The Rio Grande Rise (RGR), a possibly continental basaltic plateau and chain of seamounts now submerged in the South Atlantic Ocean some 1,200 kilometres from the coast of Brazil, was once a giant tropical island, rich in minerals and covered with vegetation, new research has found.
According to a recent study led by scientists at the University of São Paulo (USP), sediments from this formation – which is about the size of Spain – have been dated between 45 million and 40 million years ago.
In a paper published in the journal Scientific Reports, the experts explain that over 10 years, they travelled around the area in research ships and analyzed samples of seafloor sediment dredged at a depth of about 650 metres in the western RGR. They were able to characterize its mineralogical, geochemical and magnetic properties.
The samples contained mainly red clay with several minerals typical of tropical volcanic rock alterations, such as kaolinite, magnetite, oxidized magnetite, hematite and goethite.
“Our research and analysis enabled us to determine that it was indeed an island, and what’s now under discussion is whether the area can be included in Brazil’s legally recognized continental shelf,” Luigi Jovane, senior author of the article, said in a media statement.
“Geologically speaking, we discovered that the clay was formed after the last volcanic activity occurred 45 million years ago. The formation therefore dates from between 30 million and 40 million years ago. And it must have been formed as a result of these tropical conditions.”
For Jovane, the fact that a multidisciplinary team participated in the research contributed to the results.
“We have a group of the highest quality including specialists in geology, geochemistry, biology, hydrodynamics, environmental impact assessment, new energies, psychology, and law. All this accumulated science can be used to deepen our understanding of the RGR and prospect the region without affecting the local system’s synergies,” he said.
“To know whether resources can be viably extracted from the seafloor, we need to analyze the sustainability and impacts of this extraction. The ecosystem services provided by the ocean there haven’t been studied in detail, for example. When you interfere with an area, you have to know how this will affect animals, fungi and corals, and understand the impact you’ll have on the cumulative processes involved.”
Jovane and his team reconstructed the western portion of the RGR using high-resolution bathymetric mapping that showed plateaus covered with sediment and separated by a rift with a depth of more than 600 metres.
They used an autonomous underwater vehicle and a remotely operated vehicle to produce maps, videos and sonar surveys.
The AUV was capable of diving down to the seafloor and covering a pre-established area for a maximum of 12 hours. The ROV was connected to a ship by a cable as it moved while producing high-resolution images, and collecting samples of rocks and organisms with a robotic arm.
The existence of tropical soil between the volcanic lava flows detected by the researchers shows that the rocks must have been exposed to open-air weathering in a warm-wet climate in a region with active volcanoes less than 40 million years ago. The soil is similar to the “red earth” found in many parts of São Paulo state.
The RGR has been intensely studied in recent years because of its economic potential. It is in international waters and hence governed by the International Seabed Authority. In December 2018, the Brazilian government applied for an extension of its continental shelf to include the RGR, which is well beyond the limit of 200 nautical miles established for all nations by the UN Convention on the Law of the Sea.
Areas rich in cobalt, nickel and lithium, as well as tellurium and other rare earths critical to the transition from fossil fuels, one of the main drivers of global warming, to renewable energy, have been detected in the RGR.
“It’s important to understand the ecosystem services and other natural processes at work in the RGR,” Jovane said. “Only this knowledge can enable us to carry out the environmental impact assessments and calculate the mitigation measures and offsets required to protect it if economic development is permitted.”
