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

摘要

This paper aims to improve our understanding of chemical weathering processes and element transport mechanisms in the humid tropical environments. We studied the Nyong River basin (27,800 km(2)) located on the northwestern part (Ntem Complex) of the Congo craton (central Africa). The dissolved concentrations (i.e., < 0.20 mu m) of major and trace elements, dissolved organic carbon (DOC) and the Sr-87/Sr-86 ratios have been measured in more than 20 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, Th, Zr, Y, REE), silica, and DOG. The total dissolved load (TDS) is low (similar to 20 mg 1(-1)) and dominated by silica and organic matter. The comparison of different watersheds shows us that chemical weathering is more efficient in the small unit-watersheds. All the sampled rivers exhibit a wide range of Sr-87/Sr-86 ratios and high Ca/Na ratio that can be explained by the heterogeneity of the crystalline rock constituting the cratonic basement. Four selected rivers (Mengong, Awout, Soo, and Nyong) having different drainage areas and hydrological parameters were sampled over a 2-year period. Collected data show that all these rivers present the same monthly seasonal variations, with higher concentrations during rainy season and lower concentrations 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 zones where mineral dissolution is enhanced by humic substances. These swamp zones constitute a pool of organic-rich water, which can be quickly mobilized during rain seasons. In these waters, DOC and insoluble element concentrations (e.g., Al, Fe, and Th) were strongly correlated which show the key role of organic colloidal matter in the transport of some insoluble elements. Some other relationships (Al vs. Fe, REE vs. Al) were also examined in order 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 regulation even though they are likely to enhance mineral dissolution. Even if organic matter favors mineral dissolution, chemical weathering in this area is low compared to other world regions, which suggest, on a global scale, a relatively small effect of these environments on the CO2 consumption. (C) 2000 Elsevier Science B.V. All rights reserved. [References: 69]