Layer disturbances and the radio-echo free zone in ice sheets

R Drews,I Weikusat,O Eisen,S Kipfstuhl,D Steinhage,F Wilhelms,A Lambrecht,H Miller

doi:10.5194/tc-3-195-2009

Abstract

Abstract. Radio-echo sounding of the Antarctic and Greenlandic ice sheets often reveals a layer in the lowest hundreds of meters above bedrock more or less free of radio echoes, known as the echo-free zone (EFZ). The cause of this feature is unclear, so far lacking direct evidence for its origin. We compare echoes around the EPICA drill site in Dronning Maud Land, Antarctica, with the dielectric properties, crystal orientation fabrics and optical stratigraphy of the EPICA-DML ice core. We find that echoes disappear in the depth range where the dielectric contrast is blurred, and where the coherency of the layers in the ice core is lost due to disturbances caused by the ice flow. At the drill site, the EFZ onset at ~2100 m marks a boundary, below which the ice core may have experienced flow induced disturbances on various scales. The onset may indicate changing rheology which needs to be accounted for in the modeling of ice sheet dynamics.

Highlights

For over 40 years radio-echo sounding (RES) has been successfully applied to determine ice thickness and internal structure of large ice bodies
We find that echoes disappear in the depth range where the dielectric contrast is blurred, and where the coherency of the layers in the ice core is lost due to disturbances caused by the ice flow
We find that radio echoes disappear below ∼2100 m at the drill site and that this transition coincides with progressive disturbances in cloudy bands (CBs)-stratigraphy

Summary

Introduction

The absence of layering was first discussed by Robin et al (1977) and eventually named as EFZ by Drewry and Meldrum (1978) It is often characterized by an abrupt transition, and an upper onset that varies with depth. Siegert and Kwok (2000) discussed the EFZ observed in subglacial valleys West of Lake Vostok They described the ice as being potentially stagnant and suggest that recrystallization and recirculation of ice may play a role. Line-scan images (see Fig. 3) display the stratigraphy of high-scattering zones for light, called cloudy bands (CBs). They correlate with the impurity content of ice (Svensson et al, 2005) and we take them as a proxy for the layering of RES horizons. We discuss possible reasons for the absence of echoes in that zone and implications for the suitability of ice for paleoclimate research and ice-sheet modeling

Dielectric profiling and fabric analyzer

RES internal structure

Definition of EFZ-onset and system performance

Potential physical mechanisms of the EFZ

Conclusions