Chemical weathering of basaltic lava flows undergoing extreme climatic conditions:: the water geochemistry record

This study was dedicated to the early stage of the weathering of historic basaltic flows located in Mount Cameroon. The combination of high relief (i.e. 0 to 4071 m) and high rainfall range (i.e. 1.8 to 12 m/year) lead to strong climatic contrast. Spring and rivers were sampled all around the volcano. We report here the basic chemistry of the waters as well as strontium and uranium isotopic ratios. The combination of the molar proportions of solute obtained with the modal amounts of the minerals in the basalts gives a prediction of what should be the relative molar concentrations of major compounds in the weathering waters issuing from Mount Cameroon. The measured Alkalinity/Si and Mg/Si ratios are higher than the calculated ones while the measured Ca/Si ratio is equal to the calculated value. We suggest that the Si-poor waters of Mount Cameroon are due to biological pumping, trapping of Si in Fe-silicate minerals such as Si containing ferrihydrite and Si interaction with bacterial cell wall leading to the formation of allophane type minerals which were observed in Mount Cameroon soil profiles. Calcium uptake by plants explains the lower Ca/alkalinity ratios measured in the water samples. The water-rock ratio (R) calculated from the strontium isotopic compositions of the water samples, ranges from 29,452 to 367,450. The calculated weathering rates (W-R) range from 1 to 20 mm/ky and from 1 to 103 mm/ky for high and low elevations, respectively, and agree with both the thickness and the age of paleosoils found in the same area and with previously published estimates from coupled reaction-transport models. This difference emphasizes the role of vegetation and rainfall at lower elevations as compared to what happens at high elevations. (C) 2003 Elsevier B.V. All rights reserved.