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Paleobotany

The grains of time

Pollen from a crater in the city of São Paulo bears witness to climatic and environmental changes over the last 100,000 years

Beyond Parelheiros, one of the most southern districts of the city of  São Paulo, the houses, shops and scrap metal yards progressively give way to farms with vegetable gardens, pasture, palm trees, pines and a little of the Atlantic Rainforest. It is difficult to perceive that this almost flat land would be the Colonia crater, possibly formed by the impact of a comet or a meteor at least some 3 millions years ago. Its peripheral limits only become evident when one looks carefully to the horizon and notes a ring of hills circling a circular area of 10 square kilometers. The center of the crater is occupied by  marshland covered by ordinary vegetation into which not even the cattle enter.

The cattle avoid it, but the researchers love entering into this swamp. Also called a peat bog, it is formed by a layer that can reach 450 meters in width, with muddy and black sediments that slowly settled down between the edges of the crater since the supposed impact of this celestial body. The peat – an organic material under decomposition whose color, here, varies from a grayish black to a greenish black – mixes itself with pieces of stalks, bits of leaves and thorns, some fruit and grains of pollen. “These sediments can contain registrations of climatic changes over the last 4 million years in the southeast region” says the geologist Cláudio Riccomini, from the Geosciences Institute (IG) of the University of São Paulo (USP). He visited the crater for the first time in 1980 and even today would not hesitate in, once again, getting himself wet up to the waist in this marchland and collecting samples of a treasure that fascinates only scientists. “Geologically speaking” he said, “this crater is unique, since it’s still closed and isolated by its edges.”

Situated on the limits of the urban zone, some 50 kilometers from the city center, the Colonia crater is unique in the southeast region, being one of six in Brazil whose origin still needs to be attested to through more detailed studies. To these can be added a further five that, when confirmed, resulted from the impact of celestial bodies – in Latin America there are 11 and in the whole world 170 already known depressions formed by the impact of objects coming from space. With a diameter of 3.6 kilometers and edges of from 100 to 125 meters in height, the Colonia crater yet again gains prominence by reason of a study done with the 130 types of pollen grain found in a sedimentary column of 7.8 meters removed from the middle of the marchland.

The forest tentacles
In this study, published in the magazine Quaternary Research, researchers from Brazil and France, based on the sequencing of the diversity and abundance of the pollens, concluded how the vegetation had changed, according to the climatic alterations. Throughout the last 100,000 years, the limit that corresponds to the proximate age of the sediments that make up the column, the forest advanced and receded sometimes in a radical manner, gaining or losing space as if it were an octopus opening or closing its tentacles. According to the study’s coordinator, the French paleobotanist Marie-Pierre Ledru, a researcher with the Research Development Institute (IRD in French acronym), and a visiting professor at USP’s Geosciences Institute between 1998 and 2003, during these 100,000 years the Atlantic Rainforest  expanded eight times and retracted twice, in reply to climate, whether it had been hotter and more humid, or colder and drier.

When humidity and temperature had shown themselves to be more favorable towards the reproduction of plants, during one of the inter-glacial periods between 130,000 and 85,000 years ago, the forest showed three cycles of growth. The trees fed themselves comfortably on light and water, forming closed rainforest similar to that found along the São Paulo coast. But a long period of hostile climate followed – the glacial period, which lasted 73,000 years, from 85,000 to 12,000 years ago. The average temperature fell by around 5 Celsius degrees – enough to deregulate the plants – reproductive cycles, which often died out without leaving descendents. Little by little, in the place of the high and dense forest, meadowland vegetation sprung up, open and low, with trees only on the banks of the rivers. In the opinion of Marie-Pierre, probably during this period there were strong winds, capable of knocking over the highest of the most fragile trees.

The forest recomposed itself in moments of a milder climate. According to the analysis of the pollens throughout the sedimentary column, the rainforest expanded between 55,000 and 43,000 years ago and severely retracted between 43,000 and 28,000 years ago. But it again gained ground between 28,000 and 23,000 years only to shrink once again to the point of yet again almost disappearing between 23,000 and 12,000 years ago. During the most recent interglacial period, which began some 12,000 years ago and is going on until today, the trees yet again found themselves with more favorable conditions. The Atlantic rainforest also spread itself out during three moments over the last 12,000 years, reproducing the closed and dense forest, rich in species. Climate and vegetation changes registered in the Colonia crater coincide with those verified in caves, one  in the state of São Paulo and another in the state of Santa Catarina, in which this type of study had been carried out. They also check with the evidence from the ice of Greenland and Antarctica.

A garden of conifers
Each time that the forest shrunk, new species of tree sprung up, while others disappeared. During one of the moments of forest shrinkage, of around 80,000 years ago, trees of the Weinmannia species spread themselves out. One of the current representatives of this species, the Weinmannia paulliniifolia, a tree that grows up to 16 meters in height and is also known as gramimunha or gramoinha, with a bark rich in tannin widely used for curing leather, is normally found on the tops of hills. At the same time, as a consequence of climate change, the representatives of the Myrsine family, formed by almost one thousand species, almost died out and today are found in the tropical regions of the planet. During the colder periods few examples remained, contained in refuges, probably close to the rivers. “These refuges expanded some 15,000 years ago because of the favorable climatic conditions” related professor Marie-Pierre.

According to professor Marie-Pierre, during the last 12,000 years, in the surroundings of the future metropolis, conifers such as the Araucaria and the Podocarpus also grew in abundance. Their descendents, such as the Parana pine (Araucaria angustifolia) and the pinheiro bravo (Podocarpus lambertii), formed more dense populations in the southern states and in the mountainous regions of the Mantiqueira ridge  – currently only punctuating the city of São Paulo. “This phenomenon of the withdrawal of the conifers is very interesting” says Marie-Pierre, “because it’s not due to man’s actions, as we arrived a lot later. It could be the result of the evolutionary history of the ancient conifers, which no longer found favorable climatic conditions in their expansions.”

The crater also attracts researchers because of the uncertainties about its origin. After having eliminated other possibilities such as erosion or a volcano, because of the ground’s geological characteristics, the idea that this depression could be the result of the impact of a celestial body is acceptable. Nevertheless, “scientifically only discarding the other possibilities is not enough” commented the geologist Álvaro Crósta, a professor at the Geology Institute of the State University of Campinas (Unicamp), who has been studying craters for three decades. “This is a fragile point of scientific work” observed the astronomer Oscar Matsuura, a retired professor from USP?s Astronomy, Geophysics and Atmospheric Sciences Institute (IAG). Years before the North American geologist Eugene Shoemaker, one of the greatest specialists on asteroids and comets, examined the data concerning the Colonia crater and commented to his Brazilian colleagues: “I’ve no doubt that one is dealing with the impact of a celestial body. But, you are going to have to prove it.” Shoemaker died in a car accident in 1997 while on holiday in Australia.

This hypothesis will only be proven if the researchers were to be able to find signs of the impact that could be accepted as conclusive, since the object that came from space broke up after having crashed into the surface. It would be necessary to collect rock samples that sustain the layer of sediments and to have the luck to find deformations in minerals such as quartz or tiny bits of metals from the platinum group such as iridium, which only form outside of the earth.

It was the iridium detected in 1980 in 65 million year old rocks in distant points such as Italy and China that suggested to geologists the possible occurrence of a gigantic impact during this era on someplace on the planet. Only in 1991 was it that they found the crater named Chicxulub, submerged in the Gulf of Mexico of some 180 kilometers in diameter. The impact of an asteroid of around 10 kilometers in diameter must have generated a dense cloud of dust that spread throughout the planet, blocked the passage of solar light, made the temperature drop abruptly around 10 Celsius degrees and contributed to the mass extinction of many forms of life up until then, including the dinosaurs.

A dagger of ice
Riccomini believes that the impact of the Colonia crater also formed a cloud of dust and generated a shock wave and heat wave, although must smaller, but wide and sufficiently dense to cause the deaths of animals that lived at a radius of 50 kilometers. He was the one who estimated that the collision could have occurred at a time between 5 million and 3 million years ago – and, from his point of view, might well not have been caused by a rocky object such as a meteorite or an asteroid, but by a comet, which, being made of ice, would not leave any vestiges of evidence. “It would be like a dagger of ice, which disappears after a crime” he compared.

The researchers are a little disturbed, above all because the region is progressively being taken over by dwellings. Around there some 30,000 people are living. It is feared that the disordered occupation disfigures the edges, alters the composition of the peat sediments or makes future digs difficult. The first inhabitants arrived in the region at the end of the 18th century, when the Emperor Don Pedro I authorized the installation of country farms by German colonists – the crater’s name comes from that. The large farms persisted up until two decades ago, when their owners began to sell off their lands, required for a prison inaugurated in 1987, and afterwards for housing. Since 2001 the crater has been included in the Capivari-Monos Environmental Protection Area (APA), but the houses continue advancing upon the hills and onto the original vegetation of the crater.

Nature and culture
“This region has a clear vocation, which could be made use of by way of a thematic park that would attend to the entire city” suggests Matsuura. The park that he talked of explores not only the natural patrimony – the diverse forms of vegetation – and the anthropology: close by there are two villages of the Tupi-Guarani Indians.

There are notable examples of how to conserve and explore these places. In the United States the Barringer family built a geology and astronomy museum close to a 50,000 year old crater in the dessert of Arizona. In Germany, a medieval town, Ries, grew up in the interior of a crater of 25 kilometers in diameter and maintains itself through the income generated by tourism.

In Brazil there are only signs of the desire to explore craters, such as the 30 meter tower build a few years ago for the visitors to appreciate the Vargeão Dome, a 12 kilometer diameter crater in the west of the state of Santa Catarina State. During 2005,  Crósta was once again in the Araguainha Dome, a 40 kilometer diameter crater in Mato Grosso. He participated in the inauguration of a marker installed in the crater’s center, gave lectures in schools and talked to the town mayors and aldermen of Araguainha and Ponte Branca, located in the crater’s interior.

For almost 20 years Crósta crisscrossed through there and he remembers how it was difficult to explain to the residents what he did and what the circular structure cut by the river Araguaia actually was. But he didn’t hesitate in showing his maps and satellite images. Shortly afterwards he managed to prove that Araguainha was not a volcanic structure, as had been thought, but an impact crater – the oldest and largest in South America, some 245 million years old.

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