Chronology of Terra Firme formation in Amazonian lowlands reveals a dynamic Quaternary landscape

Abstract

In the Amazonian lowlands, the shift from a large wetland dominated by flooded forests (Várzea) to the modern incised valleys bounded by extensive areas of non-flooded forests (Terra Firme) is considered a key driver of the Amazonian mega-biodiversity. Dating the sedimentary beds covered by Terra Firme forest is crucial to constrain the timing of such landscape change. Here we determined the formation ages of widespread regions of Terra Firme substrates in central Amazonian lowlands combining optically stimulated luminescence (OSL) dating, magnetostratigraphy and palinostratigraphy. Our data suggest a very dynamic fluvial system leading to a recent expansion of Terra Firme forests and retraction of Várzea forests during the late Pleistocene. The shift from an extensively flooded to non-flooded surface resulted from river incision around 45-35 ka, which potentially influenced the distribution of taxa by expanding available habitat, creating new barriers to dispersal, or changing the permeability of previous barriers. Transitions between flooded and non-flooded landscapes may have driven diversification and caused changes to species distributions, contributing to the high species diversity and biogeographic patterns currently found in the region. The late Quaternary evolution of Terra Firme substrates reveals that physical landscape changes play a major role in shaping biotic habitats even in the 104–105 years time-scales.