A gradual change in climate—from dry to moist—and in rainfall patterns, with more intense and frequent rains falling in the inland regions of South America during the past 6,000 years, is believed to have altered the influence and the discharge of sediments from the torrential Rio de la Plata into the Atlantic Ocean. Recent studies by oceanographers and geologists from Brazil, Germany, and Uruguay indicate that the fine grains of sand, mud, and organic material carried by the river and later caught up in ocean currents began to arrive in this era—and are still arriving—as far as the ocean floor near the island of São Sebastião on the coast of the state of São Paulo, 2,000 km from Montevideo, capital of Uruguay and one of the last cities bathed by the river before its waters mix with those of the sea.
For more than ten years, the team headed by Michel Mahiques, professor at the Oceanographic Institute of the University of São Paulo (USP), has been analyzing samples of sediments taken from the ocean floor along the Brazilian coast and from the estuary of the Rio de la Plata in order to confirm that theory. Now the results obtained from the physical and chemical analyses of the sediments indicate the existence of two distinct regions, known as geochemical provinces: one to the south and one to the north of São Sebastião.
The physical and chemical characteristics of the sediments south of São Sebastião closely resemble those recorded in the Rio de la Plata and differ from the samples obtained between the island of São Sebastião and Cabo Frio, on the coast of Rio de Janeiro State. Mahiques and his colleagues have concluded that it is likely that the Doce and Paraíba do Sul Rivers are the two principal sources contributing to the deposition of sediments north of São Sebastião, while the Rio de la Plata supplies sediments to the southern coast and part of Brazil’s Southeast, up to São Sebastião.
Even though the Rio de la Plata is situated more than a thousand kilometers from São Paulo, it may be important for marine management to learn about its possible influence on the São Paulo coast. “Any environmental impact that occurs in the Rio de la Plata basin,” Mahiques observes, “will certainly have an impact on São Paulo’s coastline.” He is referring to the potential for currents to carry radioactive elements of manmade origin, which have already been measured. The same reasoning is valid, he says, in the event of some event that might interfere with marine life resources in southern and southeastern Brazil.
To differentiate the types of sediment detected, oceanographers and geologists employed several laboratory analyses. For example, they measured the amounts of clay, organic materials, cesium, neodymium, and lead found in the marine sediments, as well as the size of the sediments themselves. In this case, samples were collected at three sites: off the coast of Santos and adjacent cities; near the city of Cananeia on the southern coast of São Paulo; and along Itajaí, in the state of Santa Catarina. The sand or mud from the upper portion of the collected samples represents the most recent sediments, because they were the last to be deposited, while the material from the seabed is very old, because it arrived earlier at the sampled sites.
Witnesses of the River
Each particle of sediment bears a sort of geochemical signature, as the researchers like to say. In this case, sets of matching signatures form a single group. When those groupings are laid out on a map, the two major provinces appear, divided in geographical terms along the northern coast of São Paulo State. The geochemical map that resulted from the assessments demonstrated the differences and, therefore, the differing origins of the sediments.
“The sources of the grains, i.e., the rocks that were worn away by rain or wind in order to create the sediments that were later carried down the river, are different,” Mahiques says. To confirm the results obtained from samples of the sediments taken at the surface, the research groups consulted what are known as geological witnesses that can generate data from thousands of years ago, in this case columns of sand and mud removed from the sea floor using a metal pipe.
Ana Paula Campos“You can see a remarkable change from the deposition pattern of sediments that arrived about 2,800 years ago,” Mahiques says. The analysis of the witnesses indicated that while the Rio de la Plata was discharging a heavy load of sediments along the Santa Catarina coast up until 2,800 years ago, it was only after that period that sand generated by the Paraná River basin, in the form of sediments from basalts that are typical of that region, began to appear up near the city of Santos. “Because the Paraná basin had begun to receive more rain, the system of marine currents had more water, which it could use to carry the sediment further north,” he says. “And so we have been able to determine the point at which the Rio de la Plata started to influence the coast of São Paulo.”
Having answered one question, others arise. If a lot of mud is exported by the Rio de la Plata to São Paulo in areas close to the coast, could that material not also reach the deep ocean, down to a depth of 3,000 meters? The researchers hope to find the answer in the coming years, now that they have the valuable assistance of the oceanographic research vessel Alpha-Crucis. They began to use it this year to collect sediments from areas that are deeper and lie farther off the São Paulo coast. (See Pesquisa FAPESP Issue No.206).
Preliminary surveys conducted by using equipment from the Alpha-Crucis identified a structure that resembles a huge slope over which mud and the sand from the flatter region, the continental shelf, could reach deeper areas. The slope would help explain why sediments have accumulated beyond the flatter regions of the São Paulo coastline, which are known to be poor in terms of sediments. Mud and sand have apparently slid hundreds of meters through large underground canyons, scientists believe.
1. Halocene changes in the paleo-productivity of the southeastern continental shelf of Brazil (03/10740-0) Grant mechanism Regular line of Research Project Award. Coord. Michel Michaelovitch de Mahiques – IO/USP: Investment R$321,619.03 (FAPESP).
2. Increase of the research capabilities in oceanography and related sciences in São Paulo State, Brazil; (10/06147-5) Grant mechanism Research Program on Global Climate Change Coord. Michel Michaelovitch de Mahiques – IO/USP: Investment R$15,451,697.60 (FAPESP).
BURONE L. et al. A multiproxy study between the Rio de la Plata and the adjacent Southwestern Atlantic inner shelf to assess the sediment footprint of river vs. marine influence. Continental Shelf Research v. 55, pp. 41-54. 2013.