Seismic observations reveal a hard-bedded drained subglacial lake basin beneath Isunnguata Sermia, West Greenland

Through August-September 2015, a surface depression  was observed in ArcticDEM data near the south lateral margin of Isunnguata Sermia (IS), 0.88 km² in area and with 30 m maximum deflection. This ‘surface anomaly’, alongside flooding and sediment deposition in the IS foreland, was interpreted as the surface expression of a draining subglacial lake, with subsequent multi-year recovery of the surface elevation occurring as the lake recharged was inferred. Geophysical surveys were conducted through 2023-2025 to investigate lake dynamics over and around the surface anomaly. Reported here is the combined interpretation of active- and passive-source seismic analyses, aiming to image basal topography and determine the presence of a recharged subglacial lake.  

The active-source array comprised 48 vertical-component geophones installed in a 2D profile, recording seismic energy made at a surface impact source; the passive acquisition included a dense grid of Fairfield 3-component seismic nodes, buried in a shallow augur-hole to optimise coupling. By combining active- and passive-source seismic reflectivities, amplitude-versus-angle (AVA) curves can be populated across 0-50° angle range, thus allowing the material properties either side of the glacier bed to be characterised.  Subglacial water, either as a deep lake body or in saturated sediment, would produce negative-polarity reflectivity, given the acoustic softness of water versus the overlying ice. 

Rather than the planar and specular reflectivity often associated with subglacial lakes, seismic profiles show a sloping and rugose bed. Seismic reflection polarities suggest hard basal conditions. AVA analysis, extending from near-zero incidence to the ~50° critical angle, shows consistent positive polarities across all identified bed reflections, suggesting a substrate that is acoustically harder than the overlying ice – for example, consolidated sediment or bedrock. This insight is supported with constraint from passive seismic analysis of seismic velocities beneath the ice, something not typically possible with active-source data alone. These AVA responses are observed inside and outside of the surface anomaly.  Seismic analysis therefore suggests no evidence of significant water saturation or, indeed, a lake beneath the glacier bed. However, with vertical resolution no better than ~10-15 m, we cannot exclude the possibility of water films or small sediment pockets immediately beneath the glacier bed. 

We conclude that any water feature previously inferred from ArcticDEM data was not present during seismic surveying and is therefore transient. Beyond IS, this work questions the degree to which examples in the wider archive of active subglacial lakes may be similarly transient, and expands the range of subglacial settings in which water can accumulate.