“Mud, mud, glorious mud; Nothing quite like it for cooling the blood,” runs the chorus of the 1950’s era Hippopotamus Song by the comic duo Flanders and Swan. And, oceanographers find, there’s nothing quite like it for uncovering the history of continents and their changing weather systems.
That’s been the ten-year mission of the Oceanographic Institute of the University of São Paulo (USP). It has been analysing samples of sediments taken from the ocean floor along the Brazilian coast and from the estuary of the River Plate that forms the boundary between Uruguay and Argentina, and flows out past the Uruguayan capital Montevideo.
Brazilian researchers led by Michael Michaelovitch de Mahiques, a USP professor, have been using the oceanographic survey vessel Alpha-Crucis to cruise the continental shelf from the River Plate up to Cabo Frio north of Rio de Janeiro. In February 2013, one of these cruises covered nearly 2,000 kilometres 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.
The USP scientists’ work has been published in a number of scholarly journals, and the composition of the Brazilian continental shelf is the topic for one chapter of a new book published by the Geological Society in London. Continental Shelves of the World profiles 23 selected zones, including southern Brazil, where “complex interactions among climate, sea level, tectonics, oceanography and sediment input have formed distinctive sediment packages on each shelf and provide a guide to the interpretation of older shelf sequences throughout the geological record.”
Mahiques was 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 on research findings about oceanographic processes observed on the Brazilian continental shelf. Some of the studies were carried out with FAPESP funding.
You can read the articles by clicking below:
- “Oceanographic processes associated with the main continental shelf waters off South and Southeastern Brazil”, the special issue is available at sciencedirect.com/science/journal/02784343/89.
- “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 sciencedirect.com/science/article/pii/S0278434313000058.
What scientists have found is that over the last 6,000 years, changing rainfall patterns have led to a vast accretion of mud carried down the continent’s rivers and into the sea. From there, this huge plume of mud has been carried some 2,000 km northward along the coast as far as Sao Sebastião in São Paulo state. This mud, and the way it has settled, provides a valuable historical record of continental weather change.
The record indicates a gradual variation 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. 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.
According to Mahiques, a remarkable change in the deposition pattern of sediments began around about 2,800 years ago. That was when 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 Brazilian city of Santos. So in the centuries following, it spread further north, driven by a rainier and a windier continent. This drove sediments another 1,200 kilometres northward all the way to the coast of the state of São Paulo.
At present, São Sebastião (around 100 km further along the coast from Santos) marks the dividing lines between two geochemical provinces on the Continental shelf of southern Brazil. To the south is the province created by the outflow of the River Plate. To the north, nearer Cabo Frio on the coast of Rio de Janeiro State, the geological composition is different. Mahiques believes two other major rivers flowing from Brazil, the Rio Doce and Paraíba do Sul are the two principal sources.
To differentiate the types of sediment detected, oceanographers collected clay, organic materials, cesium, neodymium, and lead found in the marine sediments. Sand or mud from the upper portion of the collected samples represents the most recent sediments, because they were the last to be deposited. Collection points were: off the coast of Santos; near Cananeia, and off the coast at Itajaí, in Santa Catarina state.
Having plotted the shallow coastal waters and their deposits, researchers now plan to use the Alpha Crucis to explore deeper waters and find out if sediments can be recovered from 3,000 metre deep water.