ALBERT

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: This data set consists of 1930s, 1960s and 1980s orto-mosiac of East Greenland in particular 260 km-long section of coastline between 66.3 and 68.4◦N The archival orthophotomaps were produced with the use of Agisoft Metashape (Agisoft Metashape, 2020). We used 3 data sets: a) British Arctic Air Route Expedition (BAARE), aerial oblique images taken between July 1930 and August 1931. Photogrammetric reconnaissance was done with the use of two De Havilland DH.60 Moth planes with Gipsy 1 engines equipped for taking vertical and oblique photographs with Williamson P14 camera with a lens of known focal length of 2209.8 mm (7.25'), and 127 mm ×101.6 mm (5' ×4') glass plates with envelope adaptors for changing slides in daylight were used. For our study we used 73 images obtained during the summers of 1930 and 1931. The data set was obtained from Scott Polar Institute and is licensed by this organization. b) CORONA satellite mission, 1959-72, mostly consists of satellite stereo pair images. It was lead by the CIA and the U.S. Air Force, aimed at gathering spatial data for the creation of maps of vast remote areas for intelligence purposes. It was declassified in 1995. We used images taken on 24.09.1966. For this mission the KH-4M stereoscopic camera with 75% overlap was used. The KH-4A (Keyhole-4A) carried two J-1 (in earlier missions KH-3 cameras of 3.66 m resolution) panoramic cameras, with a focal length of 61 cm, and a ground resolution of 2.7 m to 7.6 m. The J-1 cameras were placed on an M (Mural) mount, one pointing 15° aft from the vertical and the other 15° forward. The minimal flight height was 180 km and the duration of each mission was 14-15 days. The panoramic cameras used work on the general principle that during the scanning process the lens and the scan arm moves while the film remains stationary. In this case the lens rotates around the second nodal point allowing the cylindrical focal plane to keep the image of distant objects sharp. As a result a 'bow-tie' shaped region is photographed and becomes compressed into a rectangular image. This effect creates significant panoramic image distortions. No additional meta data is available. The Corona data can currently be obtained (as digital high-resolution scans; 7 μm) from the EarthExplorer website (U.S. Geological Survey). c) Greenland 1:15000 scale, vertical aerial images. Mission was carried out between 1978-87 by the Geodetic Institute, the National Cadastre and Survey of Denmark, and the Danish Geodata Agency. A WILD RC10 camera with a nominal focal length of 87.72 mm was used to collect super-wide-angle photographs at planned flying heights of 13000 m. The images were captured on photographic film, in black and white and with 8 fiducial marks on each image. For our study we used 58 images obtained on 30th of July and 14th of August 1981. Data was obtained from Agency for Data Supply and Efficiency (SDFE). Since we did not have sufficient data to select the images with the best overlap or with the best light conditions, it was decided to use all the available images for this procedure. Initially we divided the 1930s data set into 5 regions for the production of 5 orthophotomaps. However, the 1960s and 1980s datasets were significantly different in terms of their extent and overlap. This forced us to divide our area into different sub-regions in order to produce mosaics of the same glaciers as covered by the 1930s images. For all of the images we found the corresponding areas on the ArcticDEM model and created GCPs on stable, non-ice covered bare ground, which we assume to be fixed over the time period covered. The GCP placement accuracy calculated during the processing of the mosaics was around 1 pixel and in 90% of cases was smaller than 0.8 pixel. The spatial accuracy in metres varied, but in most cases was less than 20 m, and did not exceed 15 m in the X and Y directions separately. This result can be considered satisfactory when taking into account the quality of the ground truth model, problems with the definition of the stable areas for GCPs, and the age and type of the archival images.

Description: This data set consists of satellite based study of East Greenland in particular 260 km-long section of coastline between 66.3 and 68.4◦N, used to explore glacier change from 1985 to 2019. Imagery were downloaded from the USGS website EarthExplorer (U.S. Geological Survey USGS, 1995). We sought images with minimal cloud-cover that were gathered in July/August, when we expect surface snow cover to be at a minimum. We sourced imagery from Landsat-5 for the years 1985 and 1995, from Landsat-7 for the year 2005, and from Landsat-8 for the years 2015 and 2019 (all Landsat imagery courtesy of the U.S. Geological Survey). For each year for which we had data, composite images were generated. The data set consist of shape files of fronts of glaciers in 5-year intervals. Data set used in: Unravelling the long-term, locally-heterogenous response of Greenland glaciers observed in archival photography Michael A. Cooper, Paulina Lewińska, William A. P. Smith, Edwin R. Hancock, Julian A. Dowdeswell, and David M. Rippin, 2021