Controls on intense silicate weathering in a tropical river, southwestern India

Abstract

The Silicate Weathering Rate (SWR) and associated Carbon dioxide Consumption Rate (CCR) in tropical silicate terrain is assessed through a study of the major ion chemistry in a small west flowing river of Peninsular India, the Nethravati River. The specific features of the river basin are high mean annual rainfall and temperature, high runoff and a Precambrian basement composed of granitic-gneiss, charnockite and minor meta-sediments. The water samples (n=56) were collected from three locations along the Nethravati River and from two of its tributaries over a period of twelve months. Chemical Weathering Rate (CWR) for the entire watershed is calculated by applying rainwater correction using river chloride as a tracer. Chemical Weathering Rate in the Nethravati watershed is estimated to 44t.km−2.y−1 encompassing a SWR of 42t.km−2.y−1 and a maximum carbonate contribution of 2t.km−2.y−1. This SWR is among the highest reported for granito-gneissic terrains. The assessed CCR is 2.9·105mol.km−2.y−1. The weathering index (Re), calculated from molecular ratios of dissolved cations and silica in the river, suggests an intense silicate weathering leading to kaolinite–gibbsite precipitation in the weathering covers. The intense SWR and CCR could be due to the combination of high runoff and temperature along with the thickness and nature of the weathering cover. The comparison of silicate weathering fluxes with other watersheds reveals that under similar morpho-climatic settings basalt weathering would be 2.5 times higher than the granite–gneissic rocks.