In 2017, geologist Roberto Ventura Santos, of the University of Brasília (UnB), spent four days with his team in a plastic pool assembled at a hotel in Puerto Maldonado, an eastern Peruvian city of 40,000 inhabitants.
Assisted by hotel staff, they emptied sacks of earth collected from up to 80 meters (m) deep on the banks of the Huallaga River, one of the Amazon river’s tributaries. They then filled large screens with the sediments that floated, separating those that might contain a very strong, brown mineral: zircon. Over the following weeks, back at the UnB laboratories, they recorded the chemical element proportions (mainly uranium and lead) and defined the age and origin of the 46 samples of the zirconium silicate, as it is also known.
This work led them to increase to 65 million years the window of knowledge on the geological and environmental history of Amazonia—until their breakthrough, meticulous studies such as theirs went back as far as 20 million years—and contributed to detailing the mechanisms generating the Amazon biome’s biological wealth, estimated at 40,000 species of plants, 2,500 of fish, and 425 of mammals. In an article published in April in the Journal of South American Earth Sciences, Santos’s group highlighted the importance of the Andes mountain range in forming Amazonia’s geological structures, which in turn determined their biological diversity, and added a new variable: seawater, with the organisms it carries, may have also come from the south and not just from the north—the latter was already known—and that must have occurred millions of years later.
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Dating of the zircon indicated that some 60 million years ago when the Andes range was just starting to emerge, water and organisms from the South Atlantic, via the River Plate, may have reached lands now swathed by forest in the Madre de Dios region of Peru, close to the border with Brazilian state Rondônia. According to Santos, pollen found in the sediments in Peru bear out popular thinking on the origin of Amazonia and the Pantanal wetlands region, also thought to have formed as a result of the Andes rising. One of the species identified from the pollen is of the Araucariaceae family, today a typical plant in the cold climates of the country’s South.
Comprising 104 mountains, with an average altitude of 4,000 meters (m), and 8,000 kilometers (km) of extension from northern Colombia to the south of Argentina, the Andes still regulate the function and biodiversity of most of the Amazon basin in two different ways.
Springs in the range, and their continuous erosion, provide water and sediments (earth and sand), which feed the rivers in the west of the Amazon region. The lower volume of water arriving at the mountains may have contributed to the intensive drought this year in the area, normally only attributed to lower rainfall due to the El Niño phenomenon. According to a study published in April 2022 by researchers from Peru and Brazil in Remote Sensing, in response to global warming the area of glaciers in the northern Andes melted from 2,429 km² to 1,409 km² (42%) between 1990 and 2020, thus providing less water to the rivers.
HOORN, C. et al. Annual Review of Earth and Planetary Sciences. 2023
Pink dolphins indicate that seawater was once present in the Amazonian interiorHOORN, C. et al. Annual Review of Earth and Planetary Sciences. 2023“To this day around 80% of the sediments carried by Amazonian rivers to the ocean comes from the Andes,” says geologist Maurício Parra, of the University of São Paulo’s Institute of Geosciences (IGc-USP) and a published author on the subject. “Over 60 million years, sediments carried from the Andes formed layers of decreasing thicknesses, from 2 km deep in the west of Amazonia, and 800 m on the island of Marajó, to the east.”
Furthermore, the Andes influenced the region’s climate by blocking moisture coming off the Atlantic and diverting it into the forest, increasing rainfall levels. “The climate in Peru’s capital Lima is very dry, because the moist air mass does not get there,” remarks Santos.
An immense wetland
“The uplift of the Andes was not continual, but happened in pulses,” observes geologist Michele Andriolli Custódio, of the Federal University of Amazonas (UFAM). According to the researcher, the mountains reorganized the landscape as they grew. When they raised one side of the lands flooded by water from the Caribbean sea, they forced the rivers, which were still outfalling at the base of the Andes, to run toward the east and the Atlantic. In turn, this reversal of river flows brought together areas and populations that had until then lived in isolation, or, conversely, separated those that had been coexisting together, giving rise to the formation of new plant and animal species.
Cristian Dimitrius / INPA
Pollen found in sediments demonstrate vegetation changes (clockwise): Crototricolpites annemariae, Grimsdalea magnaclavata, Malvacipolloides, Rhoipites irregularisCristian Dimitrius / INPA“As the current catchment was only formed between 10 million and 9 million years ago, most of the time the rivers ran west—the reverse of what we know today,” comments biologist Carlos D’Apolito, of the Federal University of Acre (UFAC), coauthor of a study into ancient catchments in the region published in Sedimentary Geology in July.
“The Andes created a vast, low-lying space in western Amazonia, which became a wetland of continental dimensions,” he adds. “There were alligators, turtles and fish, all of considerable size. Among the plant life, the buriti trees were certainly the most common, because these grow in flooded areas, of which there were plenty between 20 million and 7 million years ago.” Seashell fossils found today on the banks of the Solimões river and along its tributaries testify to the one-time presence of the sea, which receded and is today more than 1,000 km distant (see Pesquisa FAPESP issue nº 329).
The pink dolphins (Inia geoffrensis), manatees (Tricherchus inunguis), and rays (Potamotrygon spp.) that still thrive in rivers in the region, corroborate the idea that the interior of the forest was once covered by saltwater. Ancient populations of these species may have been trapped when connections with the sea ceased to exist. Over the generations, they adapted to their new environment, coming to differ from their marine relatives.
“Many of the plant and animal species in the Amazon today emerged in the last 5 million years, despite being from very ancient families that were in Amazonia some 60 million years ago,” says botanist Lúcia Lohmann, of USP and the University of California at Berkeley, in the US. According to the researcher, the responses of plants to environmental changes indicate that the Amazon is as much a refuge as it is a cradle of biodiversity.
In 2016, Lohmann coordinated the sampling of 10 plant species growing on the banks of two rivers, Rio Negro and Rio Branco, to the north of the Amazonia state capital Manaus. Genetic analysis has demonstrated that the rivers may have different effects on the formation of new species, known as speciation. The Rio Negro, an older, wider river, favored the genetic differentiation of species that grew on its banks, while the Branco—younger and narrower—did not present any relevant impact on such differentiation among the plants examined, although this may have been a factor in the diversification of bird and primate populations.
ippbio.inpa.gov.br
Buriti trees on the banks of the Juruá river, in the state of Acre: a plant lineage preserved over millions of yearsippbio.inpa.gov.brThere are no simple rules. Rivers as barriers, and altitude or temperature variations, may sustain the formation of new species for certain plant and animal groups, but not for others. “The same factors may trigger different evolutionary processes, impacting the biogeographical and diversification history of Amazon organisms in different ways,” she says.
An analysis of pollens and plant genes demonstrated the deep-seated relationship between geology and biodiversity, evident in the compositions of the forest, which have at times been thicker, and at other times sparser, over the last 23 million years. “The periods over which the Andes uplifted more significantly correspond to periods of greater diversification for several plant groups,” comments Lohmann, one of the coordinators of a summary on the history of Amazon vegetation formation over this period, published in the Annual Review of Earth and Planetary Sciences in May.
According to this study, continuous forest that occupied almost all of South America was divided around 30 million years ago by an area of dry climate, forming the western region of Amazonia and the Atlantic Forest to the east. There were then changes in the structure and composition of the forest: between 23 million and 16 million years ago, Amazonia was home to a wide range of vegetation, from mangroves to dry-land forests, in an estuarine environment where sea and river waters merged.
The pollen found in sediments on the banks of the rivers indicates that at least 48 plant families lived in Amazonia at that time. The number of families grew to 79 between 16 million and 12 million years ago; it subsequently fell to 25 between 12 million and 6 million years ago, giving rise to an open forest, which once again expanded between 5 million and 2 million years ago, reaching a total of 117 families. “Over thousands of years, Amazon vegetation has adapted to much bigger geoclimatic changes than we imagined. Perhaps this is the reason why so many Amazonian species that migrated to the Cerrado (wooded savanna) region, the Atlantic Forest, and Central American forests managed to survive in such diverse environments,” she says. “The Amazon vegetation background teaches us how species adapted to climate change on a scale of millions of years, which is crucial information today.”
But not all is rosy. In a January 2023 article in Science, Lohmann and other researchers from Brazil, the US, and other countries, looked at the impact of 11 types of changes caused by humans, such as urban and agricultural expansion, and 21 natural, such as the elevation of the Andes and the disengagement of South America from Africa. According to this analysis, such anthropic changes are occurring more quickly than the plants’ capacity to adapt to new environments. If this is not checked it could lead, among other effects, to a reduction in the amount of rainfall in Brazilian midwestern and southeastern farming areas.
Projects
1. Tectonostratigraphic evolution of intermontane basins associated with forearc environments using the Preandean Depression in the Central Andes of Chile as a case study (nº 19/13349-8); Grant Mechanism Regular Research Grant; Principal Investigator Mauricio Parra Amézquita (USP); Investment R$117,223.39.
2. The structure and evolution of the Amazonian biota and its environment: An integrative approach (nº 12/50260-6); Grant Mechanism Thematic Project; Principal Investigator Lúcia Garcez Lohmann (USP); Investment R$6,291,421.95.
3. Transamazon Drilling Project: Origin and evolution of the rainforests, climate, and hydrology of the South American tropics (nº 18/23899-2); Grant Mechanism Thematic Project – Program for Research on Global Climate Change; Principal Investigator André Oliveira Sawakuchi (USP); Investment R$766,559.95.
Scientific articles
CAYO, E. Y. T. et al. Mapping three decades of changes in the tropical Andean glaciers using Landsat data processed in the Earth engine. Remote Sensing. Vol. 14, no. 1974, pp. 1–21. Apr. 2022.
CUSTÓDIO, M. A. et al. New stratigraphic and paleoenvironmental constraints on the Paleogene paleogeography of Western Amazonia. Journal of South American Earth Sciences. Vol. 124, 104256. Apr. 2023.
HOORN, C. et al. Neogene history of the Amazonian flora: A perspective based on geological, palynological, and molecular phylogenetic data. Annual Review of Earth and Planetary Sciences. Vol. 51, pp. 419–46. May 31, 2023.
RODRIGUES, M. de A. et al. New insights into the Cretaceous evolution of the Western Amazonian paleodrainage system. Sedimentary Geology. Vol. 453, 106434. July 15, 2023.
ALBERT, J. S. et al. Human impacts outpace natural processes in the Amazon. Science. Vol. 379, no. 6630. Jan. 27, 2023.
