Rio Grande Rise

Rio Grande Rise. It is a large, submerged oceanic plateau located in the South Atlantic Ocean, west of the Mid-Atlantic Ridge and roughly east of the coast of Brazil. The feature is a major bathymetric high, with its shallowest portions reaching depths of less than 600 meters below sea level, separated from the South American continent by the deep Brazil Basin. Scientific interest in the rise is driven by its complex geological origins, its role in paleoceanography, and its potential for polymetallic nodule and cobalt-rich crust resources.

Geography and geology

The Rio Grande Rise is an expansive, mostly submerged feature covering an area comparable to the size of New Mexico. Its morphology is complex, consisting of a larger western portion and a smaller eastern portion, sometimes separated by a saddle. The western plateau is shallower and features seamounts, while the eastern section is more fractured. Key bathymetric features include the summit known as the Alpha Crucis Carbonate Bank and associated guyots. Geologically, it is composed primarily of alkali basalt and tholeiite formations, overlain in places by thick sequences of pelagic sediment and carbonate platform deposits. Rock samples retrieved by expeditions like those of the RV Marion Dufresne and the JOIDES Resolution have been crucial for its study.

Formation and tectonic setting

The rise is believed to have formed as a volcanic large igneous province during the Cretaceous period, approximately 80 to 70 million years ago, in association with the Trindade hotspot. Its formation is intimately linked to the early opening of the South Atlantic Ocean and the tectonic separation of the South American Plate from the African Plate. Initially formed near the Mid-Atlantic Ridge, the rise was later carried westward onto the South American plate due to seafloor spreading. Some theories also associate its formation with a mantle plume or the activity of the Walvis Ridge, making it a key feature for understanding South Atlantic tectonic evolution and the breakup of Gondwana.

Oceanographic significance

The rise acts as a significant barrier to deep-water currents in the South Atlantic, influencing the flow of major water masses like the Antarctic Bottom Water and the North Atlantic Deep Water. This interaction affects global thermohaline circulation patterns. Furthermore, its shallow summits have periodically been above sea level, creating isolated paleo-islands that impacted ancient oceanic circulation and served as potential stepping stones for biogeographic dispersal. Studies linked to projects like the International Ocean Discovery Program have used sediment cores from the rise to reconstruct past climate conditions and changes in the Brazil Current.

Exploration and research

Initial mapping of the rise was conducted during early oceanographic expeditions in the mid-20th century, including those by the RV Vema. Significant scientific investigation began in earnest with cruises by the Deep Sea Drilling Project and later the Ocean Drilling Program, which recovered critical basement rock and sediment samples. Recent decades have seen advanced surveys using multibeam sonar and submersibles by institutions like the University of São Paulo and the Brazilian Navy. The rise was a primary target of the 2013 IODP Expedition 352, which aimed to test its hotspot origin, cementing its status as a natural laboratory for studying intraplate volcanism.

Economic potential

The Rio Grande Rise has attracted attention for its potential deep sea mining resources. Surveys have identified extensive deposits of ferromanganese crusts rich in cobalt, nickel, platinum, and rare-earth elements on its slopes and summits. The area may also contain phosphorite deposits and fields of polymetallic nodules. In 2015, Brazil submitted a claim to the United Nations Commission on the Limits of the Continental Shelf to extend its continental shelf to include the rise, which, if approved, would grant Brazil exclusive rights to explore and exploit these resources. This has spurred environmental research into its unique benthic ecosystems to assess potential mining impacts.