Integrated Borehole, Radar, and Seismic Velocity Analysis Reveals Dynamic Spatial Variations Within a Firn Aquifer in Southeast Greenland

S F Killingbeck,P W Livermore,J Guandique,R R Forster,O L Miller,S Burdick,S R M Ligtenberg,L J West,L S Koenig,L N Montgomery,C Miège,D K Solomon,A D Booth,N C Schmerr,A Legchenko

doi:10.1029/2020gl089335

Abstract

AbstractPerennial water storage in firn aquifers has been observed within the lower percolation zone of the southeast Greenland ice sheet. Spatially distributed seismic and radar observations, made ~50 km upstream of the Helheim Glacier terminus, reveal spatial variations of seismic velocity within a firn aquifer. From 1.65 to 1.8 km elevation, shear‐wave velocity (Vs) is 1,290 ± 180 m/s in the unsaturated firn, decreasing below the water table (~15 m depth) to 1,130 ± 250 m/s. Below 1.65 km elevation, Vs in the saturated firn is 1,270 ± 220 m/s. The compressional‐to‐shear velocity ratio decreases in the downstream saturated zone, from 2.30 ± 0.54 to 2.01 ± 0.46, closer to its value for pure ice (2.00). Consistent with colocated firn cores, these results imply an increasing concentration of ice in the downstream sites, reducing the porosity and storage potential of the firn likely caused by episodic melt and freeze during the evolution of the aquifer.

Highlights

Warming of polar ice sheets changes the hydrological structure of surface firn and ice
Perennial water storage in firn aquifers has been observed within the lower percolation zone of the southeast Greenland ice sheet
ground‐penetrating radar (GPR), seismic, and firn cores were acquired along a flow line of Helheim Glacier, ~50 km upstream of its terminus, between July and August 2016 (Figure 1a) over a known firn aquifer (Forster et al, 2014; Miège et al, 2016)

Summary

Introduction

Warming of polar ice sheets changes the hydrological structure of surface firn and ice. In Greenland, extensive firn aquifers have been observed in high‐accumulation, high‐melt (24–50 cm/yr; Miller et al, 2020) regions in the southeast in the last decade (e.g., Forster et al, 2014; Koenig et al, 2014; Kuipers Munneke et al, 2014; Miège et al, 2016). Meltwater stored within a firn aquifer is either drained from the system, though fracture/crevasse networks, or refreezes into ice. Refreezing densifies the firn column (Hubbard et al, 2016) and has been implicated as a precursor for the catastrophic collapse of Antarctic ice shelves (Kuipers Munneke et al, 2014). Densification complicates the relationship between surface elevation change and changing ice mass, since an increase in surface density can cause a decrease in elevation (Reeh et al, 2005)

Methods

Results

Conclusion