Cryokarst–induced dynamics of the Gangotri glacier, central Himalaya

Abstract

The heterogenic debris-covered glacier surfaces are subject to differential ablation, favouring the formation of cryokarst features (such as supraglacial ponds and ice-cliffs). These features are investigated to explain the comparable and accelerated mass loss patterns for the Himalayan debris-covered and clean glaciers and can also exacerbate glacial hazards. This study provides the first cryokarst inventory (at 2.5–15 m spatial resolutions) for the extensively debris-covered Gangotri glacier, central Himalaya, with the focus on assessing their dynamics and contribution to the glacier mass loss. The inventory reveals that ponds cover about 7 % of the glacier ablation zone between 2000 and 2018, with a 43 % increase in both their count and area. However, ice-cliff coverage is relatively low (1 %), exhibiting 33 % reduction in their count and about 21 % reduction in their area between 2006 and 2018. Only few ice-cliffs are pond-associated and show 7 % reduction from 2006 to 2018. The frequency and coverage of these cryokarst features follow a similar decreasing pattern with increasing surface elevation, surface slope and feature size. These features are more frequent in lower ablation zone (LAZ: 4060–4700 m) and on gentle (4–8°) to moderate (8–16°) slopes as compared to upper ablation zone (UAZ: 4700–5100 m) and on gentler slopes. Tiny (<0.001 km²) to small (0.001–0.002 km²) cryokarst features are more frequent than larger (>0.0243 km²) ones, but area contribution is more from semi-medium (0.002–0.004 km²) to medium-sized (0.004–0.008 km²) features. Significant pond growth observed at higher elevations from previous to recent decades are associated with rising air and surface temperatures. Overall, supraglacial ponds and ice-cliffs contribute to about 21 % and 16 % of net ablation, respectively, in the debris-covered Gangotri glacier surface during 2000–2018. The observed higher mean mass loss of LAZ (showing maximal mean debris thickness of 0.32 m) than that of UAZ (with minimal mean debris thickness of 0.09 m) is further explained by higher prevalence of cryokarst features in LAZ than in UAZ.