Ice-stream flow switching by up-ice propagation of instabilities along glacial marginal troughs

Abstract. Ice-streamnetworks constitute the arteries of ice sheetsthrough which large volumes of glacial iceare rapidly delivered from the continent to the ocean. Modifications in ice-streamnetworks have a major impact on ice sheetmass balance and global sea level. Reorganizations in the drainage network of ice streamshave been reported in both modern and paleo- ice sheetsand usually result in icestrea ms switching their trajectory and/or shutting down. While some hypotheses for the reorganization of icestrea ms have been proposed, the mechanisms that control the switching of icestrea ms remain poorly understood and documented. Here, we interpret a flow switch in an ice-strea m system that occurred prior to the last glaciation on the northeastern Baffin Island shelf (Arctic Canada) through glacial erosion of a marginal trough, i.e., deep parallel-to-coast bedrock moats located up- iceof a cross-shelf trough. Shelf geomorphology imaged by high-resolution swath bathymetry and seismo-stratigraphic data in the area indicate the extension of icestrea ms from Scott and Hecla & Griper troughs towards the interior of the Laurentide Ice Sheet. Up- icepropagation of icestrea ms througha marginal trough is interpreted to have led to the piracy of the neighboring icecatchment that in turn induced an adjacent ice-strea m flow switch and shutdown. These results suggest that competition for icedischarge between the two icestrea ms, which implies piracy of ice drainage basinsvia marginal troughs, was the driving mechanism behind iceflow switching. In turn, the enlargement of the icecatchment by piracy increased the volume and discharge of Scott IceStrea m, allowing it to erode deeper and flow farther on the continental shelf. Similar trough systems observed on many other glaciated continental shelves may be the product of such competition for icedischarge between catchments.