Apr 2, 2020
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Retreating glacier over the deepest canyon of Antarctica

Satellite radar data have established that the Denmen Glacier, the largest in the east of Antarctica, retreated behind 04 for 5 kilometers, and the configuration of the continental bed under it makes it unstable compared to other glaciers East Antarctica and more prone to destruction under the influence of warm currents.

In a study published by March) d. In Geophysical Research Letters , authors from the Jet Propulsion Laboratory and the University of California in Irvine (University of California, Irvine, UCI) summarized the data 10 years of satellite observations of the evolution of the Denmen Glacier. An updated relief model explains its asymmetric retreat along an superdeep canyon and predicts a possible transition to irreversible melting with increasing ocean temperature.

Denman Glacier (Denman glacier) is located in East Antarctica, on the Land of Queen Mary. It is wide 04 kilometers and is located over a deep mainland canyon. As part of the work to create a new digital elevation model of Antarctica, BedMachine Antarctica revealed that the canyon below it is the deepest structure on land with a depth of about 3500 meters (for mapping the ice surface of Antarctica see our note). The BedMachine Antarctica project is intended not only to refine the map of the Antarctic terrain, but also as a direct application. It allows you to identify areas where the glacier is particularly unstable due to terrain features (as a rule, we are talking about spurs of mountains and smaller forms of rocky terrain): we can expect that when the ice is thawing, such areas will be the most vulnerable.

The volume of ice in the Denmen Glacier is so significant that if it was completely melted, the level of the World Ocean would rise by 1.5 m (for comparison: if the whole of Antarctica melted, the rise in level would be 22 meters).

Glacier Denmen on the new digital elevation model (DEM) BedMachine Antarctica.

The position and migration of the ( lean line of the glacier (grounding line) - the border dividing the land and its floating parts. This line is not constant even on a stationary glacier due to tidal fluctuations in ocean level: at high tide, the glacier rises due to the strength of Archimedes, and the line of leaning retreats deeper into the continent, and during low tide it shifts toward the ocean. Therefore, they talk about the overlapping zone - the area over which the glacier overlay line migrates with the frequency of tides. When studying the evolution of a glacier over periods of years or decades, such high-frequency fluctuations must be taken into account, averaging accordingly. If we are talking about radar data, then for each satellite passage, the tidal phase at that time could be different. To correctly account for all components of the system (ice sheet, mainland bed, bathymetry, etc.), the tide position at each moment in the past must be reconstructed from additional models, which significantly complicates such tasks. However, if you record data for at least several spans during the year, you can determine the overlying zone and then more confidently determine its drift over time.

The first data on The definition of the Antarctic glacier overlapping zone refers to 1540 and was received by radar imagery from ERS-1 / 2 of the European Space Agency - the first European satellites to study the Earth in a near-polar orbit. Thanks to these archival data, researchers were able to track the evolution of the glacier over the next two decades. New radar data for 1984 - 2018 years were obtained using the COSMO-SkyMed group of Italian reconnaissance satellites, which also surveyed the Earth from the polar orbit at an altitude of about 500 km. They made it possible to clarify the relief model in the vicinity of this glacier, as well as to establish that the glacier is expected to melt: over twenty years its line of deposition has shifted by about 5 kilometers, and ice losses amounted to 205 billion tons.

Mapping of glacier overlapping zones Denmen and smaller glaciers in its environs by radar survey 1984 and 1996 - 2016 From: Brancato et al., Geophys.Res.Lett. (2018)) .

In addition to this predictable statement, the conclusions obtained were non-trivial. with detailed mapping of the mainland bed. The glacier retreat is asymmetric due to its unusual location with respect to the ocean. The eastern part faces the ocean, but the glacier is tilted to the west, hanging over the deepest mainland canyon of the same name. The eastern side of the glacier practically does not recede and is kept at the expense of an ice ridge wide that fixes it like reinforcement km, at the same time, the western part has the ability to freely move along the canyon.

The longitudinal section of the Denmen Glacier and the retreat of its line of incidence deep into the superdeep mainland canyon. Brancato et al., Geophys.Res.Lett. (2018)) .

This configuration makes the glacier more unstable with respect to possible warming: when the overlay zone moves down the canyon, the advancing mass of warm ocean water comes into contact with an ever-increasing surface of the glacier (positive feedback). If the mass of water becomes warmer, the process will accelerate. Now the canyon is practically separated from the ocean, however, with a further retreat of the glacier, a bifurcation point may occur, after which a warm near-polar underwater current will gain access to the ice mass filling the canyon, and the ice melted as a result of this will be connected with the ocean.

The retreat rate of the Denmen Glacier in East Antarctica is an order of magnitude lower than that of glaciologists of interest in the West Antarctic glaciers, in particular, the glaciers of the Amundsen Sea Bay and others. The latter are washed by warm currents, and Therefore, their melting is more tangible. Against this background, the ice cover of East Antarctica looks stable. As this study shows, the evolution of the glacier is highly dependent on the topography and in certain cases, like this one, it may be at risk when the temperature of the ocean water rises. The volume of ice in large eastern glaciers is greater than in the west of the continent, and therefore their destruction even at lower speeds is also significant for taking into account the effects of warming.

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