Geochemistry and specific features of manganese ore formation in sediments of oceanic bioproductive zones

Processes of authigenic manganese ore formation in sediments of the northern equatorial Pacific are considered on the basis of study of the surface layer (<2 mm) of ferromanganese nodule and four micronodule size fractions from the associated surface sediment (0–7 cm). Inhomogeneity of the nodule composition is shown. The Mn/Fe ratio is maximal in samples taken from the lateral sectors of nodule at the water-sediment interface. Compositional differences of nodules are related to the preferential accumulation of microelements in iron oxyhydroxides (P, Sr, Pb, U, Bi, Th, Y, and REE), manganese hydroxides (Co, Ni, Cu, Zn, Cd, Mo, Tl, W), and lithogenous component trapped during nodule growth (Ga, Rb, Ba, and Cs). The Ce accumulation in the REE composition is maximal in the upper and lower parts of the nodule characterized by the minimal Mn/Fe values. The compositional comparison of manganese micronodules and surface layers of the nodule demonstrated that the micronodule material was subjected to a more intense reworking during the diagenesis of sediments. The micronodules are characterized by higher Mn/Fe and P/Fe ratios but lower Ni/Cu and Co/Ni ratios. The micronodules and nodules do not differ in terms of contents of Ce and Th that are least mobile elements during the diagenesis of elements. Differences in the chemical composition of micronodules and nodules are related not only to the additional input of Mn in the process of diagenesis, but also to the transformation of iron oxyhydroxides after the removal of Mn from the close association with Fe formed in the suspended matter at the stage of sedimentation.     

Geochemistry of rare earth elements in micro-and macronodules from the pacific bioproductive zone

Concentrations and compositions of rare earth elements (REE) in three micronodule fractions (50–250, 250–500, and >500 μm), coexisting macronodules, and host sediments are examined. The samples were collected from three sites (Guatemala Basin, Peru Basin, and northern equatorial Pacific) located in elevated bioproductivity zones of the surficial water. The influence of micronodule size is dominant for REE compositions and subordinate for REE concentrations. For example, the Ce concentration inversely correlates with the micronodule fraction dimension and drops to the lowest value in macronodules and host sediments. The Ce decrease is generally accompanied by the Mn/Fe increase in micro- and macronodules. Hence, the role of diagenetic source of material directly correlates with the micronodule dimension. The contribution of diagenetic source is maximal for macronodules. The REE signature distinctions of micronodules and macronodules can be attributed to variations of hydrogenic iron oxyhydroxides and diagenetic (hydrothermal) iron hydroxophosphates that are the major REE carriers in ferromanganese ore deposits. The relationship and general trend in the chemistry of coexisting macronodules suggest that they can represent products of the initial stage of nodule formation.     

Ferromanganese oxide deposits from the Central Pacific Ocean,II. Nodules and associated sediments

Bulk chemical, mineralogical and selective leach analyses have been made on a suite of abyssal ferromanganese nodules and associated sediments from the S.W. equatorial Pacific Ocean. Compositional relations between nodules, sediment oxyhydroxides and nearby ferromanganese encrustations are drawn assuming that the crusts represent purely hydrogenetic ferromanganese material. Crusts, δMnO₂-rich nodules and sediment oxyhydroxides are compositionally similar and distinct from diagenetic todorokitebearing nodules. Compared to Fe-Mn crusts, sediment oxyhydroxides are however slightly enriched, relative to Mn and Ni, in Fe, Cu, Zn, Ti and Al, and depleted in Co and Pb, reflecting processes of non-hydrogenous element supply and diagenesis. δMnO₂ nodules exhibit compositions intermediate between Fe-Mn crusts and sediment oxyhydroxides and thus are considered to accrete oxides from both the water column and associated sediments.Deep ocean vertical element fluxes associated with large organic aggregates, biogenic calcite, silica and soft parts have been calculated for the study area. Fluxes associated with organic aggregates are one to three orders of magnitude greater than those associated with the other phases considered, are in good agreement with element accumulation rates in sediments, and are up to four orders of magnitude greater than element accumulation rates in nodules. Metal release from labile biogenic material in surface sediments can qualitatively explain the differences between the composition of Fe-Mn crusts and sediment oxyhydroxides.Todorokite-rich diagenetic nodules are confined to an eastwards widening equatorial wedge. It is proposed that todorokite precipitates directly from interstitial waters. Since the transition metal chemistry of interstitial waters is controlled dominantly by reactions involving the breakdown of organic carbon, the supply and degradation rate of organic material is a critical factor in the formation of diagenetic nodules. The wide range of (trace metal/Mn) ratios observed in marine todorokite reflects a balance between the release of trace metals from labile biogenic phases and the reductive remobilisation of Mn oxide, both of which are related to the breakdown of organic carbon.     

Geochemistry of rare earth elements in bottom sediments of the Brazil Basin, Atlantic Ocean

The sedimentation and ore formation were studied in sediments from nine stations located in the 24°W profile in the Brazil Basin of the Atlantic Ocean. The sediments are represented by mio- and hemipelagic muds, which are variably enriched in hydrothermal iron and manganese oxyhydroxides. As compared to the sediments from other basins of the Atlantic Ocean, these rocks are marked by extremely high manganese contents (up to 1.33%) and maximal enrichment in Ce. It was shown that the positive Ce anomaly is related to the REE accumulation on iron oxyhydroxides. Influence of hydrothermal source leads to the decrease of Ce anomaly and LREE/HREE ratio. In the reduced sediments, preservation of positive Ce anomaly and/or its disappearance was observed after iron and manganese reduction. The REE contents were determined for the first time in the Ethmodiscus oozes of the Brazil Basin. Ore deposits of the Brazil Basin are represented by ferromanganese crust and ferromanganese nodules. Judging from the contents of iron, manganese, rare, and trace elements, these formations are ascribed to the sedimentation (hydrogenic) deposits. They are characterized by a notable positive Ce anomaly in the REE pattern. The extremely high Ce content (up to 96% of total REE) was discovered for the first time in the buried nodules (Mn/Fe = 0.88).     

Geochemistry of Rare Earth Elements in Ferromanganese Micro- and Macronodules from the Pacific Nonproductive Zone

Ferromanganese micro- and macronodules in eupelagic clays at Site 35 of the South Basin were examined in order to check the REE distribution during the ferromanganese ore formation in nonproductive zones of the Pacific Ocean. We studied host sediments and their labile fraction, ferromanganese micronodules (fractions 50–100, 100–250, 250–500, and >500 m) from eupelagic clays (horizons 37–40, 105–110, 165–175, and 189–190 cm), and buried ferromanganese micronodules (horizons 64–68, 158–159, and 165–166 cm). Based on phase analysis data, the anomalous REE enrichment of eupelagic clays from Site 35 is related to the accumulation of rare earth elements in iron hydroxophosphates. The Ce concentration, generally linked to manganese oxyhydroxides, is governed by the oxidation of Mn and Ce in oceanic surficial waters. Micronodules (Mn/Fe = 0.7–1.6) inherit compositional features of the labile fraction of sediments. The Ce, Co, and Th concentrations depend on the micronodule dimension. The enrichment of micronodules in hydrogenic or hydrothermal substance is governed by their dimension and the dominant source of suspended oxyhydroxide material. The study of buried ferromanganese micronodules revealed general regularities in the compositional evolution of oxyhydroxide matrices of ferromanganese micro- and macronodules. The compositional variation of micro- and macronodules, relative to the labile fraction of sediments, in the Pacific nonproductive zone dramatically differs from the pattern in bioproductive zones, where micronodule compositions in larger fractions are similar to those in associated macronodules and labile fractions of the host sediment as a result of the more intense suboxidative diagenesis.     

Geochemistry of Rare Earth Elements in the Ocean

This work briefly outlines modern ideas on geochemistry of rare earth elements (REE) in the ocean. Sources of REE and chemical properties of these elements, which govern their migration ability in natural processes, are considered. The REE behavior in the river water–seawater mixing zone is analyzed. The fractionation of dissolved and suspended REE in oceanic water in both aerobic and anaerobic conditions is also considered. It is shown that the variability of REE composition in pelagic sediments reflects the fractionation of these elements in the oceanic water as a consequence of material differentiation in the ocean. The REE distribution in terrigenous, authigenic, hydrothermal, and biogenic constituents of sediments, such as clay, bone debris, barite, phillipsite, Fe–Mn oxyhydroxides (ferromanganese nodules and micronodules), Fe–Ca hydroxo-phosphate, diatoms, and foraminifers, is considered.     

Diagenetic cycling of trace elements in the bottom sediments of the Swan River Estuary,Western Australia

Teflon strips were used in-situ in the bottom sediments at two sites in the Swan River Estuary to collect diagenetic Fe–Mn oxyhydroxides and monitor monthly changes in their morphology and trace element geochemistry. This study demonstrates that substantial concentrations of trace elements accumulate at the redox front during the formation of diagenetic Fe–Mn oxyhydroxides. It is likely that the Fe–Mn oxyhydroxides initially nucleate and grow on the Teflon strips via bacterial activity. Trace element geochemistry of the diagenetic Fe–Mn oxyhydroxides is influenced by changes in the supply of trace elements from either the bottom sediments and/or water column or changes in the physico-chemical status of bottom and porewaters. If sufficient diagenetic Fe–Mn oxyhydroxides are preserved in the upper layer(s) of the bottom sediment it is possible that diagenetic (secondary) trace element enrichment profiles may be produced which modify the historical input of natural or anthropogenic trace element sources. Alternatively, partial or complete dissolution of the diagenetic Fe–Mn oxyhydroxides in response to temporal changes in the redox status of the bottom sediment may lead to a substantial underestimate of trace element fluxes in historical bottom sediment profiles. This study highlights that considerable care must be taken when interpreting short- to long-term geochemical profiles in bottom sediments due to the possible occurrence of rapid, seasonally mediated diagenetic processes.     

Rare elements and Nd and Sr isotopic composition in micronodules from the Brazil Basin,Atlantic Ocean

The Sr isotope stratigraphy of the biogenic apatite was used to determine the age of pelagic sediments in the Brazil Basin (Station 1541) that contain ferromanganese micronodules, nodules, and coatings on the weathered volcanic rocks. The age of sediments at horizons 0–5 and 86–90 cm was estimated at 24.1 ± 0.2 Ma and 24.8 ± 0.2 Ma, respectively. The average sedimentation rate in the Late Oligocene was about 13 mm/ka. The hydrogenous Fe–Mn nodule on the sediment surface with the Mn/Fe value of 1.05–1.95 was formed at a rate of 1.2–2.4 mm/Ma, which is 1000 times lower than the growth rate of buried nodule (Mn/Fe 0.4) at depth of 83 cm. Diagenesis provoked changes in the mineral composition of the buried nodule (asbolane-buserite partially replaced by goethite), leading to the loss of a part of Mn, Ni, Li, and Tl but accumulation of trace elements linked with iron oxyhydroxides (Ce, Th, Be, As, and V) were retained. The composition of manganese micronodules at two studied depths in sediments evolved in the course of two stages of ore formation: related to the oxic and suboxic diagenesis. The Sr isotopic composition in manganese micronodules from both horizons do not differ from that of dissolved Sr in the ocean water. The ¹⁴³Nd/¹⁴⁴Nd ratio, which reflects the Nd isotopic composition in the paleocean during the micronodule formation, varies in manganese micronodules from different horizons and is constant in different size fractions.     

Rare earth element zonation in Pacific ferromanganese nodules

The lower surfaces of ferromanganese nodules from the north equatorial Pacific Ocean, which are enriched in Mn, Cu and Ni, and the upper surfaces, which are enriched in Fe, P and Co, have been analyzed for La, Ce, Nd, Sm, Eu, Gd, Dy, Er and Yb. The REE contents are lower and the Ce anomaly is smaller in the lower surfaces than in the upper surfaces. The magnitude of the Ce anomaly increases with decreasing $\text{Mn}\text{Fe}$ ratio, indicative of a seawater origin. The zonal distribution of the other REE supports the conclusion derived previously from inter-nodule and nodule/sediment relationships that diagenetic fixation of rare earths in sediments affects their enrichment by nodular iron oxyhydroxides.     

Geochemistry of ferromanganese nodules from DOMES site a,Northern Equatorial Pacific: Multiple diagenetic metal sources in the deep sea

The major and minor element composition of ferromanganese nodules from DOMES Site A has been determined by X-ray fluorescence methods. Three phases appear to control the bulk compositions: Mn and Fe oxyhydroxides and aluminosilicates. Relatively wide compositional variations are evident throughout the area. Nodules with high Mn/Fe ratios, high Cu, Mg, Mo, Ni and Zn concentrations and high todorokite/δ-MnO₂ ratios have granular surface textures and are confined to an east-west trending depression with thin Quaternary sediment cover. Nodules with low Mn/Fe ratios, high concentrations of As, Ca, Ce, Co, La, P, Sr, Ti, V, Y and Zr and low todorokite/δ-MnO₂ ratios have smooth surfaces and are confined to shallower areas with relatively thick Quaternary sediment to the north and south of the depression.All nodules in the area have compositions which are influenced by diagenesis, but those with the most marked diagenetic signature (high Mn/Fe and Cu/Ni ratios, low Ce/La ratios and more todorokite) are found in areas of very slow or non-existent sedimentation; many of these nodules are actually in contact with outcropping Tertiary sediment. This paradox may be resolved by postulating, by analogy with some shallow-water occurrences, that the nodules accrete from bottom waters which have enhanced particulate and dissolved metal contents derived from diagenetic reaction in areas remote from the site of nodule formation. The metals are supplied in a bottom flow (probably Antarctic Bottom Water) which also erodes, or prevents modern sedimentation in, the depression. Nodules on the flanks of the depression are not evidently affected by this flow and derive at least pan of their constituent metals from diagenetic reaction in the underlying Quaternary sediment.Apparently, abyssal diagenetic nodules can have an immediate and a remote diagenetic metal source. Metal fluxes derived from pore water dissolved metal gradients may not be relevant to particular accreting nodules if a significant fraction of their metals is derived from outside the area in which they form.     

Geochemistry of the authigenic ferromanganese ore formation in sediments of the Northeast Pacific Basin

Authigenic ferromanganese formations in sediments from two horizons (0–10 and 240–250 cm) located in the low/high bioproductive transitional zone of the Pacific Ocean were studied. In addition to the compositionally different two types of micronodules, crusts and ferromanganese nodules were detected in the surface horizon (0–1 cm). Three size fractions (50–100, 100–250, and 250–500 μm) of manganese micronodules were investigated. In terms of surface morphology, color, and shape, the micronodules are divided into the dull round (MN1) and angular lustrous (MN2) varieties with different mineral and chemical compositions. The dull MN1 are enriched in Mn and depleted in Fe as compared with the lustrous MN2. The Mn/Fe value in the dull MN1 varies from 13 to 14. Asbolane-buserite and birnessite are the major manganese minerals in them. The lustrous MN2 is mainly composed of vernadite with Mn/Fe = 4.3–4.8. Relative to the dull MN1, fraction 50–100 μm of the lustrous MN2 is enriched in Fe (2.6 times), W (1.8), Mo (3.2), Th (2.3), Ce (5.8), and REE (1.2–1.8). Relative to counterparts from the dull MN1, separate fractions of the lustrous MN2 are characterized by a greater compositional difference. For example, increase in the size of micronodules leads to decrease in contents of the following elements: Fe (by 10 rel %), Ce (2 times), W (2.1 times), Mo (2.2 times), and Co (1.5 times). At the same time, one can see increase in contents of other elements: Th and Cu (2.1 times), Ni (1.9 times), and REE (1.2–1.6 times). Differences in the chemical and mineral compositions of MN1 and MN2 fractions can be related to alternation of oxidative and suboxidative conditions in the sediments owing to the input of a labile organic matter, which serves as the major reducer, and the allochthonous genesis of MN2.     

Geochemical features of REE and Y accumulation in the subcolloid fraction of sediments from the northern part of Amur Bay (Sea of Japan)

The average contents of REE, Y, Fe, Mn, and clay minerals were determined in the subcolloid fraction of bottom sediments from the northern part of Amur Bay. The positive correlation of REE and Y contents with Fe and Mn is related to their flocculation and sorption on Fe, Mn oxyhydroxides and clay minerals (hydromica, smectites). The sediments are subdivided into three groups (sediments from the influence zone of the Razdol’naya River, sediments located along the eastern and western shores of the bay, and sediments weakly subjected to the river influence) with different average contents of REE, Y, Fe, and Mn. It is shown that REE and Y are mainly incorporated (98–100%) in clay minerals.     

Geochemistry of ferromanganese nodule–sediment pairs from Central Indian Ocean Basin

Fourteen ferromanganese nodule–sediment pairs from different sedimentary environments such as siliceous ooze (11), calcareous ooze (two) and red clay (one) from Central Indian Ocean Basin (CIOB) were analysed for major, trace and rare earth elements (REE) to understand the possible elemental relationship between them. Nodules from siliceous and calcareous ooze are diagenetic to early diagenetic whereas, nodule from red clay is of hydrogenetic origin. Si, Al and Ba are enriched in the sediments compared to associated nodules; K and Na are almost in the similar range in nodule–sediment pairs and Mn, Fe, Ti, Mg, P, Ni, Cu, Mo, Zn, Co, Pb, Sr, V, Y, Li and REEs are all enriched in nodules compared to associated sediments (siliceous and calcareous). Major portion of Si, Al and K in both nodules and sediments appear to be of terrigenous nature. The elements which are highly enriched in the nodules compared to associated sediments from both siliceous and calcareous ooze are Mo – (307, 273), Ni – (71, 125), Mn – (64, 87), Cu – (43, 80), Co – (23, 75), Pb – (15, 24), Zn – (9, 11) and V – (8, 19) respectively. These high enrichment ratios of elements could be due to effective diagenetic supply of metals from the underlying sediment to the nodule. Enrichment ratios of transition metals and REEs in the nodule to sediment are higher in CIOB compared to Pacific and Atlantic Ocean. Nodule from red clay, exhibit very small enrichment ratio of four with Mn and Ce while, Al, Fe, Ti, Ca, Na, K, Mg, P, Zn, Co, V, Y and REE are all enriched in red clay compared to associated nodule. This is probably due to presence of abundant smectite, fish teeth, micronodules and phillipsite in the red clay. The strong positive correlation (r ? 0.8) of Mn with Ni, Cu, Zn and Mo and a convex pattern of shale-normalized REE pattern with positive Ce-anomaly of siliceous ooze could be due to presence of abundant manganese micronodules. None of the major trace and REE exhibits any type of inter-elemental relationship between nodule and sediment pairs. Therefore, it may not be appropriate to correlate elemental behaviour between these pairs.     

Ferromanganese nodules of freshwater reservoirs of Ol’khon Island (Baikal) and the Kulunda Plain (West Siberia)

Morphology and mineralogical and geochemical compositions of freshwater ferromanganese nodules of Ol’khon Island (Sasa Formation) and Kulunda Plain (Lake Porozhnee) were studied. The study has shown rhythmic structures of the nodules, formed by macro- and microlayers with mineralized microflora. The layers are composed of either crystalline Mn mineral phases and finely dispersed Fe phases (Lake Porozhnee) or, on the contrary, crystalline goethite and X-ray amorphous Mn phases (Ol’khon Island). Separation of Mn and Fe mineral phases in the nodules proceeded during their formation and diagenesis. The freshwater nodules show both high (Lake Porozhnee) and low (Ol’khon Island) Mn/Fe ratios. The predominance of Fe phase in the Ol’khon nodules accounts for their high contents of REE, including Ce. The Porozhnee nodules grew, most likely, more rapidly, which is reflected in their low REE contents and Ce anomaly. The examined chemical and mineral compositions, textures, and structures of the nodules testify to the low-temperature hydrothermal source of their ore substance.     

Mineralogy of morphogenetic types of ferromanganese deposits in the world ocean

The mineralogy and structural features of the main types of ferromanganese deposits—nodules, micronodules, Co-bearing crusts, crustlike nodules, and low-temperature hydrothermal manganese crusts and ferruginous ochers—are considered. The correlation between their mineral composition and structure is shown. The proposed classification of mineral types is based on characteristic assemblages of Fe and Mn minerals.     

Managanese and iron in the White Sea: Sedimentation and diagenesis

Iron and manganese in bottom sediments studied along the sublatitudinal transect from Kandalaksha to Arkhangelsk are characterized by various contents and forms depending on sedimentation environments, grain size of sediments, and diagenetic processes. The latter include redistribution of reactive forms leading to enrichment in Fe and Mn of the surface sediments, formation of films, incrustations, and ferromanganese nodules. Variations in the total Fe content (2–8%) are accompanied by changes in the concentration of its reactive forms (acid extraction) and the concentration of dissolved Fe in the interstitial water (1–14 μM). Variations in the Mn content in sediments (0.03–3.7%) and the interstitial water (up to 500 μM) correspond to a high diagenetic mobility of this element. Changes in the valence of chemical elements results in the redox stratification of sediment strata with maximum concentrations of Fe, Mn, and sulfides. Organic matter of sediments with a considerable terrestrial constituent is oxidized by bottom water oxygen mainly at the sediment surface or in anaerobic conditions within the sediment strata. The role of inorganic components in organic matter oxidation changes from surface sediments, where manganese oxyhydroxide dominates among oxidants, to deeper layers, where sulfate of interstitial water serves as the main oxidant. Differences in river runoff and hydrodynamics are responsible for geochemical asymmetry of the transect. The deep Kandalaksha Bay serves as a sediment trap for manganese (Mn content in sediments varies within 0.5–0.7%), whereas the sedimentary environment in the Dvina Bay promotes its removal from sediments (Mn 0.05%).     

Role of organic matter in the accumulation of platinum in oceanic ferromanganese deposits

In order to elucidate possible processes leading to platinum accumulation in ferromanganese deposits, we analyzed published data on the interaction of dissolved platinum species in different valence states with iron and manganese oxyhydroxides under oceanic conditions and experimentally studied the kinetics of sorption of inorganic and organic complexes of platinum (II) and platinum (IV) on synthetic iron and manganese oxyhydroxides and natural materials (marine colloids, and ferromanganese crust samples). The role of dissolved and suspended particulate aquatic organic matter in the sorption accumulation of platinum was evaluated. Possible reasons for the preferential (compared with other noble metals) accumulation of platinum in oceanic ferromanganese deposits were discussed.     

Investigation on the role of microorganisms in manganese mineralization from Abadeh-Tashk area,Fars Province,southwestern Iran by using petrographic and geochemical data

Hydrothermal manganese and ferromanganese deposits associated with Neyriz ophiolite colored mélange occurred as small ore deposits in the Abadeh-Tashk area, SE of Fars Province, SW Iran. The deposits are found in three types: a) banded syngenetic ores, b) massive boudin and lens shaped diagenetic ores and c) vein and veinlet epigenetic ores. Microtextural, geochemical and mineralogical data associated with petrographic Raman, FTIR and SEM studies indicated that the primary Fe compounds formed series of microbially mediated biomats and Mn compounds were precipitated as an amorphous oxide on an active oxide surface accompanying silica gels. Field relationships between ore and host rock, high Mn/Fe ratio (17.43 to 40.79), ΣLREE, positive Eu and negative Ce anomalies in syngenetic ore types reveal that the ores were formed by hydrothermal fluid in an oceanic floor environment. Manganese was fractionated from iron due to physicochemical changes as well as microbial activities in the sedimentary environment. Microbial remains as filamentous beads with regular circular shapes, vermiform structures, series of Fe-rich biomats, traces of embedded organic material besides trace metals and REE concentrations in Mn ores emphasize the role of microorganisms in Fe and Mn precipitation. Syngenetic mineralization took place under suboxic neutrophilic conditions, while diagenetic processes resulted in variably reduced Fe- and Mn-oxides via organic matter decomposition, forming rhodochrosite as the end product. Braunite formation occurred most probably as a biogeochemically mediated early diagenetic product. Diagenetic and epigenetic Mn ores were formed when primary Mn deposits underwent subsequent diagenetic and remobilization–redeposition events respectively.     

Comparison of elemental accumulation rates between ferromanganese deposits and sediments in the South Pacific Ocean

Rates of accumulation of Fe and Mn, as well as Cu, Ni, Co, Pb, Zn, Hg, U and Th have been determined for five ferromanganese deposits from four localities in the South Pacific Ocean.Manganese is accumulating in nodules and crusts at a rate roughly equivalent to that found to be accumulating in sediments in the same area. Iron shows a deficiency in accumulation in nodules and crusts with respect to sediments, especially near the continents, but also in the central and south-central Pacific. Copper is accumulating in nodules and crusts at a rate one order of magnitude less than the surrounding sediments.This is interpreted as meaning that most of the Mn is supplied as an authigenic phase to both sediments and nodules while Fe is supplied mostly by ferromanganese micro-nodules and by detrital and adsorbed components of sediments; and Cu is enriched in sediments relative to nodules and crusts most probably through biological activity.     

Composition and characteristics of the ferromanganese crusts from the western Arctic Ocean

Layered ferromanganese crusts collected by dredge from a water depth range of 2770 to 2200 m on Mendeleev Ridge, Arctic Ocean, were analyzed for mineralogical and chemical compositions and dated using the excess ²³⁰Th technique. Comparison with crusts from other oceans reveals that Fe-Mn deposits of Mendeleev Ridge have the highest Fe/Mn ratios, are depleted in Mn, Co, and Ni, and enriched in Si and Al as well as some minor elements, Li, Th, Sc, As and V. However, the upper layer of the crusts shows Mn, Co, and Ni contents comparable to crusts from the Atlantic and Indian Oceans. Growth rates vary from 3.03 to 3.97 mm/Myr measured on the uppermost 2 mm. Mn and Fe oxyhydroxides (vernadite, ferroxyhyte, birnessite, todorokite and goethite) and nonmetalliferous detrital minerals characterize the Arctic crusts. Temporal changes in crust composition reflect changes in the depositional environment. Crust formation was dominated by three main processes: precipitation of Fe-Mn oxyhydroxides from ambient ocean water, sorption of metals by those Fe and Mn phases, and fluctuating but large inputs of terrigenous debris.