Climatic influence on late Quaternary entrenchment of Amazon rivers
Abstract
The rise of the Andes fundamentally transformed South American surface drainage by establishing the east-flowing transcontinental Amazon River. Subsequent interactions between climate and tectonics have combined to shape the landscape of the continent in a number of ways, driven by Andean sediment supply, late Cenozoic climate change, and global sea-level cycles. Untangling the roles of these various factors is difficult, yet can reveal the dominant controls on Earth's largest rivers and their depositional systems. The Amazon basin features widespread incision of modern river channels by tens of meters into recent fluvial deposits which juxtaposes perennially unflooded uplands with active floodplains, a characteristic that may have influenced the development of pre-Colombian agriculture and settlement in the region. The switch from aggradation to incision may have been driven by a number of factors. We use a case study of the upper Xingu River, a southern Amazon tributary, to isolate the influence of climate on Amazon rivers. Like much of the main-stem Amazon, the Xingu features a dissected surface of recent fluvial deposits. However, this system is located in a cratonic basin that does not receive Andean sediment, and it is buffered from sea-level change by downstream bedrock rapids and waterfalls. Thus transitions between aggradation and incision are likely due largely to changes in precipitation, which are recorded by paleoclimate proxies on Milankovitch timescales. We use the modern topography of the upper Xingu basin to reconstruct the original depositional surface and map spatial patterns of fluvial incision. We supplement our topographic analysis with forward modeling of river profile evolution under a changing discharge regime, which allows us to assess the magnitude of aggradation and incision, the timescales needed to create the observed relief, and the evolution of the system over repeated glacial cycles. We apply our results to the wider Amazon basin to explain the potential relative contribution of hydrologic changes to the observed dissection of past fluvial deposits.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMEP23A..04G
- Keywords:
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- 1815 Erosion;
- HYDROLOGY;
- 1824 Geomorphology: general;
- HYDROLOGY;
- 8175 Tectonics and landscape evolution;
- TECTONOPHYSICS;
- 8177 Tectonics and climatic interactions;
- TECTONOPHYSICS