A study led by scientists at the University of São Paulo (USP, Brazil) has shown that the Rio Grande Rise (RGR), a possible continental basaltic plateau and seamount chain now submerged in the South Atlantic Ocean about 1,200 km off the coast of Brazil, was once a giant tropical island, rich in minerals and covered with vegetation. Geologists have dated the sediments of the formation to between 45 and 40 million years ago.

A paper describing the results of the study, which involved nearly ten years of research, is published in the journal Scientific Reports with new information on the geology of the RGR, which is about the same size as Spain.

The researchers analyzed samples of seafloor sediments dredged to about 650 m depth in the western RGR and characterized their mineralogical, geochemical and magnetic properties. The samples contained mainly red clay with several minerals typical of alterations of tropical volcanic rocks, such as kaolinite, magnetite, oxidized magnetite, hematite and goethite.

In 2018, the group postulated that the RGR was once an island, based on discoveries made by scientific expeditions to the region, where they collected the sample described in the article. They traveled on the RRS Discovery, operated by the UK’s National Oceanography Center (NOC), and on the Alpha Crucis, USP’s oceanographic research vessel. The expeditions were part of a thematic project supported by FAPESP. The researchers were from USP’s Oceanographic Institute (OI) and the University of Southampton, UK.

Our research and analysis allowed us to determine that it was indeed an island, and what is now under discussion is whether the area can be included in Brazil’s legally recognized continental shelf. Geologically, we found that the clay was formed after the last volcanic activity, 45 million years ago. Therefore, the formation dates back 30 to 40 million years. And it must have formed as a result of these tropical conditions, Luigi Jovane, last author of the article and professor at IO-USP, explained to Agência FAPESP.

For Jovane, principal investigator of a FAPESP-funded project, the fact that the research involved a multidisciplinary team contributed to the results. We have a very high quality group that includes specialists in geology, geochemistry, biology, hydrodynamics, environmental impact assessment, new energies, psychology and law. All this accumulated science can be used to deepen the knowledge of the RGR and prospect the region without affecting the synergies of the local system. To know if it is viable to extract resources from the seabed, we need to analyze the sustainability and impacts of this extraction. For example, the ecosystem services provided by the ocean there have not been studied in detail. When you interfere in an area, you need to know how it will affect animals, fungi and corals, and understand the impact it will have on the cumulative processes involved, Jovane said.

The scientists focused on the western portion of the RGR, which they reconstructed using high-resolution bathymetric mapping showing sediment-covered plateaus separated by a rift more than 600 m deep. They used an autonomous underwater vehicle (AUV) and a Discovery remotely operated vehicle (ROV) to produce maps, videos and sonar soundings.

The AUV is capable of diving to the seafloor and covering a preset area for up to 12 hours. The ROV is connected to the ship by a cable as it travels, produces high-resolution images and collects rock and organism samples with a robotic arm. No one in Brazil has AUVs or ROVs, so the partnership with our British colleagues at NOC was critical, but the research is 100% Brazilian, Jovane said.

The existence of tropical soil among the volcanic lava flows detected by the researchers shows that the rocks must have been exposed to the elements in a hot, humid climate in a region with active volcanoes less than 40 million years ago. The soil is similar to the “red earth” (terra roxa) found in many parts of São Paulo state, according to Jovane.

The most commonly used measure of rock weathering, known as the chemical alteration index (CIA), was 93 for red clay. Most alkaline rocks have a CIA of less than 50. This high value points to their origin in the extreme weathering of lava flows and volcanic rocks during the Eocene (second epoch of the Paleogene), between 56 and 34 million years ago, when high temperatures favored the development of tropical forests, prior to the abrupt climatic changes that occurred when Australia rapidly separated from Antarctica. Open-air erosion was followed by thermal subsidence and submergence during the late Eocene and early Oligocene, between 35 million and 25 million years ago.

The RGR has been the subject of intense studies in recent years due to its economic potential. It lies in international waters and is therefore governed by the International Seabed Authority (ISA). In December 2018, the Brazilian government requested an extension of its continental shelf to include the RGR, which is well beyond the 200 nautical mile limit established for all nations by the United Nations Convention on the Law of the Sea (UNCLOS).

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 is important to understand the ecosystem services and other natural processes at work in the RGR, Jovane said. Only this knowledge can enable us to conduct environmental impact assessments and calculate the mitigation measures and offsets needed to protect it if economic development is allowed.


Sources

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) | Srivastava, P., J. Murton, B., Sant’Anna, L.G. et al. Red clays indicate sub-aerial exposure of the Rio Grande Rise during the Eocene volcanic episode. Sci Rep 13, 19092 (2023). doi.org/10.1038/s41598-023-46273-y


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