Monday, 5 September 2005

This presentation is part of: Poster Session I

Paleo-erosion rates of a granite pediment determined from depth-profile fitting of in situ produced 10Be and 26Al

Sachi Wakasa1, Yuki Matsushi1, Hiroyuki Matsuzaki2, Yukiya Tanaka3, and Yukinori Matsukura1. (1) Graduate School of Life and Environmental Sciences, University of Tsukuba, Tennodai 1-1-1, Tsukuba, Japan, (2) Department of Nuclear Engineering and Management, The University of Tokyo, Bunkyo-ku Yayoi 2-11-16, Tokyo, Japan, (3) Department of Geography, Kyunghee University, Hoegi-dong 1, Dongdaemun-gu, Seoul, South Korea

Paleo-erosion rates of a pediment surface consisted of granite bedrock in western part of Korea are estimated from in situ produced 10Be and 26Al analysis of the several outcrops. The granite bedrock had been weathered to grus and it is covered with gravel layer and several layers of loess. Gravel layer above the grus, having the maximum 2-m thickness, contains angular gneiss gravels with mean diameter of 15 cm and sand as matrix. It is seemed that these gravels were supplied from the gneiss mountain behind the pediment. Grain size analysis of the gravel layer sediments suggests that this layer was transported and deposited by sheetflood. Loess chronology suggests that the oldest loess just above the gravel layer was deposited at the MIS 5. The samples of grus for measuring 10Be and 26Al concentrations were taken at the different depths at the two outcrops. The results showed that the profile of concentrations of both 10Be and 26Al decreases exponentially with increasing the depth. These measured profiles and the above field findings allow us to assume that (1) the grus had been eroded by sheetflood in erosion stage, and (2) the history of pediment evolution changes from the erosion stage to burial stages at 125 ka, i.e., last interglacial period. Based on these assumptions we construct the model profiles to estimate paleo-erosion rates of the pediment surface. Since the model profiles depend on the parameters of paleo-erosion rate and burial thickness above the grus, the optimal values were searched by fitting the measured profile to the model ones with changing the two parameters. Best fitted data by a c2-based inversion method were obtained to be 10 ~ 15 mm/kyr for paleo-erosion rates and 400 cm for burial thickness, when c2 values became to be 1.7 ~ 22. In the previous studies using in situ produced nuclides analysis, mean erosion rate has been reported as 6 mm/kyr for bedrocks and 40 mm/kyr for terrace surfaces composed of soil or subsoil. The erosion rate of the pediment surface is significantly less than that of the terrace surfaces. Since the pediment surface is composed of grus as mentioned above, the resisting force for erosion seems to be the same as that of the terrace surfaces. The lower erosion rates of the pediment are, therefore, indicated that the pediment was eroded by weak erosional force because sheetflood erodes extensively with lower force.

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