Wittmann H., von Blanckenburg F., Guyot Jean-Loup, Laraque Alain, Bernal C., Kubik P. W. (2011). Sediment production and transport from in situ-produced cosmogenic Be-10 and river loads in the Napo River basin, an upper Amazon tributary of Ecuador and Peru. Journal of South American Earth Sciences, 31 (1), p. 45-53. ISSN 0895-9811.
Titre du document
Sediment production and transport from in situ-produced cosmogenic Be-10 and river loads in the Napo River basin, an upper Amazon tributary of Ecuador and Peru
Wittmann H., von Blanckenburg F., Guyot Jean-Loup, Laraque Alain, Bernal C., Kubik P. W.
Source
Journal of South American Earth Sciences, 2011,
31 (1), p. 45-53 ISSN 0895-9811
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 headwaters 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 Be-10 nuclide concentration of 2.2 +/- 0.5 x 10(4) at/g((Qz)) that was measured in bedload-derived quartz. Within the Napo lowlands, a significant drop in trunk stream Be-10 nuclide concentrations relative to the Andean hinterland is recorded, with an average concentration of 1.2 +/- 0.5 x 10(4) at/g((Qz)). This nuclide concentration represents a mixture between the 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 x 10(6) t/yr at the outflow of the Andes to 47 x 10(6) t/yr at the confluence with the Solimoes (upper Amazon River) is recorded. Therefore, approximately 35 x 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 similar to 0.9 x 10(4) at/g(Q) only. Under steady state surface erosion conditions, this nuclide concentration would translate to a denudation rate of the floodplain of similar to 0.47 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 denudation 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.
