Late quaternary downcutting rates of the Qianyou River from U/Th speleothem dates, Qinling mountains, China

Dating deposits in eaves formed by rivers may yield insight into rates and processes of bedrock incision. Three cave passages at different levels have developed in the walls of the Qianyou River valley in the Qinling mountains, south of the city of Man, China. Twelve speleothem samples near the position of palaeowater tables in three cave passages are dated by the Th-230 dating method. The results show that the river cut down at the rate of 0.23 +/- 0.02 mm/yr from 358,000 +/- 38,000 to 247,000 +/- 28,000 yr ago, 0.19 +/- 0.03 mm/yr from 247,000 +/- 28,000 to 118,000 +/- 19,000 yr ago, and 0.51 +/- 0.08 mm/yr from 118,000 +/- 19,000 yr ago until today. These fall very close to the glacial-interglacial transition following marine oxygen isotope stages 10, 8, and 6, respectively. The increase in downcutting rates during the interglacial period is consistent with warm, wet weather, increasing rates of erosion. This may lead us toward an underlying mechanism for modulating incision that is not in a direct response to the presence of headwater glaciers. (C) 2004 University of Washington. All rights reserved.