Genesis and Evolution of Ferromanganese Crusts from the Summit of Rio Grande Rise, Southwest Atlantic Ocean

Abstract

The Rio Grande Rise (RGR) is a large elevation in the Atlantic Ocean and known to host potential mineral resources of ferromanganese crusts (Fe–Mn), but no investigation into their general characteristics have been made in detail. Here, we investigate the chemical and mineralogical composition, growth rates and ages of initiation, and phosphatization of relatively shallow-water (650–825 m) Fe–Mn crusts dredged from the summit of RGR by using computed tomography, X-ray diffraction, 87Sr/86Sr ratios, U–Th isotopes, and various analytical techniques to determine their chemical composition. Fe–Mn crusts from RGR have two distinct generations. The older one has an estimated age of initiation around 48–55 Ma and was extensively affected by post-depositional processes under suboxic conditions resulting in phosphatization during the Miocene (from 20 to 6.8 Ma). As a result, the older generation shows characteristics of diagenetic Fe–Mn deposits, such as low Fe/Mn ratios (mean 0.52), high Mn, Ni, and Li contents and the presence of a 10 Å phyllomanganate, combined with the highest P content among crusts (up to 7.7 wt %). The younger generation is typical of hydrogenetic crusts formed under oxic conditions, with a mean Fe/Mn ratio of 0.75 and mean Co content of 0.66 wt %, and has the highest mean contents of Bi, Nb, Ni, Te, Rh, Ru, and Pt among crusts formed elsewhere. The regeneration of nutrients from local biological productivity in the water column is the main source of metals to crusts, providing mainly metals that regenerate rapidly in the water column and are made available at relatively shallow water depths (Ni, As, V, and Cd), at the expense of metals of slower regeneration (Si and Cu). Additionally, important contributions of nutrients may derive from various water masses, especially the South Atlantic Mode Water and Antarctic Intermediate Water (AAIW). Bulk Fe–Mn crusts from the summit of RGR plateau are generally depleted in metals considered of greatest economic interest in crusts like Co, REE, Mo, Te, and Zr, but are the most enriched in the critical metals Ni and Li compared to other crusts. Further investigations are warranted on Fe–Mn crusts from deeper-water depths along the RGR plateau and surrounding areas, which would less likely be affected by phosphatization.

Microbial Diversity of Deep-Sea Ferromanganese Crust Field in the Rio Grande Rise, Southwestern Atlantic Ocean

“…Three factors might be shaping the community structure and composition variance on the Fe–Mn substrates from the RGR. The first is related to the water depth and location, because the concentrations of elements in the crusts change depending on the water depth [1, 41]. The second might be related to the geochemical differences in the composition of the studied substrates, which can promote variations in the lifestyles of solid-surface attached microbes [16].…”

“…The second might be related to the geochemical differences in the composition of the studied substrates, which can promote variations in the lifestyles of solid-surface attached microbes [16]. The last factor is probably associated with the local circulation, as previously described for the Fe-Mn deposits from the Pacific Ocean and for the RGR [1, 16, 24, 31]. Further analyses are needed to clarify whether and how the physical oceanography and geochemical composition of Fe-Mn substrates in the RGR are associated with the differences observed in microbial communities.…”

Origin and Composition of Ferromanganese Deposits of New Caledonia Exclusive Economic Zone

“…Chemical changes in Fe-Mn crusts with water depth are a common phenomenon that has been identified in several studies [2,64,[79][80][81][82][83]. Fe-Mn crusts selected for this study range from 430 m to 4677 m and allow us to observe changes in chemical composition with water depth (Figure 11).…”

Abundance and Microbial Diversity from Surface to Deep Water Layers Over the Rio Grande Rise, South Atlantic

“…The Rio Grande Rise (RGR) is a wide elevation (~150,000 km 2 ) located in the southwestern portion of the Atlantic Ocean, made of complex morphologies such as plateaus, seamounts, canyons, and rifts at water depths ranging from ~700 to 4000 m (Jovane et al, 2019). In the last decades it is becoming strategic for potential mineral exploitation of ferromanganese crusts (Montserrat et al, 2019;Benites et al 2020;Bergo et al, 2021). However, the fact that deep sea is turning an official source of raw materials deserves an environmentally sustainable management (Guilhon et al, 2020) that may only be achieved with detailed biological diversity studies.…”