Large spatial variations in glacial erosion detected with detrital thermochronology

Studies of drainage basin erosion and landform evolution are often limited by not knowing where sediment is sourced from and how climate change influences catchment erosion. Detrital thermochronometer cooling ages collected from Quaternary glacial moraines and modern river sediments provide a promising tool to address these problems. We present an application of detrital thermochronology to quantify spatial variations in alpine glacial erosion during the Last Glacial Maximum (LGM). Results are compared to the distribution of recent erosion recorded in samples from modern river sediments. The elevation dependence of detrital apatite (U-Th)/He (AHe) ages is used as a sediment tracer to track the elevations where glacially eroded sediment is produced from bedrock. We measured ~204 AHe single grain ages from three moraines located between 2.3 and 3.7 km elevation in the Lone Pine catchment, Sierra Nevada, California. Ages from the lowest elevation moraine were measured on fine (<250 um) and coarse (>250 um) grained fractions of the sample to assess potential variations in sediment supply from different erosional processes. Measured AHe age probability density functions (PDFs) were compared with predicted PDFs, calculated by convolving bedrock age-elevation relationships with catchment hypsometries clipped at different elevations to reflect variable source elevations of sediment. Statistical comparison of the PDFs using a Monte Carlo approach and Kuiper test are used to evaluate the spatial distribution of erosion in the catchments. Results from the lowest elevation moraine indicate sediment is produced from the lower ~50-70% of catchment elevations at the 95% confidence level, indicating erosion near the base and sides of the glacier proportionally outweigh erosion from higher elevation head wall retreat and rock fall onto the glacier. Furthermore, grain-age distributions from the fine and coarse grain fractions are virtually indistinguishable, suggesting either both size fractions are sourced from similar elevations, and/or a significant disaggregation of coarse grained material into finer material during transport. Finally, the intermediate to high-elevation moraines within the cirque indicate glacial erosion is possibly uniform and occurs over 70-100% of the elevations above the sample location. These spatial variations in glacial erosion are in stark contrast to previously published results from the fluvially dominated Inyo Creek where a uniform distribution of erosion is observed from detrital AHe analysis of modern river sediments. Taken together, these results demonstrate: (1) a high sensitivity of detrital thermochronology to spatial variations in glacial and fluvial erosion processes, and (2) an increase in topographic relief and/or significant valley widening during glaciation as supported by the abundance of sediment sourced from lower catchment elevations.