Cosmogenic Nuclide ～(10)Be And Its Application In Estimating Erosion Rates Of Yangtze Catchment

In this study, cosmogenic nuclide ¹⁰Be method was first applied to the research on sediments in Yangtze River catchment. The main purposes of this application were to spatially estimate erosion rates of Yangtze catchment from source to sink, and were to introduce a new relatively quantitative method for studying erosion rate and sediment generation rate.Since 1980s, the application of terrestrial cosmogenic nuclides to study the age when the earth surface formed and the rate at which landscape evolved has been one of the greatest breakthroughs in geomorphology. Terrestrial cosmogenic nuclides, including ³He, ¹⁰Be, ¹⁴C, ²¹Ne, ²⁶Al and ³⁶Cl etc., of which ¹⁰Be is used predominately, are produced by nuclear interactions between secondary cosmic particles and target atoms of rock and soil in the very uppermost layer of earth's surface, ¹⁰Be is predominately produced by spallation in the depth of 1m of the upper surface of rock or soil, and its concentration in rock depends on cosmogenic nuclide production rate and erosion rate of the rock. When production rate is determined, ¹⁰Be can be used to estimate erosion rate of earth's surface. Actually, ¹⁰Be-derived erosion rate represents the rate at which erosion removes a rock depth equivalent to one attenuation length of ¹⁰Be, and its time scale ranges from 10³ to 10⁵yr. Since ¹⁰Be concentration records long term erosion rate and is insensitive to short term erosion rate, the significance of applying ¹⁰Be to estimate erosion rates in Yangtze Catchment is to provide base-line data for evaluating short-term human activities' effects on erosion and soil loss. Moreover, in the view of the earth system science, quantitatively studying earth surface erosion is crucial to understand geology processes, and is important research content of the earth system science.Based on measurement and analysis of ¹⁰Be concentrations in river sediments, it is found that ¹⁰Be concentrations in the main channel of Yangtze River decrease downstream from west to east, with highest concentration of 1.1×10⁶atoms·g^-1 in the Jinsha sub-catchment, upper Yangtze and lowest concentration of 0.27×10⁶ atoms·g^-1 on the delta. The higher ¹⁰Be concentrations in upper Yangtze are associated with higher elevation and lower erosion rate of plateau from headwater to upper Jinsha. As sediments of lower ¹⁰Be concentration in tributaries are discharged and mixed into the main channel, the ¹⁰Be concentration of Yangtze River is diluted and deceases from headwater to estuary. Compared with concentrations in the main channel, the ¹⁰Be concentrations in tributaries' sediments are much lower than that in main channel, for instance, the ¹⁰Be concentration of one major tributary-Min Jiang, is as low as 0.04×10⁶atoms'g^-1, nearly one thirtieth of that of Jinsha Jiang. The lower concentrations of ¹⁰Be in western tributaries are strongly connected with higher erosion rates, while, the lower concentrations of ¹⁰Be in eastern tributaries are associated with lower production rate because of lower altitude.It is revealed by tracing of geochemical elements and ¹⁰Be isotope that ¹⁰Be isotope are more stable than geochemical elements as tracer of sediments. Based on two-component mixing principle of ¹⁰Be isotope, it is shown that about 53.6% sediments come from Yalong Jiang at the junction between Yalong and Yangtze, and about 26.4% enter from Min-Daduhe at the junction between Min-Daduhe and Yangtze, and Jialing Jiang and Wujiang contribute 25% sediments in the reach from Chongqing to Yichang, and 30.8% sediments of Yangtze delta are contributed by eastern tributaries. As revealed by geochemical elements and ¹⁰Be, the sediment source of Yangtze delta has not changed greatly during Holocene, and there are about 30-34% sediments, including modern and reworked sediments, originated from eastern tributaries.According to statistics on elevation data and relationships between ¹⁰Be production and altitude and latitude, ¹⁰Be concentration and erosion rate models were established for eight sub-catchments. Average erosion rates were estimated by putting the measured ¹⁰Be concentrations into the relevant models, the results are as followed: the average erosion rate of Jinsha Jiang is 207m·Ma^-1, Yalong Jiang is 250 m·Ma^-1, Daduhe-Min Jiang is 460 m·Ma^-1, Jialing Jiang is 75 m·Ma^-1, Wu Jiang is 51 m·Ma^-1, Han Jiang is 75 m·Ma^-1, Dongting Lake is 15 m·Ma^-1, Poyang Lake is 20 m·Ma^-1. The dominate factors controlling erosion in the Yangtze cachment are relief and tectonic activity. For example, the highest erosion rate occurs in Daduhe- Min Jiang sub-catchment with active tectonics, high relief, and deep gorges, while the lowest erosion rate appears in Dongting Lake and Poyang Lake sub-catchments with weak tectonics, lower relief, and floodplains.Comparison between gauge-based and ¹⁰Be-derived erosion rates indicates that the results derived from the two methods agree well in magnitude, particularly in Jinsha Jiang and Poyang Lake, gauge-estimated erosion rates are almost equal to ¹⁰Be-estimated erosion rates, which indicates that it is reliable to estimate erosion rate using ¹⁰Be concentration, ¹⁰Be estimated erosion rates in Yalong Jiang and Daduhe-Min Jiang are distinctly higher than that based on suspended sediment flux, which is probably due to the systematic under-representation of high-magnitude, low-frequency transport events in the gauging records which cover less than a century. It may also be the case that sediments in Daduhe-Min Jiang have already deposited in Sichuan Basin before they pass gauging stations. The ¹⁰Be-derived erosion rates of Jialing Jiang, Wujiang, Hanjiang, and DongTing Lake are much lower than that estimated from gauge data, which indicates human have played important roles on erosion in these sub-catchments. Compared with ¹⁰Be-derived background erosion rates, human activities may have increased erosion rates by 4.0, 3.3, 1.8, and 4.3 times respectively.