Flow and retreat of the Late Quaternary Pine Island‐Thwaites palaeo‐ice stream, West Antarctica

Multibeam swath bathymetry and sub‐bottom profiler data are used to establish constraints on the flow and retreat history of a major palaeo‐ice stream that carried the combined discharge from the parts of the West Antarctic Ice Sheet now occupied by the Pine Island and Thwaites glacier basins. Sets of highly elongated bedforms show that, at the last glacial maximum, the route of the Pine Island‐Thwaites palaeo‐ice stream arced north‐northeast following a prominent cross‐shelf trough. In this area, the grounding line advanced to within ∼68 km of, and probably reached, the shelf edge. Minimum ice thickness is estimated at 715 m on the outer shelf, and we estimate a minimum ice discharge of ∼108 km 3 yr −1 assuming velocities similar to today's Pine Island glacier (∼2.5 km yr −1 ). Additional bed forms observed in a trough northwest of Pine Island Bay likely formed via diachronous ice flows across the outer shelf and demonstrate switching ice stream behavior. The “style” of ice retreat is also evident in five grounding zone wedges, which suggest episodic deglaciation characterized by halts in grounding line migration up‐trough. Stillstands occurred in association with changes in ice bed gradient, and phases of inferred rapid retreat correlate to higher bed slopes, supporting theoretical studies that show bed geometry as a control on ice margin recession. However, estimates that individual wedges could have formed within several centuries still imply a relatively rapid overall retreat. Our findings show that the ice stream channeled a substantial fraction of West Antarctica's discharge in the past, just as the Pine Island and Thwaites glaciers do today.