Fe-Mn Micronodules Research Articles

Rare earth element enrichment process of bioapatite in deep-sea REY-rich sediments

Rare earth element and yttrium (REY) compositions of bioapatite (e.g., fish tooth and bone) can serve as potential paleoceanographic indicators. However, the REY enrichment of bioapatite and REY transfer from FeMn micronodule to bioapatite remain unclear owing to a lack of comparative study on these processes under variable redox conditions (oxic vs. suboxic), which hampers the utility of these indicators. To address these uncertainties, we conducted in situ geochemical analyses of fish teeth and FeMn micronodules from two REY-rich sediment cores (GC01 and GC02) collected from the Clarion–Clipperton fracture zone in the central equatorial Pacific. We found that the Ce/Ce* ratios of fish teeth from GC01 (sediments ΣREY = 723 ± 274 ppm) and GC02 (sediments ΣREY = 506 ± 65 ppm) gradually increased with depth under oxic conditions, with calculated oxic pore water-derived REY increasing from ∼0–3% on the surface to ∼11–24% at 200 cmbsf. In deep sediment columns (>200 cmbsf), the suboxic pore water contributed a small amount of REY (∼4% to ∼13%) to fish teeth, as evidenced by sharp increases in Ce/Ce* ratios of fish teeth and decreases in Ce/Ce* ratios and increases in YN/HoN ratios of micronodules. Therefore, the REY-patterns of fish teeth in core-deep samples were overprinted by oxic–suboxic pore waters may be unreliable archives of ancient bottom seawater.

Depositional and circulation changes at the Chukchi margin, Arctic Ocean, during the last two glacial cycles

The shallow Chukchi-East Siberian margin of the Arctic Ocean was repeatedly impacted by Pleistocene glaciations and related changes in sea level and circulation. The depositional history across the last two Arctic glacial cycles is investigated in sediment core ARC6-C15 from the foot of the Chukchi Rise, an extension of the Chukchi Sea shelf that was impacted by the East Siberian Ice Sheet (ESIS). Our proxy data indicate a large extent of the ESIS, likely blocking the westward sediment transport from the Laurentide Ice Sheet (LIS) during the penultimate glaciation estimated to have occurred within an age span from Marine Isotope Stage (MIS) 4 to 6. The deglacial environments are characterized by enhanced sediment inputs from the East Siberian and Chukchi shelves, while overlying interstadial/interglacial sediments have a signature of the Chukchi Sea and the Beaufort Gyre. During the last glaciation, the ESIS had a smaller impact on the Chukchi margin, and the deglaciation was dominated by the LIS sediment sources. Glacial discharge from the Mackenzie area is identified in the Bølling/Allerød interval, while later deglacial pulses can be tracked to the Canadian Arctic Archipelago. In addition to glacigenic sediment inputs, both deglacial intervals contain FeMn micronodules, possibly formed under the influence of meltwater pulses during sea level rise. An increase in the chlorite content that likely marks the flooding of the Bering Strait is identified at the start of the Holocene. A pronounced oxygen and carbon isotope excursion in the early Holocene may indicate hydrographic changes in the Arctic Ocean related to the 8.2 ka meltwater discharge event.

The transformation and accumulation mechanism of rare earth elements in deep-sea sediments from the Wharton Basin, Indian Ocean

Recently, rare-earth elements and yttrium (REYs, including La-Lu and Y) have been found to be remarkably enriched in Pacific and Indian Ocean deep-sea sediments, as the extreme enrichment of critical metals have received unprecedented attention. However, the origins, transformation and accumulation mechanism of REYs in deep-sea sediments remain controversial. In this study, the mineralogical and geochemical characteristics of REY-rich sediments from the Wharton Basin, Indian Ocean (core GC34) were systematically analyzed. Our results indicate that bioapatites are the most significant host phase of REYs, ambient solutions (seawater and pore water) are the main sources of REYs in deep sea sediments. Besides being the other important host phase of REYs, Fe-Mn micronodules also act as the links between sources and sinks of REYs in deep-sea sediments. Based on the variations in the characteristic profiles of ΣREY, δCe, and MREE/MREE* with depth, combined with the fractionation characteristics of LREE, MREE and HREY in bulk sediments, bioapatites, and Fe-Mn micronodules, we present a model of REYs transformation and accumulation processes in the deep-sea sediments, which mainly occurs in two intervals. The first and dominant accumulation process is in the sediment–water interface (SWI) and shallow sediments, where Fe-Mn micronodules and bioapatites adsorb and/or substitute REYs directly from ambient solutions, resulting in the initial accumulation of REYs in bioapatites and micronodules. The secondary accumulation process in the deep subsurface sediments, due to desorption and resorption processes in bioapatites and release processes in micronodules. The significant connection between P/Th and ΣREY suggests that the physical enrichment of REY-rich materials (bioapatites and micronodules) contributed to the REYs enrichment in deep-sea sediments.

Distribution of rare earth elements and implication for Ce anomalies in the clay-sized minerals of deep-sea sediment, Western Pacific Ocean

Recent studies reveal that bioapatite and FeMn micro-nodules are predominantly hosts of rare earth elements and yttrium (REYs) in bulk deep-sea sediments by in-situ analysis and chemical extraction. However, individual clay-sized fraction (<2 μm; CSFS) could also be an important REY carriers, in which chemical species and distribution of REYs are unclear. In order to clarify the REY migration or cycle and light/heavy REE enrichment mechanism in different clay-sized minerals, this study undertook clay-sized separation, sequential chemical extraction, X-ray diffraction (XRD), X-ray fluorescence spectrometer (XRF) and inductively coupled plasma–mass spectrometry (ICP-MS) analysis of REY-rich sediment samples from two sites in the western Pacific Ocean. The results show the REY in CSFS are mainly hosted in the amorphous Fe phase (Feox1), with lesser amounts in carbonate and the crystalline Fe phase (Feox2), with average proportions of 68.0%, 27.5%, and 2.9% at site 30, and 75.2%, 16.2%, and 7.1% at site 64, respectively. The bulk sediments and CSFS show no obvious Eu anomalies, suggesting there was limited hydrothermal input. Carbonate may be an authigenic phase that inherited the REY characteristics of seawater by substituting for Ca2+, and organic degradation resulted in larger negative Ce anomalies than in seawater. The ΣREY contents increase and the heavy REEs are more readily adsorbed onto the Feox1 under oxidizing conditions. The Feox2 phase may be transformed from Feox1, which facilitates the release of light REEs. The negative Ce anomalies in Fe phase minerals are not solely caused by the depositional reduce conditions because an obvious change of Ce anomaly in different iron phase minerals.

Quantifying the controlling mineral phases of rare-earth elements in deep-sea pelagic sediments

Recent studies suggest that pelagic sediments can enrich rare-earth elements (REE) acting as a significant reservoir for the global REE budget as well as a potential resource for future exploitation. Although Ca-phosphate (e.g., bioapatite fossils) and Fe-Mn (oxyhydr)oxides (e.g., micronodule) have been considered important REE carriers in deep-sea sediments, the proportion of REE held by each mineral phase remains enigmatic. Here, we have investigated the sediments from two promising REE-rich prospective areas: the Tiki Basin in the Southeast Pacific (TKB) and the Central Indian Ocean Basin (CIOB). The mineral grains including bioapatite fossils and Fe-Mn micronodules have been inspected individually by in-situ microscale analytical methods. Correspondently, the REE bound to Ca-phosphate and Fe-Mn (oxyhydr)oxides have been sequentially extracted and quantified. The crucial role of Ca-phosphate is substantiated by sequential leaching which reveals its dominance in hosting ~69.3–89.4% of total REE. The Fe-Mn (oxyhydr)oxides carry ~8.2% to 22.0% of REE in bulk sediments, but they account for ~70.0–80.5% of Ce owing to their preferential adsorption of Ce over the other REE. Surface sediment on modern seafloor can accumulate high REE contents resulting from the REE scavenging by the host phases within the range of sediment-seawater interface. Differences between TKB and CIOB samples indicate that the REE enrichment in the deep-sea environment may be controlled by multiple factors including the productivity of overlying seawater (e.g., phosphorus flux), water depth relative to carbonate compensation depth (CCD), sedimentation rate, redox condition, and hydrothermal vent input (e.g., Fe-Mn precipitations).

Geochemistry of ferromanganese micronodules recovered from sediment-core in the western Nigeria continental margin, Eastern Equatorial Atlantic: Implications on the genesis and depositional history.

This study examines the geochemical compositions, origin and depositional characteristics of ferromanganese (Fe–Mn) micro-nodules recovered from 350 cm long sediment core DY26III-Nig-S60-GC2 at 2946 m water depth from the slope of the Eastern Equatorial Atlantic (EEA), offshore Nigeria (4°28′56′′.388′E, 3°33′10′′.764′N) within the 200 nautical miles of the Exclusive Economic Zone (EEZ). The hydrographic regime of the area of study is dominated by westward flowing North and South Equatorial Currents and the eastward flowing shallow water mass referred to as Guinea current. Observations made on the morphology of the Fe–Mn micro-nodules recovered allowed us to distinguish two distinct varieties (type-1 and type-2). The chemical composition of the type-1 Fe–Mn micro-nodules contain enriched Fe (8.55–29.05 wt%), Pb (0.03–0.35 wt%) and Co (0.04–0.68 wt%) relative to type-2 Fe–Mn micronodule that contains enriched Mn (23.85–48.75 wt%), Ni (0.07–0.75 wt%), Cu (0.25–1.55 wt%), Zn (0.05–0,56 wt%) and Ca (3.95–5.56 wt%). The geochemical investigations suggest hydrogenetic origin for the formation of the Fe–Mn micro-nodules from the EEZ, offshore, Nigeria. The sediment's depositional conditions of type-1 Fe–Mn micronodule correspond to a dry period of reduced sea-level rise (Marine isotope stage; MIS-2) during the late Pleistocene with the relative age of 16.96–12kyr. However, type-2 Fe–Mn micro-nodule corresponds to a warm period of sea-level rise and increased fluvial discharges (MIS-3) with sediment's depositional age of 47.22–41.93kyr. Based on chemical composition, discrimination plots and statistical analysis, the type-1 and 2 Fe–Mn micronodules can be classified as hydrogenetic, with additional remobilization of metals through the underlying sediments on the type-2 Fe–Mn micronodule.

Genesis of REY-rich deep-sea sediments in the Tiki Basin, eastern South Pacific Ocean: Evidence from geochemistry, mineralogy and isotope systematics

Deep-sea sediments in the eastern South Pacific Ocean were recently found to contain high concentrations of rare-earth elements and yttrium (REY). To understand the genesis of REY enrichment in the sediments, we performed detailed element and isotope geochemical analyses on samples from gravity core S028GC23 recovered from the Tiki Basin in the eastern South Pacific Ocean.The REY-rich sediments in core S028GC23 are dark-brown to black zeolite clays, with REY (∑REY) contents of 1136–2213 ppm (average of 1857 ppm). The ∑REY contents in the sediments are positively correlated with P2O5 and negatively correlated with Ce anomalies. The REY-rich sediments display obvious Ce depletion and positive Y anomalies in the post-Archean Australian shale (PAAS)-normalized REY diagrams. Bioapatite fossils contain the highest REY concentrations (average of 8921 ppm) among the constituents, indicating that they are the primary hosts of REY. The REY enter through the roots and diffuse to the tops in the bioapatite fossils. Fe–Mn micronodules are divided into two different types based on Ce and Y anomalies in the PAAS-normalized REY patterns, and they have average REY concentrations of 781 ppm and 937 ppm, respectively. The phillipsite has low REY contents (average of 106 ppm) and is associated with a low sedimentation rate. REY patterns and Sr-Nd isotopic signatures show that seawater was the main ore-forming fluids during the formation of REY-rich sediments. The sedimentation rate of the sediments calculated by the 230Thex activities was 0.8 mm/Ka. The REY accumulation process can be explained by two-stage fluid-bioapatite fossil interactions and small contributions of Fe-Mn micronodules in an oxidized environment with slow sedimentation rate.

Stratigraphic Variations of Fe–Mn Micronodules and Implications for the Formation of Extremely REY-Rich Mud in the Western North Pacific Ocean

The origin of deep-sea sediments in the western North Pacific Ocean, which are significantly enriched in rare-earth elements and yttrium (REY), and its paleoceanographic implications have been poorly constrained. Here, we investigated stratigraphic variations in the chemical compositions and textures of ferromanganese (Fe–Mn) micronodules separated from western North Pacific sediments. The characteristics of the micronodules of an extremely REY-rich mud layer vary from almost purely diagenetic to relatively hydrogenetic. This indicates the abundant supply of organic matter to the sediment together with fish debris that accumulates REY at the onset of the REY-enrichment of the mud, followed by the exposure of the seafloor to oxic water masses during the latter half of the formation of the REY-rich mud. These results support a previously proposed formation mechanism based on which enhanced bottom water currents caused pelagic fish proliferation via the upwelling of nutrients and fish debris was physically sorted and selectively accumulated on the seafloor. After the main REY-enrichment, the micronodules exhibit varying diagenetic signatures, suggesting changes in the bottom current intensities after the main REY-enrichment. However, the bulk REY contents do not increase. This implies that a sufficient increase in the fish productivity is an essential factor affecting the formation of REY-rich mud.

The Influence of Hydrothermal Fluids on the REY-Rich Deep-Sea Sediments in the Yupanqui Basin, Eastern South Pacific Ocean: Constraints from Bulk Sediment Geochemistry and Mineralogical Characteristics

Rare earth elements (REEs) and yttrium (Y), together known as REY, are extremely enriched in deep-sea pelagic sediments, attracting much attention as a promising new REY resource. To understand the influence of hydrothermal processes on the enrichment of REY in deep-sea sediments from the eastern South Pacific Ocean, we conducted detailed lithological, bulk sediment geochemical, and in situ mineral geochemical analyses on gravity core sample S021GC17 from the Yupanqui Basin of eastern South Pacific. The REY-rich muds of S021GC17 are dark-brown to black zeolitic clays with REY contents (ΣREY) ranging from 1057 to 1882 ppm (average 1329 ppm). The REY-rich muds display heavy rare earth elements (HREE) enriched patterns, with obvious depletions in Ce, and positive anomalies of Eu in Post-Archean Australian Shale (PAAS)-normalized REE diagrams. In contrast, the muds of S021GC17 show light rare earth elements (LREE) enriched patterns and positive anomalies of Ce and Eu in the seawater-normalized REE diagrams. Total REY abundances in the core show positive correlations with CaO, P2O5, Fe2O3, and MnO concentrations. In situ analyses of trace element contents by laser ablation-inductively coupled plasma–mass spectrometry (LA-ICP-MS) demonstrate that bioapatite fossils contain high REY concentrations (998 to 22,497 ppm, average 9123 ppm), indicating that they are the primary carriers of REY. The in situ Nd isotope values of bioapatites are higher than the average values of seawater in Pacific Ocean. Fe–Mn micronodules are divided into hydrogenetic and diagenetic types, which have average REY concentrations of 1586 and 567 ppm, respectively. The high contents of Fe-Mn-Ba-Co-Mo, the positive correlations between ΣREY and Fe-Mn, the ratios of Fe/Ti and Al/(Al + Fe + Mn), and the LREE-enriched patterns in the REY-rich muds, combined with high Nd isotope values shown by bioapatite fossils, strongly indicate that the hydrothermal fluids have played an important role in the formation of the REY-rich sediments in the eastern South Pacific Ocean.

Geochemical features of Fe-Mn micronodules in deep-sea sediments of the western North Pacific Ocean: Potential for co-product metal extraction from REY-rich mud

Rare earth element and yttrium (REY)-rich mud around Minamitorishima Island, Japan, in the western North Pacific Ocean has received significant attention as a novel REY resource. In REY-rich muds, industrially critical metals such as Co, Ni, and Mo occur in relatively high concentrations. We show that Fe-Mn micronodules separated from REY-rich muds are significantly enriched in these elements; we measured Co contents of almost 3000 ppm and Ni contents that exceed 39,000 ppm. Through experiments, we demonstrated that reductive chemical leaching can be used to extract major portions (generally >80%) of the metals associated with Fe-Mn (oxyhydr)oxides from REY-rich muds. The Fe-Mn micronodules were of an oxic diagenetic origin. A simple mass balance calculation indicated that the contribution of the Fe-Mn micronodules to the bulk Fe-Mn (oxyhydr)oxides in the sediment can amount to 8%–50% (median: 18%). The other metals associated with Fe-Mn (oxyhydr)oxides seem to be hosted in Mn-microparticles, probably of a hydrogenetic origin. We estimate that the most promising site for REY-rich mud development contains (9.3 ± 0.6) × 102 t, (1.6 ± 0.1) × 103 t, and (2.8 ± 0.2) × 102 t of leachable Co, Ni, and Mo, respectively, in 1 km2 × 0–10 m below seafloor. These amounts account for 0.66 ± 0.04%, 0.061 ± 0.004%, and 0.097 ± 0.007% of present annual global productions of Co, Ni, and Mo, respectively. The feasibility of extracting these metals as co-products during REY-rich mud development depends on the cost-efficiency of the recovery process, which ultimately depends on their market price in the near future.

Fe-Mn (oxyhydr)oxides as an indicator of REY enrichment in deep-sea sediments from the central North Pacific

Nowadays, pelagic sediments are globally explored as a potential resource of rare earth elements and yttrium (termed as REY), and have received unprecedented attention. However, the formation mechanism of REY-rich deep-sea mud remains controversial. The Fe-Mn (oxyhydr)oxides are ubiquitous in oxic marine sediments and potentially beneficial for the REY enrichment. The research of them is important for understanding the ore-formation of REY-rich mud, and thus can help to evaluate the role of marine sediments in global REY cycles. In this contribution, using advanced in situ analytical techniques, we provide new results on the nanoscale mineralogy and geochemistry of Fe-Mn (oxyhydr)oxides that are recovered from REY-rich deep-sea sediments in central North Pacific Ocean. In situ X-ray diffraction analyzer and scanning electron microscope reveal that the Fe-Mn minerals (e.g., vernadite, birnessite, and todorokite) occur as overgrowths on the surface of nuclei to form Fe-Mn micronodules. Transmission electron microscope (TEM) imaging reveals that these Fe-Mn minerals are composed of nanocrystals with high intercrystalline porosity, indicating a potential strong adsorption capacity. The TEM observations also imply a mineral phase transformation from birnessite (vernadite) to todorokite during the oxic diagenesis, accompanied by corresponding changes in geochemical compositions. Laser ablation-inductively coupled plasma-mass spectrometry demonstrates that Fe-Mn (oxyhydr)oxides contain high REY concentrations (varying from 439 to 1,654 ppm, with an average of 790 ppm) and can be regarded as an important host of REY. Furthermore, the PAAS-normalized and seawater-normalized REY patterns both infer that the REY contents of Fe-Mn (oxyhydr)oxides are ultimately derived from seawater and characterized by preferential scavenging of Ce. Overall, Fe-Mn (oxyhydr)oxides not only contribute a moderate proportion of REY, but also provide specific information for understanding geochemical processes related to REY enrichment in deep-sea sediments.

Enrichment of rare earth elements in siliceous sediments under slow deposition: A case study of the central North Pacific

As an important submarine rare earth elements (REE) exploration target, the REE-rich deep-sea mud in the Pacific Ocean has recently attracted much research attention, yet its metallogenic mechanism has not been sufficiently addressed. In this study, we conducted detailed grain size analysis, mineral identification and geochemical measurement for the MG026 sediment core in the central North Pacific. The >63 μm fraction of the samples mainly contains siliceous bioclastics, phillipsite accretions, fish teeth and bones and ferromanganese micro-nodules; the 4–63 μm fraction mainly contains the abovementioned biological detritus, together with ilmenite, quartz and zeolite; the <4 μm fraction mainly contains barite, clay minerals, carbonate-fluorapatite and amorphous ferric hydroxide. The rare earth elements and yttrium (ΣREY) contents in the samples can reach 810.4 ppm, and are mainly concentrated in the biological apatite (fish teeth and bones) and the Fe-Mn oxide-hydroxide on the micro-nodule surface. The grain size analysis suggests that the smaller the Mz (mean grain size), the higher the REY enrichment. The post-Archean Australian shale-normalized REE patterns are slightly HREE-enriched with significant negative Ce anomalies, indicating that the rare earth elements of the samples are mainly seawater-derived with minor terrigenous input. We conclude that the REY content is controlled by the grain size, the amount of fish teeth and sedimentation rate: With low sedimentation rate, REYs from the seawater may have mainly replaced the Ca2+ ions of biological apatite lattice in form of isomorphism, and minor REYs may have also adsorbed on Fe-Mn micro-nodules due to the scavenging effect. As a concurrent result of the low sedimentation rate, the mean grain size of sediment may have decreased, and abundant phillipsite may have been formed.

Can mima-like mounds be Vertisol relics (Far North Region of Cameroon, Chad Basin)?

Non-anthropogenic earth mounds, defined as mima-like mounds in this study, have recently been observed in non-carbonate watersheds along the Sudano-Sahelian belt in the Chad Basin. In the Diamare piedmont (northern Cameroon) they are particularly well developed within stream networks. In less eroded areas, they occur as whaleback, flattened morphologies, or even as buried features. All these shapes are composed of clay-rich sediment associated with high proportions of secondary carbonate nodules and FeMn micro-nodules. Their soil structure is prismatic to massive and vertical cracks are observed locally. Grain-size distributions emphasize the clay-rich nature of the sediment, with average clay contents of 32%±12.8% (n=186), which is significantly higher than the clay content in the adjacent sediments in the landscape (mean=10%±4%, n=21). Moreover, high proportions of smectite characterize the soil, with average contents of 34±7% (n=25). At the micro-scale, the groundmass has a cross-striated b-fabric, with embedded smooth subangular quartz and feldspar grains of the silt-size fraction. All the characteristics point to altered vertic properties in the clay-rich sediment composing the mima-like mounds. Mima-like mounds are thus interpreted as degraded Vertisols. Compared to present-day Vertisols occurring in the piedmont, mima-like mounds are located upstream. It is thus proposed that the Vertisol areas were more extensive during a former and wetter period than the present-day. Subsequent changing climatic conditions increased erosion, revealing the gilgai micro-relief by preferential erosion in micro-lows rather than in micro-highs. Mima-like mounds of the Chad Basin might thus result from pedogenesis combined with later erosion. These local processes can be inherited from regional climatic variations during the Late Pleistocene-Holocene and likely be related to the African Humid Period.

Native Hg liq in the metalliferous sediments of the East Pacific Rise (21°S)

Native Hg droplets, documented here for the first time in mid-ocean ridge metalliferous sediments, were found in the surface sediment of a magmatically robust and ultra-fast spreading mid-ocean ridge, the East Pacific Rise. The surface sediment and Fe–Mn micronodules in it are enriched in Hg. Two different processes can produce Hg 0 liq in this environment: hydrothermal and biogenic. Bio-transfer of ionic Hg dissolved in the hydrothermal fluids into Hg 0 in the sediment can account for the eventual native Hg occurrences in the close vicinity of the hydrothermal vents. The Hg 0 liq occurrences in the distal metalliferous sediments most probably have originated directly from the hydrothermal fluids and been transported by hydrothermal plumes away from the vents. Condensation of Hg 0 liq from Hg 0 vap in the vent fluids subjected to near critical phase separation seems the most plausible mode of origin of the native Hg occurrences. The high-temperature vents at the global spreading ridge chain introduce an assessed amount of 1.62–4.86 × 10 3 t/yr Hg 0 liq into the ocean. With time, the deposited Hg 0 liq will likely be dissolved in the overlaying seawater and will not survive in the geologic record.

Fe–Mn micronodules born in the metalliferous sediments of two spreading centres: the East Pacific Rise and Mid-Atlantic Ridge

A geochemical study of Fe–Mn micronodules associated with the metalliferous sediments at two spreading centres has shown that their composition depends on the site of micronodule formation. Close to the hydrothermal mounds they exhibit significant variation in elemental content related to the type of hydrothermal discharge (low- or high-temperature), the nature of primary hydrothermal matter (plume fall-out, oxidised sulfides), and the extent of diagenesis. In this environment three types of micronodules can be distinguished although not observed as pure end-members: (1) diagenetic micronodules; (2) micronodules formed generally from the plume fall-out of oxyhydroxide matter; and (3) micronodules grown on the oxidised sulfide grains supplied to the sediments by slumping or fall-out of nearby buoyant plume. Away from the active spreading centre, the hydrothermal signatures of primary precipitates are gradually masked and hydrogenous/diagenetic processes lead the micronodule formation. Composition of micronodules becomes less variable. Well-pronounced, deep rift valleys confine the hydrothermal plume, which brings the hydrothermal suspension into contact with restricted volumes of seawater and, consequently, weakens the hydrogenous influence on the primary hydrothermal matter. Shallow rift valleys do not confine hydrothermal plumes, which are scattered over hundreds of kilometres by bottom currents. This brings the hydrothermal suspended matter into contact with large volumes of seawater. Extensive scavenging occurs, which masks the hydrothermal signal away from the spreading axis and enhances the hydrogenous one. Thus, the ridge crest morphology, defined by the spreading rate, is supposed to play a certain role, though indirect, in the chemical composition of the primary precipitates and, consequently, in the composition of the micronodules formed.

Strontium, neodymium, and lead isotope variations of authigenic and silicate sediment components from the Late Cenozoic Arctic Ocean: Implications for sediment provenance and the source of trace metals in seawater

Provenance changes of silicate sediment deposited during the Late Cenozoic (5–0 Ma) on the Alpha Ridge, central Arctic Ocean are determined from variations in strontium, lead, and neodymium isotope compositions. Whereas strontium and lead isotope compositions are relatively invariant from ∼5 to 1.7 Ma, ϵ Nd values start to increase at ∼3 Ma. Subsequently, 87Sr/ 86Sr and 206Pb/ 204Pb ratios progressively increase and ε Nd values progressively decrease from ∼17. Ma to the present day. From these isotope variations, three different endmember compositions for sediment source regions are defined. The two endmember compositions defined by sediment that was deposited by sea ice from 5 to 1.7 Ma are consistent with a significant component being derived from the East Siberian Shelf. The progressive change in isotope compositions from 1.7 to 0 Ma correlates with an increase in coarse detritus deposited by icebergs over this time period. The isotope data are consistent with a progressive increase in the proportion of sediment that was deposited in the central Arctic Ocean since 1.7 Ma having been derived from the northern Canada or Queen Elizabeth Island region. Changes in the strontium, neodymium, and lead isotope compositions of Arctic seawater are determined by analyzing the oxide fractions of FeMn micronodules. Strontium isotope compositions of the micronodule oxide fractions are similar to published Late Cenozoic seawater data and indicate that >97% of the Sr is seawater derived, but minute contributions of Sr from silicate detritus prohibit using the hydrogenous fractions for chronostratigraphic purposes. Neodymium and lead isotope compositions of the micronodule oxide fractions, which reflect those of Arctic seawater, follow the isotope variations of the silicate components throughout the Late Cenozoic. These relations indicate that river water can not be the primary source of rare earth elements and Pb to the dissolved reservoir in Arctic seawater, and that ice rafted detritus, by dissolution or exchange processes, is an important source of trace elements to seawater in ice-covered oceans.

Geochemical constraints on the formation of Late Cenozoic ferromanganese micronodules from the central Arctic Ocean

In order to determine geochemical compositions of Late Cenozoic Arctic seawater, oxide fractions were chemically separated from 15 samples of hand-picked ferromanganese micronodules (50–300 μm). The success of the chemical separation is indicated by the fact that > 97% of the Sr in the oxide fraction is seawater-derived. Rare-earth element (REE) abundances of the Arctic micronodule oxide fractions are much lower than those of bulk Fe-Mn nodules from other ocean basins of the world (e.g., 33 vs. 145 ppm Nd), but the Arctic oxides are enriched in Ce relative to Nd (CeNNdN = 2.2 ± 0.5) and have convex-upward, shale-normalized REE patterns (NdNGdN = 0.61 ± 0.06, GdNYbN = 1.5 ± 0.2, NdNYbN = 0.9 ± 0.2), typical of other hydrogenous and diagenetic marine Fe-Mn-oxides. Bulk sediment samples from the central Arctic Ocean have REE abundances and patterns that are characteristic of those of post-Archean shale. Non-detrital fractions (calcite + oxide coatings) of Recent Arctic foraminifera have REE abundances and patterns similar to those of Recent foraminifera from the Atlantic Ocean.Electron microprobe analyses (n = 178) of transition elements in 29 Arctic Fe-Mn micronodules from five different stratigraphic intervals of Late Cenozoic sediment indicate that oxide accretion occurred as a result of hydrogenetic and diagenetic processes close to the sediment-seawater interface. Transition element ratios suggest that no oxide accretion occurred during transitions from oxic to suboxic diagenetic conditions. Only K is correlated with Si and Al, and ratios of these elements suggest that they are associated with illite or phillipsite. Ca and Mg are correlated with Mn, which indicates variable substitution of these elements from seawater into the manganate phase. The geochemical characteristics of Arctic Fe-Mn micronodules indicate that the REEs of the oxide fractions were ultimately derived from seawater. However, because of minute contributions of Sr from siliciclastic detritus during diagenesis or during the chemical leaching procedure, Sr isotope compositions of the oxide fractions cannot be used to trace temporal changes in the 87Sr86Sr ratio of Arctic seawater or to improve the chronostratigraphy.

The Pliocene record in the central Arctic Ocean

Pliocene sediment from the central Arctic Ocean was reported first some 25 years ago. Although the basis of the Pliocene age was tenuous, none of the data subsequently presented have invalidated the original Pliocene age report. New data confirm the earliest report as well as establish the presence of older Miocene sediment. The Pliocene sediment recovered from the central Arctic Ocean to-date includes only 2–3 m of silty lutite consisting largely of illite, kaolinite, and chlorite, with lesser amounts of expandable clays, abundant Fe-Mn micronodules, but few fossils. The Pliocene age is based on extrapolation of sedimentation rates from paleomagnetic boundaries, arenaceous foraminifera, and 87Sr/ 86Sr isotope ratios obtained from Fe-Mn micronodules and compared to established seawater 87Sr/ 86Sr isotope ratios for this time. Notably absent in all except the upper part of the central Arctic Ocean Pliocene sediment is coarser-grained glacially ice-rafted debris (IRD) or any indication of measurable surface water productivity. Early interpretations of the Arctic Ocean record included the concept that sea ice was present during the Pliocene, and that the ice-cover existed more or less continually to the present. Late Cenozoic warming events that may have temporarily removed all or part of the Arctic Ocean's ice were thought to be restricted to the Pleistocene. More recently, published data suggest that the Pliocene Arctic Ocean was a warmer, perhaps ice-free ocean. General Circulation Modeling experiments with all or parts of the Arctic ice-cover removed are cited in support of Pliocene data generated primarily from the Arctic Ocean's margins that have suggested a warmer ocean. While there is important evidence for a warmer Pliocene Earth, the data to support significant warming in the central Arctic Ocean remains ambiguous. Reevaluation of part of the data used to support supposed Pliocene warming intervals on the Arctic Ocean's margins suggests that at least some of the implied Pliocene warming events may have actually occurred during the Pleistocene. The central Arctic Ocean's Pliocene record studied to date indicates only that the ocean had very low surface water productivity and extremely low sedimentation rates, both consistent with the presence of an ice-cover. There is little evidence of organic activity of any kind. However, the sum of the data obtained is equivocal concerning the affect of a warmer Earth on the central Arctic Ocean and the presence or absence of sea ice.

A microprobe study of metalliferous sediment components

Smectite aggregates, Fe-Mn micronodules, and fish debris from the coarse fraction of a metalliferous sediment sample from the Bauer Basin have been analyzed by microprobe. Both X-ray diffraction and bulk chemical analyses reveal that the smectite, an iron-rich and aluminum-poor nontronite, is the dominant phase. A linear programming solution to a series of mass balance equations indicates that the bulk sample is 70% smectite, 4% todorokite, 13% microlaminated material, and 1% fish debris. Two episodes of micronodule development are indicated by the presence of overgrowths of microlaminated iron-rich material over a homogeneous dense todorokite core. The two micronodule compositions can be explained by reaction of hydrothermally produced amorphous Fe-Mn oxyhydroxides with biogenous opal to form the calculated phase composition. The rare earth elements in the bulk sample can be explained by a similar diagenetic process which distributes rare earth elements of a seawater origin between cerium-enriched micronodules and cerium-depleted fish debris.