Imprimir

Stone age

Rejuvenated rocks

Layers of the rocks in Central Brazil may be as much as 200 million years older than was thought

FABIO COLOMBINIExposed rocks in Bom Jesus da Lapa in Bahia, and the São Francisco River in the background: ages vary according to the method usedFABIO COLOMBINI

A historian can easily confuse a geologist if he asks the age of the vast layer of sedimentary rocks known as the Bambuí Group, which forms a small area in the states of Goiás and Tocantins a and a good part of Minas Gerais and Bahia. “It’s going to be a good discussion,” says Márcio Pimentel, a geologist from the Federal University of Rio Grande do Sul (UFRGS). Geologists began studying this region 30 years ago, but the age attributed to it is still uncertain: it varies from 740 million to 550 million years, depending on the analysis method used. On-going studies indicate that the age of the rocks may be more recent, but there are still few arguments in favor of this conclusion. The great problem when it comes to defining a more accurate date is that the rocks at the heart of Brazil are sedimentary, i.e. formed by the combination and fusion of fragments of other rocks and terrestrial or marine detritus. Other regions are formed by rocks of volcanic origin, the dating of which is simpler.

Just as interesting as the definition of a probable date is that the debates about the start of the formation of these rocks are revealing something that is rarely seen: the arduous process of constructing scientific truth by argument (generally friendly) between different research groups, who work with different techniques and present arguments that tend to be added to, transformed or eliminated. Each team defends its own position and, even though it has analyzed rocks from different depths in different places, it supposes that the conclusions could be good for this whole vast region. The different sets of sedimentary rocks of the Bambuí Group occupy a region of some 300,000 sq. km. In the east-west direction they stretch from Brasília to the Espinhaço Hills in Minas, at depths up to two kilometers. From north to south they go from Belo Horizonte to the north of Bahia.

Those who move away from the towns find a flat landscape marked by soybean plantations, pasture or a little cerrado [savannah] and caatinga [arid shrubland]. A lot of diamonds and gold have been taken from this land that is rich in caves and split in two by the São Francisco River; now oil and natural gas are being extracted, contradicting the geological precepts that determine that lands of this type should not contain hydrocarbons. Geologists argue that to know the age – or ages – of this region better it is important, among other reasons, to know what other economically valuable minerals could emerge from there.

046-049_Geologia_195

“I’d almost bet my bottom dollar that the Bambuí Group is less than 600 million years old,” says Pimentel. “We always imagined that it was older.” Pimentel and Joseneusa Rodrigues, a geologist who did her doctorate with him and now works in the Companhia de Pesquisa de Recursos Minerais (CPRM) [Mineral Resources Research Company] in Rio de Janeiro, are heading up the teams from UFRGS which is rejuvenating the Bambuí Group, as they determine the age of 25 samples of zircon crystals, a mineral which results from the modification of granite or volcanic rocks and was part of the limestone of the Bambuí Group. Zircon pinched from rocks collected in locations like Montes Claros, in the north of Minas, and the Chapéu Hill, in the central region of Bahia, must have come from the mountains that began to form around 600 million years ago and dominated the landscape of the current Central Brazil region, but were eroded to the point where, today, only modest hills remain. The team from Rio Grande do Sul found zircons up to 550 million years old, which would represent the maximum age of the limestones to which they adhered “Some samples are even younger than 550 million years,” says Pimentel, “but we are still confirming the analyses.”

This possibility brings the Bambuí Group close to the geological period known as the Cambrian, which began 544 million years ago and ended 488 million years ago. This was when, most of the animal groups appeared (previously there had only been microscopic life), possibly because of the accumulation of oxygen in the atmosphere and the oceans. But if the rocks of the Bambuí Group really were as young as the zircon samples indicate, fossils of invertebrates would have been found, which would help define the age of the rocks. “I wonder where the fossils are,” says Pimentel, for whom the only reason fossils have not been seen in the region is because the paleontologists have not carefully excavated there and the geologists keep their eyes peeled for rocks and not fossils.

“Marly Babinski questioned my data and said that the zircons were too young, but she put her team to work there and now she’s also finding more recent things,” says Pimentel. “Little by little, we’re converging on this interpretation.” The pioneering studies of Marly Babinski, from the Institute of Geosciences (IG) at the University of São Paulo (USP), indicated that the Bambuí Group could, inversely, be very old. She roamed the north of Minas and Bahia for the first time in 1989, on a two-week trip with a lot of rain and roads full of holes. Through an analysis of the proportion of extremely rare lead isotopes spread through the carbonates, a technique new at the time, she determined an age for the rocks from the region: the Bambuí Group should be 740 million years old, give or take 22 million years.

GUSTAVO MACEDO DE PAULA-SANTOS / IG-USPPossible fossil in carbonates from the Lagoa Santa region in Minas GeraisGUSTAVO MACEDO DE PAULA-SANTOS / IG-USP

“The conclusions we reached are not exclusive,” Pimentel believes. Therefore, neither his data nor Marly’s are good for the whole region, formed from sub-sets of rocks with their own characteristics. “We managed to extract zircon from the whole region where it had collected, except from the deepest layers.” Ricardo Trindade, a professor from the Institute of Astrophysics, Geophysics and Atmospheric Sciences (IAG) at USP who has been working with Marly for years, is bothered: “If these new data obtained with zircon crystals are correct we’ll have to review all we know and have done.”

Considering the two ages results in consequences on which there is still far from any consensus: either the rock deposit from the deepest to the most superficial layers was very slow or there was a hiatus at the base of the Bambuí of some 200 million years. Many geologists do not feel at ease with these possibilities, which Marly is very relaxed about: “It’s what we’re finding today,” she says. “Tomorrow we may change our opinion when faced with other evidence.” From recent collection trips to Minas and Bahia, her team has brought back rare fossils, in the form of curved strands of hair, whose age is still unknown. She herself, when collecting in 1995 in Moema and Bom Despacho, to the west of Belo Horizonte, had seen strange signs in rocks like the ones they are showing her now, “but no one thought it was important,” she recalls. The problem, she says, is that no one has managed to identify what type of organism these fossils may be and  at what period they must have been formed.

The earth covered with snow
There’s another problem that’s more difficult to solve. If the data are really correct, one of the first glaciations of the planet, around 730 million years ago, was global. “To confirm this,” says Marly, “we need more datings.” In the meanwhile this possibility contradicts the most widely accepted view of the geological history of the earth. Most geologists prefer to believe that a layer of around one kilometer thick of ice covered the whole planet, leaving everything like the Antarctic is now, only in the second great glaciation, which happened around 635 million years ago.

Alexandre Uhlein, a scientist from Rio Grande do Sul State who has been based in Belo Horizonte for almost thirty years, is heading up a team of geologists from the Federal University of Minas Gerais that challenges these two views. Through measurements of strontium isotopes of rocks collected in the municipality of Correntina, in southwest Bahia, and comparisons with equivalent rocks from Namibia, formed when Africa and Brazil were a single block, the team from Minas concluded that both the carbonate layers and the rocks of glacial origin must be around 630 million years old.

Rocks from the ocean floor that are around 740 million years old

MARLY BABINSKI / IG-USPRocks from the ocean floor that are around 740 million years oldMARLY BABINSKI / IG-USP

Therefore, the region must have started forming as the result of an accumulation of ice from this time, which coincides with a great glaciation of the planet, called the Marinoan, and already well characterized on other continents. The ice, carrying the remains of rocks, accumulated on one of the basic structures of the South American continent, the so-called San Francisco craton. When the climate got warmer, the ice melted and the water flowed to lower regions. Part of the material it carried, however, remained and layers of limestone accumulated on it, which are reminiscent of a lasagna.

“It’s improbable that more than 100 million years passed between the formation of the deepest layers and the most superficial ones in the Bambuí Group,” comments Fabrício Caxito, who is doing a doctorate supervised by Uhlein. From March to July 2011, Caxito worked at McGill University in Canada, with Galen Halverson, one of those who defend the Snowball Earth hypothesis, according to which the ice from a global glaciation could have transformed the planet into a huge snowball. He is also one of the authors of a graph on the variation of strontium isotopes on the planet. According to Caxito, Halverson says that the Bambuí Group can only be from the Marinoan glaciation, the only one that was recognized as being global.

“Different methods lead to different results,” Uhlein recognizes. “The analysis of strontium isotopes is a more efficient methodology for dating racks than analyzing lead isotopes and, of course, tells another story, which is the one we currently believe.” In the same way, Marly believes that the strontium isotopes’ technique “does not allow absolute ages to be obtained,” that it would only work for open seas and “does not apply to the Bambuí Group.” She says that this region may have been the bottom of an enclosed sea, which began where Belo Horizonte is today and expanded towards the north.

“Currently, we have more uncertainties than solutions,” says Trindade. “Over the last few decades the most highly qualified geologists in Brazil have arrived at estimates for the ages of the Bambuí Group which came nowhere close to what we’re finding.” As luck would have it, the participants in this debate are friends and seem to relish the confusion: “It’s fun,” says Pimentel. As they analyze more rocks taken from the heart of Brazil, perhaps the geologists will reach a consensus on the most appropriate techniques to be used, or adopt others that may lead to other results, or perhaps they will conclude that this vast region has different histories and different ages. “Who’s wrong at one point may be right at another,” ponders Caxito, in true Minas Gerais enigmatic fashion, “and even those who are right may not be right about everything.”

MARLY BABINSKI / IG-USPYoung researchers from USP and UFMG in a quarry in Lagoa SantaMARLY BABINSKI / IG-USP

Such are the workings of science. “We’re always looking for the truth that we hardly ever find,” says Uhlein. If they wanted the geologists could provoke the historians by asking when the Second World War began. The most probable response will be September 1, 1939, when the Germans invaded Poland. But this is a “European response,” in the view of English historian, Niall Fergusson. He says that the “real response” is July 7, 1937, when Japan invaded China, starting a war that in just a few months mobilized 850,000 soldiers. Fergusson considers other possibilities: the war may have begun even earlier, in 1931, when Japan occupied Manchuria, a Chinese territory, in a bloody episode that left 200,000 people dead, or in 1935, when Mussolini invaded Abyssinia, or even in 1936, when the Germans and Italians helped Franco contain the rebels in the civil war in Spain and were already testing the tactics they would use later against other countries. Perhaps the geologists and historians have more in common than they imagine.

Scientific articles
CAXITO, F. A. et al. Marinoan glaciation in east central Brazil. Precambrian Research. v. 200-203, p. 38-58. 2012.
BABINSKI, M. et al. Direct dating of the Sete Lagoas cap carbonate (Bambuí Group, Brazil) and implications for the Neoproterozoic glacial events. Terra Nova. v. 19, p. 401-06. 2007.

Republish