Decoding ice sheet behavior using englacial layer slopes

The complex flow fields of the Antarctic and Greenland ice sheets deform layers deposited as snow at the ice sheet surface, leaving a record of the regional flow history and/or local transitions in basal boundary conditions within the geometry of ice sheet layers. Ice-penetrating radar reveals these layers, but radar data interpretations are limited by the challenges of quantitatively and reproducibly comparing observations with model output. We present a conceptual framework that relates along-track reflector slope to gradients in the steady state velocity field of ice sheets. This method makes effective use of englacial reflectors in regions where it is challenging to image continuous layers and avoids the error propagation inherent to tracer-transport methods, developing the potential for formal radar data assimilation in future modeling studies. We apply our method to radar data collected at the grounding line of Whillans Ice Stream, where enhanced bed friction produces characteristic reflector slopes reproducible using a higher-order ice flow model. Plain Language Summary Radar imagery collected to measure ice thickness in Antarctica and Greenland provides a vast amount of information about the internal structure of the ice sheets. Most notably, these data show that the ice sheets are not simply flat layers of ice deposited as snow at the surface. They are complex, folded masses that record the history of ice flow in the geometry of their internal layers. This study establishes a new interpretation framework that allows us to identify unique characteristics of the ice structures and relate them to their formation mechanism, a task that has been historically difficult to perform. Developing methods like this one, which help to describe and interpret the internal structure of the ice sheets in an automated and quantitative way, improves our understanding of the processes that controlled historic ice sheet behavior, allowing us to make better predictions of future ice sheet behavior.