Vast ice depression in Dålk Glacier , East Antarctica

Обширная депрессия в леднике Долк, Восточная Антарктида © 2017 г. С.В. Попов1*, С.С. Пряхин2, Д.П. Бляхарский3, Г.В. Пряхина4, С.В. Тюрин4 1Полярная морская геологоразведочная экспедиция, СанктПетербург, Россия; 2Арктический и Антарктический научно исследовательский институт, СанктПетербург, Россия; 3АО Геоскан, СанктПетербург, Россия; 4СанктПетербургский государственный университет, СанктПетербург, Россия *spopov67@yandex .ru

In the afternoon of January 30, 2017, in the west ern part of the Dålk Glacier, near the Russian field base Progress1, a depression formed on the fragment of the road connecting the Russian station Prog ress and the Chinese station Zongshan with the air field and the logistic traverse to the inland Vostok and Kunlun stations .During the austral summer field season, by coincidence, aerial surveying was carried out in this area, so it is possible to assess the extent of this catastrophic phenomenon .Sections a and b of Fig . 1 show a fragment of the orthophoto of the de pression area before and after the event .In section с of the same figure, its enlarged image is displayed .Aerial photos were taken on January 20 and Febru ary 9, 2017, respectively .They were processed using Agisoft PhotoScan 1 .2software package (Agisoft LLC, Russia), which implements a modern technology of creating threedimensional models based on digital photos .This makes it possible to perform a morpho metric analysis of the natural feature under study . 1 -water channel from Boulder Lake; 2 -wet (?) blue spot; 3 -water channel near Progress1 .The white star is marking the place of the photo in Fig .2, a .Ortophoto on a was ob tained on January 20, 2017, the other ones were obtained on February 9, 2017 Рис. 1. Депрессия в леднике рядом с российской полевой базой Прогресс1 до (a) и по сле события (b), а также увели ченное изображение депрес сии (c) .The orthophoto and the threedimensional model of the feature allowed estimating the max imum linear dimensions of the forming depres sion, which were 183 × 220 m, and its area reached 40,260 m 2 .According to the preliminary assessment, the depth of the depression in the early days was 20-30 m; the maximum measured depth was 43 m .
An approximate volume of the cavern was about 884,013 m 3 .Unfortunately, the above event occurred at the end of the field season, shortly before the de parture of the seasonal team from the Progress sta tion .Thus, it was impossible to accomplish the nec essary set of geophysical and glaciological studies .However, the reconnaissance observations were car In the photo (see Fig .1, b) obtained in the course of aerial surveying, channel 1 flowing out from Boulder Lake is clearly seen adjoining the western part of the hill on which the Russian field base Progress3 is located .Further northwards, downstream, within the Dålk Glacier, there is a bright blue spot 2 (see Fig .1, b) .It lies within the feature marked as a lake in the Australian maps [1] .Possibly, after the breakthrough of Boulder Lake, water along the underice channels flowed into it, filled it to the margin, and then burst through .Water flowed further coming to the surface as drain channel 3, directly adjacent to the eastern part of the hill, near the northern slope of which the Rus sian field base Progress1 (see Fig .1, b) is located .This process was promoted by numerous crevasses in the area [2] which disturb the integrity of the gla cier .Further, water along the existing as well as the forming channels flowed under the Dålk Glacier eroding and collapsing the cavern, which, probably, had already existed there for some time .The rapid ly flowing lake water was visually observed at vari ous sites, between the field base Progress3 and the formed depression .Then, under the Dålk Glacier, it rushed to the Prydz Bay .A large underice channel is clearly seen in the photos in Fig . 2 .
A similar process of subglacial floods called using the Icelandic term jökulhlaups [3] has been repeated ly discussed in the scientific papers with reference to the subglacial hydrosphere of the Antarctica [4][5][6][7][8] .Thus, proceeding from the preliminary information and description of similar phenomena, the process of the depression formation can be presented as fol lows .In the lake basin of Boulder Lake, in the pro cess of melted glacial water inflow, water accumu lated gradually (the accumulation period could be several years) .Lakes of this type, as a rule, are not emptied by a simple overflow .When the water level reached a certain critical point, as a result of hydro static pressure and thermal expansion of the drain channels, a breakthrough in the weakest place oc curred .Due to the excess of water temperature over the temperature of the melting ice and the heat re leased during the movement of the water flow, the breakthrough developed rapidly resulting in forma tion of a channel for the drainage of lake water .In the process of lake emptying, the crosssectional area of the channel increases, and hydrostatic pressure drops as the lake water volume is reduced .Outflow from the lake occurred until the water level fell below the height of the drain threshold .Further, judging by the photo, the channel got into a grotto or a crevasse and moved further not too deep under the surface spread ing in the snow firn sequence .Presumably, water gradually filled the interglacial cavity in the area of the depression formation, which, in turn, caused a breakthrough, emptying along the interglacial chan nels towards the ocean, and, as a consequence, ice subsidence in the form of a depression, and further drainage of water into the Prydz Bay .
The described event is of great scientific inter est .In addition, it is of great practical importance in the light of the study of subglacial reservoirs and the underice hydrographic network as a whole .At simi lar unique features located near the infrastructure of the polar stations, it is possible to directly study the equivalents of the processes occurring in the interi or regions of the Antarctica .The practical interest to its study is associated with ensuring the safety of the movement of people and transport vehicles within the areas of the Antarctic glacier .
Acknowledgements.The authors thank A .V .Mirakin, V .V .Lukin, V .L .Martyanov for the support of the scientific works in Antarctica .