Online

Neilson, Thomas Bundgaard

Geology

2016

MS

This research aims to understand the natural and human influences on erosion in three tributary watersheds to the Mekong River, Yunnan Province, China and to assess the utility of a novel application of isotopic indicators of erosion. It explores how erosion varies through time and space as a function of physical characteristics of the landscape, tectonic forces, and human alteration of the landscape for forestry and agriculture. To accomplish these goals, I use four sediment-associated radionuclides: in situ 10Be, meteoric 10Be, 210Pbex, and 137Cs. These isotopes accumulate in or on sediment grains, and each accumulates to a different depth on the landscape and has a different half-life. Thus, the isotopes can be used to track sediment as it moves across Earth's surface, each providing unique insight into processes occurring over a certain time period (from ~50 to 50,000 years) or eroding to a certain depth on the landscape. The studied watersheds range from 22° to 27° N latitude, and from 200 to 2500 km2 in area. I collected 54 samples of river-borne sediment within the three study watersheds, and measured the concentration of each isotope in every sample. In addition to the measured isotopic concentrations, I utilize on over 20 years of daily sediment yield data at the outlet of each watershed, hillslope steepness, normalized channel steepness (ksn), contemporary land-use data, elevation, and 56 years of mean annual precipitation data (MAP). Long-term erosion rates scale with topographic parameters in two of the three study basins, indicating that topography, or the underlying tectonic forces responsible for topography, control erosion rates over the past 6,000 to 50,000 years. Isotopic data also show that contemporary erosion is higher in cultivated areas than un-cultivated areas, a direct result of agricultural practices. Contemporary sediment yield, however, has not increased notably due to land-use change; however, under-representation of large stochastic events and sediment trapped by agriculture have reduced sediment yield relative to the long-term average in two of the studied watersheds. Overall, the data imply changes in contemporary erosion that are consistent with Chinese policies that promoted deforestation from the 1950's to the late 1980's and conservation from the late 1990's to present. This proves to be a significant finding, as the result of the top-down approach China has taken with conservation policy has been widely called into question in previous studies. While each isotope has the potential to provide unique information regarding erosional processes, in situ 10Be and 210Pbex proved to be the most useful, while meteoric 10Be was the most challenging to utilize. Though interpretation is complex, measuring all four isotopes on the same sediment samples helps to fully realize the potential of in situ 10Be to estimate background erosion by simultaneously allowing for assessment of contemporary and human induced erosion.