Masters Thesis

Tracking cosmogenic 10Be in multiple grain sizes down

The concentration of 10Be in alluvial sediments can be used to interpret catchment-scale erosion rates in a variety of landscapes. The concentration of cosmogenic 10Be within channel sediment represents the average time-period for those clasts to have been eroded from the catchment and assumes that the transport time from the slope to the channel is negligible. Furthermore, the method typically analyzes grain sizes less than 1 mm and assumes that this size is “average.” In order to test these assumptions and determine the appropriate applications of this method, I have applied this method in a tectonically active catchment in the eastern Transverse Ranges, southern California: Millard Canyon in the San Gorgonio Pass within the San Bernardino Mountains. I use 10Be Terrestrial Cosmogenic Nuclides (TCN) concentrations in multiple grain-sizes to determine erosion rates, understand geomorphic process and sediment transport, and explore implications of clast size variable 10Be concentrations by sampling transects of the catchment and comparing 10Be concentrations across grain sizes. I present measurements of eight 10Be samples from three locations ~2 km apart from an arid catchment in the southern San Bernardino Mountains that capture the active channel over four grain sizes (0.25- 0.500 mm, 1-2 mm, 11- 22 mm, 100-250 mm). Comparing 10Be concentrations across grain sizes and transect location reveal trends. The 10Be concentration in the 0.250- 0.500 mm sand fraction of the lower catchment mid-fan site is enriched 1.4 times the catchment mouth transect, while the 11-22 mm pebble fraction of the lower catchment mid-fan site is reduced to 0.7 times the upper catchment concentration. Sand 10Be concentrations yield erosion rates twice as fast as pebble 10Be concentrations; thus, interpretation of only one grain size may yield misleading results. These data show how 10Be concentrations may vary as a function of grain-size and location, and geomorphic processes may alter the 10Be concentrations in the lower catchment. A cause for the discrepancy of 10Be concentrations in grain sizes from the same catchment may be the mixing of sediments with different 10Be concentrations in the active channel. Specifically, incorporation of older sediment can alter the measured 10Be inventory. A sediment mixing model was used to determine the percentage of sediment of a known age that must be present in the active channel to return the observed concentrations. Results considering channel-adjacent surface sediments of known age suggest between 18-44 ± 9% of sediment in active channel is sourced from adjacent surfaces; alternatively, the most likely source is aeolian transport of grains. Our data indicate that 10Be concentration, and thus erosion rates and exposure ages, vary with grain size and suggest sediment transport pathways and sources of sediment outside the catchment should be considered in any TCN study.

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