Grey Glacier. Calving front of Grey Glacier in Patagonia protrudes into water under the lake surface. Shin Sugiyama1, Masahiro Minowa1, Marius Schaefer2.
75A2282
1Institute
Calving front of Grey Glacier in Patagonia protrudes into water under the lake surface Shin Sugiyama1, Masahiro Minowa1, Marius Schaefer2
of Low Temperature Science, Hokkaido University, Japan, 2Physics and Mathematics Institute, Universidad Austral de Chile
1. Introduction
Subaqueous ice front geometry of a calving glacier is crucial information to understand the processes occurring at the ice-water interface. Recent observations at the front of tidewater glaciers in Greenland revealed undercutting of the ice cliff, which formed as a result of enhanced melting associated with subglacial discharge (Rignot et al., 2015; Fried et al., 2015). In contrast to the research effort on tidewater glaciers, few studies have been reported on the ice front geometry of freshwater calving glaciers. Freshwater calving glaciers are common, for example in Patagonia, Alaska, and New Zealand, and many of the glaciers are retreating under the influence of ice-lakewater interaction. To better understand ice-water interaction at the front of a freshwater calving glacier, we performed side-scan sonar survey at the calving front of Grey Glacier, a lake-calving glacier in the Southern Patagonia Icefield (Fig. 1).
2. Method
Grey Glacier has three separated ice front terminating in Lago Grey, Chile. In January 2016, we performed side-scan sonar and CTD measurements near the eastern terminus (Fig. 1). The lake depth near the glacier is ~300 m. We employed a side-scan sonar system (Imagenex, Model 872) operated at frequencies of 260/330/800 kHz (Fig. 2a). The transmitter was hanged in water with a cable and towed by a bloat along the ~1 km long glacier front. The boat was operated within several hundred meters from the glacier so that the ice cliff was within the range of the sonar (Fig. 2). The location of the sonar recorded with a GPS was used to transform a sonar image into real space using post-processing software (SonarWiz, Chesapeak Technology). Lake water temperature and turbidity were measured by lowering a CTD profiler (JFE Advantec, ASTD101) from a boat. Water depth at the CTD casting sites were obtained from the pressure data of the profiler.
c
50 km
Grey Glacier
Grey Glacier Boat track
b 100 m Lago Grey
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CTD sites
Figure 1. (a) Location of the study site in Patagonia (Landsat 7 14 October, 2001). (b) Ice fronts of Grey Glacier (Landsat 8, 2 February 2016). (c) Satellite image of the eastern terminus with CTD casting sites and boat track during the side-scan survey (Astrium, March 2016).
a
c
Imagenex, Model 872
Frequency: 260 / 330 / 800 kHz Max depth: ~300 m Operation: PC connected Power supply: 100/240V AC
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d
3. Results
Sonar images showed complex ice front geometry in the water (Fig. 3). Ice was not a vertical cliff, but ice protruded into water under the lake surface. Judging from the image transformed to the real space, glacier front forms under water terraces with length of ~50–100 m. The length of the terraces are spatially variable. We lowered the CTD profiler near the calving front. At the region where the ice protruded, water was much shallower (< 100 m) than the lake depth (~300 m) (GE4 and GE6 in Fig. 4). Water temperature (~ 3°C) and turbidity showed fairly uniform distribution from the surface to the bottom (Fig. 4). Similar water properties were found also in the layer above the ice terrace.
a
Figure 2. (a) Side-scan sonar used in this study. (b) Schematic diagram of the side-scan sonar measurements. (c) and (d) Photographs showing the boat measurements in front of Grey Glacier. Photographs taken in January 2016.
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■ CTD casting sites
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Glacier front on 2 February, - - - Ice front at the lake surface - - - Ice front in the water
Figure 3. (a) Side-scan sonar image of the front of Grey Glacier. (b) Side-scan sonar image transformed and mapped on real space. The dotted line indicates the glacier front position determined by a satellite image on 2 February, 2016. White and red broken lines are our interpretations of the ice front at the water surface and in the lake. Boxes are CTD casting sites
4. Discussion and Outlook
Ice cliff geometry of Grey Glacier was substantially different from recent report in Greenland. This is because water circulation and thermal structure in a lake are substantially different from those in fjord. There is no upwelling of subglacial discharge at the front of a freshwater calving glacier, because turbid subglacial meltwater is denser than lakewater (Sugiyama et al., 2016; Minowa et al., 2017). Our interpretation of the observed ice front geometry is that lake water absorbs energy from solar radiation and provides heat for melting to the upper part of the ice cliff, whereas heat transport to the deeper region is less efficient. Contradicting to this hypothesis, our CTD measurements showed a fairly uniform temperature distribution from the surface to the bottom. Further investigations on lake thermal structure and circulation are needed to understand the mechanism of the ice terrace formation, and its role for ice front melting in the lake.
Figure 4. Temperature (red) and turbidity (blue) profiles obtained by the CTD casting at GE1 and GE4–GE7.
References
- Fried et al. (2015) Distributed subglacial discharge drives significant submarine melt at a Greenland tidewater glacier, Geophys. Res. Lett., 42, 9328–9336. - Minowa et al. (2017) Seasonal Variations in Ice-Front Position Controlled by Frontal Ablation at Glaciar Perito Moreno, the Southern Patagonia Icefield. Frontiers in Cryosphere Science, 5:1. - Rignot et al. (2015) Undercutting of marine-terminating glaciers in West Greenland, Geophys. Res. Lett., 42, 5909–5917. - Sugiyama et al. (2016), Thermal structure of proglacial lakes in Patagonia, J. Geophys. Res. Earth Surf., 121, 2270–2286.
Figure 5. Schematic diagram showing the vertical cross section of the front of Grey Glacier.
Acknowledgement
We thank Gino Casassa and the field campaign members for their support in the filed. The boat was operated by Bigfoot Adventure Patagonia. This research is funded by JSPS KAKENHI grant 26550001 (2014–2017), and 16H05734 (2016–2020).
