Rise of the Rio Grande was a gigantic tropical island – 11/26/2023 – Science

Research led by scientists from USP (University of São Paulo) reveals that the Rio Grande Rise, a possibly continental portion submerged in the South Atlantic about 1,200 kilometers off the coast of Brazil, was —between 45 million and 40 million years ago— a gigantic tropical island, covered in vegetation and rich in minerals.

The result of almost ten years of work, the study recently published in the journal Scientific Reports brings new information about the geological formation formed by a set of underwater mountains that occupies an area equivalent to the territory of Spain.

The researchers found the presence of red clays with some minerals such as kaolinite, magnetite, oxidized magnetite, hematite and goethite, which are typical of tropical alteration of volcanic rocks. Mineralogical, geochemical and magnetic characteristics of a distinctive red clay sample dredged from a water depth of about 650 meters in the western area of ​​the elevation were analyzed.

In 2018, the group had already raised the hypothesis that the location was an island based on discoveries during a cruise in the region, when they found the sample that gave rise to the current article.

Carried out with the oceanographic research ships Alpha Crucis, from USP, and Discovery, owned by British royalty, the expedition was part of a Thematic Project supported by Fapesp and integrated by scientists from the Oceanographic Institute (IO) of USP and the University of Southampton, in England.

“The research allowed us to define exactly the presence of the island, and what is being discussed today is whether this area can be included in the Brazilian continental shelf. Geologically, we were able to discover that the clays were formed after the last volcanic activities recorded 45 million years ago , that is, the formation took place between 30 million and 40 million years ago. And it must have formed as a result of these tropical conditions”, IO-USP professor Luigi Jovane, supervisor of the work, tells Agência Fapesp.

For Jovane, who coordinates a project financed by Fapesp, the fact of having a multidisciplinary team participating in the research contributed to the results.

“We have a very high quality group, which involves areas such as geology, geochemistry, biology, hydrodynamics, environmental impact assessment, metallurgy, new energies, psychology and law. All this accumulated science can be used to deepen the understanding that allows prospecting without affecting the synergies of the local system. To know whether exploration at the bottom of the sea is viable we need to study sustainability very well and understand the impacts. There is no calculation of the ecosystem services of the sea, for example. The moment we change a region we need to understand how this affects animals, fungi, corals and the impact on cumulative processes”, he states.

First author of the article and currently professor at the University of Mumbai (India), Priyeshu Srivastava also had support from Fapesp through two projects (19/11364-0 and 22/02479-0).

This year alone, Jovane has already led the publication of four other articles with results from studies carried out with samples of volcanic rocks and ferromanganese crusts from the Rio Grande Rise (Frontiers in Marine Science, Journal of Materials Research and Technology, Geochemistry and Marine Geology) .

Discovering the bottom of the sea

Scientists focused the study on the easternmost part of the Rio Grande Rise. The region was reconstructed using high-resolution bathymetric mapping, making it possible to observe plains covered in sediment separated by a deep crack of more than 600 meters.

With the help of an autonomous underwater vehicle (AUV) and a remotely operated vehicle (ROV) from the National Oceanography Center (NOC), in Southampton, the group carried out a very high-resolution survey of the seabed by collecting samples, magnetic data , images and sonar.

The AUV, which emerges until it reaches the ocean floor, tracks a pre-established region for a maximum period of 12 hours. The ROV remains connected to the ship via a cable, but it also has the ability to move alone, collecting high-resolution images and samples of rocks and organisms with a robotic arm. “As we don’t have this type of equipment in Brazil, the partnership with the English at NOC was fundamental. But all the research is 100% Brazilian”, he adds.

The findings of the work show that there is a layer of tropical soils between the volcanic lavas, indicating that less than 40 million years ago the region was emerged and covered in tropical vegetation with high volcanic activity. Jovane says that, in its original stage, this soil is similar to red earth, typically found in the interior of São Paulo.

The chemical alteration index (CIA, the acronym for “chemical index of alteration”) reached a value of 93, indicating that the red clay is a product of the extreme wear of lava flows and subaerial chemical weathering of these volcanic rocks with a hot and humid climate during the Eocene — the second epoch of the Cenozoic era (between 56 million and 34 million years approximately) when high temperatures favored the development of tropical forests and then recorded sudden changes in climate, which would have been caused by the rapid separation of Australia and Antarctica. The CIA of most alkaline rocks is less than 50.

This happened before the region underwent thermal subsidence — a movement of a tectonic or thermal nature that affects the substrate of the basins — to its modern bathymetric depth.

The Rio Grande Rise was considered “international waters” and depended on the International Seabed Authority, linked to the UN (United Nations), until Brazil submitted the request to extend the Continental Shelf beyond 200 miles in this region, according to the United Nations Convention on the Law of the Sea (Unclos).

Areas rich in cobalt, nickel, lithium and rare earths, such as tellurium, have already been found at the site, target minerals for industries focused on high-efficiency energy generation — a replacement for fossil fuels, one of the causes of global warming.

“It is essential to understand the ecosystem services and natural processes that act on the Rio Grande Rise. Only by knowing them will we be able to carry out an environmental impact assessment and calculate these impacts to then have a complete study for the environmental preservation of the area, including mitigation and compensation for possible use of part of the region”, says Jovane.

The article “Red clays indicate sub-aerial exposure of the Rio Grande Rise during the Eocene volcanic episode” can be read here.