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The distant past of a great river

Erosion of the Andes may have connected ancient watersheds and formed the Amazon

Northern Brazil’s great river, the world’s largest, now flows eastward

Léo RamosNorthern Brazil’s great river, the world’s largest, now flows eastwardLéo Ramos

The debate about the origin of the Amazon River—the world’s largest, at 7,000 kilometers (km) long and 20 km wide at the city of Manaus—is as extensive as the river is long. After a team from Rio de Janeiro concluded, on the basis of fish fossils, that 2.5 million years ago an early version of the Amazon River was flowing westward and draining into a now-arid part of the Caribbean (see Pesquisa FAPESP Issue No. 216), a São Paulo geophysicist has introduced a possible new explanation for the formation of the river and the Amazon Basin. The new hypothesis suggests that these waters were already flowing eastward at a much earlier time—about 10 million years ago.

According to this new proposal, which was developed by mathematically simulating the evolution of the terrain and sediment deposits in the region, the Amazon River likely assumed its present west-to-east direction of flow not only because of changes in the Earth’s interior that triggered the uplift of western Amazonia—per the traditional approach—but also as a result of surface movements. Increased erosion of the Andean chain caused by weather disturbances is thought to have created the downward slope that extends from the Andes to Marajó Island and along which one-fifth of the world’s fluvial water drains.

“I showed that the dynamics of erosion and sedimentation was probably capable of altering and draining the region,” says geophysicist Victor Sacek, a professor at the University of São Paulo (USP), who explained his hypothesis in an article published online in the July 2014 issue of Earth and Planetary Science Letters. His conclusions concur with those of geologist Paulo Roberto Martini, whose research team established the Amazon as the world’s longest river in 2008 (see Pesquisa FAPESP Issue nº 150). “The rapid uplift of the Andes and the swift erosion caused by the Amazon along the mountain range are monumental,” Martini says. “The amount of sediment that the river transports each month to the ocean exceeds the size of an entire Sugarloaf Mountain.”

Understanding this hypothesis calls for a review of the evolution of the regional landscape. In the early Miocene geological period 24 million years ago, the headwaters of the rivers in northern South America were not located in the Andes as they are today, but rather in much less prominent relief to the west that divided the waters of the region into two different hydrographic basins. To the east of the watershed, the rivers descended towards the present-day mouth of the Amazon. To the west, they flowed in the opposite direction towards the basins at the foot of the Andes and fed into immense lakes and swamps, which formed a wetland 20 times larger than the present-day Pantanal in Mato Grosso State, known as the Pebas system.

054-057_Rio Amazonas_223-01Carina Hoorn, a geologist at the University of Amsterdam, the Netherlands, argues—on the basis of rocks and fossils collected along the banks of rivers—that the basins separated by the Purus Arch began to merge 16 million years ago. In this fashion, the Amazon River and its watershed expanded to their current size over the ensuing six million years, when the slope of the relief in the northern part of the continent caused the water of the lakes between the Andes and the Purus Arch to begin flowing mainly eastward in rivers. The team headed by Petrobras geologist Jorge de Jesus Figueiredo collected and analyzed rock samples in exploratory wells on the ocean floor near the mouth of the Amazon, and their conclusions supported Hoorn’s hypothesis.

Northern South America and the Amazon River also gained their present-day features as a consequence of the interaction of the lithospheric plates, which Sacek has been researching since his doctoral studies, completed in 2011. Along the western coastline of South America, the Nazca plate, which forms the floor of the Pacific Ocean, collides with the South American continental plate. As a result, the oceanic plate, thinner and denser than the continental plate, dips below the continent towards the asthenosphere, a layer in the Earth’s interior so hot that its rocks behave like a thick liquid flowing slowly over the course of thousands of years. “The base of the Andes grows and deepens, as its peaks concurrently rise,” Sacek says. “This is because the lithospheric plates float on the asthenosphere like icebergs float on the ocean, with only a small tip visible beneath the surface. The continental crust is 30 to 40 km thick on average, while the Andes can exceed 70 km, though the height of the mountains is no more than 7 km.”

In addition to lifting the Andes, Sacek observes, the thickening of the crust due to the collision between tectonic plates, has another effect on the relief to the east of the mountain range. The weight of the adjacent thicker crust pulls on the areas at the foot of the Andes, creating the bed of basins onto which the sediments carried by the water from the Andes and from the interior of the continent were deposited during the Miocene. Up to this point, the experts generally concur. “As the Nazca plate dipped further below the South American plate, the Andes and surrounding areas began to float on a deeper and more viscous asthenosphere,” says geophysicist Grace Shephard of the University of Oslo, Norway. In 2010, Shephard and colleagues in the United States presented a reconstitution of the changes in the asthenosphere below the continent, showing that the streams of fluid rock would have had enough force to lift the basins near the Andes and tilt all of the sub-Andean and Amazonian terrain towards the east.

Erosion in the Andes
Taking the opposite approach, Sacek disregarded the effect of the asthenosphere and used the mathematical model he had already adopted during his doctoral studies to simulate the topographic changes caused by the balance between the Andean uplift, the erosion of its rocks by rainfall and rivers, and the transport and deposition of the sediments created by the water—which Shephard had ignored. Since the combined effect of all these processes on a continental scale over the course of millions of years is much too complex to be calculated with realistic details, he had to find a balance between making a very complex, more confusing model, or making a very simplistic one that could not properly represent nature. “It wasn’t easy,” he says.

Since there is still much uncertainty about how quickly the Andes uplifted, how efficiently the erosion wore away its rocks and the water transported its sediments, Sacek tested several numerical values in his simulations. Independently of the numbers he input into the computer, however, the simulations of the geological history of the last 35 million years reproduced the reversal of the Amazon River. As the Andes became a barrier to the moisture carried by winds from the Atlantic, increased rainfall on the eastern flank of the mountain range also increased the amount of sediment transported down from the mountains. At some point, the deposited sediments completely buried the basins alongside the Andes, creating the gentle eastward slope seen in Amazonia today. One of the simulations indicated that the Amazon Basin was formed about 10.5 million years ago, as Hoorn maintains. The mathematical model, however, failed to simulate the evolution of the Pebas system and the lakes and wetlands scattered between the Andes and the Purus Arch, instead indicating periods and locations different from those arrived at by geologists.

Shephard called Sacek’s work “impressive,” but noted that only the model that she and other researchers made, which accounted for the effects of the asthenosphere, is able to correctly represent the Pebas System. “The challenge remains to find a better way to combine the geological phenomena of the Earth’s surface with those that occur in the planet’s interior,” she says. “These processes are not mutually exclusive.” Geologist Dilce Rossetti, a researcher at the National Institute for Space Research, believes that the origin of the Amazon is likely to remain an open question for many years. “The data obtained so far are too sparse and localized to be used to make extrapolations,” she affirms. “The sediment dating figures are not yet totally reliable and the units of different geological periods need to be better mapped.”

Hoorn, the Dutch researcher, doubts that new data will change her own conclusions with regard to the Amazon. “The data from the mouth of the Amazon are very clear, and other records in Suriname and Venezuela confirm the existence of a system of rivers originating in the Andes in the late Miocene,” she says. Rossetti observed that the history could be even more complicated. The Andean sediments in the samples studied by Figueiredo are absent in samples collected higher up in the well, she says. “Neither he nor I can explain why this occurs,” she acknowledges. The Amazon Region continues to be a source of puzzlement, surprise and fascination (see the article on the first photos from the Amazon).

The course of the Amazon River, by Nicolas Sanson: recognized accuracy

National Library The course of the Amazon River, by Nicolas Sanson: recognized accuracyNational Library

Rescued map

The course of the Amazon River, by Nicolas Sanson:  recognized accuracy

A work that was once deemed scarcely worthy of interest, The Course of the Amazon River, drawn in 1656 by French cartographer Nicolas Sanson, has finally been recognized as one of the first scientific maps of the Amazon Region. Jorge Pimentel Cintra, a professor at the Polytechnic School of the University of São Paulo (USP), and one of his students, Rafael Henrique de Oliveira, disregarded the criticism of French geographer Charles-Marie de La Condamine that the map had been prepared “based solely on historical information,” compared the geographical coordinates he used with current ones and acknowledged that the map was accurate to the greatest extent possible for that time.

“Our hypothesis is that this map was a rough draft that Sanson used as the basis for making world atlases,” Cintra says. In a paper published in Acta Amazonica, he and Oliveira list 14 other maps that were made based on the work of Sanson. The map, not published until 1680—13 years after the death of Sanson, France’s royal cartographer—shows villages, mountain ranges and a few Amazon tributaries, displaying an error—commonplace at the time and undetected until 1707—that places the river’s headwaters near Quito, Ecuador. Another mistake, associated with the headwaters error, was to consider the Coca and Napo rivers, now known to be its tributaries, as part of the Amazon.

Cintra and Oliveira regarded Sanson’s map as a little more accurate than another one, the Magni Amazoni fluvii, completed one year earlier in 1655 by French military engineer Blaise François Pagan, the Count of Pagan. Both maps were based on information from an account by Cristobal de Acuña, a Spanish Jesuit priest who came down the river from Ecuador in 1639 in the company of the Portuguese explorer Pedro Teixeira. “I looked at Acuña’s account, published in 1641, redid the calculations and compared the latitudes and longitudes of the two maps,” Cintra says. “At the points where I was in doubt, Sanson and Pagan were too, because the information was unclear.”

Carlos Fioravanti

Tectonic, climatic and erosional evolution in convergent margins: A numerical approach (nº 2011/10400-0); Grant mechanism Post-doctoral research grant; Principal investigator Victor Sazek (IAG-USP); Investment R$ 147,351.39 (FAPESP).

Scientific articles
CINTRA, J. P.; OLIVEIRA, R. H. de. Nicolas Sanson e seu mapa: o curso do rio Amazonas. Acta Amazonica. v. 44, n. 3, p. 353-66. 2014.
HOORN, C. et al. Amazonia through time: andean uplift, climate change, landscape evolution, and biodiversity. Science. v. 330, p. 927-31. 2010.
SACEK, V. Drainage reversal of the Amazon River due to the coupling of surface and lithospheric processes. Earth and Planetary Science Letters. v. 401, p. 301-12. 2014.