Résumé
Paleoenvironmental reconstructions based on Amazonian fluvial and marine pollen data are often hindered due to the scarcity of modern calibration data. In Amazonia, rivers are the main pathways for pollen from upland and flooded forests to continental and marine sediment sinks. Linking pollen assemblages transported by rivers to watershed vegetation is crucial to interpret palynological archives. In order to constrain the interpretation of palynological archives, we analysed the pollen content of 36 riverbed sediment samples distributed from the mouth of the Içá River (ca. 68oW) to the Amazon estuary (ca. 49oW) with support of grain size and major element analyses of the inorganic sedimentary content. The palynological analysis shows a strong imprint of the lowland floodplain vegetation as the main source of the riverborne pollen. The upper reaches of the Amazon River present abundant Cecropia, reflecting early-successional stages induced by intense floodplain disturbance. The middle and lower reaches of the Amazon River exhibit a progressive increase in herbs mainly represented by Poaceae and Cyperaceae, related to wider floodplains covered by seasonally inundated vegetation. Floodplain forests of whitewater rivers (várzeas) are palynologically characterized by Iriartea, Mauritia, Ilex, Pseudobombax, Acalypha, and Luehea/Laetia, with high frequency of Cecropia or herbs, and montane taxa. At the black- and clearwater rivers, the igapó floodplain forests provide high percentages of Symmeria, Myrtaceae, and Alchornea, and can be distinguished by Sapium/Sebastiana, Piranhea, Amanoa, Pouteria, Ferdinandusa/Genipa, Attalea/Astrocaryum, and Eschweilera which suggest conditions of higher stability and mature forests. Contribution from the non-flooded uplands, Andean Mountains, and savannas/Cerrado occur, but their signals are blurred by the high pollen production of the lowland floodplains. Our data provide a calibration set for environmental reconstructions of river-influenced deposits across the Amazon including offshore areas of the most iconic riverine dominated landscape in the world.
•Modern pollen callibrations improve vegetation reconstructions.•Pollen assemblages from the Amazonian rivers are strongly influenced by the lowlands.•Different river types are discriminated by their riverbed pollen assemblages.•Distinct increase in grass pollen possibly relate to periodically flooded environments.•Guidelines for vegetation interpretation on riverine and marine data are presented.