Distortion of isochronous layers in ice revealed by ground-penetrating radar

In addition to measuring ice-sheet thickness, ground-penetrating radar can be used to delineate reflections in ice sheets(1-3). These reflections are generally accepted to result from layers of isochronous deposition of snow and can reveal much about the dynamics of the ice now. Here we present ground-penetrating radar data from Fletcher Promontory, Antarctica, which show arches and troughs in isochronous ice layers to a depth of 100 m, We demonstrate that the origin of these features can be determined by their growth with depth, and many features result from local anomalies in accumulation rate which can be correlated with the ice surface slope. One arch appears to be the result of a local anomaly in vertical strain-rate. Its proximity to the ice divide, width, and growth with depth, indicate that this arch is one of a class of features postulated by Raymond(4) but not previously shown to exist in the field. Such a feature is an indication of the nonlinear rheology of ice and requires that palaeoclimate records from ice cores extracted from the vicinity of ice divides underlain by similar features should be specifically corrected for such effects on ice-flow.