Ice slab development in the Greenland Ice Sheet's percolation zone and their implications for surface meltwater runoff
Authors:   , Jullien Nicolas 
Published: October 2024

In the last three decades, mass losses from the Greenland Ice Sheet have contributed ∼11mm to global sea level rise. Nearly half of these losses have occurred through surface meltwater runoff, not only because melt rates have increased, but also because runoff has increasingly occurred from higher elevations, especially from areas covered by firn. Firn is a porous medium resulting from the superposition of snowpack accumulation that is at least one year old. Greenland’s firn layer covers roughly 80% of the ice sheet and is tens of metres thick. When meltwater is generated at the surface of the ice sheet, it can percolate within the firn’s pore space and refreeze. The firn layer therefore holds the potential to partially buffer Greenland’s meltwater contribution to sea level rise. However, record melting events have taken place on the ice sheet in the last two decades, generating large amounts of meltwater. This meltwater, having refrozen in the firn, created near-surface, several metres thick and spatially continuous ice bodies, hereafter ‘ice slabs’. These ice slabs rendered the deep porous space beneath them inaccessible for further meltwater refreezing, and meltwater is instead forced to run off on top of them. However, the structure of the shallow firn associated with a shift from vertical percolation to lateral runoff remains poorly understood. Ultimately, this prevents from understanding the subsurface conditions that enable the transition from a retention-dominated regime to a runoff-dominated regime. This thesis investigates the development of ice slabs in firn and how they control surface meltwater runoff. Using Accumulation Radar onboard NASA’s Operation Ice Bridge, it is documented that ice slabs have thickened and expanded inland in the last two decades. Between 2012 and 2018, their coverage increased by 37-44%. From retrievals of the uppermost locations of supraglacial rivers, it is shown that the sectors experiencing surface meltwater runoff over firn areas are underlain by ice slabs. Ice slabs at least 3–3.7m thick sampled in Accumulation Radar flightlines are associated with the onset of visible meltwater runoff and are indicative of their widespread spatial contiguity. Ice slabs mainly thicken by superimposed ice formation when ponding or slowly flowing meltwater refreezes atop. Meltwater can sometimes find a pathway through the otherwise near-impermeable ice slabs and percolate vertically to depth, locally increasing the ice slab’s thickness. Spatial heterogeneity in ice slab thickness is associated with the structure of the supraglacial hydrological system, whereby thicker ice slabs are observed at places which frequently experience visible meltwater runoff. Overall, the inland expansion of ice slabs has increased the area that can be drained by surface rivers. In light of the rapid expansion of ice slabs and their importance to ice sheet runoff, the development of an ice-sheet-wide approach to monitor their future expansion constitutes a research priority. This thesis considers whether data acquired by Sentinel-1’s C-band Synthetic Aperture Radar in Horizontally-transmitted, Vertically-received polarisation mode (σ0H V) can fulfill this task. σ0H V correlates well with observed ice content in the shallow firn. Using the ice content as a proxy for density, shallow firn density prediction from σ0H V is statistically valid. Changes in backscatter signal intensity before and after the summer 2019 melt season show that σ0H V can be used to monitor the densification of the shallow firn layer in the percolation zone. This approach also shows that largest thickening of ice slabs occur in the vicinity of the visible runoff limit. The findings in this thesis can be used to improve parametrisations associated with meltwater runoff in models by proposing a threshold above which meltwater flows laterally rather than percolating vertically. However, surface meltwater may refreeze some distance away from where it was produced (i.e. not running off). Therefore, the partition between the amount of meltwater that runs off versus the one that refreezes remains poorly known. Further research should continue investigating the processes of meltwater refreezing in firn and on top of ice slabs through in-situ measurements. In the context of a warmer future, this thesis demonstrates that combining the use of remote sensing data (especially radar) together with in-situ measurements of firn properties is highly relevant to document the process of shallow firn densification. Improving the parametrisation of runoff and refreezing in models could reduce uncertainties in surface mass balance estimates and hence the ice sheet’s future contribution of meltwater runoff to sea level rise.