Exploring mechanisms and rates of tunnel valley formation beneath deglaciating mid-latitude ice sheets using high-resolution 3D seismic data and numerical modelling

The geological record of landforms produced beneath deglaciating ice sheets offers insights into otherwise inaccessible subglacial processes. Large subglacial channels formed by meltwater erosion of sediments (tunnel valleys) are widespread in formerly glaciated regions such as the North Sea. These features have the potential to inform basal melt rate parameterisations, realistic water routing and the interplay between basal hydrology and ice dynamics in numerical ice‑sheet models; however, the mechanisms and timescales over which tunnel valleys form remain poorly understood. Here, we present a series of modelling experiments, informed by geophysical observations from novel high-resolution 3D seismic data (6.25 m bin size, ~3.5 m vertical resolution), which test different hypotheses of tunnel valley formation and calculate the rates at which these features likely form beneath deglaciating ice sheets. Reconstructions of the former British-Irish and Fennoscandian ice sheets from a 3D thermomechanical ice‑sheet model (BRITICE CHRONO version 2) are used to calculate subglacial water routing and steady-state water discharges as these ice sheets retreated across the North Sea Basin during the last glaciation. Using these simulations, we calculate potential meltwater channel erosion rates and estimate how quickly tunnel  valleys are formed beneath deglaciating ice sheets in warmer than present-day climates. We find little evidence for widespread water ponding which may have led to channel formation through outburst floods. Instead, our results demonstrate that seasonal surface melt delivered to the bed could incise large channels of comparable dimensions to tunnel valleys over timescales of several hundred years as these ice sheets deglaciated.