Erosion rates in subtropical, rapidly developing countries: An isotopic approach to measuring background rates of erosion in Brazil and China
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 Be 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 (R=0.73) and mean annual precipitation (R=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 Be 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 Be 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 yr. Modern sediment yields range from 90 to 2,879 tons km yr (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 (R 0.60) and mean basin slope (R 0.42) There is a negative but strong relationship between erosion rates and precipitation in my dataset (R 0.60) I sampled some places where Be samples had been collected before to test the methodological assumption of time invariant Be 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. ii Citations Material from this dissertation has been accepted for publication in Geomorphology on March, 1, 2016 in the following form: Sosa Gonzalez, V. Bierman, P.R. Fernandes, N.F. Rood, D.H. 2016. Long term, background denudation rates of southern and southeastern Brazilian watersheds estimated with cosmogenic Be. Geomorphology iii Dedication My work, as well as all my achievements, would have not been possible without the unconditional help and love from my family and closest friends. Carmen (mami) y Junior (papi) !esta meta alcanzada es tambien de ustedes y para ustedes! To my life partner, Paul L. Michael, who got here just in time to help me make it through this journey. iv Acknowledgements I am eternally grateful to Dr. Paul Bierman for his advice and guidance in science, research and life, for the past 6 years. Special thanks go to Dr. Andrea Lini, Dr. Walter Poleman and Dr. Joshua Farley for their help with my research and dissertation. I want to thank Dr. Amanda H. Schmidt for all of her help with my dissertation, they were a key piece of the puzzle. Thanks to Dr. Dylan H. Rood for his help with the Accelerator Mass Spectrometry Laboratory at SUERC (Scottish Universities Environmental Research Centre) for Be measurements. Big thanks to my family and dearest friends for their support during these 4 years, and for their faith in me. A special thanks go to my sister Noelia and her family for their support. Noa, thanks for keeping your auntie entertained and laughing the stress away. My research was possible thanks to funding from the US National Science Foundation awarded to Dr. Amanda H. Schmidt (NSF EAR 1114166) Dr. Paul Bierman (NSF EAR1114159) and Dr. Dylan H. Rood (NSF EAR 1114436) Table