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Meltdown of ice bridges and emergence of new islands in the Barents Sea observed by Sentinel-1 INSAR

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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.
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1
Aleksey Sharov and Dmitry Nikolskiy
Meltdown of ice bridges and emergence of new islands
in the Barents Sea observed by Sentinel-1 INSAR
The 10th International FRINGE2017 Workshop, Aalto University, Helsinki, 07.06.2017 (#29)
Aleksey Sharov and Dmitry Nikolskiy
Meltdown of ice bridges and emergence of new islands
in the Barents Sea observed by Sentinel-1 INSAR
The 10th International FRINGE2017 Workshop, Aalto University, Helsinki, 07.06.2017 (#29)
The research is a continuation of the former PhD-study (1993-1997) on radar
remote sensing and mapping of glacier changes in the European High Arctic.
The present paper is based on the main outcomes of
SMARAGD (GZ37.541) http://dib.joanneum.at/smaragd cd results
MAIRES (FP7 GMES)
EuRuCAS (FP7)
GEODESIA (AOP GLAC.249, DLR)
25-year programme of glacier change mapping in the Eurarctic, since 1993
http://dib.joanneum.at/maires online atlas
Expeditions to Novaya Zemlya (2001), Spitsbergen (2006), Franz Josef Land
(1995 / 2008) organised by Joanneum Research.
Frameworks
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2
Eurarctic ice bridges
The most significant and durable ice bridges are formed by tidewater glaciers stretching
across water bodies and connecting isolated tracts of land to the mainland (e.g. island to
island(s). Cf. river and lake ice bridges / ice roads
They can be grounded on the sea bottom or be afloat and can span distances of many
kilometers over sometimes relatively deep water bodies; their ice thickness ranges from
several tens to several hundred meters.
They are characterized with flat and smooth or wavy and fissured surface.
They are dynamically connected to parental glaciers. They facilitate migration of animals
or people over a water body (Wikipedia 2017).
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.
They belong to relatively uncommon, rapidly vanishing and very attractive objects of
present-day glaciation.
3
Location of Eurarctic ice bridges
In the Eurarctic, most ice bridges are situated in secluded bays
and channels, which are well protected from the impact of
strong winds, waves and insolation.
Solid subglacial islands (hypothetic): Franz Josef Land: 10,
Novaya Zemlya: 8, Severnaya Zemlya: 7, Svalbard: 4+
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3
Glacier islands
(after Clupperton, Sugden
and Pelto, 1987)
The ice bridge collapse leads to the emergence of one or several
new “glacier islands”. It can be
real solid island – forerunner of the „glacier-free world“;
ice island / cap – isolated part of glacier ice or large tabular
iceberg sitting on a shoal;
icy island - morainic remnant made of loose material (sand,
gravel, mud, rocks) consolidated with ice).
The majority of newly discovered icy islands does not survive for
several years and erodes away (Pelto et al. 2017).
Probably, this is the reason for that, nowadays, the appearance
of new islands due to glacial retreat is, sometimes, regarded as
“an obvious phenomenon in the main stream of popular
literature” which does not deserve close scientific attention and
cannot be a subject of scientific discovery (Ziaja & Ostafin 2015).
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Essentials
Ice thickness / Ice motion
Water depth / Surface height
Oceanic and glacioclimatic settings
Methods
Ice penetrating radar in situ
Airborne radio-echo sounding
Glacier morphology / rheology
Retrospective reconstructions
Bathymetric studies / Tide records
Topographic data analysis, DGPS
Visual observations, signs of decay
(D)INSAR, e.g. coherence analysis
Study Methods
Identification of ice bridges (subglacial channels / islands) is not trivial and most
of them remain hypothetic objects till the very end of their existence.
ASIC: RES data 1997 (Dowdeswell et 2002)
Glacier bed below sea level: -200 m
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Triple Case Study
Case studies on the use of Sentinel-1 interferometric products for documenting and
forecasting breakups of ice bridges in three different parts of the Eurarctic glaciation:
1) ice bridge breakup at Hall - Littrow Island in the eastern part of Franz Josef Land,
2) rheology and evolution of Hornbreen - Hambergbreen ice isthmus in Hornsund,
south Spitsbergen,
3) mapping new islands appeared in north Novaya Zemlya in 2014-2016.
1. Hall-Littrow Island: Exploration History
4 km
Topographic maps. by J.Payer(1874), W.Wellman (1889), Soyuzmorproject(1971)
CI: 20 – 40 m, one height spot: 11 m
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Sonklar Ice Bridge: Height & Thickness
620 MHz RES data 1994. No flight tracks
over the channel (Dowdeswell et 1996)
Elongated lowering. Surface heights: 8 - 17 m
Water depths: 25 - 85 m. Ice thickness: 100 m
AFA TE-100, 08.08.1953AFA TE-100, 08.08.1953
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Sonklar Ice Bridge: Meltdown
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Field surveys in 1990s:
Salt water and superficial currents
in the opening.
Repeated spirit levelling at the
geodetic spot “Servant”, 8.6 m asl.
Elevation decrease - 0.69 m / 45 a
Hence we revived the hypothesis
about the presence of Nordenskjöld
Channel and put the passage in the
provisional map.
Bridge Width
1953: 9.2 km
1964: 8.7 km
1993: 7.3 km
……………..
2014: 0.6 km
Satellite image KFA-1000 (08.1993): “windows” with
open water and broken ice floes
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Ice Bridge: ERS-INSAR
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ERS-1/2-INSAR 09/10.10.1995
(B= 129 m)
Moving parts disintegrated first
(tides, basal melt, local subside)
ERS-1/2-DINSAR 1995/1953 + ICESat 05
+33 m positive elevation change in slow-
mowing area north-west of the ice bridge;
-9 m negative elevation change nearby.
Vertical motions: 12.9 cm/day
Ice Bridge: Motion
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ERS-1/2-INSAR of 17/18.12.1995 (B=-43
m), non-geocoded. Zero ice flow velocity
across the channel (buttressing resistance).
ERS-1/2-INSAR phase-gradient image: ice
bridge is formed by two glaciers coming into
contact from opposite directions.
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Littrow Island by Sentinel-1A
The image of 12.10.2015 shows several floes of glacier ice still blocking the passage while the
image of 24.10.2015 represents the glacier-ice-free Nordenskjöld Channel in all its glory. The INSAR
pair was processed on the 30th of October 2015 - the date of discovery.
Sentinel-1-A
INSAR pair
12.10.2015
1012.8 mb
32.4 km/h
-1.8 C
24.10.2015
986.2 mb
68.3 km/h
-3.2 C
Sentinel-1-A/B-INSAR: Fringe
14
Sentinel-1 24.10/05.11.2016
(B= + 79.3 m)
Vertical motions at the ice coast,
north of Littrow Island (indicator).
Littrow Island is not the only
territory in Franz Josef Land
being attached to the other by a
glacier.
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Sentinel-1-A/B: Coherence
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Regular pattern of transverse
crevasses = extended ice flow
on the surface of Sonklar
Glacier conveying ice into
newly open Nordenskjöld
Channel (released from the
buttressing resistance of tha
ice bridge).
There exist several glacier-
hidden islands in the area.
Sentinel-1-A PSI-INSAR
16
20 Sentinel-1-A scenes of
2015 – 2017, descending
orbs.
Geodetic signal, ice free
Vertical displacement:
-15mm/a - 69 cm/45 a
(4 x the eustatic rise)
Melting of buried ice?
Still, few summer scenes.
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Hall-Littrow Island: Climate
In 2016, the mean annual temperature of -5.7C recorded at Krenkel Station, the northernmost
meteorological station in the Eurasian Arctic, achieved its 30-year maximum. The average wind
speed of 22.3 km/h was also close to its maximum value of 23.5 km/h (1987).
50 km northward of Littrow Island
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Littrow Island: Morphometric Data
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Characterics / Island Littrow Island Hall Island
Area, km² 58.5 0. 30. 850.7 (982.8) 5. 6.
Height, m 410 421
Coast length. km 33.6 162.0 (176.8)
Ice coast length, km 16.2 104.8 (120.3)
Straight width, km 0.5 – 1.4 km
Straight depth, m 25 – 85 m
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2. Ice Isthmus in Hornsund
A relatively low and flat ice isthmus formed by Hornbreen & Hambergbreen (H - H) is currently
bridging Torell & Sörkapp lands in South Spitsbergen. It is
• 7.5 km wide (2006) vs 25 km (1936)
• 130 m high (2006) vs 220 m (1936)
approx. 6.5 km long, 35 km² large.
Short-term vertical motions of the H – H surface.
There are no nunataks. The “ice divide” is vague.
There is strong evidence that the glacier bed lies below sea level.
Early map was compiled by the Russian-Swedish expedition (1901).
Hypothesis on
subglacial strait
by V.Koryakin (1975)
V
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Macheret et al. 1985
(440-620 MHz, 1980):
„There was no bottom
return“
Dowdeswell et al.
(MK IV 60 MHz, 1984)
„No deep trough exists
here“
Pälli et al. 2003
GPR-RAMAC, 2000):
„The low-lying glaciated
valley filled by H-H
glaciers is likely to
become a partially
inundated ice-free
isthmus in the relatively
near future“.
History of Surveys: RES vs GPR data
21
17/18.12.95, high water
07/08.12.95, mid water
09/10.04.96, low water
H-H ice bridge: Tidal effects (ERS INSAR)
Uni Oslo: tide prediction service
FRINGE 2017 Workshop
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12
DGPS surveys on the H-H ice bridge, 2006
Horizontal terms of ice flow
INSAR 09/10.04.96
Pt
No Displacement, cm
horizontal vertical
10.3 -1.9
21.1 -0.5
3 0.5 + 0.3
4 4.8 - 0.2
5 40.9 - 0.8
60.1+ 0.1
7 0.2 - 0.0
810.7- 120.3
911.9-114.9
10 11.1 - 102.7
11 31.5 - 80.2
FRINGE 2017 Workshop
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DGPS vs INSAR
23.03/06.05.94
vertical motion (tides, basal melt):
1.9 cm / day in cold season, -19C Vertical terms of ice flow
H-H in Sentinel-1-A/B INSAR: Panta rhei
FRINGE 2017 Workshop
26
Sentinel-1A/B
B= - 136.7m
31.10.2016
06.11.2016
P = 1032-1015
Unilateral gradient
of phase fringes
on either side
of the ice divide.
Vertical motion.
Ice flow across
old ice divide.
EW surface tilt.
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WV01 – 22082016
H-H Width: 5.4 km (2016) vs 8.8 km (2004)
H-H ice bridge: Meltdown Summary
The overall length of calving fronts along both bridge sides was measured
as 8.2 km, which is equal to or even longer than the total width of all
outlets feeding the icy isthmus.
We conclude that Sörkapp Land in South Spitsbergen can become a
separate island with a total land area of 1200 km² due to the
disintegration of the ice bridge in the 2020s.
This event won’t influence the migration routes for polar bears.
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 gradient of phase fringes 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 outin situ in 2006.
The intensity of motions increased drastically in the course of past 10 years.
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15
Ice bridge withdrawal at 1:70 000
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3. Identifying and mapping new islands in NZ
Data 1) State topographic maps of Novaya Zemlya at 1:100000 with CI = 20 m issued in
1971, glacier state 1952 / undulations, crevasses around subglacial islands.
2) Sentinel-1-A coherence map of north
Novaya Zemlya is characterized with
large terrestrial coverage (only two SAR
interferograms are needed to cover the
entire NNZ)
short repetition interval enabling one entire
coverage per year (complex weather);
manifold coverage is needed to reduce
“false alarms”
high contrast in representing separate
islands and ice-free islets.
3) High-resolution optical imagery, e.g.
WorldView 01-03, QuickBird 1-2.
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16
Sentinel-1 coherence map of Novaya Zemlya
Geocoded mosaic of two Sentinel-1A interferograms of
19.09/01.10.2015 + 04.10/16.10.2015, repeated in
25.10/06.11.2016
8 solid out of 11 new islands appeared in north Novaya
Zemlya in 2014 – 2016 due to glacial retreat. Is it a
discovery by lucky chance?
FRINGE 2017 Workshop
31
The largest island discovered in north NZ
Discovered in the Sentinel-1A
SAR scenes of 04.10/16.10.2015
SAR image-map composite of the NN island. Area: 6.82 km². Height: 212 m. Type: solid.
Glacier: Archangelskoy Guby
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New glacier islands appeared in 2014-2016
No. Name / Location Coordinates Area, km² Island type
Latitude, °N Longitude, °E
1Littrow / FJL 80,1917 58,1684 60,028 solid
2Mesyatseva / FJL 81.7236 62.7583 2,710 ice island
3Belousova / FJL 80.8150 57.7683 0,750 solid?
4Malyshok / FJL 80.8367 57.4919 < 0.01 icy island
5- / NNZ 75.7682 58.6743 6.819 solid
6- / NNZ 76.0300 60.8018 0.853 solid
7- / NNZ 75.9657 60.3330 0.593 solid
8- / NNZ 75.5756 58.2750 0.418 solid
9- / NNZ 74.4819 56.3621 0.214 solid?
10 - / NNZ 74.5002 56.1483 0.186 icy island
11 - / NNZ 76.0183 63.0048 0.150 solid
12 - / NNZ 75.9654 66.1015 0.073 solid
13 - / NNZ 75.9671 66.1789 0.028 icy island
14 - / NNZ 75.5711 58.2853 0.011 icy island
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Conclusions / Problems to solve
Seeking for subglacial channels and new glacier islands is an interesting job
which provides an opportunity to be an early explorer of a new tract of land – a
sort of consolation to the glaciologist who have to observe vanishing glaciers.
Most study ice bridges are characterized with similar history of their
exploration.
Still, there are nearly 30 large solid islands hidden by glaciers to be discovered
in the Eurarctic.
The Sentinel repetition interval of 6 days is still too long for studying rheology
of even slow moving glacier areas.
There are many fringes on the glacier surface and, sometimes, too many.
There exist problems with phase unwrapping, aliasing etc.
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18
Perito Moreno ice bridge
Thanks !
Ice bridge withdrawal: Nordenskjöld Channel
Catalogue of Glaciers in FJL (1965)
19
„Ice bridge“ width
?
http://dib.joanneum.at/integral
Ice bridge withdrawal: Hornsund
... Ice retreat has also resulted in the formation of a new island in the southeast of Hall Island (Sharov and Nikolskiy, 2017;Pelto, 2018). In 2002 the new island was still a peninsula with an area of 59.5 km 2 , connected to Hall Island by marine-terminating glaciers ( Figure S9). ...
Full-text available
Article
The glaciers of the Franz Josef Land (FJL) archipelago in the Russian Arctic are subjected to rapidly-warming temperatures but are small contributors to sea level. We analyze ice surface elevation data derived from satellite stereo imagery (WorldView and SPOT), radar altimetry (CryoSat-2), and a digitized 1953 cartographic map to calculate elevation change rates [Formula presented]. Mass loss from FJL doubled between 2011 and 2015 compared to 1953–2011/2015, increasing from a rate of −2.18 ± 0.72 Gt yr⁻¹ to −4.43 ± 0.78 Gt yr⁻¹. This 2011−2015 rate indicates an acceleration in ice loss from that observed in 2003–2009 by multiple studies using ICESat and GRACE. Glacier thinning rates are spatially highly variable. We observe glacier thinning rates of up to 10 m per year, and in general we see a trend of increased thinning from the NE towards the SW. Glacier retreat is widespread and has led to the creation of at least one new island. Historically, ice wastage from FJL is thought to have been relatively small, but accelerating ice loss may be the new normal for this archipelago in a warming Arctic.
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