Abstract

Climate changes have been considered as 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 50’000 and > 200’000 yrs occupy the higher portions of central Amanzonia, whereas multiple terraces next to floodplains occur at lower elevations and display ages of a few thousands of years. These lower terraces, referred to as mid-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 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 yielded in the construction of mid-lower terraces at various elevations, which however, are all situated at lower elevation than the higher terrace levels. Our model shows that dry-to-wet shifts in climate, in relation to the modern situation, yield a landscape architecture where mid-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 to high-frequency climate changes in central Amazonia, at least since the Late Pleistocene, with great implications for the distribution and connectivity of different biotic environments in the region. Because of this short time scale of response to external perturbations, we suggest that the streams in Central Amazonia possibly also respond in rapid and sensitive ways to human perturbations.

Highlights

  • The lowlands of the central Amazonia host one of the largest systems of alluvial deposits on Earth, composed of extensive Quaternary fluvial terraces with wide incised40 valleys where modern rivers flow (Fig. 1) (Rossetti et al, 2005; Pupim et al, 2019; Sioli, 1984)

  • Our results show that fast and widespread landscape changes possibly occurred in response to high-frequency climate changes in central Amazonia, at least since the Late Pleistocene, with great implications for the distribution and connectivity of different biotic environments in the region

  • According to the scenarios described the current configuration of the terraces and floodplains in central Amazonia could be explained by aggradational and incisional processes controlled by high-frequency climatic variations and local base level changes, which occurred at a lower frequency

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Summary

Introduction

40 valleys where modern rivers flow (Fig. 1) (Rossetti et al, 2005; Pupim et al, 2019; Sioli, 1984) These deposits are of great relevance for the reconstruction of both the evolution of the physical landscape and the patterns of biotic diversity in the region (Ribas et al, 2012; 2019; Pupim et al, 2019, Bicudo et al, 2019). The areas that currently display terraces were part of a large depositional system in which rivers in this region had high deposition and 45 avulsion rates in an aggradational pattern (Hoorn et al, 2010; Pupim et al, 2019; Wilkinson et al, 2010) During this stage, flooded environments covered much larger areas, because they were not restricted to incised valleys as is currently the case.

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