Development of a subglacial lake monitored with radio-echo sounding: case study from the eastern Skaftá cauldron in the Vatnaj?kull ice cap, Iceland

摘要

We present repeated radio-echo sounding (RES, 5?MHz) on a profile grid over the eastern Skaftá cauldron (ESC) in Vatnaj?kull ice cap, Iceland. The ESC is a ～ ?3?km wide and 50–150?m deep ice cauldron created and maintained by subglacial geothermal activity of ～ ?1?GW. Beneath the cauldron and 200–400?m thick ice, water accumulates in a subglacial lake and is released semi-regularly in j?kulhlaups . The RES record consists of annual surveys conducted at the beginning of every summer during the period 2014–2020. Comparison of the RES surveys reveals variable lake area (0.5–4.1?km 2 ) and enables traced reflections from the lake roof to be distinguished from bedrock reflections. This allows construction of a digital elevation model (DEM) of the bedrock in the area, further constrained by two borehole measurements at the cauldron centre. It also allows creation of lake thickness maps and an estimate of lake volume at the time of each survey, which we compare with lowering patterns and released water volumes obtained from pre- and post-j?kulhlaup surface DEMs. The estimated lake volume was 250?GL (gigalitres? = ?10 6 ?m 3 ) in June 2015, but 320? ± ?20?GL drained from the ESC in October 2015. In June 2018, RES profiles revealed a lake volume of 185?GL, while 220? ± ?30?GL were released in a j?kulhlaup in August 2018. Considering the water accumulation over the periods between RES surveys and j?kulhlaups, this indicates 10?%–20?% uncertainty in the RES-derived volumes at times when significant j?kulhlaups may be expected.