EGU2020-17749
https://doi.org/10.5194/egusphere-egu2020-17749
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

The development of a subglacial lake monitored with radio echo sounding and comparison with water volumes released during jökulhlaups: Case study from the Eastern Skaftá Cauldron in the Vatnajökull ice cap, Iceland

Eyjolfur Magnusson1, Finnur Pálsson1, Magnús T. Gudmundsson1, Thórdís Högnadóttir1, Christian Rossi2, Thorsteinn Thorsteinsson3, and Erik Sturkel4
Eyjolfur Magnusson et al.
  • 1Institute of Earth Sciences, Universtity of Iceland, Reykjavík, Iceland (eyjolfm@hi.is)
  • 2Remote Sensing Institute, German Aerospace Center (DLR)
  • 3Icelandic Meteorological Office, Reykjavík, Iceland
  • 4Department of Earth Sciences, University of Gothenburg, Sweden

We present a 6 year record of repeated radio echo sounding (RES) on a profile grid (200-400 m between profiles) surveyed over the Eastern Skaftá Cauldron (ESC). ESC is an ice cauldron produced and maintained by powerful geothermal activity (~1 GW) at the glacier bed. Beneath the cauldron and 200-400 m of ice, water accumulates in a lake and is regularly released in jökulhlaups. The maximum discharge in the river Skaftá exceeded 3000 m3 s-1 in the most recent ones in 2015 and 2018. The record starts in 2014 and consists of annual measurements, obtained in June each year; the last on June 2019. Comparison of the repeated RES profiles (2D migrated) reveals the margin of the lake at different times and enables a classifying of traced reflections into lake and bedrock measurements. The bedrock measurements were obtained with the lake close to its minimum size in 2016, 2017 and 2019 (£~1 km2 compared to 4.0 km2 in 2015), hence it is possible to obtain fairly accurate digital elevation model (DEM) of the glacier/lake bed. This DEM is further constrained by two borehole measurements of the lake bed elevation at its centre. The traced lake reflections and comparison with the bedrock DEM enables creation of a lake thickness maps and an estimate of the lake volume for each survey. The lake thickness maps and volumes in June 2015 and 2018 are compared with the surface lowering pattern and water volumes drained in the jökulhlaups in October 2015 and August 2018. The drained water volume was derived by integrating the surface lowering during the jökulhlaups and adding estimated volume of crevasses formed in the events. The lowering in the 2015 jökulhlaup was obtained from TanDEM-X DEMs of September 23rd and October 10th, shortly before and after the jökulhlaup. The lowering in the 2018 jökulhlaup was derived from dense set of airborne altimetry profiles acquired on August 9th, a few days after the jökulhlaup, compared with a DEM in June 2018 (ArcticDEM in July 2017 corrected with dense GNSS profiles in June 2018). The lake volume estimate from the RES data is 240x106 m3 in June 2015 but 320±20x106 m3 drained from the cauldron in October. In June 2018 a relatively dense RES profile grid (~200 m between profiles) reveals a lake volume of 180x106 m3 while 210±30x106 m3 drained from the cauldron in August. This comparison demonstrates the applicability of our survey approach to monitor the water accumulation in the lake and thus better constrain potential hazard in jökulhlaups.

How to cite: Magnusson, E., Pálsson, F., Gudmundsson, M. T., Högnadóttir, T., Rossi, C., Thorsteinsson, T., and Sturkel, E.: The development of a subglacial lake monitored with radio echo sounding and comparison with water volumes released during jökulhlaups: Case study from the Eastern Skaftá Cauldron in the Vatnajökull ice cap, Iceland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17749, https://doi.org/10.5194/egusphere-egu2020-17749, 2020

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Presentation version 1 – uploaded on 01 May 2020
  • CC1: Comment on EGU2020-17749, Eva Eibl, 01 May 2020

    This is a pretty cool work. Thanks for sharing.

    So radio echo sounding is not only possible to resolve the bedrock-ice or bedrock-water interface but also the water-ice interface if low frequencies are used? Is this based on the density difference?

    • AC1: Reply to CC1, Eyjolfur Magnusson, 04 May 2020

      Radio echo sounding was for a long time the main tool to identify subglacial lakes beneath Antarctica Ice Sheet; the lakes where identified in the radar profiles as flat and bright reflectors (reflections both from bed-ice interface and water-ice interface is caused by contrast in the dielectric properties between the two media). But not all subglacial lakes reveals this identity in RES data including the one beneath the Eastern Skaftá Cauldron. It is probably not clear enough in the pdf we submitted but here the reflections from lake roof is pure identified by intercomparison of repeated RES-profiles (see slides 8-15, in particular slide 14-15). In the case where altitude of traced reflection, at given location in a repeated profile, correspond to the same minimum for two or more years, these reflections are expected to be bedrock reflections, hence outside the lake margin for corresponding years. Reflections traced at significantly higher altitude at same location in the profile for any other year corresponds to reflections from a lake roof, hence within the lake margin for corresponding year. This intercomparison of the data reveals that the edge of the lake is usually characterised by relatively steep side walls, which further helps identifying the margin of the lake where data was limited (the full length of some of profiles could e.g. not be driven for all years due to crevasses).