Jean-Loup Guyot

Trace element geochemistry in the upper Amazon drainage basin (Bolivia)

Abstract The distribution of trace elements (Sr, Mo, Ba, Rb, U, Mn, Ni, Cu, Zn, and Cd) was investigated in surface waters and associated particulates in two upper Amazonian basins (Mamore and Beni). Dissolved Sr, Ba, Mo, Rb, U and to a lesser extent Zn and Cd correlate with major ions and appear to be predominantly derived from soluble rocks (carbonates, evaporites or sulfides essentially contained in shales). These elements are conservative in waters except in highly alkaline or sulfate-rich waters where calcite and barite can precipitate. The concentrations of transition element are generally not correlated with major ion chemistry. Their behaviour can be largely explained by equilibrium reactions in solution and solid–liquid exchanges. Additionally, the distribution of Mn and Cu reflects also the lithological differences between the Mamore and Beni basins. The occurrence of mineralisation in the Beni basin explains the high Zn, Cd as well as the concentrations of Cu in some rivers of the Beni basin. The higher content of Mn in the Mamore basin may reflects the predominance of carbonate rocks. The fractionation of trace elements in SPM has been attributed to the sorting of feldspars during transport in water, and to the mixing between clay minerals and quartz. These minerals, refractory to weathering and introduced in water rivers by mechanical erosion, have not contributed to the dissolved load.

The significance of suspended sediment transport determination on the Amazonian hydrological scenario

Rivers play an important role in continental erosion as they are the primary agents of transferring erosion products to the ocean. Understanding rivers and their transport pathways will improve the perception of many processes of global significance, such as biogeochemical cycling of pollutants and nutrients, atmospheric CO2 drawdown, soil formation and their erosion, crust evolutionin short the interaction between the atmospheric and the lithospheric compartment of the Earth s system (Allen, 2008). This interaction is characterised by the relative proportions of mechanical degradation vs. chemical weathering, whose products are, in dissolved or solid form, transported by rivers. The sediment load of rivers is thereby controlled by catchment relief, the channel slope and its connectivity to the hill slope, but also by climatic factors such as precipitation. The latter, together with temperature, exert control over chemical weathering that is dependent on physical erosion to a degree that is yet unknown (Anderson et al., 2002; Gaillardet et al., 1999; Riebe et al., 2001). Both mechanical erosion and chemical weathering, are, however, governed by tectonic activity, which drives processes of landscape rejuvenation and preconditions the fluvial transport regime (von Blanckenburg et al., 2004). On the shorter time scale, humans may act as geomorphic agents by constructing dams and reservoirs, and changing land use by deforestation and mining (Hooke, 2000; Syvitski et al., 2005; Wilkinson and McElroy, 2007). In tropical regions around the globe, large river basins are especially concentrated, and their behaviour plays an important role in river sediment transport (Latrubesse et al., 2005). For example, the tropics represent 25% of the total continental lands and contain 57% of the