The role of the Béni floodplain on the chemical weathering fluxes in the upper Madeira basin, Bolivia

Abstract

Institute GEMOC Key Centre, Department of Earth andPlanetary Sciences, Macquarie University, Sydney, NSW2109, AustraliaOn a global scale, several authors have reported good correla-tions between physical erosion fluxes and chemical weatheringfluxes (Gaillardet et al., 1999). This linkage have several origins,one being that the active floodplains can act as a biogeochemicalreactor in which the sediments produced in mountainous zonesand transported by the rivers, once deposited in foreland basins,are deeply weathered.We have tested this idea in the floodplain of the Rio Beni, inthe upper Madeira Basin, in Bolivia.Extreme physical denudation rates are found in the BolivianAndes, in particular due to the abundance of poorly lithifiedshales and of extensive outcrops of fluviolacustrine deposits inthe Upper Andes. These river sediments are exported and 40%is deposited in the foreland basin of the Beni R. where extensivefloodplains are submitted to wet tropical conditions.We compare here monthly collected river samples upstream(Rurrenabaque) and downstream (Riberalta) the Rio Beni flood-plain and averaged dissolved fluxes (over one year). Concentra-tions of major elements are corrected from atmospheric inputs.The main result is that the floodplain has almost no effect onthe chemical denudation rates of the Rio Beni, except loweringthem because of an increase in the drainage surface area.Although long-term weathering in the floodplain is more intensethan in the Andes, the flux of dissolved material released doesnot impact the Rio Beni chemical denudation rates. The differentelements show particular behavior that we will discussed in detail.We conclude that the foreland basin of the Rio Beni acts aboveall as a deposition area and does not contribute to the actualchemical weathering rates. This conclusion supports the idea thatchemical erosion is essentially active in mountainous areas andthat the high temperatures of the plain do not increase the chem-ical denudation rates. In the Beni river basin, tectonic character-istics (uplift and subsidence zone) appear as the first-orderfactors controlling chemical weathering fluxes.