Caldera Lake Öskjuvatn, Iceland: Unexpected ice loss in winter and multidecadal property changes

The caldera Lake Öskjuvatn, is at 1050 m elevation in Iceland’s interior. It is a deep (217 m) dimictic lake formed after an eruption in 1875. Geothermal activity with gas and liquid inflows, down to 84 m depth, maintains a ∼0.13 km2 permanent ice opening in winter. Remote sensing data revealed a progressively disappearing ice cover in the winter 2012. Physical and chemical conditions were explored in April 2012 (ice-free) and April 2013 (ice-cover). Measurements included CTD profiles and continuous temperature records. Meteorological observations from an automatic station show frequent southwesterly winds in the first quarter of 2012. Near-linear temperature increase with depth in April 2012 indicated effective whole lake vertical mixing. In contrast, the lake was weakly stratified in April 2013 with heat stored below 60 m depth. Moored temperature records in the winters of 2013 and 2014, revealed sustained under ice temperature, and hence a density rise in the upper 60 m, which is half the lake volume. The April 2012 concentrations of geochemical temperature indicators gave no indications of enhanced thermal activity. However, the concentrations of dissolved mineral constituents had decreased since 1975. Chloride and lithium decreased by 30\% but the geothermal indicators, silicate and sulphate, had decreased less, at ∼15\%. The inflowing water, from local precipitation, had not changed. The estimated annual lake-air flux of carbon dioxide, 19000 tons in 2012, had substantially decreased. The unexpected winter ice loss in Lake Öskjuvatn in February-March 2012 was driven by a complex interplay of wind stress, the lake’s seasonal deep water heat storage and geothermal activity. Inflow of geothermal gas at 84 m depth off the western shore enhances the vertical transport of heat to the lake’s upper layer which, together with frequent southwesterly wind stress 2012, eventually induced vertical instability, whole lake turnover and a complete ice melt.