Rocks found in northeastern Brazil and in African countries indicate that mountains as massive as the present-day Himalayas uplifted 610 million years ago on the supercontinent known as Gondwana. The emergence of this ancient chain of mountains, which spanned some 2,500 kilometers, marked the first time in the Earth’s history that a mountain range of that scale was able to form, and it may have had far more than geological repercussions. The nutrients that washed down from these supermountains are thought to have reached the oceans and set in motion the evolution of multicellular organisms, the first complex living beings.
The long-ago existence of the “Afro-Brazilian Himalayas” was described in a paper published in the journal Nature Communications in October 2014, collaboratively authored by scientists in Brazil, Australia and France. The paper’s first author is Carlos Ganade de Araújo of the Geological Survey of Brazil (SGB) in Rio de Janeiro. Araújo recently completed his PhD at the University of São Paulo (USP) under the guidance of Umberto Cordani, a co-author of the paper, after conducting research at the laboratory headed by Daniela Rubatto of Australian National University, and Renaud Caby of Montpellier 2 University. In addition to the Brazilian samples, the geologists studied rocks found in the West African countries of Togo and Mali.
“The association between the formation of mountains of this type and the emergence of complex life has been proposed in a number of earlier papers,” Araújo says. “The novelty of our work lies in having shown that the emergence of this arc of mountains was synchronous—within the margin of error of the techniques used to measure the timing—with the emergence of the Ediacaran fauna.” [Ediacaran is the name of the geological period during which multicellular life forms first began to flourish].
The chain of mountains is thought to have uplifted when two great blocks of the Earth’s crust collided. On one side was a section of blocks of rock that now form part of Central Africa, the Sahara and the São Francisco River Basin; on the other side was a region of connected blocks that constitute present-day West Africa and the Amazon River Basin. Araújo and Miguel Basei, also of USP, collected samples of these rocks in Forquilha, a municipality near Sobral, in the scrublands of the Brazilian state of Ceará; in a stretch of tropical forest near the town of Lato, in Togo; and in rocky outcrops in the midst of the sands of the Sahara, in Mali. These three sites lie along a deep geological fault known as the Transbrasiliano-Kandi Lineament, which has been studied in depth by the USP team and the group led by Reinhardt Fuck and Márcio Pimentel of the University of Brasília—the result of a collision between blocks of rock that are thought to have formed the Afro-Brazilian Himalayas.
Relatively accurate dating of such ancient minerals is possible only because of the presence of zircons—crystals rich in radioactive chemical elements such as uranium. As each zircon specimen forms, it functions as a kind of time capsule. Once it has been transformed into a solid crystal after the magma cools, the material then contains a certain concentration of uranium which, through the slow and continuous loss of subatomic particles typical of radioactive elements, produces lighter chemical elements, such as thorium and lead, at a known rate. “After the mineral crystallizes, it does not exchange uranium atoms with the external environment. It’s as if the crystalline structure were a closed system,” explains Lêda Maria Fraga, who also works at the SGB and has authored a study that identified some of the world’s oldest zircons (see box on page 52).
The researchers then measure the concentration of the varieties (isotopes) of the chemical element lead that are byproducts of uranium and calculate how long ago the zircon was formed. It was also possible to determine that the Brazilian and African rocks that contain the zircons were formed in very high-pressure environments, at depths of more than 90 kilometers. These rocks, known as eclogites, originate in ultra-high-pressure situations that result from the collision of two tectonic plates—the immense blocks of rock that form the Earth’s crust.
In such a collision, the edge of one plate slides beneath the other, in a process known as subduction. The part that submerges is subjected to extremely high pressures which alter its rocks. Some of these altered rocks later return to the surface—or are exhumed, in geologists’ terms, and this process can create enormous chains of mountains, such as the Himalayas of today and their Ediacaran counterpart.
According to Araújo, this is the other important point of the study: tracing the oldest evidence of a plate tectonics mechanism like the ones we know today. “Although there are indications that the tectonic plates may have been active to some extent since the Archean Eon more than three billion years ago, not until the Ediacaran Period do we see signs of subduction deep enough to cause the continental plates to descend very much, and consequently trigger the formation of mountains as high as the present-day Himalayas.”
This geological phenomenon, possibly unprecedented until then, may have had equally unprecedented consequences for the evolution of life on Earth, the researchers say.
Around the time when the Afro-Brazilian Himalayas formed, life forms emerged on Earth that even today are surrounded by an aura of mystery. Known as the Ediacara biota, they were multicellular organisms whose relationship to the groups of living beings that would later evolve is still rather unclear, although some scientists have identified the precursors of the present-day Cnidarians (jellyfish and corals) among the fossils of creatures that lived during that period.
The Ediacara biota are generally soft-bodied creatures with a discoid appearance similar to stalks of algae. There are also fossilized trails (ichnofossils) left by the animal’s movements that appear to indicate the presence of wormlike creatures that crawled along the ocean floor. Other fossil data indicate that, around that time, there were already organisms with complex embryonic development, which is essential for multicellular organisms to produce tissues that are specialized for different functions, such as muscles or ganglia. These macroscopic life forms were exclusively aquatic. Microorganisms had already colonized Earthly environments, although animals and plants did not begin to leave the oceans until the Cambrian Period 540 million years ago, with the gradual emergence of adaptations for withstanding water loss.
The rise of the mountain range that spanned part of present-day Africa and Brazil may have triggered the evolution of these creatures by inundating the oceans with food. These high mountains are thought to have undergone an unprecedented erosion process that transported the nutrients in the rocks to the ocean. This mineral “banquet” likely led to the proliferation of marine microorganisms that conduct photosynthesis and produce oxygen, thus increasing the availability of that gas in the oceans and the atmosphere. With greater oxygenation, these environments would have been more favorable for sustaining the metabolism of complex living beings.
“We can also compare the effects of the emergence of the mountains in the Ediacaran with what happened after the rise of the Himalayas,” Araújo points out. In fact, that mega-chain of mountains helped shape the topography and climate of Asia by, for example, controlling the annual monsoon rains that fall over the Indian subcontinent and preventing cold Arctic winds from reaching South Asia. The curious thing is that processes of this kind appear to have occurred again some tens of millions of years later, during the so-called Cambrian Explosion, a large-scale evolutionary event that began about 540 million years ago and was even more significant than the genesis of the Ediacara biota. The Cambrian Explosion marks the appearance of nearly all of the large animal groups known today in the fossil record, including the first arthropods (now the largest and most diverse animal populations on the Earth, such as insects and crustaceans), and the first chordates (a group that includes vertebrates, such as humans).
According to Araújo, supermountains that coincide with the explosion of life during the Cambrian Period emerged in eastern Gondwana, when an ocean disappeared from an area that is now Mozambique. “We could look at this event to the east as a continuation of the process that had already been promoting the evolution of complex life since the Ediacaran,” the researcher comments.
Pieces of Hades
Another geological study based on the analysis of zircons found vestiges of a fragment of what may have been the oldest continent on Earth, on the Brazil-Guyana border. This continent is thought to have existed about 4.2 billion years ago—when the Earth was about 300,000 years old—in the Hadean Eon, the first and most turbulent period in the planet’s history.
Until a few decades ago, scientists believed that during that period—named after Hades, the mythological Greek god of the underworld and the dead—the surface of the Earth was dominated by oceans of liquid rock and craters formed by the impact of celestial bodies. But geological records found in the past decade suggest that, in the distant past, there were already mechanisms that could produce continents comparable to those in existence today.
The most recent indication that such primitive continents may have existed is a 4.2 billion-year-old zircon extracted from volcanic rocks collected in Guyana in 2011 by teams from the Geological Survey of Brazil and the Guyana Geology and Mines Commission. “Finding traces of when and how the “first” continental crust or something like it was formed is always a great discovery,” explains Brazilian geologist Lêda Maria Fraga, who coauthored an article discussing the finding in the Brazilian Journal of Geology. “As far as I know, that zircon is the oldest mineral in South America,” she says.
Zircons, composed of three chemical elements (zirconium, oxygen and silicon), are very hard minerals that can survive the transformations undergone by the rocks that form the continents. The ancient zircon from Guyana, for example, remained intact even after the rock that originally contained it melted about two billion years ago, when the part of the planet where Amazonia now exists was dominated by volcanos. Scientists think that one such volcano erupted and brought up from the depths of the Earth molten material in which older zircon crystals floated.
To date, zircon specimens of that age have been found in fewer than ten places in the world. The oldest such specimen—4.4 billion years—was found in Jack Hills, Australia. Minerals from Canada, China and the United States are comparable in age or a little younger than those identified in southern Guyana.
The Guyana zircon studied by Fraga and Serge Nadeau, first author of the paper in the Brazilian Journal of Geology, was dated based on the radioactive decay of the chemical element uranium—the same technique used to determine the age of the zircons from the Afro-Brazilian Himalayas. Looking back to such a distant era, what is surprising is that it has survived the transformations undergone by the continental crust since then.
Geochronological and thermochronological constraints on the high grade rocks related to the neoproterozoic orogenesis in the vicinities of the Transbrasiliano-Kandi Lineament (NE Brazil – NW Africa) (nº 2012/00071-2); Grant mechanism Regular Line of Research Project Award; Principal investigator Umberto Giuseppe Cordani (IGC-USP); Investment R$ 144,331.80 (FAPESP).
GANADE DE ARAUJO, C. E. et al. Ediacaran 2,500-km-long synchonous deep continental subduction in the West Gondwana orogen. Nature Communications. 16 Oct. 2014.
NADEAU, S. et al. Guyana: the lost Hadean crust of South America? Brazilian Journal of Geology. v. 43, p. 601-6. Dec. 2013.