PERSISTENCE OF THE PLEISTOCENE GLACIAL BUZZSAW IN THE ST. ELIAS MOUNTAINS, ALASKA

In southeastern Alaska, the coastal Chugach-St. Elias Mountains coincide with some of the highest modern erosion rates on Earth, driven by large temperate glaciers and ice sheets. These glacial systems currently occupy fault-controlled valleys that act to funnel ice from high-elevation accumulation regions to sea-level marine ablation zones (Enkelmann et al., 2015). These mountains have topographic characteristics that have been explained as a prime example of a glacial climate interacting with a rapidly uplifting maritime orogen (Meigs and Sauber, 2000). Recent mass-balance measurements of sediment efflux from the margin reveal a doubling of mass loss coincident with the onset of the mid-Pleistocene climate intensification, indicating the increased role of glacial erosion in modifying the orogenic landscape (Gulick et al., 2015). As glacial erosion is maximized at the equilibrium line altitude (ELA), Pleistocene cooling should drive erosion to cover a wider elevation range during glacial-interglacial transitions. We examine this hypothesis using silt-fraction mineralogy and elemental concentrations from Integrated Ocean Drilling Program Expedition 341 boreholes across the Gulf of Alaska continental margin. Onshore bedrock elemental data is used to create a Bayesian mixing model for Gulf of Alaska sediments accumulating offshore the Bering-Bagley ice fields. We find that glacigenic sediment production for the mid-late Pleistocene (<0.7 Ma) has been concentrated within bedrock found in the ~200-1500 m elevation band coincident with the modern to proposed-LGM ELA range, but in rock more common to the higher elevations. The relative proportions of bedrock endmembers have changed very little over this time period, with minor perturbations as the ice stream advances and retreats through the Bagley ice valley. A >1-km thick continental shelf has been constructed from this material over this relatively short time period (Montelli et al., 2017), indicating that the Bering-Bagley glacial system has been supplying consistently high volumes of rock derived from a relatively narrow, likely fault-controlled region of the orogen.