OCEANOGRAPHY y

Revelations from a submerged archipelago

Beaches, rivers, and mangroves covered a mountain range that was above sea level 40 million years ago

Carlos Fioravanti

PUBLISHED IN AUGUST 2019

ntil it sank into the Atlantic Ocean some increasing study over recent years because of 40 million years ago, the Rio Grande its economic value. In 2015, the International Rise, the largest submerged mountain Seabed Authority (ISA) granted the Geological Urange of the Brazilian continental mar- Survey of Brazil (CPRM), a state-owned com- gin, was most likely an archipelago with geo- pany linked to the Ministry of Mines and En- graphic features similar to those found today ergy, a 15-year right to investigate the potential along the coast. While on an expedition with for economic exploitation of the ferromanga- the Alpha Crucis research vessel in January and nese crusts in the rock of the rise, which is rich February 2018, a team from the University of in cobalt, nickel, molybdenum, niobium, plati- São Paulo Oceanographic Institute (IO-USP) num, titanium, tellurium, and other chemical mapped the flattened tops of the rise and identi- elements. In a preliminary survey presented at fied stepped terraces with river channels, dunes, a 2015 congress in Florida in the United States, caves, and fossil remnants of mangroves. The the CPRM reported the identification of 9,729 dredges brought up samples of volcanic rocks square kilometers with high reflectance (radia- and coral and sponges that grew on the walls of tion reflection), indicating the likely presence of a rift that cuts through the center of the three ferromanganese crusts. In December 2018, the parts of the Rise. The results of the trip were Brazilian government requested that the United detailed in an article published in May in the Nations extend the legal boundaries of Brazil's scientific journal Frontiers in Marine Science. continental shelf beyond 200 nautical miles (370 Located 1,300 kilometers (km) from Porto km) to include the rise. Alegre the Rio Grande Rise covers approximately “We needed technical data to consolidate the 150,000 square kilometers, an area three times Brazilian claim,” says geologist Roberto Ventura the size of the state of Rio de Janeiro, and has Santos, former director of CPRM and a profes- depths ranging from 700 to 2,000 meters. Fur- sor at the University of Brasília (UnB). In turn,

thermore, the Rio Grande Rise has received researchers from public research institutions NOC / UK

56 z MARCH 2020 Deep-sea fish of the genus Allocyttus and black coral of the genera Stichopathes (top) and Bathypathes (left) in the central plateau region of the Rio Grande Rise

PESQUISA FAPESP z 57 in Brazil and other countries, such as 1 the United States Geological Survey and Kingston University in the United The top Kingdom, launched expeditions with the purpose of exploring the region. As of the a result, the history of the submerged region is gradually being uncovered. mountain What today is a mountain range under The terraced the sea must have once been part of the supercontinent Gondwana. As a conse- marine steps quence of tectonic processes and the rise (above) could of heat from the Earth's interior to the result from coastal location where the Tristan da Cunha erosion, and the archipelago lies today near the middle canyon (below) of the Atlantic Ocean, Gondwana began could result from to break apart between 146 million and ancient rivers. At 100 million years ago, forming present- the top of the Rio day South America and Africa. Grande Rise, the Based on the analysis of ocean floor canyon ranges fractures and tectonic plate movements from 600 to 800 in a study conducted at the State Univer- meters deep sity of Rio de Janeiro and the University of Liverpool, England, CPRM geophysi- cist Michelle Graça concluded that up to 80 million years ago, the Rio Grande Rise could have been linked to—or at in an article published in March in Ma- in the region close to Tristan da Cunha least be close to—the Walvis Ridge, a rine and Petroleum Geology. ranging from 1 million to 30 million years 3,000 km long mountain range 600 km “The two elevations were separated old and rocks up to 114 million years old from Namibia, Africa. The movements 50 million years ago,” notes Graça. She on the Namibian coast (see map). of the tectonic plates and magma jets believes the Rio Grande Rise has stopped According to analyses by Santos's from the Earth's interior separated the forming and, according to recent sam- group, published in May in the journal two elevations, giving rise to their pres- ples, contains boulders approximately 83 Terra Nova, the elevation's most recent ent features. The Walvis Ridge formed a million to 47 million years old. In con- lava expulsion must have occurred from mountain chain, and the Rio Grande Rise trast, the Walvis Ridge continues to form 44 million to 40 million years ago when formed a flat-topped plateau, as detailed as a result of rising magma, with rocks the mountain range was still above sea level. At that time, it began to submerge “probably as a result of the weight of a volcano, the lava, and the movement of tectonic plates, which caused the crust to sink,” says geologist Luigi Jovane of IO- USP, who was also a coordinator of the Alpha Crucis expedition to the region, in collaboration with the British and the Americans. “However, we can still see very little,” he notes with frustration. The USP expedition examined only the shallowest part of one of the three rocky masses of the Rio Grande Rise. Interestingly, according to the most recent studies, the area that the Brazil- ian government is laying claim to is made up of ancient masses from the current African continent. “The rocks of the Rio Grande Rise belonged to the Afri- can side of Gondwana,” Graça states. Santos, from UnB, came to the same con- Anemone of clusion: "The composition and age of the the genus Paraphelliactis rocks from the Rio Grande Rise that we 2 from the rise analyzed are more similar to those of the

58 z MARCH 2020 3 Two submerged elevations

Brasília The Rio Grande Rise and Walvis Ridge must have been joined 80 million years ago BRAZIL

São Paulo ANGOLA

Mid-Atlantic Porto Alegre Ridge Walvis Ridge ~1.500 km NAMIBIA

URUGUAY

ARGENTINA Rio Grande Rise

Tristan SOUTH da Cunha AFRICA Atlantic Ocean

African continent." The idea that this lationship between the sponge Saroste- Starfish of the mountain range could have been part gia oculata and the anemone Thoracactis family Solasteridae caught on the of Africa emerged in 2011, when CPRM topsenti, which form coral-like branches. bank of a canyon geologists collected granite and gneiss Another of the institute’s biologists, Pau- samples up to 2.2 billion years old, older lo Corrêa, described the region's biodi- than the rocks of the sea floor, which are versity, which potentially includes new a maximum of 240 million years old. The species. The region is a fragile environ- discovery changed the view of this re- ment, “which renews itself very slowly,” gion, which had previously been believed notes Sumida. to be the result of ocean floor formation Bacteria capable of producing oxygen and volcanic eruptions. from ammonia and sulfate appear to be the main energy sources for the organ- FRAGILE ENVIRONMENT isms on the Rio Grande Rise, accord- Two years later, in 2013, Brazilian and ing to IO-USP biologist Vivian Pellizari. Japanese researchers used a deepwa- “Ferromanganese crusts on rocks har- GOOGLE EARTH

3 ter research mini-submarine to collect bor their own microbial communities, other continental rock samples. Their with lower biological diversity than the analysis reinforced the hypothesis that nearby sediments,” she says, based on a this South Atlantic region was indeed study by the Institute's Natascha Bergo, 4 a piece of the continent that was sub- a doctoral student researching the ori- merged during the separation of South gins of the Rise's crusts. Project America and Africa. According to Santos, Researchers are concerned about the Marine ferromanganese deposits: A major resource of the two elevations may be the missing possible impacts of commercial exploi- E-tech elements (No. 14/50820-7); Grant Mechanism pieces needed to complete the puzzle tation, although technologies to extract Thematic Project; Principal Investigator Frederico Pereira Brandini (USP); Investment R$8,823,844.62. of the junction of South America and ores from the seabed do not currently

PAULO VINICUS FERRAZ2 AND 4 PAULO CORRÊA / IO-USP Africa. While the northeast coastline of exist. “Many organisms found there re- Scientific articles Brazil fits western Africa well, the south- flect life in the oceans millions of years JOVANE, L. et al. Multidisciplinary Scientific Cruise to the east and south of Brazil do not match up ago when there was no oxygen in the Rio Grande Rise. Frontiers in Marine Science. v. 6, a252, p. 1–7. May 24, 2019. with southern Africa. atmosphere,” says IO biologist Frederi- GRAÇA, M. C. et al. Crustal thickness mapping of the “It is a peculiar ecosystem,” says IO- co Brandini, who noted that the waters central South Atlantic and the geodynamic develop- USP biologist Paulo Sumida, who par- above the Rio Grande Rise are poor in ment of the Rio Grande Rise and Walvis Ridge. Marine and Petroleum Geology. v. 101, p. 230–42. March 2019. RAYLLA SOUZA SILVA / IO-USP SOUZA RAYLLA SILVA

1 ticipated in the expedition to the region. nutrients. Sumida added: "Any interven- SANTOS, R. V. et al. Dating Gondwanan continental crust His team found one of the first examples tion could easily bring these organisms at the Rio Grande Rise, South Atlantic. Terra Nova. Online.

PHOTOS in the South Atlantic of the symbiotic re- to extinction." n May 2, 2019.

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