Rapid glacial erosion at 1.8 Ma revealed by 4He/3He thermochronometry.

Alpine glaciation and river incision control the topography of mountain ranges, but their relative contributions have been debated for years. Apatite 4He/3He thermochronometry tightly constrains the timing and rate of glacial erosion within one of the largest valleys in the southern Coast Mountains of British Columbia, Canada. Five proximate samples require accelerated denudation of the Klinaklini Valley initiating 1.8 +/- 0.2 million years ago (Ma). At least 2 kilometers of overlying rock were removed from the valley at >/=5 millimeters per year, indicating that glacial valley deepening proceeded >/=6 times as fast as erosion rates before approximately 1.8 Ma. This intense erosion may be related to a global transition to enhanced climate instability approximately 1.9 Ma.