Weathering indices as climate proxies. A step forward based on Congo and SW African river muds

Despite the influence of other geological and geomorphological factors, chemical weathering at the Earth's surface is strongly controlled by climate. Thus, a measure of weathering intensity determined from soils or sediments should provide information about the climatic conditions associated with their formation. Available geochemical and mineralogical data on modern fluvial and marine muds from different regions of southern Africa and its Atlantic continental margin are used to review the links between sediment composition and climatic properties together with the possible causes of variance. Although river muds may not be generated exclusively in a single sedimentary cycle and erosion and weathering processes do not necessarily take place in a spatially homogeneous way, significant relationships between mineralogical and geochemical signatures of river mud and rainfall in the corresponding catchment area were recognised. Our study shows that the composition of clay is strongly influenced by climatically-driven weathering, whilst coarser mud fractions tend to be more affected by provenance, grain size, hydraulic sorting, and recycling. In the marine environment the climatic signal may be lost even in clay, because of hydraulic fractionation, authigenic mineral growth and mixing with foreign particles. Given the ubiquitous character of fluvial muds, and the easy and non-expensive methods available for separating and analysing clay fractions, their geochemical fingerprints represent a most precious source of information concerning climate. Any geochemical parameter used as a regional proxy of climate, however, still requires that the diversity of geological, geomorphological, and biological factors that affect its value are cautiously considered.