Ferromanganese Crust: Is a Type of Cenozoic Black Stromatolite in Seabed? The Case of the Rio Grande Rise, South Atlantic Ocean

Volcanism

The basement at this drilling site consists of tholeiitic basalt which was Ar-Ar dated by Rohde, et al. [37] at 80-87 Ma - Santonian (Late Cretaceous) - in good agreement with the predicted age of the oceanic crust for this region based on the magnetic anomaly data. This also suggests that the WRGR formed at or close to a spreading center [35, 36, 38]. It is worth noting that this age coincides with the intrusion ages of most of the onshore alkaline intrusions in Brazil (e.g. the Poços de Caldas region) [39]. In contrast, rocks dredged from the guyots protruding from this platform have alkali- basaltic composition as characteristic for intra-plate ocean island basalts [40], with one of these samples that was Ar- Ar dated by Rohde, et al. [37] at only 46 Ma (Mid Eocene). The occurrence of ash layers and volcanic breccia in cored sediments of the same age at DSDP Sites 516 and 357 is another indication that a widespread volcanic-tectonic event affected the WRGR region during the Mid Eocene. It is believed that the RGR has been uplifted by up to 700 m during this event which coincides with the formation of the transtensional Taubaté Basin onshore and the formation of alkaline volcanic rocks in the area of Rio de Janeiro and within the Eocene sequence of the Santos and Campos Basins [39]. During this event parts of the WRGR plateau were uplifted above sea level and several short-lived volcanic islands were created, which were later submerged as the plateau subsided [36, 38]). The associated paleoenvironmental changes, i.e. ocean circulation pattern and water column characteristics (depth, temperature, dissolved oxygen concentration, luminosity, salinity), may have played an important role in the development of favorable conditions for the formation of stromatolites and the deposition of Co-rich ferromanganese crusts.

While the current geological understanding for the origin of the RGR, as mentioned above, mainly argues in favor of a thick oceanic crust made mostly of basalts, recent geophysical studies indicate that it may also be partially made of continental crust [38, 41]. This hypothesis has been strengthened by the recovery at the WRGR of continental crustal rocks (granite, gneiss, granulite) of Precambrian age and the direct observation from the Japanese submersible Shinkai 6500 of a porphyritic granite rock pavement with more than 5 m in extension [42]. These important new findings imply to revise the interpretation on the origin of the Rio Grande Rise and on the South Atlantic tectonic evolution as a whole. During continental rifting, fragments of material from the continental margins may be detached and surrounded by oceanic crust, as microcontinents in the ocean basins. Seismic studies and drilling results have shown that the Kerguelen Plateau in the Indian Ocean is underlain at least in part by continental crust [43, 44, 45]. Since the RGR (Atlantic Ocean), Kerguelen (Indian Ocean) as well as the Jan Mayen (Arctic Ocean) microcontinents are associated with hotspots, this suggests that deep mantle plumes may play a role in their formation by weakening the local lithosphere prior to breakup. Alternatively, detachment of continental material during breakup may control the location and composition of subsequent intraplate volcanism [46]. A new petrographic and geochemical study is being carried out on a more complete set of dredged volcanic rocks which should help in refining the interpretation of the RGR formation (Christian Lacasse pers. comm., January 2015).

Sedimentation

Sediments on the RGR have been described in details from the cored sections of DSDP Hole 516F, located on the northeastern flank of the RGR. The sequence begins with basal sediments of about 82 Ma old (Late Cretaceous). The stratigraphic succession mainly comprises limestones that are divided into eight lithostratigraphic units that are distinguished by their lithification and diagenesis degree. The first seven units are represented by a continuous succession of pelagic carbonates interbedded with volcanogenic sediments while the lowermost unit (Unit 8) is in contact with basalts of the bedrock [47]. According to these authors, the carbonate rocks sequence is almost continuous from Santonian-Coniacian to Recent, with only one observed discontinuity in the Upper Miocene. Several important geological boundaries were identified such as the Cretaceous/Tertiary, Eocene/Oligocene, and glacials/ interglacials of the Lower Pliocene.

The stratigraphic succession can be summarized upward as follows: i) basal unit (Unit 7) between 1270- 1240 m below seafloor (mbsf), composed of ferruginous cherts, limestones, glauconite-rich sandstones, turbidites, and shallow water microfossil (1252-1240 mbsf); ii) Unit 6, between 1240-1000 mbsf, composed of malm limestones intercalated with micritic limestones containing nannofossils, microcrystalline limestones and shales; iii) Unit 5, between 1000-874 mbsf, formed by limestone and calcareous malm, gray in color, with some thin layers alternatively enriched in organic matter or carbonate; iv) Unit 4, between 874-634 mbsf, mainly consisting of foraminiferous and calcareous nannofossils, interlayered with greenish gray montmorilonitic volcanic ash; v) Unit 3, between 634-332 mbsf, formed by chalk, composed of nannofossils occasionally mixed with foraminifers; vi) Unit 2, between 332-193 mbsf, composed of mud with nannofossils and foraminifers, with parallel laminations; vii) and Unit 1, between 193-0 and mbsf, composed of calcareous mud flats with nannofossils and foraminifers.

Sample Collection

Before collecting the crust samples on the Rio Grande Rise, multi-beam surveys of high definition (bathymetry, sub-bottom and back scattering) were performed ​and completed by micro seismic profiles. The samples were collected during R/V Marion Dufresne cruises Leg 1 and Leg

2, using rectangular and circular dredges with the assistance of a TV Grab system which allowed additional sampling and filming of the seabed.

The locations of the dredged samples are shown in figure 1c and the dredging coordinates and sample description are summarized in Table 1.

Optical and Electronic Petrography

The textures and structures were characterized in thin sections by optical microscopy using transmitted and reflected light and scanning electron microscopy. Hand specimens were previously classified in two morphological groups based on macroscopic observations, between plate- like (flat and concretion) and nodule-like types.

The analysis by scanning electron microscopy (SEM) was done on a Zeiss LS15 microscope in the mineral analysis laboratory (LAMIN-BE) of the Belém Regional Superintendence of the Geological Survey of Brazil. The polished slides were covered in high vacuum with a 20 m thick carbon film. The emission filament is tungsten and the high vacuum mode (3.0 - 1.5 x 10 -5 mPa) was used for electronic imaging and chemical analysis. Backscattering electron (BSE) images of minerals and textures were obtained at a voltage of 20 kV, inrush current of 60 to 90 pA, working distance of 8.5 mm, and magnification between 60 and 2000 times. The chemical composition of the minerals was analyzed by energy dispersive spectrometry (EDS) on an Oxford Instruments X-Act SSD 10mm2 detector. Analytical results were also acquired at a working distance of 8.5mm and with a voltage of 20kV, but with an incoming current between 170 and 200 pA to maintain an output count rate of about 2000 cps

Litogeochemical Analysis

The chemical analyses were performed at Acme Analytical Ltda., in Vancouver, Canada. Concentrations of traces elements and rare earth elements were determined by ICP-MS, after fusion with lithium metaborate and digestion with nitric acid. The major oxides and some metals elements were determined by X-ray diffractometry (PANalitical Axios) after fusion with lithium metaborate, lithium tetraborate and sodium nitrate. Later, using optical microscope and geochemistry analyzes, the samples were classified between Fe-Mn-rich and Ca-P-rich crusts.

87Sr/86Sr Ratio Measurement

87Sr/86Sr ratios of the substrate on ferromanganese sample HS-145a and on a carbonate vein in sample ML11D were determined through TIMS (Thermal Ionization Mass Spectrometry), in a Finningan MAT 262 at the University of Brasília. Samples were digested in H3PO4 and the residue kept in a HNO3 (2.9 N) solution for column chromatography. The solution containing Sr was collected, dried and then dissolved in 1 ml HNO3 (50%) for analysis. Procedures followed steps used by Alvarenga, et al. (2007). NBS-987 was used as reference material, and yielded a 87Sr/86Sr ratio of 0.71028 ± 1 (1σ).

Petrography

The petrographic characterization of the ferromanganese crusts was performed on the basis of both their morphological (plate-like vs. nodule-like) and geochemical (Co-rich vs. P-rich) classifications. The micro stratigraphic sequences consist of the P-rich crusts at the base and Co-rich crusts at the top (Figure 3). Internal structures can be grouped either as sedimentation structures or as discontinuities. Sedimentary structures can be wavy-layered (or plane- parallel stratification), parallel columns, branching columns, botryoidal and nodular. The transition from one structure to another may be gradual or abrupt. When abrupt, the transition can represent change in the characteristics of the sedimentary environment (changed facies) but also a depositional hiatus.

Wavy-layered Structure: Is very similar to algal mats of common stromatolites. Its typically exhibits undulations and is defined by the intercalation of light and dark layers generating a micro-banding (Figure 4a). They are more common on the basis of Co-rich crusts, and also on the basis of various depositional cycles, marking the change of a depositional cycle. It often contains materials that may represent micro detrital carbonate. In the Co-rich crusts the clear layers predominate whereas in the P-rich crusts the proportion of darker layers increases. Gradual facies variations were observed between P-rich and Co-rich crusts. Columnar Structure: Is also very similar to that occurring in common stromatolites. It exhibits simple columns (parallel) or has a branching (composite) or more complex (random) aspect (Figure 4b and 4c). Usually they occur in the middle layer of the crusts, having wavy-layered structure at the base, characterizing the beginning of a new depositional cycle. Despite this columnar aspect a plan-parallel layering can be observed on a larger scale. The micro-banding, defined by the intercalation of light and dark layers, is similar to bacterial mats observed in common stromatolites. Botryoidal Structure: It mainly occurs in P-rich crusts and is formed by the intercalation of light and dark layers, the latter being more predominant. This type of structure is very similar to the common botryoidal morphology observed on the surface of ferromanganese crusts. It can also appear scattered between the layers of P-rich crusts (Figure 4d). The limestones layers are formed by deposition of fossiliferous sediments. Micro-nodule Structure: This structure is more common on top of the ferromanganese crusts. It is generally observed with a gradational change from random structures to nodules. Initially the shapes are very irregular and gradually change to a more regular aspect (Figure 4e). This type of structure occurs in Co-rich crusts and is formed by intercalation of light and dark concentric thin layers that materialize the shape of the oolites. Apparently these structures do not have a germination nucleus. Discontinuities: These structures are apart from those described before and mark changes in depositional environment and/or interruption of sedimentation cycle (Figure 4f). They can be either plane-parallel (concordant) in aspect, reflecting changes in the depositional environment, corresponding to erosional surfaces (discordant), marking the beginning of a new cycle or depositional hiatus, or representing a lateral change of facies.

Click to enlarge

Figure 4: Photomicrographs of the optical microscopy showing the main sedimentary structures of ferromanganese crusts. a) wavy-layered structures marked by undulation nd intercalation of dark and light layers; b) parallel columns; c) branching columns and random structures; d) botryoidal structures with biofilms; e) micro-nodules or oolites structures; f) unconformity between two depositional cycles.

Stratigraphy

Their stratigraphic succession comprises an older phosphatized crust (P-rich crust) at the base and a younger non-phosphatized crust (Co-rich crust) at the top, both being separated by a discontinuity (depositional hiatus) (Figure 3a). In the younger non-phosphatized crust, at least three layers of phosphate-carbonate rock, several millimeters thick, were identified. They may represent depositional hiatuses that have occurred in the South Atlantic Ocean.

Click to enlarge

Figure 5: Main sedimentary structures observed on scanning electron microscopy. a) and b) Columnar structures in the plate- like planar Fe-Mn crust, composed mainly of Fe-vernadite, and containing hydrogoethite at the edges of the columnar structure and vugs containing fragments of ferruginous material and detrital silicates; c) and d) detail of the columnar structure showing the space between the columns filled by fragments and detrital crust and silicates, composed of Fe-vernadite, amorphous Fe- Mn oxyhydroxide at the edges and vugs containing fragments of ferruginous material and detrital silicates; e) and f) details of the micro nodular structures in the nodule-like cobaltiferous crust. The vug is filled with fragments of the Fe-rich crust, and carbonate; g) and h) columnar structures on top of film-like crusts containing detrital material in the boundary zone of the columnar structures.

The old phosphatized crusts correspond to the basal section of ferromanganese crusts of the Rio Grande Rise. They have a maximum thickness of 10 cm, a smooth botryoidal irregular surface, banded internal structure with up to seven layers that can be visually distinguished (Figure 6.). A series of parallel fractures, filled with carbonate- phosphatic material, intersect the crusts. Observation through the microscope shows slight banding defined by the alternation of ferromanganese layers and carbonate- phosphate bioclastic layers. These layers enriched in bioclastic carbonate-phosphatic material are repeated throughout the crust. The main structures of P-rich crusts are botryoidal and wavy-layered. Some structures are delineated by thin layers of a very dark material that can be organic matter. In a complete thin section, seven layers which were subdivided into twenty-three facies characterized by alternation of depositional cycles that begin with bioclastic carbonate deposition and carbonaceous material, followed by the deposition of ferromanganese crust (Figure 6). The carbonate portions are composed mainly by foraminifers.

Click to enlarge

Figure 6: Images of an old phosphatized crust. a) hand specimen (scale of 15 cm) showing banding structures of P-rich crust; b) and c) black biogenic structures in reflected and transmitted light under the microscope; d) detail of black biogenic structures in reflected light under the microscope; e) and f) profiles of crust with interpretation of depositional cycles.

The young non-phosphatized crusts can be plate-like flat, plate-like concretion and Nodule-like. Plate-like flat show botryoidal surficial morphology, inner columns and wavy- layered structures with at least five distinctive depositional layers, more or less well defined, with a total thickness ranging between 55 and 90 mm (Figure 3). Micro nodular structures predominate at the base of these crusts and are progressively replaced upward by columnar structures. The columnar structures have a parallel lamination with attenuated color variations that may reflect the involvement of biogenic components. The portions of crusts, appearing clearer under the reflected light of the microscope, indicate an environment with the precipitation of manganese oxides whereas the darker portions of the crusts may represent algal mats with relics of bacterial films deposited between manganese oxide layers. Another obvious structure is an erosive discordance that may reflect a depositional hiatus between two different crusts.

Plate-like concretion exhibit a smooth botryoidal morphology at their surface including five distinct wavy layers (Figures 7a and 7b). Their total thickness is about 140 mm. The core consists of several fragments of carbonate- phosphate-rich rocks. Analysis of a complete polished thin section indicates nine individual depositional cycles with predominance of columnar, and more rarely wavy- layered, brotryoidal, oolite or micro-nodule structures (Figures 7c and 7d).

Nodule-like are concretions of various forms, from flattened oval to even irregular, with variable diameters, containing the carbonate-phosphate core (Figure. 7f and 7g). They occur at depths between 900 and 1500 m and have diameters between 5 and 20 cm. Under the microscope, the crust displays columnar, wavy-layered, botryoidal and micro-nodular structures. In this type columnar structures predominate and can be parallel and branching. The transition from one structure to another can be abrupt or transitional. When abrupt, we considered that can be a depositional hiatus. Near to the core of the nodule, the layers are very irregular and, toward the top, the structures become more regular. In micro-stratigraphic profile was possible to distinguish different layers and also different facies, which may reflect changes in the characteristics of the depositional environment of the ferromanganese crust.

Click to enlarge

Figure 7: Images of an young non-phosphatized crust: a) and b) hand specimens of plate-like crust (scale of 15 cm) with botryoidal surficial habit and plate-like crust section, respectively; c) thin section profile in reflected light under the microscope of plate-like crust with hiatus surface; d) and e) columnar structure and detail of columnar structure, respectively, in reflected light under the microscope; f) and g) hand specimens of (scale of 15 cm) nodule-like crust and section; h) and i) wavy-layered and columnar structures of nodule-like crust and micro-nodule structure of crust, respectively, in reflected light under the microscope.

Mineralogy

XRD data obtained by Souza, et al. [48] in Fe-Mn crusts of Rio Grande Rise revealed that non-phosphatized layers of the samples are mainly composed of Fe-vernadite (δ-MnO2), and calcite and quartz may be present to a lesser extent. Older phosphatized samples are composed of a stable 10 Å Mn-phase (todorokite) instead of a Fe-vernadite. Carbonate

fluorapatite (CFA) and calcite are present in all phosphatized samples.

Geochemistry

Chemical analyses of major, minor and rare earth elements (REEs) were carried out on 49 crust samples, including 42 of Co-rich and 7 of “P-rich” type (Table 2). The predominant components of young non-phosphatized crust are Mn and Fe, and in old phosphatized crust are CaCO3 and P2O5. The ore forming elements are Co, Ni, Pb, Ti, Cu and Ce.

a) Old phosphatized crusts

b) Young non-phosphatized crusts Table 2: Chemical composition of ferromanganese crusts.

Mn and Fe are the main elements in young non- phosphatized crust, with an average concentration (wt %) of 19.04 and 16.07, respectively. The Fe/Mn ratios average 0.85 which indicates the hydrogenous origin of these deposits. In these crusts the average concentrations (wt %) of Co, Ni, Pb, Ti, Cu and Ce are 0.92, 0.34, 0.25, 0.03, 0.92 and 0.15, respectively. However, in old phosphatized crust, the main elements are CaO and P2O5, with an average of 34.77% and 15.93%, respectively. In this type of crust the ore forming elements content is lowest and only Ni is higher (0.41wt %), whereas the average concentrations (wt %) of Co, Ti, Cu, Pb and Ce are 0.18, 0.04, 0.2, 0.02 and 0.02, respectively. The geochemical signature of Fe–Mn crusts is interpreted using the Fe - Mn - (Ni+Co+Cu) x 10 diagram, with all our data points plotting in the field of hydrogenic crusts (Figure 8). The (Mn+Fe)/Ti ratios in samples of young non-phosphatized crust vary from 31.72 to 49.86 and in old phosphatized crust between 46.63 and 87.19. The average growth rate of non- phosphatized crust were calculated using empirical law [49], with values of 1.75 mm/Ma, 2.24 mm/Ma and 1.70 mm/ Ma for plate-like concretion, plate-like flat and nodule-like crusts, respectively.

Click to enlarge

Regarding major oxides young non-phosphatized Fe-Mn- rich crusts are enriched in SiO2, Na2O, K2O that concentrate in the clastic fractions (quartz and feldspar) as described from thin sections. The concentrations of trace elements in young non-phosphatized crusts are also higher than in old phosphatized crusts, except for phosphate. In comparison, the old phosphatized crusts are enriched in carbonate, phosphate and yttrium.

According Bau, et al. [53], “The CeSN/CeSN* ratio is the simplest approach to quantify redox related decoupling of Ce from the other REY, which produces Ce anomalies in REYSN patterns and the YSN/HoSN ratio reveals decoupling of the geochemical twins Y and Ho, which produces YSN anomalies in REYSN patterns. The bivariate diagram of CeSN/CeSN* ratio vs YSN/HoSN ratio show different groups of ferromanganese deposits, i.e., hydrogenous, diagenetic and hydrothermal”. On the Rio Grande Rise crusts, young non phosphatized plot in hydrogenous and old phosphatized plot in diagenetic type (Figure 9). This diagram show that old phosphatized crusts is diagenetic origin.

Click to enlarge

The content of REE varies greatly between the young non-phosphatized and the old phosphatized crusts. In young non-phosphatized crusts the total REE contents vary from 2041 to 2866 ppm, with an average of 2420 ppm. The average abundances of light-REE and heavy-REE are, respectively, 2100 ppm and 264 ppm. The LREE/HREE ratios vary from 5.23 to 8.13, with an average of 6.68. Among all REE elements Ce is the most abundant, with an average content of 1514 ppm. In old phosphatized crusts the total REE contents vary from 424 to 1341 ppm, with an average of 919 ppm. The average abundances of light-REE and heavy-REE are, respectively, 590 ppm and 328 ppm. The LREE/HREE ratios vary from 1.24 to 3.48, with an average of 1.8. The PAAS- normalized REE patterns of ferromanganese crusts from the Rio Grande Rise differ significantly between the young non- phosphatized and old phosphatized types, e.g., flat pattern of the other REE elements, with high positive Ce anomaly and enriched approximately ten times the PAAS (Figure 10). However the phosphatized crusts do not have Ce anomalies and have fewer REE elements.

Click to enlarge

Also according Bau, et al. [53], “The REY distribution in hydrogenetic Fe–Mn crusts represents an exchange equilibrium suggests that (i) at a given Fe/Mn ratio, the absolute REY concentration in a hydrogenetic Fe–Mn crust is largely controlled by the dissolved REY concentration in ambient seawater; and (ii) hydrogenetic Fe–Mn crusts are characterized by very similar REY distribution patterns regardless of their place of origin as long as there was no significant difference between dissolved REY concentrations and distributions in their depositional environments during growth.” In Rio Grande Rise crusts, young non phosphatized show positive anomaly of Ce and negative anomaly of Y and old phosphatized show positive Y anomaly, which distinguish the two crusts types (Figure 10).

Microbial Diversity

On the Fe-Mn crusts samples of Rio Grande Rise, the prevalent classes identified were Gammaproteobactéria (57- 24%), Alphaproteobactéria (17-16%) and Nitrosospharia (17-7%), and in the Fe-Mn encrusted coral skeletons, the prevalent class was related to Gammaproteobacteria (27%), Alphaproteobacteria (20%), Planctomycetacia (7%), Nitrososphaeria (5%), and Deltaproteobacteria (5%), according to the studies carried out by Bergo et al. (2021).

87Sr/86Sr indirect dating

87Sr/86Sr analyses on phosphatized ferromanganese crusts samples yielded ratios of 0.70833 ± 2 and 0.70880 ± 2. Comparing the 87Sr/86Sr ratio of the samples to the curve of the Sr isotope composition of the oceans through the Cenozoic, we suggest that analyzed materials crystallized between 21.7 and 14.5 Ma ago (Figure. 11 and 12, Table 3). These Sr isotope ratios represent isotope ratios without an age correction to subtract radiogenic 87Sr produced by internal decay of 87Rb after the time of deposition. Phosphate minerals and calcite have a low 87Rb/86Sr ratio (<0.01) with a high concentration of unradiogenic Sr [54, 55]. The age correction is almost negligible for the Sr isotope. However, data must be interpreted with care because Sr ages of carbonate and carbonate fluorapatite are susceptible to diagenetic effects: incorporation of Sr during the crystallization of authigenic minerals [48].

Click to enlarge

Click to enlarge