LUMINESCENCE AGES OF FLUVIAL TERRACES IN EASTERN AMAZONIA

The evolution of the Amazon transcontinental river system remains hampered by the lack of reliable absolute ages to constrain sediment deposition in a hundred thousand to few million years timescales. Luminescence techniques have providing chronological control for late Quaternary sediments, but the method is limited to the last ~150 ka. In order to extend the age range of luminescence dating, this study tested thermally transferred optically stimulated luminescence (TT-OSL) signal of quartz for dating of fluvial terraces in eastern Amazonia. Terraces at the flank of the lower Xingu River are deposits 3-10 m higher than the present river level, mainly composed of coarse sand and gravels with finning upward pattern. Clay and silt are related to overbank deposits and long-time of intensive weathering. Two profiles were sampled and firstly measured using the OSL signal. As expected, seven from eight samples had saturated OSL signal. TT-OSL protocol got suitable natural doses, providing the first absolute ages for the Quaternary Amazon sediments overlaying Cretaceous rocks. The ages for ALC-01 profile were 211, 343, 262 and 211 ka; and for ALC-02 were 1.6, 107, 234 and 200 ka, from top to bottom. However, some age uncertainty still remain, as dose rate heterogeneity (1.4-3.1 Gy/ka), which could explain age inversions or overlap. Dose rate heterogeneities might be due to changes in water table, loss or uptake of radionuclides and translocation of clays. In addition, the thermal lifetime of the TT-OSL traps is a function of the mean annual air temperature through time, suggesting a maximum lifetime of 200-700 ka at 20^oC. Thus, the oldest ages can be underestimates. Assuming that ages are consistent, ages between 200-350 ka appears to represent the oldest Quaternary deposits in the lower Xingu valley, suggesting a minimum age for the Xingu River under similar dimensions as today. These ages are in agreement with speciation events of Psophia, which indicate that Xingu River was established as isolating barrier at 800-300 ka (Ribas et al., 2011). After this time, environmental changes resulted in vertical incision and development of the modern Xingu Ria. Futures geochronological efforts might shed light about the role of the Amazonian rivers for bio-paleogeography during the late Cenozoic. (FAPESP grant 14/23334-4; 2012/50260-6)