Georadar‐derived estimates of firn density in the percolation zone, western Greenland ice sheet

Joel Brown,W Tad Pfeffer,Joel Harper,Neil Humphrey,Ellen Mosley‐Thompson,John Bradford

doi:10.1029/2011jf002089

Abstract

Greater understanding of variations in firn densification is needed to distinguish between dynamic and melt‐driven elevation changes on the Greenland ice sheet. This is especially true in Greenland's percolation zone, where firn density profiles are poorly documented because few ice cores are extracted in regions with surface melt. We used georadar to investigate firn density variations with depth along a ∼70 km transect through a portion of the accumulation area in western Greenland that partially melts. We estimated electromagnetic wave velocity by inverting reflection traveltimes picked from common midpoint gathers. We followed a procedure designed to find the simplest velocity versus depth model that describes the data within estimated uncertainty. On the basis of the velocities, we estimated 13 depth‐density profiles of the upper 80 m using a petrophysical model based on the complex refractive index method equation. At the highest elevation site, our density profile is consistent with nearby core data acquired in the same year. Our profiles at the six highest elevation sites match an empirically based densification model for dry firn, indicating relatively minor amounts of water infiltration and densification by melt and refreeze in this higher region of the percolation zone. At the four lowest elevation sites our profiles reach ice densities at substantially shallower depths, implying considerable meltwater infiltration and ice layer development in this lower region of the percolation zone. The separation between these two regions is 8 km and spans 60 m of elevation, which suggests that the balance between dry‐firn and melt‐induced densification processes is sensitive to minor changes in melt.

Highlights

[2] Temporal variations in firn density can partially explain observed changes in ice sheet surface elevation [Holland et al, 2011] and can substantially influence mass balance calculations based on surface elevation observations [Zwally et al, 2005; Helsen et al, 2008]
[3] Densification of firn in regions of the Greenland ice sheet (GrIS) accumulation area that do not melt is primarily driven by overburden, with spatial variations in densification rates linked to temperature and accumulation rate [Herron and Langway, 1980]
[19] To validate the accuracy of our method, we compared the results of both inversions of georadar data collected at Crawford Point with a 120 m core drilled in the same year within 1 km of our Common midpoint (CMP) (Figure 7)

Summary

Introduction

[2] Temporal variations in firn density can partially explain observed changes in ice sheet surface elevation [Holland et al, 2011] and can substantially influence mass balance calculations based on surface elevation observations [Zwally et al, 2005; Helsen et al, 2008]. Detailed shallow core and snow-pit studies of the upper few meters of firn within the percolation zone [e.g., Benson, 1960; Fischer et al, 1995; Parry et al, 2007; Dunse et al, 2008] reveal seasonal high-density layer boundaries Throughout this layered structure are ice lenses and ice pipes. [5] Cores collected in the upper regions of the percolation zone [e.g., Mosley-Thompson et al, 2001] span the full depth of the firn column from the annual snow layer to the theoretical firn close-off density of $830 kg/m3 [Paterson, 1994] These cores have been used to calculate long-term average accumulation rates, density versus depth relationships (dr/dz), and densification rates (dr/dt). The Dix inversion, which solves for layer velocities using only stacking velocities and zero-offset traveltimes [Dix, 1955], is the most common method of calculating interval velocities

Methods

Results

Conclusion