Abstract
Primarily dependent on the lithology of parent rocks, sediment composition is controlled by several superposed processes, including chemical weathering of minerals crystallized at higher temperatures and pressures in the Earth's crust. In different climatic regimes and geomorphological settings, weathering operates with different modalities and consequences that are best investigated in modern, natural environments. Excellent conditions in this regard are provided by the subequatorial Niger River, sourced in hot-humid Guinea and flowing in a great arc across the southern edge of the hyperdry Sahara Desert to eventually reach hyperwet coastal Nigeria, a peculiar geometry conditioned by late Mesozoic rifting and diachronous opening of the Atlantic Ocean. In this study, we combine optical observations of mineral dissolution and replacement with petrographic, heavy mineral, clay mineral, elemental geochemistry, and Nd and Hf isotope geochemistry data. Our aim is to determine which techniques and mineralogical or chemical parameters are more likely to indicate weathering rather than provenance effects as well as the climatic conditions under which sediment composition is prone to be profoundly modified by the breakdown of labile minerals. In entirely first-cycle sand derived from the Archean craton in humid Guinea, all garnet and at least 85%-90% of plagioclase but less than or equal to 10% of K-feldspar grains have been lost. Unweathered feldspars are virtually all crosshatched microcline, confirming the order of tectosilicate durability: quartz > microcline > orthoclase > plagioclase. Half of plagioclase and most K-feldspar and garnet grains are instead preserved in the Benue catchment. In contrast with parameters overwhelmingly affected by recycling (i.e., quartz/feldspar; weathering index of Parker; zircon, tourmaline, and rutile), reliable indicators of weathering are based on feldspar (plagioclase/feldspar, kaolinite%, chemical index of alteration [CIA]) and garnet (garnet/garnet + staurolite + kyanite + andalusite + sillimanite) weatherability. Rather than reflecting current erosional regimes, however, a scarcity of plagioclase and garnet, high CIA, or kaolinite may be inherited through recycling of older siliciclastic rocks or paleosols. The comparison among all major river systems in sub-Saharan Africa indicates that weathering processes can and do drastically alter sand composition in the extreme hot-wet equatorial conditions from Guinea to the Congo and the Rwanda-Burundi rift highlands (Guineo-Congolian bioclimatic domain). In the subequatorial Sudanic domain, sand composition is less strongly affected in areas with steeper topographic relief (e.g., Benue drainage basin). In dry tropical areas, weathering is minor to negligible.