Abstract
AbstractLandforms produced beneath former ice sheets offer insights into inaccessible subglacial processes and present analogues for how current ice masses may evolve in a warming climate. Large subglacial channels cut by meltwater erosion (tunnel valleys [TVs]) have the potential to provide valuable empirical constraints for numerical ice-sheet models concerning realistic melt rates, water routing, and the interplay between basal hydrology and ice dynamics. However, the information gleaned from these features has thus far been limited by an inability to adequately resolve their internal structures. We use high-resolution three-dimensional (HR3-D) seismic data (6.25 m bin size, ∼4 m vertical resolution) to analyze the infill of buried TVs in the North Sea. The HR3-D seismic data represent a step-change in our ability to investigate the mechanisms and rates at which TVs are formed and filled. Over 40% of the TVs examined contain buried glacial landforms including eskers, crevasse-squeeze ridges, glacitectonic structures, and kettle holes. As most of these landforms had not previously been detected using conventional 3-D seismic reflection methods, the mechanisms that formed them are currently absent from models of TV genesis. The ability to observe such intricate internal structures opens the possibility of using TVs to reconstruct the hydrological regimes of former mid-latitude ice sheets as analogues for contemporary ones.
Highlights
Throughout the Quaternary, the growth and retreat of ice sheets across high- and mid-latitude continental shelves drove major changes to topographic relief and global sea level (e.g., Batchelor et al, 2019)
We use novel high-resolution 3-D (HR3-D) seismic data from the North Sea to examine the infill of tunnel v alleys (TVs) in unprecedented detail and discuss the implications for TV genesis
DATA AND METHODS We examined six HR3-D seismic data sets, covering ∼60 km2, from the central North Sea (Figs. 1A and 1B)
Summary
Throughout the Quaternary, the growth and retreat of ice sheets across high- and mid-latitude continental shelves drove major changes to topographic relief and global sea level (e.g., Batchelor et al, 2019). The six HR3-D seismic data sets image 19 cross-cutting incisions that are 300–3000 m wide, up to 300 m deep, and possess undulating thalwegs We interpret these as TVs formed by subglacial meltwater based on their distinctive morphology (e.g., Stewart et al, 2013). The fact that the ridges are confined to the TV rather than extending onto the surrounding banks, combined with their irregular rims, suggests these landforms do not result from permafrost processes Rather, based on their morphological similarity to features on glaciated terrains elsewhere (Ottesen and Dowdeswell (2006); Fig. S2), we interpret these landforms as crevasse-squeeze ridges that formed through sediment injection upwards into basal fractures beneath grounded ice (Rea and Evans, 2011; Evans et al, 2016). Several TVs contain chaotic and displaced reflections along the sides and bases, which we interpret as evidence of slumping, faulting, and glacitectonic thrusting (Figs. 1D and 3H)
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