Uranium isotopes in a subtropical mountainous river of Taiwan: Insight into physical and chemical weathering processes

Abstract

The correlation between physical and chemical weathering in mountainous rivers is debatable. A reliable geochemical tracer for physical erosion and chemical weathering is necessary to obtain crucial information on the relationship between weathering processes and environmental changes on the Earth’s surface. Small mountainous rivers (SMRs) of Taiwan are characterized by extremely high physical and chemical weathering, and therefore, they provide the foremost natural laboratory for studying weathering processes in tectonically active high-standing islands. In this study, water samples collected from the Kaoping River, the largest river catchment in southern Taiwan, were analyzed for elemental concentrations, as well as for U and Li isotopes, to understand the weathering processes and to evaluate the use of the 234U/238U activity ratio, hereafter (234U/238U), as a probe for weathering. The average (234U/238U) of the Kaoping River is significantly higher than that of other rivers worldwide, and variations in (234U/238U) reflect the degree of physical and chemical weathering. The weathering environment corresponds to the weathering-limited regime, and the mountain area is a natural example of the kinetic-limited control on chemical weathering by intense physical erosion. High physical erosion predominates in the mountainous region, which increases (234U/238U), relatively reduces U concentrations, and inhibits chemical weathering. The decreased (234U/238U) and increased Na/Li and δ7Li downstream clearly indicate enhanced chemical weathering in the alluvial plain. Our results demonstrate that dissolved (234U/238U) in the SMRs of Taiwan can reflect the relative contributions of physical and chemical weathering within the river catchment. The coupling between chemical weathering and physical erosion can be observed downstream and the relationship becomes less pertinent in the mountain area due to changes in weathering kinetics. A positive correlation between (234U/238U) and uplift rate in the Kaoping River suggests that the riverine (234U/238U) is mainly controlled by uplift-induced physical erosion, which may be influenced by landslide activities in the rapidly eroding mountainous river of Taiwan.