Dredging Up Clues to the Geologic History and Mineral Resources of the Rio Grande Rise, Southwest Atlantic Ocean

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USGS scientists focused on finding iron-manganese crusts, whose chemistry reveals how the ocean has changed throughout millennia. The crusts also contain valuable elements like cobalt, molybdenum, and the very rare metals tellurium and platinum, which make them of economic interest.

This article is part of the March 2018 issue of the Sound Waves newsletter.  

May of the ocean floor showing the depth which is labeled in meters, and an Earth globe shows a dot to locate the study area.

The Rio Grande Rise comprises two major seafloor features: a north-south string of seamounts that forms the eastern Rio Grande Rise, and an elliptical plateau that forms the western Rio Grande Rise, the focus of the January–February research cruise. Black rectangles show areas where the cruise dredged for rock samples. m, meters; km, kilometers.

USGS scientists joined a January–February research cruise to the western Rio Grande Rise, an underwater plateau in the southwest Atlantic Ocean between the Mid-Atlantic Ridge and the coast of Brazil. Professor Luigi Javane of the University of São Paulo led the expedition. Participants sought to better understand the seafloor topography of the rise, its sedimentary history, and the processes that have formed mineral crusts rich in iron and manganese on its top and flanks.

The iron-manganese (Fe-Mn) crusts were the focus of the USGS participants—research geologist James Hein and Ph.D. student and USGS intern Kira Mizell of the Pacific Coastal and Marine Science Center in Santa Cruz, California. Hein leads a team studying the three main types of seafloor mineral deposits: sulfides that form around seafloor hot springs at mid-ocean spreading ridges, nodules that form on sediment-covered abyssal plains, and Fe-Mn crusts that form on sediment-free surfaces, typically on seamounts (underwater mountains) and larger features like the Rio Grande Rise.

Fe-Mn crusts, also called “seafloor pavements,” precipitate slowly from seawater, growing just a few millimeters, layer by layer, over thousands to millions of years. Their chemistry reveals how the ocean has changed throughout millennia. The crusts also contain valuable elements like cobalt, molybdenum, and the very rare metals tellurium and platinum, which make them of economic interest. Several hundred kilograms of rocks collected during the January–February cruise are already providing clues to the unusual history of Fe-Mn crusts at the Rio Grande Rise.

A series of photo graphs of rock samples that have been cut in half to reveal the inside structure.

Cut rock samples from the Rio Grande Rise show Fe-Mn crusts (black and gray) growing on various types of iron-rich substrate rocks (pale to dark brown). Photo credit: Kira Mizell, USGS.

The cruise was conducted from 28 January to 21 February aboard the University of São Paulo research vessel (R/V) Alpha Crucis. Funded by FAPESP (São Paulo Research Foundation (São Paulo Research Foundation), the cruise was part of the Marine E-tech program, a UK-led international, multidisciplinary consortium of universities, research centers, and industry contributors with two main goals: (1) to understand the cycling of cobalt, tellurium, and the rare-earth elements in Fe-Mn crusts, and (2) to determine how to reduce the possible environmental impacts of deep-ocean mineral extraction and metal recovery from seafloor mineral deposits. Hein, a world expert in marine minerals, was consulted in the development of this E-tech program, which is thus based on his USGS Global Ocean Mineral Resources project. Hein was invited to participate in the cruise to share his knowledge of the mineral deposits as well as dredging techniques.

Of the 22 days aboard ship, 15 were packed with various operations, including multibeam mapping to measure seafloor depths, backscatter mapping to gain information about seafloor roughness and composition, rock and sediment sampling, seawater sampling, and laboratory work to measure physical, biological, and chemical properties of various samples, and to study the mineral resource potential of sampled crusts. Hein led the team that used dredges to collect rocks from the seafloor. His guidance met one of the goals of the cruise: to train Mizell and the University of São Paulo researchers on dredging logistics and methods they had not used previously, and to advise them on current understanding and techniques for studying the genesis of Fe-Mn crusts.

Three photographs show people working on the deck of a ship to collect and process samples dredged from the bottom of the ocean.

Dredging operations aboard the research vessel Alpha Crucis.

  • Top: The first dredge with successful rock recovery is wrangled back on deck. Photo credit: Kira Mizell, USGS.
  • Bottom left: James Hein collects samples emptied from a fresh dredge. Photo Credit: Chico Vicentini, University of São Paulo.
  • Bottom right: Kira Mizell cuts ferromanganese crusts with a diamond-blade saw to expose their inner structure. Photo credit: James Hein, USGS.
In two photos side-by-side, on left: 4 men standing on ship deck near winch shack, on right is two women standing on deck.

James Hein (white hardhat) monitors dredge operations from the winch shack and explains dredging techniques to Chief Scientist Professor Luigi Jovane (red hardhat) and the ship's crew. Photo credit: Kira Mizell, USGS. Right: Kira Mizell (left) and colleague Mariana Benites (University of São Paulo) pose with marine mineral samples after the seventeenth and final dredge of the cruise. Photo credit: Isobel Yeo, National Oceanography Centre, UK. Photo credit: Isobel Yeo, National Oceanography Centre, UK

In total, the team completed 17 dredge hauls—mostly from the upper parts of the plateau (600- to 900-m water depths—using a variety of 1-m-wide dredges that recovered several hundred kilograms of an amazing variety of rocks. The samples include ferromanganese crusts, phosphorites, volcanic rocks, limestones, and other rock types. More than 250 samples were cut on a diamond-blade rock saw and described in detail onboard.

The rock samples proved very interesting and offered much insight into the history of seawater chemistry and seafloor shape, or morphology, in the region. The Fe-Mn crusts are particularly unusual in that they appear to be heavily phosphatized, which offers an additional perspective on oceanic environmental changes. Also, some very curious rocks were recovered, such as granitic gneiss, a rock typical of continental crust which would not be expected on the plateau. Follow-up studies using a remotely operated vehicle (ROV), cameras, and larger, heavier dredges will be important to expand on these exciting shipboard findings.

Hein and Mizell are currently working with University of São Paulo scientists to analyze the Fe-Mn crust samples for mineralogy, chemical composition (including the concentration of potentially economic elements), and age dating. They are conducting these analyses in collaboration with and in support of the Ph.D. thesis of University of São Paulo student Mariana Benites, whom Hein is advising with Professor Jovane. The team looks forward to co-authoring papers about their findings in this unique region.

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