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Geology

The glaciers turned into backlands

300 million years ago, the landscape of the future northeast of Brazil was dominated by ice

ANTONIO ROCHA CAMPOS / IGC-USPCurituba, in Sergipe: a rut 25 meters long left by an iceberg, sliding over a rock had been before the bottom of a lake or sea ANTONIO ROCHA CAMPOS / IGC-USP

Today, the backlands of the Northeast are marked by the mandacaru cactus, by frequent droughts and by intense heat, but it has not always been like that. Some 300 million years ago, when South America, Africa, Southeast Asia, Australia and Antarctica formed a single supercontinent located close to the South Pole, a vast portion of what is today the northeast of Brazil used to be covered by glaciers, the edges of which would release giant blocks of ice, like the icebergs that one sees today in the environs of Antarctica. In the less inhospitable portions of the terrain, where there was no ice, shrubs and small trees would grow, distant relatives of pines and araucarias, to form a countryside similar to today’s Iceland, now very close to the North Pole.

A team from the Geosciences Institute (IGc) of the University of São Paulo (USP) has managed to reconstitute this scenario and to prove, for the first time, that there was indeed glaciation in the Northeast – viewed before as just a hypothesis awaiting confirmation – based on the analysis of rocks on which the glaciers left scars or grooves, as they slid down to the sea. In search of clues of the ancient ice, in an authentic detective work that started 25 years ago, the researchers found that the relief itself keeps a memory of those times, the end of the era called Paleozoic, when the major part of the continents of today’s Southern Hemisphere were united in an immense bloc, Gondwana, which was found to be covered by ice.

“In those days, more than half the future Brazilian territory was under a glacial climate”, avers geologist Antonio Carlos Rocha Campos, the coordinator of the group that examined the area of some 10,000 square kilometers that comprises the states of Sergipe, Bahia and Alagoas.For some time, the signs of glaciers have been known in the southeastern, southern and central-western regions, particularly in São Paulo and in Paraná, but in the northeast there were merely vestiges of this frozen period.

The most recent and conclusive marks of glaciation in the northeastern territory were discovered in October last year: several ruts and shallow excavations, of up to 40 centimeters in depth and 25 meters long by 3 meters in width. Found in the neighborhood of Santa Brígida, 412 kilometers to the north of Salvador, in Bahia, and close to Nova Canindê de São Francisco and Curituba, in Sergipe, 213 kilometers to the west of the capital, Aracaju, the excavations show the typical characteristics left by the movement of icebergs on the bottom of lakes or shallow seas, in a similar way to the ruts that are seen today on the continental platform of the arctic region of North America.

The marks registered by the blocks of ice were the pieces that were missing to complete the jigsaw puzzle. “No more doubts remain that the glaciation at the end of the Paleozoic age, known as the ice age of Gondwana, reached Brazil too”,explains Rocha Campos, who heads the team made up of researchers from the IGc, the Federal University of Rio Grande do Norte (UFRN), the São Paulo State University (Unesp) and the University of Minnesota, in the United States.If the more consistent proofs of the presence of ice in the Northeast in remote times are recent, the first evidence arose 25 years ago, amidst a series of coincidences.

On an afternoon at the end of the 70s, in the heat of almost 40 degrees, Rocha Campos was traveling towards Igreja Nova, in the interior of Alagoas, 183 kilometers to the west of Maceió, when a shining spot, at the verge of the road, a little before the entry to the town, called his attention. He immediately asked the car to stop and ran to the place. As he was not carrying digging equipment, he borrowed a broom and a hoe and set about clearing a small area, of one meter by two, while the locals thronged around him brought cans of water to help to take the earth from on top of the rock.

In the rectangle he had dug out, Rocha Campos discovered a polished rocky surface, with parallel grooves: they were erosion marks, possibly caused by the passage of some iceberg. On the surface, there was a layer of tillite, a solid, grayish colored rock, made up of grains of different diameters – from the finest (grains of clay) to the thickest (of sand) and even pebbles -, more evidence that the material had a glacial origin.

Formed by the movement of glaciers, which crushed and dragged along fragments of the rocky ground over which they slid, tillite is a sedimentary rock equivalent to till, a sediment that is also chaotically blended that always appears close to present-day glaciers, forming lateral or frontal lines. The conclusion: if it is like that today, it ought to have been so in the past too. “The layer of tillite was more evidence that there had been ice there”, comments the researcher.

Rocha Campos had found the right place, at the right time. “It was five o’clock in the afternoon, the sun was setting, and the rays were hitting the rock at an angle”, the Geologist says. “If I had been passing there at midday, I wouldn’t have found the grooved surface and the tillite. At least, not at that time.” The proofs had been found. The details had yet to be analyzed.Involved in other projects, Rocha Campos had to wait another 20 years before returning to the Northeast. In 2001, he went back to Igreja Nova and found a quarry where tillite was extracted, to be used as gravel for building.

He widened the region that he was to study and found this and other glacial rocks occurring in Santa Brígida, in the interior of Bahia, in Nova Canindê de São Francisco and Curituba, in Sergipe. In these places, in the middle of the scrubland, he also found the presence of ruts and lacerations left by the icebergs that, pushed by wind or aquatic currents, hollowed out the sand from the bottom of a lake or the sea, like plows that imprint their mark on the terrain and accumulate part of the sediments in excrescencies to the sides. At last, the researchers were able to assert, with coherence and consistent proofs, that glaciers had made part of the natural scenario of the region of the Northeast.

According to the geologist, the white covering probably lasted between 15 million and 30 million years in the territory that was to become Brazil, during the transition from the Carboniferous period to the Permian one. This glaciation was one of the longest and most intense phases of refrigeration of the planet that have ever been heard of. The future Brazilian territory was then occupied by more primitive plants, the gymnosperms (with bare seeds and without any fruit), which in millions of years originated today’s conifers and pine trees.

While gymnosperms ought to inhabit the higher regions, in accordance with the scenario that the researchers are beginning to outline, in the lower and humid areas would grow the plants that are relatives of present-day ferns and brackens, from the group of pteridophytes. In the frozen seas, small invertebrates would live – like mollusks, brachiopods and echinoderms, such as the sea lilies -, as well as primitive fishes. Rocha Campos’s own history with the glacial eras that affected Brazil is an old one. Since the end of the 60s, in collaboration with researchers from Brazil and from other countries, the geologist has been investigating – and has now detected – various signs of the remote existence of glaciers in the states of São Paulo, Paraná, Santa Catarina and Rio Grande do Sul.

With these findings, the researcher from USP expanded the indications of another geologist, an American called Orville Derby, who in1888 collected the first records of the ice from Gondwana in the region of the Paraná Basin – a geological terrain that covers the states of Mato Grosso, Mato Grosso do Sul, Goiás, Minas Gerais, São Paulo, the whole region of the South, besides a part of Paraguay and Uruguay. “If the signs of glaciers covered such a vast area, their effects ought to extend to other regions of Brazil”, Rocha Campos thought. In search of answers, the researcher selected four areas for study: Mato Grosso and Rondônia, the south of Amazonas, the northwest of Minas Gerais, and the Bahia-Sergipe-Alagoas circuit.

Ice in Minas Gerais
Because of the original suspicions, caused by the polished rocks discovered on the verge of the road in Igreja Nova, and with the assistance of studies published by other researchers, the team from the IGc not only found traces of glaciation from the end of the Paleozoic Era in the Northeast, but confirmed that in those days there were also glaciers in the northwest of Minas Gerais, in the environs of towns like Santa Fê de Minas and Canabrava, 400 kilometers from Belo Horizonte.

It is believed that these glaciers used to form a mass of ice distinct from the one in the Northeast, and may have extended from where the capital of Minas is today up to the frontier with Bahia and Goiás, where there is a depression known as the Sanfranciscan Basin, in a reference to the São Francisco River. In Mato Grosso and Rondônia, the other areas investigated by the team, the works are still at the initial stage and have to carry on until the end of the year. The data analyzed so far suggests, though, that the glaciation of Gondwana may have extended over these regions.

There are still many uncertainties, as there are regions still to be studied, and the preliminary analyses are limited to studying each region in isolation. The next step in the work is precisely to establish the connections between these different areas and to form a joint view of the era when the future Brazilian territory lived under the cover of ice. But, according to Rocha Campos, it can now be said with certainty that the glaciation of 300 million years ago was due to the closeness to the South Pole, not to tectonic movements that rearranged the land surface and originated mountains that, due to the altitude, sheltered glaciers.

The geologists would say: this is a phenomenon related to latitude, those imaginary horizontal lines that run parallel to the equator. The blocks that used to make up Gondwana did not merely break up, but they also shifted in a northerly direction. Those that nowadays make up the Brazilian territory, today between 10 degrees and 35 degrees latitude south, 300 million years ago were to be found some 30 degrees to the south of their current position, almost 3,300 kilometers closer to the pole.

Other causes
The glaciers that reached the Northeast of Brazil may have come from Gabon, in West Africa, then stuck to Brazil. Those that arrive at the Parana River Basin and Minas Gerais are regarded as extensions of a mass of ice that covered Namibia. According to the researchers, the higher latitude should not have been the only factor to prompt glaciation. Variations in the orbit of the Earth that interfered in the intensity of the sunlight that reached the planet, associated with changes in the atmosphere, may have collaborated.

The gains from this work may be not only scientific, by helping to retell the climatic and geological history of Brazil, but also economic. In the Paraná Basin, it has now been discovered that the rocks with a glacial origin are intercalated with layers of coal and are part of aquifers or reserves of natural gas, and in the Northeast something similar might be occurring.

The Project
Neo-paleozoic Glaciation External to the Parana Basin: Southeast and Northeast of Brazil (nº 00/12125-2); Modality Regular Research Assistance; Coordinator Antonio Carlos Rocha Campos; Investment
R$ 103,856.25

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