–C are presented in Figure . (b) RSE with geothermal heat flux (GHF) contours at 2‐mW/m 2 intervals (Martos et al, ), radar predictions for basal water (Jordan et al, ), and locations of “basal ice units” (Bell et al, ). (c) RSE with basal slip ratio transparency (MacGregor et al, ), values of 0.8 (dashed), 1.0 (black), and 1.2 (dotted) are delineated.…”

“…An increase in observations of ice thickness, obtained using airborne ice‐penetrating radar (IPR), has enabled improved representation of Greenland's bed topography and, consequently, the production of progressively accurate and spatially “complete” (without holes) bed digital elevation models (DEMs; e.g., Bamber, Griggs, et al, ; Bamber et al, ; Morlighem et al, , ). IPR data, therefore, provide a necessary input to ice sheet modeling and also allow for the assessment of more specific basal and englacial properties, including subglacial roughness and its anisotropy (e.g., Jordan et al, ; Lindbäck & Pettersson, ; Rippin, ); ice rheology, through the measurement of englacial stratigraphy (e.g., Karlsson et al, ; MacGregor, Fahnestock, et al, ); englacial temperature from radar attenuation (e.g., MacGregor, Li, et al, ); and determining the distribution of basal water from bed‐echo reflectivity (e.g., Jordan et al, ; Oswald et al, ). Despite the influence of basal processes upon ice dynamics being theoretically well constrained (Cuffey & Paterson, ; van der Veen, ), our understanding of spatial variation in subsurface conditions and processes remains restricted by the paucity of IPR data; consequently, observation‐led interrogation is required to understand the magnitude of their effects in situ.…”

“…We compare results inferred from surface data with BM3 and highlight the importance of topographic anisotropy and scale both in the planning of geophysical surveys and spatial interpolation. Finally, to understand the relationship between the surface expression and basal processes we compare with complementary data sets: basal water (Jordan et al, ), geothermal heat flux (Martos et al, ), basal slip (MacGregor et al, ), and ice velocity (Joughin et al, , ).…”