Does chemical index of alteration (CIA) reflect silicate weathering and monsoonal climate in the Changjiang River basin?

Abstract

Chemical weathering of continental silicates significantly influences global climate change, earth surface processes, material cycling and oceanic chemical composition. How to quantitatively reconstruct chemical weathering history has become an important issue in global change research. Chemical index of alteration (CIA) has been widely used as a quantitative indicator for estimating the degree of silicate weathering. However, its method of calculation and the limitations of its application are not well understood. In this study, we calculated CIA values from suspended particulate matter collected from the mainstream and major tributaries of the Changjiang River. The values yielded considerable variations at different temporal and spatial scales. The average CIA values increased from the upper to middle-lower reaches, and were lower in the suspended samples taken during the flood than in the dry season. The spatial variation in the CIA is predominantly controlled by basinal monsoon climate. In contrast, the temporal variation in the Changjiang River basin is controlled mainly by the changing provenance of suspended samples in relation to the shift of the precipitation zone. The CIA probably indicates the integrated weathering history in the river basin, and thus, cannot be used as a reliable proxy of instantaneous chemical weathering. Furthermore, the calculation method and hydrodynamic sorting also influence the CIA values. Therefore, caution should be taken when using the CIA as a proxy for studying chemical weathering from different regions.