Rio Grande Rise may have been a volcanic island

Brazilian and British researchers have found evidence that part of the seabed in the Southwestern Atlantic may have been created and submerged by volcanic lava eruptions.

By Elton Alisson  |  Agência FAPESP – Lava flows forced up from Earth’s mantle 80 million years ago by the separation of the African and South American tectonic plates that had begun 120 million years ago created an island like Iceland today.

The lost land of dinosaurs, forests, beaches, ravines, and plateaus the size of Wales began subsiding 40 million years ago. It is now drowned 3,000 meters under the Southwest Atlantic and 1,500 kilometers offshore Rio de Janeiro, Brazil (hence in international waters).

This hypothesis regarding the history of the Rio Grande Rise, as the oceanic ridge is known, has been reinforced by discoveries made by Brazilian and British researchers who conducted a scientific cruise to the region in late October and early November 2018 on the Royal Research Ship (RRS) Discovery.

The expedition was part of a project supported by FAPESP. The researchers are affiliated with the University of São Paulo’s Oceanographic Institute (IO-USP) in Brazil and the University of Southampton in the UK.

The aim of the project is to understand the processes underlying the formation, distribution and preservation of ferromanganese crusts – mineral deposits rich in critical metals for which there is growing demand from the electronics industry and which are vital to new low-carbon technologies. These include cobalt, used in rechargeable batteries for electric vehicles, and tellurium, used in solar power cells.

“We’re studying how these ferromanganese crusts were formed in terms of biology, geology, paleoceanography and paleoclimate,” Luigi Jovane, a professor at IO-USP and a coprincipal investigator for the project, told Agência FAPESP.

These metals and other critical materials known as e-tech elements are increasingly scarce on land. Some are highly concentrated in ferromanganese modules and crusts on the seabed. Tellurium is common on deep-ocean plateaus and seamounts, for example.

Many countries are eager to develop these mineral reserves. China, France, Germany, Norway, Japan, Russia, South Korea and the UK, among others, are on the verge of starting to mine the seabed. This activity will clearly have a significant environmental impact and cannot be considered a viable option unless it is sustainable.

“We will identify the processes that result in high-quality mineral deposits, develop a predictive model for their occurrence, and study ways of minimizing the environmental impact of their extraction,” Jovane said.

To obtain a better understanding of the processes underlying the formation and composition of these mineral deposits on the ocean floor, the researchers conducted three cruises in small deep-water basins off North Africa and Southeast Brazil.

The first cruise took place in October 2016, when RRS James Cook explored the abyssal plains of Madeira in the North Atlantic. In the second cruise, the oceanographic research ship Alpha Crucis, acquired by FAPESP for IO-USP in 2012, focused on the Rio Grande Rise in February 2018.

The Discovery sailed from the Port of Santos, São Paulo State on October 20, 2018, for the third cruise with a team of ten Brazilian and ten British researchers, returning 18 days later on November 8.

“In the third cruise, we went back to the areas studied in the second cruise in February, as these aren’t covered by the program run by the Brazilian Geological Service [CPRM] to explore cobalt-bearing crusts in the Rio Grande Rise,” Jovane said.

In 2014, the International Seabed Authority (ISA) granted Brazil 15-year rights to explore the mineral potential of 150 lots belonging to the Rio Grande Rise. CPRM must present the results to the ISA, which was established by the United Nations Convention on the Law of the Sea (UNCLOS) to regulate mineral-related activities in the international seabed area beyond the limits of national jurisdictions.

“We set out to study areas that don’t overlap with those being explored by CPRM, in order to understand the processes that formed the Rio Grande Rise and gave it the observed morphology,” Jovane said.

Autonomous and remotely operated underwater vehicles

The researchers used sonar to map the seabed on the Rio Grande Rise with sufficient resolution to pinpoint rocks only a few dozen centimeters in diameter.

Based on the maps produced using high-frequency sound waves, they chose five areas of particular interest and sent down an autonomous underwater vehicle (AUV) called Autosub6000 to record black-and-white videos.

Having mapped the areas with sonar and the Autosub6000, they then sent down HyBIS, a remotely operated vehicle (ROV) connected to the ship by wire and equipped with cameras and spotlights, to record closeups of the seabed and collect samples using a manipulator arm.

These technologies enabled the researchers to view, in the central portion of the Rio Grande Rise, a shallow plateau with a depth of 800 m divided by a huge rift over 1,400 m deep and 250 km long that cuts the Rise in two.

The Great Rift is 24 km wide and has deep troughs with vertical sides like a canyon. The basalt scarps are up to 600 m in height.

Large areas of highly eroded ferromanganese crust were detected in the basaltic walls of the canyon. “We saw that the crust and subsoil are eroding in many parts of the Rise, but we don’t yet know why,” Jovane said.

Almost 1 km back from the edge of the Great Rift, the researchers found growing numbers of chunks of black ferromanganese crust on the calcareous sandstone seafloor. This could mean the seabed there was once a beach.

The edges of the plateau near the Great Rift were strewn with large round basalt boulders, which can form only in areas where there is very high energy, such as the beds of fast-flowing rivers, or the sea shore where waves crash and tear at the cliffs, dislodge chunks of rock and roll them around until they form smooth boulders.

“This is evidence that the Rio Grande Rise subsided. The volcanic lava from the eruptions produced by separation of the African and South American tectonic plates became sea cliffs, which the waves broke up to create the escarpments of the Great Rift,” said Bramley Murton, a professor at the University of Southampton and the expedition’s chief scientist.

The researchers also observed a bed of red clay at the bottom of the scarps under the basalt lava. They believe that this clay deposit merged into the top of another lava flow beneath the first.

Geologists recognize this type of mud as a bole, the top of a lava flow that has been weathered by humid subtropical conditions, oxidizing the lava, turning its iron content red and transforming the rock into clay minerals. “This is an ancient soil, or a paleosoil, as we call it,” Jovane said.

One hypothesis is that after volcanic lava erupted to produce the island now called the Rio Grande Rise, sun and rain eroded the top and transformed it into soil. Thousands of years later, the volcanoes erupted again, burying the landscape in another flow of white-hot lava, and everything was incinerated.

As it cooled, the lava contracted to form the Great Rift. This was probably followed by further cycles of weathering, soil formation, plant growth, animal grazing, lava eruptions and incineration, according to Murton.

“Finding evidence that the Rio Grande Rise, which lies under deep water, was once dry land was totally unexpected,” he said.