Half-century evolution of the debris cover on the Djankuat Glacier, the Caucasus

Abstract

The ongoing degradation of the Djankuat Glacier is also reflected in the expansion of the debris cover on the ice surface. During the 56 years since the start of direct measurements in 1968, the debris-covered glacier area has grown from 2% to 20%. The layer of superficial moraine changes the structure of the heat balance of the glacier surface, significantly affecting ice ablation. A thin (< 7 cm) cover can lead to increased melting of sub-debris ice, whereas as the debris layer thickens further, melting progressively weakens until complete vanishing after the debris cover thickness exceeds 1.5 m. Based on the results of a field survey of the debris cover in 2022, another, fourth map of the debris thickness was compiled, continuing a series of similar maps as of 1983, 1994 and 2010. The mean debris thickness varies greatly by altitudinal belts, and currently it reaches on average 60 cm throughout the glacier, which is more than twice the average all-glacier value for 1983. Thus, all the 4 debris surveys conducted over the years indicate that the hydrological role of the debris cover has always come down to an unambiguous effect of a general melt-rate weakening for the glacier as a whole. The total volume of moraine material increased 4‑fold over the 39‑year-long period 1983–2022, up to 275 thousand m3 , despite the fact that the glacier area has significantly decreased over the same period by more than 1,5 times for both the physical surface and its orthogonal projection. The acceleration of debris mass growth over the last decade is demonstrated. Activation of denudation processes due to progressive deglaciation of the rock revetment above the firn basin causes a more intensive influx of colluvial material to the glacier. Together with the rise of the kinematic equilibrium line, this leads to an increase in the upper boundary of the debris-covered surface on the glacier.