COMBINING MODERN BERYLLIUM-10 COSMOGENIC RADIONUCLIDE INVENTORIES WITH DETRITAL ZIRCON URANIUM-LEAD GEOCHRONOLOGY IN SOUTH-CENTRAL CHILE: AN INVESTIGATION OF EROSION DYNAMICS AND SEDIMENT DISTRIBUTION

This thesis applies detrital in-situ Beryllium-10 cosmogenic radionuclides and detrital zircon uranium-lead analyses to quantify the magnitude and sources of erosion and track the distribution of quartz and zircon in modern landscapes. Based on the results of this thesis, “double-dating” samples from the sedimentary archive might be used to reveal changes in erosion rate and source terranes because distinct erosion rates and zircon provenance exist within the source area of the deposit. This is accomplished in two steps: an initial calibration of quartz and zircon fertility and mixing along modern rivers using samples collected above and below major river confluences (this study), followed by sampling and analysis of sand from the sedimentary archive (future work). The focus of this study was on the Mataquito River and its tributaries, the Teno and Lontué, adhering to a sampling geometry where a sample is collected above a confluence from each tributary and below the confluence. Fertility ratios for both quartz and zircon were calculated using the abundance of 10Be and the detrital zircon age distributions of each sample, as well as a mixing model. The results indicate that quartz and zircon fertilities are predominantly sourced from the southern (Lontué) tributary, while erosion rates are an order of magnitude higher in the northern (Teno) tributary. We observed a pitfall of the current method in that the mixing of sediments from the Lontué and Teno tributaries was likely incomplete below the confluence of these two rivers. We explore several potential mixing scenarios to both demonstrate that this lack of mixing is most likely responsible for the observed data, as well as to explore the sensitivity of our inferences of quartz and zircon fertility to the poorly mixed sample. Future work will include a detailed empirical exploration of the effect of sediment mixing below the river confluence to improve quartz and zircon fertility estimates, as well as an analysis of geologic archives of sediments to infer the past magnitude and distribution of erosion rates within the Andes.