Temporal and spatial variations in glacial erosion from detrital zircon (U-Th)/He thermochronology: Patagonian Andes
Abstract
Alpine glacial erosion is commonly thought to exert a first order control on mountain topography and sediment production. However, understanding where glacial sediment is eroded from and downstream mixing of sediment has been difficult to quantify due to uncertainties in the spatial distribution of subglacial erosion, and temporal variations in glacial mechanics that influence basal sliding/erosion. Detrital `tracer' thermochronology works on the premise that thermochronometer bedrock ages increase with elevation and grains from a detrital sample downstream can be used to infer the sediments' source elevations. Here we present 600 new single grain detrital zircon (U-Th)/He analyses from 6 detrital samples in the Leones catchment, Northern Patagonian Icefield. The samples include one terminal moraine (~2.5 ka; sand material mixed from 4 locations of the moraine) and three locations from the active glacial outwash channel. Three of the outwash samples are from sand size material, and two from pebble size material to identify potential grain size variations in erosion. The spatial and temporal evolution of catchment erosion is determined using the upstream hypsometry of the catchment and an elevation profile of 18 bedrock samples. The distribution of observed detrital ages is interpreted by comparison to predicted grain-age distributions for different erosion scenarios such as uniform, slope-, and elevation-dependent erosion.
Results suggest: (1) Comparison of the moraine and downstream samples indicates paleo-erosion rates (~2.5 ka) were enhanced at elevations in the lower ~500 m above the moraine relative to present. Hence, temporal variations in the pattern of catchment erosion may occur on syn- and post-glacial timescales. (2) Small differences between age distributions from the grain size fractions show that coarser material is likely sourced from lower elevations. (3) Over the distance of 13 km separating the modern channel samples, no systematic variation in age distributions can be observed. This suggests a robust erosion pattern traceable from the more proximal glacial outwash downstream over at least this length scale.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMEP0300006F
- Keywords:
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- 1199 General or miscellaneous;
- GEOCHRONOLOGY;
- 1815 Erosion;
- HYDROLOGY;
- 1824 Geomorphology: general;
- HYDROLOGY;
- 1862 Sediment transport;
- HYDROLOGY