Geologist Adriana Camejo Aviles, from the State University of Campinas (Unicamp), was impressed with the quantity and variety of plant remains accumulated 14 meters deep in the soil of the Colônia crater.
The site is a circular area with 3.6 kilometers in diameter formed between 30 million and 5 million years ago and currently occupied by shrubs and grasses, houses, vegetable plots, trails and waterfalls in the extreme south of São Paulo, in the neighborhood of Vargem Grande, in Parelheiros district.
His work —along with another, with the discovery of aquatic organisms—recorded signs of life in this region and indicates what was the predominant vegetation in more recent times.
In just 0.5 gram of soil, Aviles found a set of pollens and spores representing 115 plant species, with some groups growing in cold environments, typical of Araucaria forest. Detailed in an article published in the journal Grana in July 2021, the survey gathers 1 algae, 10 fern species, 45 flowering plants and 2 pine trees.
The age of the pollen samples, between 180,000 and 135,000 years, corresponds to the penultimate glaciation, when an ice sheet covered a vast expanse of the planet. Its discovery indicates that a vast araucaria forest (Araucaria angustifolia) must have covered the territory that today corresponds to the city of São Paulo and probably large areas of the state.
It would be a landscape similar to the one found today in the interior of Paraná, Santa Catarina and Rio Grande do Sul and in the higher and colder regions of the states of São Paulo, Rio de Janeiro and Minas Gerais.
“Eighteen species of plants that lived in the crater at that time disappeared from the region. The two pine species persist only in the araucaria forests of the South and Southeast regions”, says Aviles.
A pollen classified in the genus Acaena or Polylepis, from the Rosaceae family, the same as the rose bushes, apple trees and Andean shrubs, represents a highly threatened species with the rise in temperature in recent decades. According to her, the local extinction and the permanence of plants in other regions reflect climate changes over the last 180,000 years.
“The vegetation began to change about 135,000 years ago, when the glacial period ended and the influence of the summer monsoon winds increased, which enter South America from the north and still bring moisture from the Amazon to the Southeast region today” , explains Colombian biologist Paula Rodríguez-Zorro, from the National University of Colombia, author of an article published in Scientific Reports in April 2020 on the changes in climate in the region.
Until the arrival of the monsoons, cold winds from the south passed through the current state of São Paulo throughout the year and favored the growth of plants such as araucaria. With the increase in temperature and humidity, the vegetation thickened and formed the tropical forest known today as the Atlantic Forest.
In the last glaciation, between 70,000 and 14,000 years ago, with the coldest and driest climate, natural fires — as indicated by the coal detected in the samples — opened clearings that formed large fields of altitude.
Because of these changes, araucaria populations suffered continuous reductions until they disappeared completely from the region about 45 thousand years ago, when the monsoons had already made the
warmer and humid climate, favorable to the tropical forest.
algae
“It is exciting to think that these microscopic beings lived 1.5 million years ago”, says botanist Gisele Carolina Marquardt, from Univeritas University (UNG), in Greater São Paulo, when showing the diatom fossils, aquatic organisms collected in the crater, as a result of a postdoctoral internship at the Institute for Environmental Research (IPA).
Situated at the bottom of the food chain and serving as a food source for other organisms, diatoms are single-celled microscopic algae with silica shells that fit neatly, like the lid on a jewelry case. Its composition facilitates preservation in sediments.
“Because environmental changes affect their composition, diversity and abundance, diatoms are excellent records of changes in the characteristics and water level of a place,” says Marquardt.
With her colleagues at the IPA and the Luxembourg Institute of Science and Technology (List), she described two new glacial cold-adapted fossil species, one named Pseudostaurosira crateri after the crater. The two species are detailed in articles published in the journals Fottea and Phytotaxa in April 2021.
“I found about 200 varieties of diatoms in samples of 5 g of soil and we estimate that 30 of them could be new species”, says Marquardt.
The algae fossils were preserved in the deeper layers of the sediment sample that accumulated in the ancient crater lake, now covered by swampy terrain. They were found in 2017 through a 50-meter borehole that removed a cylinder of earth, a type of sample called sedimentary core.
“The humidity of the place helps to preserve the pollen in the sediments, preventing oxygenation, which would lead to decomposition”, comments French paleobotany Marie-Pierre Ledru, from the Research Institute for Development (IRD), co-author of the two articles.
“There are other craters in South America, but they are in a dry environment, in which microfossils oxidize and disappear.”
Dedicated to the region for two decades, she says that the residents of Vargem Grande were excited about diatoms, which she presented in 2021.
“The identification of fossil plants reveals important biological phenomena in the region, which are part of a set of climate changes in Brazil”, emphasizes paleontologist Paulo Eduardo de Oliveira, from the University of São Paulo (USP), who was not involved in the studies.
According to him, the cold climate of about 25,000 years ago also transformed the current Federal District, now occupied by savannah, into a forest with a cold climate, but without the araucarias. In the same way, he adds, the sea level has dropped 100 meters, exposing the continental shelves, which are now covered by water, and allowing the growth of a forest typical of cold and humid regions.
In 2003, the site was listed by the Council for the Defense of Historic Heritage (Condephaat) and listed in the Earth Impact Database, a list of the 190 impact craters (resulting from the collision of a meteor or comet) in the world, 11 of them in South America. .
In 2009, it was recognized as one of the five geological monuments in the state of São Paulo. With the discovery of microorganisms, the ancient life of the crater began to gain clearer outlines.
Last year, residents, researchers and government officials decided to remove three streets leading into the crater. A neighborhood association helped resettle residents who were undocumented.
“Everything happened in an organized way and the three streets were given back to the park and to scientific research”, celebrates Ledru.
origin uncertain
“The crater is the only known place with a continuous record of the natural history of South America over the last 30 to 5 million years”, emphasizes geologist Álvaro Crósta, from Unicamp, who has also studied the region for more than three decades.
There, each centimeter of sediment represents up to hundreds of thousands of years of ancient vegetation and climate records.
He estimated the depth of the sediment layer at about 250 meters, followed by a rock layer at about 150 meters, as detailed in a paper published in 2019 in the journal Meteoritics & Planetary Science.
The origin of the crater remains uncertain. It may have formed after the collision of a celestial body or by some other geological process.
“Any of these hypotheses still needs to be confirmed,” says Crósta. According to him, it will only be possible to clarify how the crater was formed by drilling to its base, about 400 meters below the surface, eight times deeper than what was achieved with the study that led to the discovery of diatoms.
