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All the newsScientific discovery: the Amazon basin was very dynamic during the climatic cycles of the Quaternary period
The Amazon is a giant. Occupying 40% of the surface area of South America, it is the largest river basin in the world, composed of a hydrographic web of more than 1,000 rivers that feed the Amazon itself and increase its discharge, making it the most powerful of all rivers. The humidity emanating from it favours the expansion of a tropical forest rich in biodiversity, and the power to model landscapes.
Despite its exceptional characteristics, this river has proven to be highly reactive in comparison to other river systems. A recent study concluded that it was highly dynamic during the glacial cycles of the Quaternary period, which led to considerable fluctuations of discharges and sediment flows from the erosion of the Andes mountain range. The evolution of the latter has had a substantial impact on that of the Amazon basin.
It is known, for example, that the Amazon basin was occupied during its history by shallow inland seas formed when the levels of tectonic activity and sediment influxes from the Andes were low. These marine environments progressively vanished, allowing sediment to fil the basin and the landscape to become continental.
The transformation of the Amazon basin in a several dozen thousand years
Glacial cycles (or periods of a 100,000 years) have dominated the recent climate of Earth though scientists still do not understand whether these cycles affect rivers. Indeed, for many large rivers, the process of erosion and the incision of river beds exceeds these cycles and therefore does not react to these rapid changes of climate.
The Amazon provides certain answers. The young and vast alluvial terraces[1] show that rapid geological changes have taken place, potentially linked to changes in runoff during the climatic cycles of the Quaternary period.
To verify this hypothesis of causality, scientists from Massachusetts Institute of Technology used a combination of theoretical analyses, empirical measurements and topographical analyses to estimate the speed at which a river profile reacts to changes in discharge and sediments deposits. They then compared these models with the river terraces observed and with the age of burial of the sediments. The results obtained are surprising: the Amazon basin could have evolved from erosion to formation in only several dozen thousand years, a much shorter duration than the 100,000-year period of glacial cycles and very fast for such a huge river.
What will be the reaction to climate change in the future?
This new discovery confirms that the evolution of the fluvial dynamics of the Amazon has been affected by changes in the glacial climate and the tectonic movements. It reveals that these changes occurred at a relatively fast pace. According to the authors, this “high sensitivity” is explained by the considerable sediment load in the river basin, the narrowness of the flood plains and the shallow slopes.
This discovery regarding the past raises questions about the Amazon’s future: what impact will climate change, this time generated by anthropic activities, have on river ecosystems? At the moment, Brazil, which has the largest quantity of freshwater in the world, is already faced with recurrent water crises (see our article on the Parana published in July 2021).
[1] Ancient banks formed when the river incised downwards under the effect of successive massive deposits of sediment followed by a period of erosion.