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Fifteen years ago we carried out dGPS field surveys and mapped surface changes in the Hornbreen-Hambergbreen (H-H) icy isthmus, also known as Mendeleev Plain in South Spitsbergen, Svalbard, using satellite interferometry (ERS-1/2) and altimetry (ICESat) data. Local vertical movements with amplitudes of a few cm per day, measured on the flat and smooth glacier surface, were associated with accelerated basal melting and tidal effects and showed that a relatively large part of the H-H system was afloat. We therefore assumed that there was a subglacial strait and predicted the collapse of the H-H ice bridge by 2020. Now is 2021 and the H-H ice bridge that connects Sörkapp Land to the main island of Spitsbergen is still intact, although it is getting thinner and narrower over time. To explain the reason for our failed long-term forecast, we decided to repeat remote sensing surveys of the ice bridge. This time this was done using the repeat-pass Sentinel-1-A/B SAR interferograms controlled with concurrent altimetry data from the ICESat-2 satellite. Preference was given to the ICESat-2 ATL-06 land ice elevation data recorded on the days with low snow cover on the glacier surface and SAR interferograms with horizontal polarization, short spatial (several tens of meters) and temporal (6 days) baseline. All elevation tracks were corrected for the geoid height (31.8 m at 77.0°N and 16.8°E) and co-registered to SAR interferograms using a straightforward transformation, precise orbits and the Sentinel-SAR sensor model implemented in the ENVI SARscape software. In the resultant composite products every height spot within each altimetric transect is given corresponding interferometric phase and coherence values (See Annex). Such a combination made it possible to reliably estimate the length of interferometric baselines, to determine interferometric phase offsets and to remove them over homogeneous glacier surfaces. In addition, the process of geocoding and visual interpretation of the interferometric motion phase was supported with precise glacier elevation values, which is a luxury for ice bridge surveys. The current height of the ice walls on the flanks of the bridge was measured as 40 m a.s.l. in the west and 50 m a.s.l. in the east. Bathymetric data show the maximum depths of 96 m in Hambergbukta and 93 m in Brepollen, which means that the ice thickness of the front parts of Hambergbreen and Hornbreen does not exceed 130 m and 150 m, respectively. The maximum height of the ice bridge along its lowest axis was measured as 111 m compared to 130 m in 2006. We determined the mean value of the surface lowering in the area of the ice bridge to be 20 m or 1.5 m / a, which is three times the average lowering rate for present-day glaciation in southern Spitsbergen specified by J. Bamber et al. in 2005. The width of the ice bridge along its arcuate lower axis decreased from 8.8 km (2004) to 5.4 km (2016) and 4.8 km (2019). The present minimum width is 4.5 km. Hornbreen's current frontal velocity of 28.8 cm/day, measured in Sentinel-1 interferograms of 2020 with a transferential approach, is slightly lower than that determined in the field at 31.5 cm/day in 2006. The overall length of calving fronts along the bridge’s flanks decreased from 8.2 to 7.8 kilometres, which is still 1.2 times longer than the total width of all inlets feeding the icy isthmus. Under the current climate warming and the significant loss of ice due to calving, surface ablation and basal melting, the H-H ice bridge is melting much faster than the inland glacier parts. It is very likely that there is a narrow subglacial channel that connects the Greenland and Barents seas. In composed interferometric products, points with low interferometric coherence and greatest changes in elevation demarcate the approximate position and configuration of this subglacial strait. The width of the potential strait in the narrowest part is less than one kilometre. The interferometric analysis of SAR data shows that, currently, the entire area of the ice bridge is set in motion as a result of glacier flow and tidal effects. Several small islands and banks support the H-H ice bridge in its central part, which explains the durability of this object and leaves open the question of the possible appearance in the region of a new island (Sörkappland) with a total area of around 1270 km². We discuss the effects of long-term glacioclimatic and oceanographic trends, tidal effects, hydrometeorological conditions and inherent limitations of satellite radar interferometry and lidar altimetry on the validity and value of our results. Some differences in the results of our remote measurements from previous terrestrial surveys in the Mendeleev Plain with ground penetrating radar from A. Pälli (2003), M. Grabiec (2017) et al. are explained by methodological differences and natural changes in the region.
Mass balance of Russia's northernmost ice caps is poorly known and scarcely mapped. Thorough information about glacier fluctuations in the outer periphery of Russian shelf seas is both lacking and highly desired since it may constitute the relevant benchmark for judging and projecting climate change impacts in the entire Arctic. The present study is focussed on geodetic measurements and medium-scale mapping of the mass balance on a dozen insular ice caps, some large and some smaller, homogeneously situated along the Eurasian boundary of Central Arctic Basin. The study region extends for approx. 2.200 km from Victoria and Arthur islands in the west across Rudolph, Eva-Liv, Ushakova, Schmidt and Komsomolets islands in the north to Bennett and Henrietta islands in the east thereby comprising the most distant and least studied ice caps in the Russian Arctic. The situation of insular ice masses close to the edge of summer minimum sea ice proved helpful in analysing spatial asymmetry of glacier accumulation signal. The overall mapping of glacier elevation changes and quantification of mass balance characteristics in the study region was performed by comparing reference elevation models of study glaciers derived from Russian topographic maps 1:200,000 (CI = 20 or 40 m) representing the glacier state as in the 1950s-1960s with modern elevation data obtained from satellite radar interferometry and lidar altimetry. In total, 14 ERS and 4 TanDEM-X high-quality SAR interferograms of 1995/96 and 2011 were acquired, processed in the standard 2-pass DINSAR manner, geocoded, calibrated, mosaicked and interpreted using reference elevation models and co-located ICESat altimetry data of 2003-2010. The DINSAR analysis revealed the existence of fast-flowing outlet glaciers at Arthur, Rudolph, Eva-Liv and Bennett islands. The calculation of separate mass-balance components is complicated in this case because of generally unknown glacier velocities and ice discharge values for the mid-20th century. Hence only net balance values were determined for those ice caps. Other ice caps belong to the category of slow-moving or passive glaciers with simpler estimation of mass balance characteristics. Glacier elevation changes on several study glaciers were repeatedly determined with ICESat GLA06 data releases 28 and 29, and statistically compared. The root mean square difference between test determinations was given as less than 1 m rms and the lidar oversaturation effect was neglected in further work. Modern outlines of maritime glacier faces were corrected with the high-resolution optical quicklook imagery obtained from WorldView and QuickBird satellites. The research revealed the reduction of glacier area and general lowering of the glacier surface on most ice caps. Several new islets were discovered due to the glacial retreat in northern parts of Eva-Liv, Schmidt and Komsomolets islands. The cumulative mass budget in the study region remained negative while individual rates of volume change varied from -0.09 km3/a to +0.04 km3/a. Positive values of average mass balance with the maximum accumulation signal of approx. 0.9 m/a were determined on Ushakova, Schmidt and Henrietta ice caps. The results were represented in the form of glacier change maps with 50-m grid at 1:200,000 scale. The vertical accuracy of glacier change maps proved on several small and large ice caps was given as ± 0.3 m/a rms. Several resultant maps can be accessed at http://dib.joanneum.at/MAIRES/index.php?page=products. Further sub-regional comparison of glacier change maps with climatological, oceanographic, rheological, gravimetric and other ground-truth and EO data showed that spatial changes of insular glaciers are closely dependent on the frequency of precipitation events, water depth, sea ice regime, polynyas and gravity anomalies nearby. New opportunities for validating mass changes on the largest study glaciers and determining their bulk density are expected from the next release of GOCE gravity field data and CryoSat-2 radar altimetry data announced by ESA for 2012.
Meltdown of Ice Bridges and Emergence of New Islands in the Barents-Kara Region Observed by Sentinel-1 INSAR Aleksey Sharov1, Dmitry Nikolskiy2 1JOANNEUM RESEARCH, Austria; 2Sovzond, Russian Federation Natural ice bridges formed by glaciers stretching across water bodies and connecting isolated tracts of land to the mainland belong to relatively uncommon, rapidly vanishing and very attractive objects of present-day glaciation. Under current climate warming and essential ice loss due to calving, surface ablation and basal melting, low-lying ice bridges melt down much faster than inland glacier parts. Rapid disintegration of ice bridges in the Arctic reveals new, as yet uncharted islands, bays and capes formerly covered by glacier ice. Publications periodically announce the appearance of new typically small islands along Arctic ice coasts. Some of these findings turned out to be drying shoals, unlithified morainic remnants or grounded icebergs. Satellite radar interferometry (INSAR) helps a lot in identifying subglacial straits, studying rheology of ice bridges, mapping and forecasting their changes. The present paper discusses the use of Sentinel-1AB INSAR data series for determining rheological characteristics of the largest ice bridges and their parental glaciation, and mapping new large islands emerged in the Franz Josef Land, Novaya Zemlya and Svalbard archipelagos due to glacial retreat. Wide terrestrial coverage and relatively short repetition interval of Sentinel-1 IW data with accordingly high quality and detail of SAR interferograms obtained in the period of X.2015 – XI.2016 allowed several case studies on the use of Sentinel-1 interferometric products for studying breakups of ice bridges to be carried out in three different parts of the Barents-Kara glaciation: 1) observational study of glacier changes and new islands appeared in north Novaya Zemlya in 2014-2016, 2) detailed study of the ice bridge breakup at Hall Island and the emergence of Littrow Island and several other islets in the eastern part of Franz Josef Land, 3) in-depth study of the rheology and evolution of Hornbreen-Hambergbreen icy isthmus (35 km²) in Hornsund, south Spitsbergen, the largest ice bridge in the region. Our Sentinel-1 IW data set included • Sentinel-1AA INSAR pair of 19.09 - 01.10.2015 and Sentinel-1BB INSAR pair of 25.10 – 06.11.2016 representing the entire Northern Island of Novaya Zemlya; • Sentinel-1AA and -1BB INSAR pairs of 12.10 – 24.10.2015 and 24.10 – 05.11.2016 taken over the Franz Josef Land archipelago; • Sentinel-1AA INSAR pair of 20.01 – 01.02.2015 and Sentinel-1AB INSAR pair of 31.10 - 06.11.2016 obtained over south Spitsbergen. The Sentinel-1 data interpretation was supported with previously processed ERS-1/2 INSAR time series of 1993-2004 and geodetic, oceanographic and glaciological data obtained during field surveys in the 2000s. The main results and conclusions from the research were summarized as follows: 1. In total, 14 new islands with the overall land area of 73 km² formerly attached to the larger lands by glaciers were detected and mapped along the Barents- and Kara coasts of Novaya Zemlya and Franz Josef Land; their geodetic coordinates and main topographic characteristics were determined and validated. 2. So, Littrow Island in Franz Josef Land specified as a peninsula in contemporary maps, was – for the second time after the American Wellman polar expedition (1898) – discovered to be separated from Hall Island by the Nordenskjöld Channel. The present width of Nordenskjöld Channel was measured as 1 km and the total area of Littrow Island was given as 60 km². This is the largest island emerged in the study region because of the ice bridge collapse. 3. The breakup of ice bridges led to the increase of ice flow velocities on tributary glaciers. Several formerly quiescent ice streams conveying ice into newly opened water channels “woke up” and their flow velocities increased essentially. The regular patterns of transverse crevasses detected with the aid of Sentinel-1 coherence images on the surface of tributary glaciers released from the buttressing resistance of icy lintels indicate the extended character of glacier motion. 4. The Hornbreen - Hambergbreen icy isthmus with the maximum height of 130 m asl (2006) is still intact although its width decreased from 8.8 km (2004) to 5.4 km (2016). Currently, the entire area of the ice bridge is set in motion as a result of glacier flow, tidal effects and basal melting. The unilateral phase gradient on either side of the ice divide observed in winter Sentinel-1 SAR interferograms proves the prevalence of vertical motions in the central part of the ice bridge, which complies with the results of dGPS surveys carried out in situ in 2006. The intensity of motions increased drastically in the course of past 10 years. The overall length of calving fronts along the bridge’s flanks was measured as 8.2 km, which is equal to or even longer than the total width of all outlets feeding the icy isthmus. Hence we concluded that Sörkapp Land in South Spitsbergen might become a separate island with a total land area of 1270 km² due to complete disintegration of this ice bridge. 5. The impact of long-term glacioclimatic and oceanographic trends, seasonal effects, hydrometeorological conditions and inherent limitations of satellite INSAR on the validity and worth of our results was discussed.
The present investigation was aimed at studying spatial variability and directional dynamics of inland ice/snow fields in relation to lateral variations of surface gravity at the sub-regional scale of tens to hundreds of kilometres. The main goal was to evidence, both theoretically and empirically, the existence and significance of gravitational impacts and associated atmospheric effects on the status and fluctuations of Eurasia's northernmost insular ice caps using space-borne radar altimetry and interferometry data from the Earth Explorers and Sentinel missions in conjunction with GOCE satellite gradiometry, ground-based meteorological time series and glaciological observations. EO-based geodetic and cryospheric models describing the amount of solid precipitation, snow drift and accumulation, glacier elevation change and ice flow pattern were generated, homogenized and combined in the terrain-following reference frame. The models were then verified and applied to the diagnosis of glacioclimatic differences in arctic lowlands with strong gravity anomalies. It was ascertained that surface gravity gradients modulate the atmospheric circulation and stability thereby influencing the intensity of local precipitation and glacier growth and flow patterns in a cascade fashion.