Peering Beneath the Ice: Tectonic and Climatic Controls on Erosion at Hubbard Glacier, Alaska
Abstract
Climate exerts a first-order control on glacial erosion through changes in glacier mass balance, but bedrock properties such fracturing and faulting also can influence the spatial distribution and magnitude glacial erosion. Glacial erosion-mass balance coupling appears to exist with most sediment derived from where ice velocity is the fastest and/or near the equilibrium line altitude (ELA), where ice flux is often assumed to be the greatest. We investigate modern spatial distribution of erosion within the tidewater Hubbard Glacier, in the tectonically active St. Elias Range, Alaska. Provenance of sediment deposited by Hubbard Glacier into Disenchantment Bay (DB) is used to test the hypothesis that subglacial erosion is primarily influenced by ice flux/velocity, resulting in higher erosion rates at/below the modern ELA, where ice flux and ice velocity is observed to be the greatest for the Hubbard (Enkelmann, E. et al., 2015, GRL; Fahnestock, M. et al., 2016, RSE). Spatially varying subglacial erosion rates are determined from sediment volume differences (1999-2004) in DB following method of Hallet et al. (1996, QSR), and sediment provenance. Provenance is established using geochemical abundances in the silt fraction from DB following D'Haen et al. (2013, HP), with onshore source rocks characterized from published and unpublished data. Provenance determined by an inverse Bayesian mixing model reveals non-uniform modern erosion rates. Rates are greater (~30± 11mm/y)in the area at/below the ELA within Chugach Metamorphic Complex and Yakutat Group (CMC-YG) rocks, compared to ~8 ± 2 mm/y in the Alexander Terrane located above the ELA. High erosion rates in the CMC-YG spatially correspond with enhanced ice flux and velocity below the ELA. Inverse provenance-modeled erosion rates are ~50% lower in the CMC-YG and ~60% higher in the AT when compared to rates when source is partitioned from an ice-flux-driven forward erosion potential model (Enkelmann, E. et al., 2015, GRL). This discrepancy may reflect temporal variability in sediment provenance that influences inverse modeled rates, temporal variability not captured in the ice flux model, or bedrock characteristics of more easily erodible, fault-controlled weakened rock within the AT underlying the Hubbard not captured by the simple ice flux erosive model.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMEP23A..08J
- Keywords:
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- 1815 Erosion;
- HYDROLOGY;
- 1824 Geomorphology: general;
- HYDROLOGY;
- 8175 Tectonics and landscape evolution;
- TECTONOPHYSICS;
- 8177 Tectonics and climatic interactions;
- TECTONOPHYSICS