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Geology

Where does the sand on our beaches come from?

Radioactive chemical elements tell the story of the formation of Brazil's south eastern beaches

ROBERTO MEIGIKOS DOS ANJOSChemical composition indicates the origin of the sand in Tabatinga and Ubu, in the state of Espírito SantoROBERTO MEIGIKOS DOS ANJOS

Anyone who sees a huge pile of sand in front of a building that is under construction can hardly imagine that it hides secrets from a time when the beaches began to be formed by sediment that was dragged back and forth at the mercy of fluctuations in  the sea level.  It is the composition of sand that tells the tale and when this happened, as physicist Roberto Meigikos dos Anjos, from the Fluminense Federal University (UFF)  discovered. Meigikos, who comes from Sao Paulo and is a graduate of the University of São Paulo (USP), exchanged the laboratories of the Sao Paulo state capital for the beaches of Rio de Janeiro. In Niterói, his work began with the measurement of the natural radioactivity of sand from beaches and an assessment of the risks in using this sand in civil construction. More recently he and his team started writing a type of history of the formation of the Brazilian coast.

Some parts of the north coast of Rio de Janeiro State have a concentration of radioactive chemical elements that may expose the population to a natural dose of radiation that is three to five times greater than the worldwide average, an effect which researchers normally call an anomaly. Occasional contact with this radiation is not harmful for those who just go to the beach now and again, but if this sand is used in large quantities for building a house, for example, it may cause health problems for those who live in it, because the people are permanently exposed to the radiation emitted by the elements enclosed in the walls. Intrigued with the level of radiation detected in sand taken from beaches like Guaxindiba, in the municipality of São Francisco de Itabapoana, Meigikos decided to analyze other places along the coast. On various trips, often using his own automobile, he and his students collected samples of sand from 50 beaches on a stretch of coast that goes from the north of Espírito Santo to the south of São Paulo. The target, therefore, was no longer anomalies per se, but to discover the origin of this sediment and the mechanisms that carried it there.

Studying the correlation between the radioactive chemical elements, thorium, uranium and potassium,  researchers can trace the mineralogical properties of beach sand, estimate the type of rock formation from which it originated and say if this sediment reached there by wind or river action or if it was carried there by sea water. It also allows them to assess whether the sediment that is today deposited on the shore had remained for a long time on  the land  or if it was submerged in deep or shallow water. This is relevant information, since in the Brazilian case sea level fluctuations were important to shape the coastal plains. The beaches began to form during the last 18,000 years (during the Quaternary geological period) and are still undergoing transformation. During this period there have been drastic variations in the sea level, which sometimes exposed large areas of the continental shelf and at others left them submerged. “This rise and fall of the ocean works like a filter, reprocessing the sediment that gives rise to the sand on the beaches”, says Meigikos.

Generally speaking, sand contains light minerals that are spread throughout the surface water and heavy minerals that are concentrated on the bottom of the ocean. But on some of our beaches waves and marine currents have brought together the heaviest (and most interesting, economically) minerals, such as ilmenite and rutile that are used in the production of pigments, zircon, which is used in the steel industry, and monazite, used for making catalysts. These heavier minerals contain high concentrations of thorium and uranium, while the lighter ones, like quartz and feldspar have high levels of potassium.

These minerals are all mixed up on the beach. The color of the sand usually helps identify them; the darkest sand, with a shade between red and black, indicates a greater presence of heavy elements, while sand that is light in color represents the lighter mineral elements – except it is not so easy to say which came from where. This is where Meigikos’ radiometry technique comes into its own, because the identification of radioactive elements helps determine from which type of rock this sediment originated.

After analyzing the sand from 50 beaches, the group from UFF calculated the ratios of the concentrations of thorium and uranium and thorium and potassium in the samples. The first figure helps establish the main means of transport and the time the sediment spent under water. This is because a part of the uranium undergoes oxidization and takes on a more soluble form in contact with air, and therefore the sediment that is exposed for more time to the air has a lower uranium concentration, while thorium is very stable.

As a consequence of the different behaviors of these elements, if the division of thorium by uranium results in a high number, this is a sign that the uranium spent a long time out of the water and underwent an intense oxidization process. Meigikos evaluates this relationship using a scale that goes from 0 to 7. When the result is greater than 7 this means that the sediment spent a lot of time out of water, in other words, the uranium oxidized a lot. Between 2 and 7 means that it spent a long time in shallow water environments, such as rivers or lakes. A result below 2 indicates that the sediment spent most of the time in deep water where the level of oxygenation is lower.

The relationship between thorium and potassium, in turn, allows another part of the story to be told. Almost all the sediment that forms the sand on beaches comes from the decomposition and erosion of rocks over thousands of years. The enigma, however, is to find out how it arrived on the beach. It may have been carried by wind and deposited directly on the beach or washed down rivers to the sea, where it spent some time being dragged from one side to another until it ended up on the beach. The researchers noticed that if the sand has a large amount of potassium this sediment probably came directly from the rock to the shore. But if the amount of potassium is low it went through various stages that led to the decomposition of this chemical element.

Narrower beaches, like those in the region between Caraguatatuba, on the north coast of São Paulo, and Angra dos Reis, to the south of Rio de Janeiro, have a level of potassium that is comparable to that of granitic rocks. For Meigikos, this is a sign that the sand in this region came mainly from the Serra do Mar[coastal range of hills], a chain of very old granite rocks that was formed more than 500 million years ago, and was carried to the coast by the wind. But there are exceptions. In Caraguatatuba and Ubatuba the sand was carried down rivers and spent a long time submerged in deep water before being deposited on the beaches.

In areas where the stretch of sand is wider, which is common to the north of Rio and in Espírito Santo, the potassium level is considerably lower. The explanation is that the sand there came from vast sedimentary deposits that accumulated between 65 million and 2 million years ago some kilometers inland from the coast. Rivers, like the Paraíba do Sul and the Doce, carry this sediment to the ocean where it remains for long periods before reaching the beaches.

In addition to clarifying aspects of geological history,  an understanding of how beaches were formed may also resolve doubts about how the coast was occupied by the first Brazilians. Understanding the variation of sea levels may help in understanding the conditions that facilitated or made it difficult for humans to settle on the coast long before the Europeans arrived.

The main archaeological records of the presence of people in the region are the “sambaquis”, mounds up to 30 meters high, formed from shells and sand or earth and constructed all along the coast by the first nomadic people to live in the place. Dating the “sambaquis” indicates that the region was occupied a maximum of 6000 years ago, the date most accepted by archaeologists and anthropologists. But recent studies suggest that “sambaquis” may be as much as 8000 years old.

Critics of this work allege that this date is improbable because, in theory, the first humans to inhabit the coast did not arrive that long ago. They also allege that the place where the supposedly oldest “sambaqui” was found, on the Camboinhas beach in Niterói, was completely under water 8000 years ago. Some date studies indicate that the spit of land on which this mound of shells is to be found is only 5000 years old. Other date studies done on turf found at the bottom of the lagoon indicate that there was fresh water there a long time before, which makes the presence of humans possible. The use of Meigikos’ radiometry technique may bring an end to this impasse.

In partnership with archaeologist, Tania Andrade Lima, from the National Museum of the Federal University of Rio de Janeiro (UFRJ), who identified the Algadão “sambaqui” in Angra dos Reis, the physicist hopes to be able to say whether Camboinhas was, indeed, under water. “We’re going to use the correlation between thorium and uranium. If the result is between 2 and 7 or greater than 7, it’s possible that there were people living there at that period”, he explains. If the sediment was in deep water it is impossible for the place to have been occupied by human beings, because it is believed that these groups were formed by people that did not go off hunting and depended basically on fish and shellfish. For this reason it is probable that they preferred to stay close to shallow water that made it easier for them to collect their food. “Apparently these two places, identified as the oldest, were exceptionally favorable for human occupation”, says Tania.

Meigikos expects not only to help Tânia resolve this impasse about Camboinhas, but also, in partnership with archaeologist Ângela Buarque from UFRJ, to explain why some regions on the Rio de Janeiro coast have no “sambaquis”. “The Lagos region is one of the richest in these shell mounds, which are common in Búzios, Cabo Frio, Arraial do Cabo and Saquarema. But there’s not one “sambaqui” in Araruama”, says Meigikos. “Academics have always asked themselves why. My hunch is that this region, for some specific  condition of nature, remained very much above sea-level, which we intend to answer after analysis of the  level of uranium in the sand that comes from there.”

In the final instance this study may strengthen the idea that humans first arrived in the Americas much earlier than has been imagined. The discovery of the Luzia fossil in Minas Gerais, by the group from USP working with anthropologist, Walter Neves, has already pushed the occupation of the inland region of Brazil back to 11,500 years ago, almost 3000 years earlier than had been thought. If the data from Camboinhas and Algodão are confirmed it is probable that the coast was inhabited more than 6000 years ago.

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