Climate changes and the formation of fluvial terraces in central Amazonia inferred from landscape evolution modeling

[摘要] Climate changes have been considered an essential factor controlling the shaping of the recent alluvial landscapes in central Amazonia, with implications for explaining the biogeographic patterns in the region. This landscape is characterized by wide floodplains and various terrace levels at different elevations. A set of older terraces with ages between 50 and >200  ka occupy the higher portions of central Amazonia, whereas multiple terraces next to floodplains occur at lower elevations and display ages of a few thousand years. These lower terraces, referred to as middle–lower terraces, reveal what can be perceived as a stochastic pattern both in space and time. Despite the widespread occurrence of these geomorphic features, no process-oriented analysis has been conducted to explain their formation. Here, we develop a landscape evolution model referred to as SPASE (Sedimentary Processes and Alluvial Systems Evolution) to explicitly account for fluvial erosion and deposition in combination with lateral channel migration to explore the controls on terrace development. The model results show that the higher terraces were deposited under the condition of a higher base level for the basins upstream of the confluence between the Solimões and Negro rivers. The subsequent decrease in the base level initiated a phase of gradual incision, thereby resulting in the current fluvial configuration.The model also predicts that high-frequency climate changes resulted in theconstruction of middle–lower terraces at various elevations which, however, areall situated at lower elevation than the higher terrace levels. Our modelshows that dry-to-wet shifts in climate, in relation to the modern situation, yield a landscape architecture where middle–lower terrace levels are better preserved than wet-to-dry changes in climate, again if the current situation is considered as reference. Finally, our results show that fast and widespread landscape changes possibly occurred in response tohigh-frequency climate changes in central Amazonia, at least since the LatePleistocene, with great implications for the distribution and connectivityof different biotic environments in the region. Because of this short timescale of response to external perturbations, we suggest that the streams incentral Amazonia possibly also respond in rapid and sensitive ways to humanperturbations.

[学科分类] 土壤学