In-Situ Measurements of Three-Dimensional Deformation in the Greenland Ice Sheet

Abstract:

Investigating the three-dimensional flow of an ice sheet can reveal spatial variations in internal deformation and demonstrate how ice and bed characteristics affect ice flow. While several studies have explored deformation in the longitudinal direction, few have considered ice flow in the transverse plane due to the challenges of in-situ measurement. Further, existing studies have examined ice flow in complex valley glaciers but not in ice sheets such as the Greenland Ice Sheet (GrIS). We address this knowledge gap by investigating ice flow in the ablation zone of the GrIS in a location where the ice flows over a hard bed. In the 2014 and 2015 melt seasons, we drilled nine boreholes ~670 m to the bed and installed a dense array of englacial sensors. The sensors contain inclinometers and magnetometers that measure the tilt and the azimuth of tilt as the ice deforms. We use these measurements to calculate longitudinal and transverse strain rates to reveal the three-dimensional flow field in this region of the GrIS. Although deformation only accounts for ~4% of the total ice motion measured from surface GPS stations, the three-dimensional flow field shows transverse deformation in the basal ice that is likely an effect of variations in the bed topography. Ignoring this transverse motion leads to an overestimate of deformation velocity at our site by 19 to 39%. In areas with even higher rates of deformation, this overestimate is likely to be even more consequential for the partitioning of surface motion into its deformation and sliding components. This study shows that not only is transverse behavior detectable, but it can also skew deformation measurements if overlooked. As such, transverse deformation should be considered in models that seek to incorporate realistic ice behavior.