Selected hydrogenetic ferromanganese crusts were characterized at a microstratigraphic scale across various NW Pacific seamounts. The crusts encompassed over 100 sites, and were mostly found on old ...rock outcrops. The crusts also exhibit spatial compositional variation and are largely enriched in Co and Ni. However, the parameters controlling this compositional variation remain unclear. To address this issue, we characterized the microstructure, mineralogical and chemical compositions of seven carefully selected samples from topographic highs in the mid-latitudes on the Philippine Sea and Pacific plates and revealed the following trends: (1) the lower phosphatized layer formed in the middle Miocene or earlier and only on the Pacific Plate, (2) an increasing Co/Mn ratio with younger age from the substrate to the surface, and generally higher in the Pacific Plate than the Philippine Sea Plate, and (3) an increase of detrital quartz and plagioclase since approximately 5 Ma, with high Al/Mn ratios near the continental margin and to a lesser extent in pelagic areas. These results are partly congruent with those of other sediment cores from the other NW Pacific basins. Furthermore, the secular increase of Co and detrital minerals in the crust samples suggests a strong influence of oceanographic and tectonic conditions, especially the expansion of the oxygen minimum zone (OMZ) for the former and a significant input of detrital materials from the continents for the latter.
•Secular changes in chemical and mineralogical compositions are well correlated among the hydrogenetic ferromanganese crusts in the NW Pacific.•The trends of the changes are characterized by phosphatization, expansion of the oxygen minimum zone, and detrital materials input.•The changes are ideal markers of long-term oceanographic and tectonic conditions during the ultra-slow and uninterrupted growth of the crusts.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The distribution of Mo between seawater and marine ferromanganese oxides has great impacts on concentration and isotopic composition of Mo in modern oxic seawater. To reveal the adsorption chemistry ...of Mo to ferromanganese oxides, we performed (i) detailed structural analyses of Mo surface complexes on δ-MnO
2, ferrihydrite, and hydrogenetic ferromanganese oxides by L
3- and K-edge XAFS, and (ii) adsorption experiments of Mo to δ-MnO
2 and ferrihydrite over a wide range of pHs, ionic strengths, and Mo concentrations. XAFS analyses revealed that Mo forms distorted octahedral (
Oh) inner-sphere complexes on δ-MnO
2 whereas it forms a tetrahedral (
Td) outer-sphere complex on ferrihydrite. In the hydrogenetic ferromanganese oxides, the dominant host phase of Mo was revealed to be δ-MnO
2. These structural information are consistent with the macroscopic behaviors of Mo in adsorption experiments, and Mo concentration in modern oxic seawater can be explained by the equilibrium adsorption reaction on δ-MnO
2. In addition, the large isotopic fractionation of Mo between seawater and ferromanganese oxides detected in previous studies can be explained by the structural difference between
MoO
4
2
-
and adsorbed species on the δ-MnO
2 phase in ferromanganese oxides. In contrast, smaller fractionation of Mo isotopes on ferrihydrite is due to little change in the Mo local structures during its adsorption to ferrihydrite.
The structures of Mo species adsorbed on crystalline Fe (oxyhydr)oxides, goethite, and hematite were also investigated at pH 8 and
I
=
0.70
M (NaNO
3). Our XAFS analyses revealed that Mo forms inner-sphere complexes on both minerals:
Td edge-sharing (46%) and
Oh double corner-sharing (54%) for goethite, and
Td double corner-sharing (14%) and
Oh edge-sharing (86%) for hematite. These structural information, combined with those for amorphous ferrihydrite and δ-MnO
2, show the excellent correlation with the magnitude of adsorptive isotopic fractionation of Mo reported in previous studies: the proportion of
Oh species or their magnitude of distortion in Mo surface complexes become larger in the order of ferrihydrite
<
goethite
<
hematite
<
δ-MnO
2, a trend identical to the magnitude of isotopic fractionation.
Based on the comparison with previous reports for Mo surface species on various oxides, the chemical factors that affect Mo surface complex structures were also discussed. The hydrolysis constant of cation in oxides, log
K
OH (or the acidity of the oxide surfaces, PZC) is well correlated with the mode of attachment (inner- or outer-sphere) of Mo surface complexes. Furthermore, the symmetric change in Mo species from
Td to
Oh is suggested to be driven by the formation of inner-sphere complexes on specific sites of the oxide surfaces.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Hydrogenetic ferromanganese crusts in the northwestern (NW) Pacific seamounts are considered as potential future sources of strategic metals. The details of their regional and fine-scale variations ...in composition and structure are currently unknown. We attempted to characterize the variation patterns in a selected area of the NW Pacific seamounts, based on geological and geochemical analyses, which were performed with a TV-monitored drill machine and remotely operated vehicles, after full bathymetric mapping and an acoustic survey. Subsequent chemical, mineralogical, and structural analyses of the well-preserved crust cores indicated that the hydrogenetic crusts in the seamount area have similar distinct fine-scale variations throughout the growth layers from the substrate to the surface. The crust cores show almost continuous stratigraphic growth since the middle Miocene (approximately 15 Ma ago) or earlier, with significant variations in metal concentration, growth structure, and mineralogy. The fine-scale stratigraphic variations are surprisingly well-correlated across the seamount areas of approximately 1000 km apart; thus, the bulk compositional variation becomes much smaller than a secular variation when compared within the seamount area. Stratigraphic analysis is crucial for the compositional characterization of ferromanganese crusts on various scales.
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BFBNIB, DOBA, GIS, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Ferromanganese nodules are cm‐sized, authigenic, abyssal manganese‐iron‐hydroxide concretions. They grow very slowly at rates of a few mm per million years. Although their ages are older than ...millions of years, they are often found half buried on the modern sediment surface. The mechanisms for the nodules' persistence at the surface without complete burial could be linked to their occasional motion or agitation. Here, we report evidence for the rotation of a nodule from the Penrhyn Basin, South Pacific detected by paleomagnetism. The paleomagnetic inclinations of specimens from the nodules' surface are consistent with the recent geomagnetic fields. The paleomagnetic directions from the surface to the core show successive changes and form a great circle with a pole at (azimuth = 53.9°, dip = 32.1°). This suggests that the nodule rotated along its pole while successively recording magnetizations. As the nodule was found on a gentle slope at the foot of an abyssal hill, it may have moved downslope due to bottom current underwashing. Rock magnetic analyses of the nodule suggest the presence of magnetite in single domain and vortex states. Low temperature magnetometry revealed that magnetite grains were heavily oxidized to maghemite, especially close to the core of the nodule. The rotation may have exposed the rising part of the nodule to oxidative pore water. Oxygenated Antarctic Bottom Water might have caused remagnetization due to low temperature oxidation of magnetite. The rotation would also facilitate the omnidirectional growth of the nodules' mixed layer of diagenetic buserite and hydrogenetic vernadite.
Plain Language Summary
Ferromanganese nodules are deep marine mineral resources that grow very slowly at a speed of few mm per million years on the seabed. They are very old, but often remain on the sediment surface. It remains unclear, why they were not buried by deep‐sea sedimentation. A nodule recovered from the Penrhyn Basin in the South Pacific was measured to reveal its fossil magnetization by the ancient Earth's magnetic field. Each specimen from the nodule showed a stable magnetization that is linearly decreasing toward the origin during alternating field demagnetization. The dip of the magnetization for the surface is consistent with the recent Earth's magnetic field, while the magnetization directions toward the nodule's center form a great circle. This suggests that the nodule rotated along a pole while successively recording magnetizations. The nodule may have moved downslope due to sediment removal by deep current. The rotation may have made the pore water within the nodule more oxidative, causing loss of primary magnetization and acquisition of secondary magnetization by removing Fe2+ from magnetite (Fe3O4). The rotation of the nodule would also stimulate the growth of its two interlayered mineral phases, of which one grows better in sediment and the other in seawater.
Key Points
A ferromanganese nodule of the South Pacific showed stable magnetizations, whose directions form a great circle suggesting rotation with time
The major magnetic mineral is magnetite, and its oxidation to maghemite may have caused acquisition of secondary magnetization during rotation
Rotation is important for the chemistry of nodules as it permits the growth of a mixed layer with diagenetic buserite and hydrogenetic vernadite
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Thick hydrogenetic ferromanganese (FeMn) crusts from the northwest and central Pacific seamounts often show a distinct dual structure composed of a typical hydrogenetic porous, friable upper part of ...FeMn oxides (Layer 1) and the underlying dense, hard phosphatized growth generation of FeMn oxides (Layer 2 in this study). Layer 2 always appears above the substrate rock and composes the lower part of the crust; it is never found as the upper crust layer in contact with seawater. The chemical composition of Layer 2 clearly differs from the younger Layer 1 hydrogenetic FeMn oxides, and is depleted in Fe, Al, Ti, and Co, and enriched in Ni, Cu, and Zn relative to Layer 1. The Be isotope age models of the crusts were refined with paleomagnetic and paleontological information, and applied to selected crust samples. The age model indicates fairly continuous growth from the substrate to the surface and fairly constant growth rates during the past 17Ma. The growth rate from the Miocene to the present has varied by a factor of two, about 2–4mm/Myr in Layer 1, while Layer 2 has similar but more variable growth rates than Layer 1.
The calculated age for the base of Layer 1, and possibly the age of termination of phosphatization, is never younger than the late Miocene. The age seems to vary with water depth, shallower-water crusts (between 991 and 1575m) showing a younger age of about 10Ma whereas the deeper-water (2262m) crusts have extrapolated ages for the base of Layer 1 of be 17.1±2.5Ma. This trend indicates that phosphatization took place in a less-oxidizing environment during growth of Layer 2, followed by a weakened oxygen-minimum zone or intensified AABW during growth of Layer 1.
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•Phosphatized older layer was found from hydrogenetic Fe–Mn crusts from more than 60 samples in the NW and central Pacific.•The ages of the base of non-phosphatized layer seem to vary from 17.1Ma to 7.8Ma accordingly with water depth.•The chemical composition of phosphatized layer is of characteristic nature of early-diagnenetic buserite.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Marine ferromanganese crusts and nodules as potential mineral deposits have received increasing attention. However, much less knowledge is available concerning the incorporation and enrichment ...mechanisms for antimony (Sb) and arsenic (As) in marine ferromanganese oxides. In this study, the surface complexations of Sb(V) and As(V) on synthetic ferrihydrite and Mn oxides (δ-MnO2 and birnessite) were investigated by a combination of adsorption experiments, extended X-ray absorption fine structure (EXAFS) analyses, and quantum chemical calculations. The speciation, distribution, and local structure of Sb and As in different types of natural marine ferromanganese oxides were determined by X-ray absorption near edge structure (XANES) and EXAFS analyses to reveal the enrichment mechanisms for the two elements in ferromanganese oxides at the molecular level. To the best of our knowledge, the Sb EXAFS analyses for different types of marine ferromanganese oxides are herein reported for the first time. Results showed that Sb(V) is preferentially adsorbed on Mn oxides through energetically favorable bidentate–mononuclear complexation because of the structural similarity between the octahedron SbV(OH)6− and MnO6 unit, although bidentate–binuclear (corner-sharing) and bidentate–mononuclear (edge-sharing) complexes can be formed on ferrihydrite and Mn oxides for the adsorption of Sb(V). By contrast, tetrahedral AsVO43− is mostly adsorbed on ferrihydrite and Mn oxides with the formation of bidentate–binuclear complexes. In natural marine ferromanganese oxides, Sb and As can be retained by Fe and Mn (oxyhydr)oxide components, and the disparate distribution of the two elements to Mn oxides may largely depend on the Mn/Fe ratio and constituent minerals. The larger enrichment factor of Sb than that of As in marine ferromanganese oxides may result from their preferential attachment modes onto the Fe and Mn phases and different inhibition effects from coexisting anions in seawater. Compared with As, a part of Sb may be strongly associated with the lateral sites in Mn oxides via the formation of bidentate edge-sharing complexes, with which anions such as sulfate in seawater do not significantly compete. The findings from this study provide the molecular-scale insights into the enrichment processes and mechanisms of Sb and As in marine ferromanganese oxides. Our study also helps elucidate the incorporation mechanisms and geochemical behaviors of other oxyanions in marine and surface environments.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
We present the first report of the concentrations and stable isotope compositions of Te in the surface layer of ferromanganese crusts collected from the slope of two seamounts in the Northwest ...Pacific Ocean, Takuyo-Daigo and Takuyo-Daisan, which cover water depths from 1000 to 5500 m. The Te concentration in the surface layer of the ferromanganese crusts sharply decreases with increasing depth at shallow regions, but this decrease becomes more gradual at deeper regions. The Te isotope composition exhibits two trends that become (1) lighter with increasing water depth at shallow depths and (2) heavier with increasing water depth at greater depths. These profiles are the same for the two seamounts and indicate two types of correlations between the concentration and isotope composition with water depth. The turning points are located at approximately 2000 m for Takuyo-Daigo and 3200 m for Takuyo-Daisan, which likely correspond to different oxygen minimum zones. Co-precipitation processes with Fe and oxidation involving Mn may be responsible for the variation of Te concentration and isotope composition with water depth. These processes are also related to a change of the dissolved oxygen concentration in ambient seawater. The Te isotope compositions in ferromanganese crusts may therefore serve as a potential proxy for changes of the oxygen minimum zone of paleoceans.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Hydrogenetic ferromanganese crusts (hereafter referred to as “crusts”) on Pacific seamounts are formed by the precipitation of iron–manganese oxides from seawater on volcanic and biogenic substrate ...rocks. As crusts grow continuously and have very slow growth rates of between 1 and 10 mm/m.y., they can potentially be used as records of the Neogene paleoceanographic and paleoclimatic conditions. Crusts can be considered as compressed sediment cores containing biogenic, volcanogenic, and terrestrial particles that include eolian dusts and the partly weathered products of substrate acquired during its growth. In this study, selective leaching experiments were conducted on a sample of ferromanganese crust, which had been obtained from the Federated States of Micronesia at a water depth of 2262 m. Chemical leaching experiments were conducted using oxalic acid buffered with ammonium oxalate on the crushed crust samples, which is an optimization of previously proposed sequential leaching procedures. The applied method was found to be effective in separating the major mineral phases of crusts from associated metallic components, thereby providing concentration of the residual fraction for use in analysis following the leaching experiment. Using this method, polygenetic particles were extracted from the crust and identified using optical and electron microscopes. They were found to be of various origins and included volcanogenic, biogenic, terrestrial, and extraterrestrial material. In addition, well-sorted prism-shaped chained magnetic particles were observed in residual fractions. Rock magnetic experiments support the idea that the magnetic particles are magnetites and originated from fossil magnetotactic bacteria. The fossil magnetotactic bacteria might have been living on the crust at the time of crust formation. Alternatively, fossil magnetotactic bacteria could have been transported by deep sea currents from the sediment where magnetotactic bacteria originated.
The tungsten (W) species in marine ferromanganese oxides were investigated by wavelength dispersive XAFS method. We found that the W species are in distorted Oh symmetry in natural ferromanganese ...oxides. The host phase of W is suggested to be Mn oxides by μ-XRF mapping. We also found that the W species forms inner-sphere complexes in hexavalent state and distorted Oh symmetry on synthetic ferrihydrite, goethite, hematite, and δ-MnO2. The molecular-scale information of W indicates that the negatively-charged WO42− ion mainly adsorbs on the negatively-charged Mn oxides phase in natural ferromanganese oxides due to the strong chemical interaction. In addition, preferential adsorption of lighter W isotopes is expected based on the molecular symmetry of the adsorbed species, implying the potential significance of the W isotope systems similar to Mo.
Adsorption experiments of W on synthetic ferrihydrite and δ-MnO2 were also conducted. At higher equilibrium concentration, W exhibits behaviors similar to Mo on δ-MnO2 due to their formations of inner-sphere complexes. On the other hand, W shows a much larger adsorption on ferrihydrite than Mo. This is due to the formation of the inner- and outer-sphere complexes for W and Mo on ferrihydrite, respectively. Considering the lower equilibrium concentration such as in oxic seawater, however, the enrichment of W into natural ferromanganese oxides larger than Mo may be controlled by the different stabilities of their inner-sphere complexes on the Mn oxides. These two factors, (i) the stability of inner-sphere complexes on the Mn oxides and (ii) the mode of attachment on ferrihydrite (inner- or outer-sphere complex), are the causes of the different behaviors of W and Mo on the surface of the Fe/Mn (oxyhydr)oxides.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Deep-sea ferromanganese crusts are found ubiquitously on the surface of seamounts of the world's oceans. Considering the wide distribution of the crusts, archaeal and bacterial communities on these ...crusts potentially play a significant role in biogeochemical cycling between oceans and seamounts; however little is known about phylogenetic diversity, abundance and function of the crust communities. To this end, we collected the crusts from the northwest Pacific basin and the Philippine Sea. We performed comprehensive analysis of the archaeal and bacterial communities of the collected crust samples by culture-independent molecular techniques. The distance between the sampling points was up to approximately 2,000 km. Surrounding sediments and bottom seawater were also collected as references near the sampling points of the crusts, and analyzed together. 16S rRNA gene analyses showed that the community structure of the crusts was significantly different from that of the seawater. Several members related to ammonia-oxidizers of Thaumarchaeota and Betaproteobacteria were detected in the crusts at most of all regions and depths by analyses of 16S rRNA and amoA genes, suggesting that the ammonia-oxidizing members are commonly present in the crusts. Although members related to the ammonia-oxidizers were also detected in the seawater, they differed from those in the crusts phylogenetically. In addition, members of uncultured groups of Alpha-, Delta- and Gammaproteobacteria were commonly detected in the crusts but not in the seawater. Comparison with previous studies of ferromanganese crusts and nodules suggests that the common members determined in the present study are widely distributed in the crusts and nodules on the vast seafloor. They may be key microbes for sustaining microbial ecosystems there.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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