CHEMICAL WEATHERING IN THE ZAMBESI BASIN: ASSESMENT OF THE CARBON DIOXYDE COMSUMPTION BY THE KARU BASALT PROVINCE

The quantification of the role of weathering in the carbon cycle and its interaction with climate and tectonics at the geological time scale is one of the key questions of the geoscientists. The consumption of atmospheric CO₂ by silicate weathering indisputably plays the central role in the long term carbon budget and consequently on mean global climate. Through the composition of major elements in river waters, CO₂ consumption by the alteration of continental rocks can be estimated.

The aim of this study is to estimate of the chemical weathering rate of the Zambesi basin and the impact of Karoo basalt province on chemical atmospheric consumption, evaluated from a database of major elements. The Karroo basalts outcrop erupted around 183 +/2 2 10⁶ take place in the Upper and the Middle Zambezi, covering a surface of 9600 km2. The Zambesi Basin, located between 8° and 20° south latitude and between 16.5 and 36 east longitude, is the fourth largest in Africa. The catchment has a total area of some 1,281,000 km2, the mean annual temperature is 19,3°C and the annual rainfall varies from nearly 2 000 mm to 600 mm. During the sampling period, the annual runoff at Victoria Fall gauging Station ranged between 50 to 2000 m3/s ie 6.9 to 0.6 l/s/km2. The consumption rate of atmospheric CO2 associated with the chemical weathering was calculated from riverine HCO3- concentrations. During the weathering of volcanic rocks, all dissolved carbonates originate from atmospheric/sil CO2. Values of CO2 consumption rates are relatively high, about 0.024 10¹² mol/yr, and are comparable to Deccan Traps consumption rates.