Abstract
AbstractThe effect of spatial variations in ice thickness, accumulation rate and lateral flow divergence on radar-detected isochrone geometry in ice sheets is computed using an analytical method, under assumptions of a steady-state ice-sheet geometry, a steady-state accumulation pattern and a horizontally uniform velocity shape function. By using a new coordinate transform, we show that the slope of the isochrones (with a normalized vertical coordinate) depends on three terms: a principal term which determines the sign of the slope, and two scale factors which can modify only the amplitude of the slope. The principal term depends only on a local characteristic time (ice thickness divided by accumulation rate minus melting rate) between the initial and final positions of the ice particle. For plug flow, only the initial and final values have an influence. Further applications are a demonstration of how the vertical velocity profile can be deduced from sharp changes in isochrone slopes induced by abrupt steps in bedrock or mass balance along the ice flow. We also demonstrate ways the new coordinate system may be used to test the accuracy of numerical flow models.
Highlights
Radio-echo sounding of the cold Greenland and Antarctic ice sheets dates back to the 1970s
The discovery of past local disruption of layers observed in Kamb Ice Stream (Ice Stream C), West Antarctica, allowed Jacobel and others (1996) to conclude that there had been an abrupt stoppage of the ice stream a few centuries ago
The principal term =ð1 À Þ depends on the characteristic time T 1⁄4 H=ða À mÞ between the initial and final horizontal positions of the particle
Summary
Radio-echo sounding of the cold Greenland and Antarctic ice sheets dates back to the 1970s Such observations provide the ice thickness and basal conditions and show internal layering. Internal layers can be traced over very long distances and are commonly regarded as isochronal layers, i.e. layers with the same age (Bailey and others, 1964; Robin and others, 1969) They are flowmarkers, and potentially provide significant information about ice rheology and dynamics, ice boundary conditions such as accumulation rate or basal sliding, ice flow divergence or convergence, the effect of bedrock irregularities, etc. They provide a strong constraint on, and a test of, ice-sheet models. The discovery of past local disruption of layers observed in Kamb Ice Stream (Ice Stream C), West Antarctica, allowed Jacobel and others (1996) to conclude that there had been an abrupt stoppage of the ice stream a few centuries ago. Ng and Conway (2004) have deduced that the stagnant Kamb Ice Stream flowed at velocities of over 350 m a–1, from the internal layer stratigraphy
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