Abstract
Abstract. We present repeated radio-echo sounding (RES, 5 MHz) on a profile grid over the eastern Skaftá cauldron (ESC) in Vatnajökull 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 jökulhlaups. 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 km2) 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-jökulhlaup surface DEMs. The estimated lake volume was 250 GL (gigalitres = 106 m3) 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 jökulhlaup in August 2018. Considering the water accumulation over the periods between RES surveys and jökulhlaups, this indicates 10 %–20 % uncertainty in the RES-derived volumes at times when significant jökulhlaups may be expected.
Highlights
Subglacial lakes have been directly and indirectly observed beneath both temperate and cold-based glaciers
The minimum area of 0.5–0.6 km2 was observed less than a year after the 2015 and 2018 jökulhlaups, while the maximum of 4.1 km2 was observed in June 2015, ∼ 4 months prior to a jökulhlaup
The radio-echo sounding (RES) surveys indicate lake volumes < 50 GL in 2016 and 2019, less than a year after jökulhlaups, which strongly suggests that the lake drained completely or was reduced to an insignificant volume in the preceding jökulhlaups
Summary
Subglacial lakes have been directly and indirectly observed beneath both temperate and cold-based glaciers. Significant reduction in basal sliding over a period of years has been related to persistent leakage from such a lake (Magnússon et al, 2010). The detection of subglacial lakes has been achieved using a combination of radio-echo sounding (RES) and satellite remote sensing, but routine monitoring of such lakes remains a difficult task. The first such RES observation was made more than 50 years ago (Robin et al, 1970), when RES data, acquired near the centre of East Antarctica, revealed a ∼ 10 km long unusually flat subglacial surface with high reflectivity “attributed to a thick water layer beneath the ice”. Many subglacial lakes actively drain and fill and as a result are difficult to distinguish in RES data (Carter et al, 2007; Siegert et al, 2014); synthetic aperture
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