The past in grains of sand
Radioactive material allows the geological origin of the beaches on the southern coast of Rio de Janeiro to be tracked
Beaches with pale dappled sand stained with black sand did not always form the southern coast of the state of Rio de Janeiro. They took on their current shape in the last 10 thousand years, through a slow exchange of caresses between the ocean and the continent. In addition, only part of the sand from these beaches came from the seabed. A substantial portion of the sediment that currently forms the Angra dos Reis and Parati coast was brought from far away by the rivers that flow down the Serra do Mar mountain range.
The team of physicist Roberto Meigikos dos Anjos reached this conclusion after analyzing almost 600 samples of sand from a 25-kilometer stretch of beaches that run from the town of Angra dos Reis to the town of Parati, on the so-called Green Coast (Costa Verde) of the state’s shoreline. Carried by rivers such as the Mambucaba, which forms the border between the two municipalities, very old crystalline rock sediments from the Serra do Mar mountain range reach the ocean continuously, where they are tossed by the waves here and there before ending up on the shore.
Meigikos and the physicist Carla Carvalho, a former PhD student of his at the Federal Fluminense University (UFF), only discovered the origin of these sediments after deciphering the history recorded in the chemical composition of the grains of sand. Using a device that measures the concentration of radioactive elements, they calculated the proportion of potassium, thorium and uranium that is naturally found in the sediments.
These elements are unstable and tend to turn into other elements, emitting gamma rays. Through chemical reactions and physical processes, atoms of these elements combine with other more stable ones and are incorporated into the molecular structure of the minerals. Light minerals (quartz and feldspar)are pale in color and rich in radioactive potassium. Heavy minerals, such as monazite and ilmenite, on the other hand, contain more uranium and thorium.
Meigikos and Carla noticed that the darker sediments tended to accumulate to the north of the mouth of the Mambucaba river, in particular at the Histórica and the Goiabeiras beaches, beyond the side of the Algodão island that faces the continent, whereas the paler sediments were more common at other beaches. The same pattern was found 10 kilometers south from there, on the beach of Tarituba, where sediments rich in thorium and uranium were concentrated on strips of sand toward the north, where the São Gonçalo river flows out.
In both the pale and the dark beaches, the thorium and uranium content varies depending on the distance from the water line. There is an explanation for this. “The ocean works like a filter that carries away the lighter sediments while leaving the heavier ones on the beach,” says Meigikos, who is coordinating a project to recreate the geological history of the 2 thousand kilometers of beaches in Brazil’s southeast (see Pesquisa FAPESP issue 138).
While working at Angra and Parati, the researchers not only investigated the composition of the sand and outlined the inverse path of the grains, they also collected sediments from the Mambucaba river up all the way to a place near its source, in Arapeí (São Paulo state), which lies at the top of the Serra do Mar mountain range. Comparing the composition of the sediments, they concluded in an article published this year in the Journal of Environmental Radioactivity, that the sand in the southern part of the state of Rio de Janeiro’s coast could only have come from the crystalline rocks of the Serra do Mar mountain range, formed over 500 million years ago. “The Mambucaba and São Gonçalo rivers are major carriers of the heavy minerals found in the sand on those beaches,” says Meigikos.
The analysis of the levels of radioactive uranium, thorium and sodium, proposed in 2006 by Meigikos as a strategy for tracking the origin of the sediments, has proven to be useful not only for geology, but has also helped archeologists to retell the history of human occupation along the Brazilian coast. Long before the arrival of Europeans in 1500, the descendents of the earliest inhabitants of South America had already crossed savannas and forests and established themselves on the coast of what would later become Brazil. Proof of this are the sambaquis: mounds as much as 30 meters high and 200 meters long, formed by an accumulation of stones, sands, shells and the remains of crustaceans that are generally believed to have been built by humans.
In 1981, the archeologist Lina Kneip, from the National Museum of the Federal University of Rio de Janeiro, dated the sambaqui on Camboinhas beach in the city of Niterói: 7,950 years old. Another archeologist from the same museum, Tania Lima, estimated in 2002, also based on radioactive carbon, that the sambaqui on the island of Algodão in the bay of Angra was 7,860 years old. If the data are correct, these would be among the oldest sambaquis in Brazil.
However, many doubted this. Geological information suggests that at that time the sea was five meters higher than it is today, and that it remained so for 3 thousand years, which would have made living there impossible. After covering 200 kilometers of the Rio de Janeiro state coast and collecting sand from 16 beaches, Meigikos and his team from the UFF Radioecology Laboratory obtained new evidence indicating that both Lina and Tania were right.
The radiometric analysis shows that both at Camboinhas and on the island of Algodão the sea was, indeed, higher, but not so high as to cover the land on which these sambaquis stand. “Comparing thorium and uranium levels gives us some idea of how long the sediments spent under water,” explains Meigikos. “At that time, these areas were swampy at most, creating a propitious environment for occupation.”
However, what survived the waters was unable to withstand humans. The Niterói sambaqui was destroyed by the advance of the city. That on the island of Algodão, protected by the National Institute of Historic and Artistic Heritage, is now right next to a hotel that should never have been built there.
CARVALHO, C. et al. Application of radiometric analysis in the study of provenance and transport processes of Brazilian coastal sediments. Journal of Environmental Radioactivity. v. 102, p. 185-92. Feb. 2011.
ANJOS, R. M. et al. Correlations between radiometric analysis of Quaternary deposits and the chronology of prehistoric settlements from the southeastern Brazilian coast. Journal of Environmental Radioactivity. v. 101, p. 75-81. Jan. 2010.