Is chemical index of alteration (CIA) a reliable proxy for chemical weathering in global drainage basins?

Abstract

The chemical weathering of silicate rocks in continents as an important sink of atmospheric CO₂ is of great significance for global environmental change. Rivers play a key role in earth surface processes and are regarded as the most important carrier of terrigenous materials into the sea. The chemical index of alteration (CIA) has been widely used as a proxy for chemical weathering in sediment source area. In this paper, the CIA values of suspended particulate matters from 44 rivers worldwide are recalculated. The CIA values vary significantly, with the highest average value occurring in African rivers and the lowest in North American rivers. The correlation analyses suggest that on a global scale the CIA is sensitive to land surface temperature, latitude at river mouth and soil depth in the drainage basins, but poorly correlated with drainage area, precipitation, average elevation and runoff. However, the CIA of the rivers draining east China are closely related to temperature, precipitation, latitude and runoff, primarily reflecting the dominant control of monsoon climate on chemical weathering in the catchments. It is therefore unrealistic to find a simple law of regulating chemical weathering in continents. The CIA actually reflects the integrated weathering history in the drainage basins and therefore, caution should be taken while using it as a direct and quantitative proxy for evaluating the intensity of instantaneous chemical weathering in continents.