Major and trace element abundances, and strontium isotopes in the Nyong basin rivers (Cameroon): constraints on chemical weathering processes and elements transport mechanisms in humid tropical environments

This paper aims to improve our understanding of chemical weathering process es and element transport mechanisms in the humid tropical environments. We studied the Nyong River basin (27,800 km(2)) located on the northwestern pa rt (Ntem Complex) of the Congo craton (central Africa). The dissolved conce ntrations (i.e., < 0.20 mu m) of major and trace elements, dissolved organi c carbon (DOC) and the Sr-87/Sr-86 ratios have been measured in more than 2 0 rivers draining watersheds with various surface areas (similar to 1 to 28 ,000 km(2)). All these rivers exhibit low major cations concentrations (i.e ., Na, Mg, K, Ca) but high concentrations of some trace elements (Al, Fe, T h, Zr, Y, REE), silica, and DOG. The total dissolved load (TDS) is low (sim ilar to 20 mg 1(-1)) and dominated by silica and organic matter. The compar ison of different watersheds shows us that chemical weathering is more effi cient in the small unit-watersheds. All the sampled rivers exhibit a wide r ange of Sr-87/Sr-86 ratios and high Ca/Na ratio that can be explained by th e heterogeneity of the crystalline rock constituting the cratonic basement. Four selected rivers (Mengong, Awout, Soo, and Nyong) having different drai nage areas and hydrological parameters were sampled over a 2-year period. C ollected data show that all these rivers present the same monthly seasonal variations, with higher concentrations during rainy season and lower concen trations during dry season. This implies that the weathering and transport mechanisms of small watersheds can be extended to the whole Nyong basin. In the small unit-watersheds, chemical weathering mainly occurs in swamp zone s where mineral dissolution is enhanced by humic substances. These swamp zo nes constitute a pool of organic-rich water, which can be quickly mobilized during rain seasons. In these waters, DOC and insoluble element concentrat ions (e.g., Al, Fe, and Th) were strongly correlated which show the key rol e of organic colloidal matter in the transport of some insoluble elements. Some other relationships (Al vs. Fe, REE vs. Al) were also examined in orde r to get information about chemical weathering or element transport. Based on these data, it has been concluded that the chemical composition of these river waters is controlled by geomorphic and historic factors (e.g., thick cation-poor soil). In contrast, the present day climatic parameters (high rainfall and temperature) play a minor role in water chemistry regula tion even though they are likely to enhance mineral dissolution. Even if or ganic matter favors mineral dissolution, chemical weathering in this area i s low compared to other world regions, which suggest, on a global scale, a relatively small effect of these environments on the CO2 consumption. (C) 2 000 Elsevier Science B.V. All rights reserved.