According to a study, the Rio Grande Rise undersea volcanic plateau could have been a tropical island roughly 45 million years ago.
Signs of Dry Land
The seafloor surrounding the Rio Grande Rise was the subject of exploration by Brazilian and British scientists in 2018. Back then, the specialists were able to notice some rocks that looked like they were from dry land.
As they watched the footage taken by their remotely operated submersible around 650 meters below the surface, they were struck by odd red clay layers. Marine geologist Bramley Murton from the National Oceanographic Center in Southampton explains that red clay isn't typically found on the seabed. Murton adds that the deposits appeared like they were tropical soils.
In the "Red clays indicate sub-aerial exposure of the Rio Grande Rise during the Eocene volcanic episode" study, the specialists revealed that the unique mineral makeup of the clay could have only been formed by open-air weathering in tropical humidity and heat. This is the most recent discovery that hints that the ocean patch used to be an island.
Ocean Patch Could Have Been Tropical Island
Murton and the team think that the tropical island could have been as big as Iceland, roughly one-fifth of the total area of Rio Grande Rise.
The plateau's origins can be traced back to 80 million years ago. A huge mantle plume sat under the mid-ocean ridge of the South Atlantic, leading to great volcanism bursts. The rise that resulted prompted life as Iceland's Cretaceous version was nearer to the mid-ocean ridge than present-day South America. As time passed, volcanic activity went down. The plateau also ended up drifting west through the Atlantic and sinking under the waves.
However, roughly 40 million years ago, the mantle plume exhibited its last volcanism gasp. It was isolated from the rise of the western portion. It was in this region that the specialists discovered the red clays sandwiched between lavas, thought to be around 45 million years old.
Marine geologist Luigi Jovane from the University of São Paulo, who is also a co-author of the study and who has been leading Rio Grande Rise investigations for over a decade, notes that the result is outstanding. The red clays serve as conclusive evidence that the area used to be an island.
This study serves as the culmination of two expeditions to the area in 2018. During the first one, researchers were aboard the Alpha Crucis, a Brazilian research vessel. They mapped the underwater terrain of the rise with the help of sonar. This project was initially meant to characterize the ferromanganese crusts, which are rich in minerals, that are known to happen on the rise's seafloor. The mapping shed light on a 30-kilometer-long, steep-sided canyon that bisected the rise. It also revealed wave-cut platforms, prehistoric beach terraces, and drowned waterfalls.
After eight months, the team went back aboard the RRS Discovery of the National Oceanography Center. Such a vessel had a ROV (remotely operated vehicle), enabling them to snap footage of the rocks that were exposed. The ROV also had a robotic arm for sample collection.
With the red clay samples, the researchers gauged their mineral compositions in the lab. They found that the clay mostly consisted of a type of mineral known as kaolinite, which is dominant in tropical soils due to its resistance toward grave chemical weathering. Jovane explains that the clays are mineralogically and chemically the same as terra roxa, or red earth, that can be found across Brazil.
Marine geologist Tony Watts from the University of Oxford, who did not participate in the study, notes that the dataset is robust. Watts agreed with the interpretation of the team that the area used to be above sea level. He further notes that the study yields crucial implications for understanding the history of the rise's magma and subsidence.
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