Author's personal copy Sediment production and transport from in situ-produced cosmogenic 10 Be and river loads in the Napo River basin, an upper Amazon tributary of Ecuador and Peru H. Wittmann a, * ,1 , F. von Blanckenburg a,1 , J.L. Guyot b , A. Laraque c , C. Bernal d, e , P.W. Kubik f a Institut für Mineralogie, Universität Hannover, Callinstr. 3, Hannover, Germany b LMTG (CNRS-IRD-UT3-OMP), IRD, CP 7091, Lago Sul, Brasilia, Brazil c LMTG (CNRS-IRD-UT3-OMP), IRD, BP 64501, Montpellier, France d LMTG (CNRS-IRD-UT3-OMP), Avenue Edouard Belin, Toulouse, France e Ministerio de Energia y Minas, Quito, Ecuador f Laboratory of Ion Beam Physics, ETH Zurich, 8093 Zurich, Switzerland article info Article history: Received 18 May 2010 Accepted 24 September 2010 Keywords: Erosion Denudation Cosmogenic beryllium-10 In situ-produced 10 Be River loads Andes Sediment transport Napo basin Oriente basin Channel-floodplain exchange abstract Cosmogenic nuclide-based denudation rates and published erosion rates from recent river gauging in the Napo River basin (Peruvian Amazonia) are used to decipher erosion and sedimentation processes along a 600 km long transect from the headwaters to the lowlands. The sediment-producing head- waters to the Napo floodplain are the volcanically active Ecuadorian Andes, which discharge sediment at a cosmogenic nuclide-based denudation rate of 0.49  0.12 mm/yr. This denudation rate was calculated from an average 10 Be nuclide concentration of 2.2  0.5  10 4 at/g (Qz) that was measured in bedload-derived quartz. Within the Napo lowlands, a significant drop in trunk stream 10 Be nuclide concentrations relative to the Andean hinterland is recorded, with an average concentration of 1.2  0.5  10 4 at/g (Qz) . This nuclide concentration represents a mixture between the 10 Be nuclide concentration of eroded floodplain deposits, and that of sediment eroded from the Andean hinterland that is now carried in the trunk stream. Evidence for addition of sediment from the floodplain to the trunk stream is provided by published decadal-scale sediment flux measurements from gauging stations operated in the Napo basin, from which an increase from 12  10 6 t/yr at the outflow of the Andes to w47  10 6 t/yr at the confluence with the Solimões (upper Amazon River) is recorded. Therefore, approximately 35  10 6 t of floodplain sediment are added annually to the active Napo trunk stream. Combined with our nuclide concentration measurements, we can estimate that the eroded floodplain deposits yield a nuclide concentration of w0.9  10 4 at/g (Qz) only. Under steady state surface erosion conditions, this nuclide concentration would translate to a denudation rate of the floodplain of w0.47 mm/yr. However, we have no geomorphologic explanation for this high denudation rate within the low relief floodplain and thus suggest that this low-nuclide concentrated sediment is Andean- derived and would have been deposited in the floodplain at a time when erosion rates of the Andes were elevated. Therefore, the recently eroded floodplain sediment provides an Andean “paleo denu- dation rate” of 1.2 mm/yr that was calculated for high Andean production rates. A likely period for elevated erosion rates is the LGM, where climate and vegetation cover of the Andes differed from that of the Holocene. A possible cause for the erosion of the floodplain is the tectonic uplift of the Eastern Andes, which progressively shifts the Napo River northwards. Hence, the river cuts into ancient lowland sediment, which is admixed to the Andean sediment carried in the main Napo River. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction The aim of this study is the evaluation of sediment production and transport in the Napo River basin that is located at the Ecua- dorianePeruvian border. The Napo is an anastomosing river with steep slopes; the steep Napo morphology is attributed to uplift of * Corresponding author. Tel.: þ49 (0) 331 288 2820; fax: þ49 (0) 331 288 2852. E-mail address: Wittmann@gfz-potsdam.de (H. Wittmann-Oelze). 1 Present Address: GeoForschungsZentrum Potsdam, Telegrafenberg, Germany. Contents lists available at ScienceDirect Journal of South American Earth Sciences journal homepage: www.elsevier.com/locate/jsames 0895-9811/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsames.2010.09.004 Journal of South American Earth Sciences 31 (2011) 45e53