Scientists are studying sediments to reconstitute the past of the continental shelf
December 17, 2014
By Elton Alisson
Agência FAPESP – Brazil’s southern and southeastern coast is bordered by a continental platform – a portion of the continent that extends over the ocean – measuring approximately 1,155 million square kilometers. Its width ranges from 200 kilometers in the region of Santos, along the coast of the state of São Paulo, to 90 kilometers in Cabo Frio, along the coast of the state of Rio de Janeiro.
A group of researchers from the Oceanographic Institute of the University of São Paulo (IO-USP) is studying the sediments found in this ocean region to identify the oceanographic processes that occurred in this crust, known as the south-southeastern Brazilian continental shelf.
Conducted with FAPESP funding and using the oceanographic research vessel Alpha-Crucis, which was acquired by the Foundation in 2012 for the IO-USP, some of the findings are described in a chapter of a book about the world’s continental shelves published in early November of 2014 by the U.K.’s Geological Society.
“With the sediments collected during scientific cruises of the Alpha-Crucis, we were able to make a number of inferences about ocean circulation in the past with regard to the south-southeastern Brazilian continental shelf,” said Michael Michaelovitch de Mahiques, a professor at the IO-USP and study coordinator, in comments to Agência FAPESP.
“In that way, we were, in a manner of speaking, able to reconstitute the climate and environment of this ocean region for the past few thousand years,” he said.
The sediments collected in Santos and Cananeia on the southern coast of the state of São Paulo and in Itajaí in the state of Santa Catarina, are fine grains of sand, in addition to mud and organic material deposited at the bottom of the sea.
On cruises made in 2013, the IO-USP researchers collected sediments at depths of more than 1,000 meters – an impossible task in the past due to the lack of Brazilian equipment and suitable vessels.
“The Alpha-Crucis has allowed us to conduct research better than ever before. By utilizing this vessel, we are able to spend more time at sea, get to places that are farther away, and perform ongoing surveys,” Mahiques said.
In February 2013, one of these cruises traveled nearly 2,000 kilometers from the south-southeastern continental shelf where they collected hundreds of samples of sediments in waters at depths of as great as 1,400 meters.
Advance of the plume
Based on chemical and physical analyses of the samples, researchers discovered, among other things, that the water plume from the Rio de la Plata, which carries sediments from the estuary formed by the Paraná and Uruguay Rivers, reached the coast of the state of São Paulo 3,000 years ago, as far as the island of São Sebastião (read more at http://revistapesquisa.fapesp.br/en/2014/03/14/mud-south/?).
Up to 4,500 years ago, the plume stretched over a narrow band along the coast of Uruguay and southern Brazil and reached only to the north of the Island of Florianópolis in the state of Santa Catarina.
One hypothesis to explain the advance of the plume would be the gradual change in the climate – from dry to moist – and in rainfall patterns, with more intense and frequent rains falling on the inland regions of South America during the past 3,000 years.
Because the Paraná basin had begun to receive more rain, there was more water in the system of marine currents from the south-southeastern Brazilian continental shelf to carry the sediment further north, over 1,200 kilometers away.
In addition, there would also have been in increase in the strength and frequency of winds. “The main forcing element that carries this plume to the South and Southeast regions of Brazil is wind,” Mahiques said. “Associated with the increased discharge of water from the Rio de la Plata, South America’s second largest drainage basin, winds contribute to the arrival of sediments from the Rio de la Plata on the coast of the state of São Paulo,” he said.
The researchers also determined that the physical and chemical characteristics of the sediments south of São Sebastião resemble those found in the Rio de la Plata and differ from the samples collected between the island of São Sebastião and Cabo Frio on the coast of Rio de Janeiro State.
One hypothesis that explains this difference is that the Doce and Paraíba do Sul Rivers are the principal sources contributing to the deposition of sediments north of São Sebastião, whereas the Rio de la Plata supplies sediments to the southern coast and part of Brazil’s Southeast up to São Sebastião.
“Understanding these past sedimentary processes allows us to evaluate the coastal regions that potentially favor the accumulation of sediments produced by natural processes as well as by anthropogenic materials [discarded by man],” he said.
According to Mahiques, the existence of anthropogenic materials has also been observed in some columns of mud from the south-southeastern Brazilian continental shelf at a depth of 100 meters.
“Until recently, the export of organic and inorganic anthropogenic materials to deep ocean areas had been overlooked. We have found these materials in areas of the continental shelf,” he said.
The IO-USP professor is also one of the editors of a special edition of the journal Continental Shelf Research, recently published by Elsevier, about the south-southeastern Brazilian continental shelf.
The publication’s other two editors are professors Áurea Ciotti from the Marine Biology Center (CEBIMAR) of USP and Osmar Olinto Möller Junior of the Oceanographic Institute of the Federal University of Rio Grande (Furg).
The special edition of the journal is a collection of 10 articles that report the research findings on oceanographic processes observed on the south-southeastern Brazilian continental shelf. Some of the studies were carried out with FAPESP funding.
“We issued a call for proposals to select unpublished works related to this region of the Brazilian continental shelf,” Mahiques said.
“The special edition of the journal is a good indication of the quality of research studies being carried out in the field of oceanography in Brazil,” he said.
Entitled “Oceanographic processes associated with the main continental shelf waters off South and Southeastern Brazil”, the special issue is available at www.sciencedirect.com/science/journal/02784343/89.
The book Continental shelves of the world: Their evolution during the last glacio-eustatic cycle, with a chapter by Mahiques and colleagues, may be purchased at www.geolsoc.org.uk/M0041.
The article “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” (doi: 10.1016/j.csr.2013.01.003) by Mahiques and colleagues may be read in the journal Continental Shelf Research at www.sciencedirect.com/science/article/pii/S0278434313000058.
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