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Sosa-Gonzalez, Veronica

Natural Resources

2016

PhD

Erosion, a surface process, can be quantified over long-term (assumed to be the natural erosion rate of the landscape) and contemporary (modern) timeframes. My research used the rare cosmogenic isotope 10Be in sand and cobbles collected from rivers in southeastern Brazil (Santa Catarina and Rio de Janeiro states) and southwestern China (Yunnan province) to quantify long-term, background rates of erosion and sediment supply. These measurements will also increase number of such measurements in tropical and subtropical climates. I assessed the relationship between landscape parameters (topographic and climatic) and background erosion rates in order to understand factors related to erosion. My data from so far unsampled states in Brazil shows that background erosion rates range between 13 and 90 m/Myr. I found that mean basin slope (R2=0.73) and mean annual precipitation (R2=0.57) are strongly correlated to erosion rates. Steep, escarpment-draining basins in Brazil erode faster than lower gradient basins draining the highlands. Comparing the isotopic concentration of river sand and cobbles, my data show that these grain sizes are sourced from different parts of the landscape. I compiled all published Brazilian cosmogenic 10Be data, and compared them to erosion rates from similar tectonic settings. While the erosion rates in Brazil are relatively low, they are similar to those in southeastern North America, but faster than rates measured on escarpments in southern Africa. In China, I tested the human effects on denudation by comparing long-term erosion rates derived from in-situ 10Be concentration and the modern sediment yield of 22 watersheds in Yunnan. Background erosion rates range between 17 and 386 m/Myr; long term sediment yields based on these erosion rates range from 79 to 893 tons km-2 yr-1. Modern sediment yields range from 90 to 2,879 tons km-2 yr-1 (data from Schmidt et al., 2011). In most watersheds, the modern sediment yield is 2-3X higher than long-term rates, likely the effect of a long history of land use in Yunnan. I found a statistically significant, positive relationship between erosion rates and both area (R2 = 0.60) and mean basin slope (R2 = 0.42). There is a negative but strong relationship between erosion rates and precipitation in my dataset (R2 = 0.60). I sampled some places where 10Be samples had been collected before to test the methodological assumption of time-invariant 10Be concentration. Concentrations generally agree on samples taken 6 months apart and in samples from the active channel and from floodplains, but not in samples collected a decade and centuries apart.