Fe-Mn Crusts Research Articles

Distribution and accumulation rate of Hg in the upper quaternary sediments of the Deryugin Basin, Sea of Okhotsk

The Hg distribution and some mineralogical-geochemical features of bottom sediments up to a depth of 10 m in the Deryugin Basin showed that the high and anomalous Hg contents in the Holocene deposits are confined to a spreading riftogenic structure and separate fluid vents within it. The accumulations of Hg in the the sediments were caused by its fluxes from gas and low-temperature hydrothermal vents under favorable oceanological conditions in the Holocene. The two mainly responsible for the high and anomalous Hg contents are infiltration (fluxes of hydrothermal or gas fluids from the sedimentary cover) and plume (Hg precipitation from water plumes with certain hydrochemical conditions forming above endogenous sources). The infiltration anomalies of Hg were revealed in the following environments: (1) near gas vents on the northeastern Sakhalin slope, where high Hg contents are associated only with Se and were caused by the accumulation of gases ascending from beneath the gas hydrate layer; (2) in the area of inferred occasionally operating low-temperature hydrothermal seeps in the central part of the Deryugin Basin, in which massive barite chimneys, hydrothermal Fe-Mn crusts, and anomalous contents of Mn, Ba, Zn, and Ni in sediments develop.

A mid miocene flood basalt event observed in the osmium isotope record of seawater

Two Fe–Mn crusts from the central Pacific and one from the central Atlantic were dated by osmium (Os) isotope stratigraphy. All three crusts show a pronounced Os isotope minimum around 12 Ma. The Os/Os decreased from 0.8 at 15 Ma to 0.7 at 12 Ma and reached a value of 0.8 again at 9 Ma indicating symmetrical shoulders of this excursion. In the modern ocean, the concentration of dissolved Os as well as the Os/Os of 1.06 are essentially homogenous, which is accurately reflected by recent marine sediments in several depositional settings. The observed negative Os isotope excursion at 12 Ma, which has not been observed in earlier studies of pelagic and metalliferous sediments, thus must have been caused by an event that changed the osmium isotope signature of global seawater. Possible interpretations for the Mid-Miocene decrease include changes in continental weathering intensity i.e. a reduction of the radiogenic Os flux, or an increase in the unradiogenic input fluxes, such as intensified weathering of ultramafic rocks, exhalations derived from large igneous flood basalts or their weathering, a meteorite impact, or a combination of several of these reasons. In the Mid-Miocene there was at least one larger meteorite impact, the Nordlinger Ries Crater and at the same time the large Columbia River igneous flood basalt province formed. Application of a simple mass balance model RSW = Rr (fr) + Ru (1 fr) where fr is the ratio of riverine flux of Os to the total flux of Os, and were subscripts sw, r and u refer to seawater, riverine and unradiogenic, respectively, shows that exhalations from a large igneous flood basalt are the most likely cause of the observed minimum. Therefore the most viable explanation of the unradiogenic Os is the eruption of the Columbia River flood basalts. Such an interpretation is also possible for the two other more pronounced minima in the Cenozoic Os isotope record of seawater that have previously been reported for the Cretaceous/Tertiary and Eocene/Oligocene boundaries. The exhalations of the Deccan Traps and the Ethiopian Flood Basalts were large enough to release sufficient amounts of unradiogenic Os into the ocean to cause the observed minima.

Helium isotopes in ferromanganese crusts from the central Pacific Ocean

Helium isotopes have been measured in samples of two ferromanganese crusts (VA13/2 and CD29-2) from the central Pacific Ocean. With the exception of the deepest part of crust CD29-2 the data can be explained by a mixture of implanted solar- and galactic cosmic ray-produced (GCR) He, in extraterrestrial grains, and radiogenic He in wind-borne continental dust grains. 4He concentrations are invariant and require retention of less than 12% of the in situ He produced since crust formation. Loss has occurred by recoil and diffusion. High 4He in CD29-2 samples older than 42 Ma are correlated with phosphatization and can be explained by retention of up to 12% of the in situ-produced 4He. 3He/ 4He of VA13/2 samples varies from 18.5 to 1852 R a due almost entirely to variation in the extraterrestrial He contribution. The highest 3He/ 4He is comparable to the highest values measured in interplanetary dust particles (IDPs) and micrometeorites (MMs). Helium concentrations are orders of magnitude lower than in oceanic sediments reflecting the low trapping efficiency for in-falling terrestrial and extraterrestrial grains of Fe-Mn crusts. The extraterrestrial 3He concentration of the crusts rules out whole, undegassed 4–40 μm diameter IDPs as the host. Instead it requires that the extraterrestrial He inventory is carried by numerous particles with significantly lower He concentrations, and occasional high concentration GCR-He-bearing particles.

Sorption of heavy metal cations by low-temperature deposits of Pacific hydrothermal fields

Cation exchange reactions with participation of heavy metals Mn, Co, Ni, Cu, Zn, Cd, Ba, and Pb were studed in oceanic low-temperature hydrothermal deposits of various mineral compositions and in hydrogenic Fe-Mn crusts. Individual minerals and their assemblages differ significantly in absorptive capacity, which increases in the following order: hematite ≪ Si-protoferrihydrite < protoferrihydrite < geothite < nontronite ≪ Fe-vernadite + Mn-feroxyhyte < Fe-free vernadite < bernessite + Fe-free vernadite < bernessite; i.e., it successively increases from the mineral with a coordination type of lattice to minerals with a layer-type structure. The exchange complex of all minerals includes Na+, K+, Ca2+, and Mg2+, i.e., the main cations of seawater. In Mn minerals, Mn2+ is the main exchange component. The contribution of all the mentioned cations to the exchange capacity of minerals is as high as 90–98%. The highest absorptive capacity among the examined low-temperature oceanic deposits is characteristic of hydrothermal Mn minerals. Their capacity exceeds substantially that of hydrothermal oxides, hydroxides, Fe-aluminosilicates, and hydrogenic Fe-Mn minerals. The absorptive capacity of all examined Mn minerals relative to heavy metals increases in the same order: Ni < Zn < Cd < Mn < Co < Pb < Cu.

Hafnium and neodymium isotopes in seawater and in ferromanganese crusts: The “element perspective”

Abstract Hafnium and Nd isotopes are increasingly used as paleoceanographic proxies. Comparing the “mantle–crust array” and the “seawater array” in plots of e Hf vs. e Nd , it has been observed that for a given e Nd value the corresponding e Hf value is higher for seawater than it is for terrestrial rocks. While this difference had initially been explained by significant hydrothermal input of mantle Hf into seawater, the currently favoured explanation is incongruent weathering of continental rocks producing radiogenic riverine Hf input. We here address this topic from the perspective of the behaviour of these two elements in seawater and in ferromanganese (Fe–Mn) crusts. We distinguish between a “truly dissolved” and a “dissolved” Hf and Nd pool, the latter being comprised of truly dissolved and colloid-bound (“colloidal”) Hf and Nd. While there exists a hydrothermal pathway for colloid-bound dissolved mantle Hf into the oceans, there is, in marked contrast to Nd, no important riverine pathway for colloidal or truly dissolved continental Hf. Owing to their respective chemical speciation in seawater, there exists truly dissolved Nd in the ocean, while the amount of truly dissolved Hf is insignificant. Neodymium is in exchange equilibrium between local seawater and both, the hydrous Fe and Mn oxides hydrogenetic Fe–Mn crusts are composed of. Due to continuous ad- and desorption there is continuous isotopic re-equilibration and the isotopic composition of Nd in a Fe–Mn crust reflects that of truly dissolved Nd in local ambient seawater. In contrast, Hf is only associated with the hydrous Fe oxides on which it forms surface precipitates that do not exchange with seawater. Due to this lack of isotopic re-equilibration, the isotopic composition of Hf in a Fe–Mn crust is the average of that of all the Hf scavenged during the lifetime of the hydrous Fe oxide particles. Since the Hf-bearing hydrous Fe oxides in a Fe–Mn crust do not form from local ambient seawater at the crust's growth site but are advected as colloids or fine particles, their Hf isotopic composition depends on the origin and migration pathway of these colloids. Hence, while Nd isotopes in Fe–Mn crusts provide reliable information on truly dissolved Nd in local ambient seawater, Hf isotopes rather indicate the origin and pathway of hydrous Fe oxide colloids, and might differ from truly dissolved Hf in local ambient seawater. This may explain the occasional decoupling of Nd and Hf isotopes in Fe–Mn crusts and supports the notion of a significant hydrothermal mantle signal of Hf in seawater.

U-Th chronology and paleoceanographic record in a Fe-Mn crust from the NE Atlantic over the last 700 ka

Hydrogenetic ferromanganese crusts (Fe-Mn crusts) provide a secular record of the variations of seawater composition responding to changes in ocean circulation and erosion processes. In this respect, the acquisition of an absolute and reliable chronology in Fe-Mn crusts is a prerequisite. Here we combine four different and complementary chronometers ( 10Be, 230Th ex, 230Th ex/ 232Th, 234U/ 238U) in a Fe-Mn crust dredged at ∼2000 m depth in the east Atlantic to first establish a reliable chronology over the Quaternary period. Then, we use EDS chemical analysis to look for correlation between major element chemistry and climate changes. ( 230Th ex), ( 230Th ex/ 232Th), and Be data give very consistent growth rates. In particular, the good match between ( 230Th ex) and ( 230Th ex/ 232Th) data indicates that at the location of crust 121DK, 230Th and 232Th fluxes in the water column change simultaneously and suggests that the normalization of 230Th ex to 232Th makes ( 230Th ex/ 232Th) a better chronometer. Our best-fit model suggests that crust 121DK experienced changes in growth rates at ∼122 and 312 ka and a growth with a constant 230Th initial flux. This chronology returns an age of 680 ka for the uppermost 1.5 mm. The ( 234U/ 238U) depth profile, however, was clearly affected by diffusion of 234U in the porous crust and can therefore not be used to derive a reliable chronology. One part of the crust seems isolated from pore water diffusion and can be physically recognized as a zone of very small porosity. On the basis of the ( 230Th ex/ 232Th) chronology, major element chemistry is shown to be linked to climate change. Mn/Fe variations compare well with those in a Fe-Mn crust from the Pacific, showing systematic maxima during glacial stages 2 and 4. High Mn/Fe are tentatively interpreted to reflect expansion of the oxygen minimum zone during glacial periods, resulting from higher bioproductivity. In addition we note that the surface ( 230Th/ 232Th) activity ratio of crust 121DK is entirely consistent with advection of deep water from the western toward the eastern Atlantic basin.

Mercury- and Silver-Rich Ferromanganese Oxides, Southern California Borderland: Deposit Model and Environmental Implications

Mercury- and silver-enriched ferromanganese oxide crusts were recovered at water depths of 1,750 to1,300 m from La Victoria knoll, located about 72 km off the coast of northern Baja California. No other ferromanganese precipitate found so far in the modern ocean basins is similarly enriched in Hg and Ag. The precipitates consist of submetallic gray, brecciated, Mn oxide layers overlain by brown earthy, laminated Fe-Mn oxide crusts. Both oxide types are rich in Hg (to 10 ppm) and Ag (to 5.5 ppm). The Mn-rich layers are composed of δMnO 2 , with lesser amounts of 10A and 7A manganates, whereas the Mn phase in the Fe-Mn crusts is solely δMnO 2 . The Fe phase in both layers is X-ray amorphous. Established criteria for distinguishing hydrothermal versus hydrogenetic crusts indicate that the Mn-rich layers are predominantly of low-temperature hydrothermal origin, whereas the Fe-Mn crusts are hydrogenetic, although there is some overlap in the source of chemical components in both types. La Victoria knoll is uplifted continental basement rock with basalt, andesite, and schist cropping out at the surface; the knoll may have an intrusive core. The Hg and Ag were derived from leaching by hydrothermal fluids of organic matter-rich sediments in basins adjacent to La Victoria knoll and, to a lesser extent, from continental basement rocks underlying the knoll and adjacent basins. Both rock types are notably enriched in Ag and Hg. Faults were the main fluid transport pathway, and hydrothermal circulation was driven by high heat flow associated with thinned crust. Other elements derived from the hydrothermal fluids include Tl, Cd, Cr, and Li. The main host for Hg and Ag is FeOOH, although MnO 2 likely hosts some of the Ag. Minor sulfide and barite also may contain small amounts of these metals. Possible analogs in the geologic record for this deposit type are found in the Basin and Range province of the western United States and Mexico. The discovery highlights the fact that fluids circulating along faults in the offshore California borderland are transporting potentially toxic metals (Hg, Ag, Tl, As, Cd, Cr, Pb, and Ni) and depositing them on and just below the ocean floor.

Osmium isotope stratigraphy of a marine ferromanganese crust

Ferromanganese crusts provide records of long term change in ocean circulation and continental weathering. However, calibrating their age prior to 10 Ma has been entirely based on empirical growth rate models using Co concentrations, which have inherently large uncertainties and fail to detect hiatuses and erosional events. We present a new method for dating these crusts by measuring their osmium (Os) isotope record and matching it to the well-known marine Os isotope evolution of the past 80 Ma. The well-characterised crust CD29-2 from the central Pacific, was believed to define a record of paleooceanographic change from 50 Ma. Previous growth rate estimates based on the Co method are consistent with the new Os isotope stratigraphy but the dating was grossly inaccurate due to long hiatuses that are now detectable. The new chronology shows that it in fact started growing prior to 70 Ma in the late Cretaceous and stopped growing or was eroded between 13.5 and 47 Ma. With this new technique it is now possible to exploit the full potential of the oceanographic and climatic records stored in Fe–Mn crusts.

Vartion of the Fe/Mn ratio of ferromanganese crusts from the cetral North Pacific: implication for paleoclimate changes

Abstract Contentsn of Fe, Mn and other elements in four ferromaganese crusts recovered from the ceteral. North Pacific are analyzed at high depth-resolution by electron microprobe for reconstruting factors controlling their deposition. Maganese (Iv) in hydrogenetic ferromaganese crusts is mainly supplied as collidal precipitates from the Oxygen Minimum zone (OMZ), which consentrates high amounts of disolved Mn (II). The iron is derived from carbonate dissolution and silcate particles of eolian dust. An increase in paleoprodutivety during cooler climate would potentially lead to adecrease in Mn despostion and due to enlargement of the OMZ which has a “temporary storage” effect for Mn. On the other hand, not affected by the OMZ, the iron enering the Fe-Mn crust would likely increase with the eolian dust input and surface productivity at glacial stages. As a result, the increasing FE/Mn ratio shoud indicate a cooling climate. This is a sopported by the following opservations. In the profile of the past 1 Ma,...

Compositional variation and genesis of ferromanganese crusts of the Afanasiy—Nikitin Seamount, Equatorial Indian Ocean

Eight ferromanganese crusts (Fe-Mn crusts) with igneous and sedimentary substrates collected at different water depths from the Afanasiy-Nikitin Seamount are studied for their bulk major, minor and rare earth element composition. The Mn/Fe ratios < 1.5 indicate the hydrogenetic accretion of the Fe-Mn hydroxides. These Fe-Mn crusts are enriched in Co (up to 0.9%, average ∼ 0.5%) and Ce. The Ce-content is the highest reported so far (up to 3763 ppm, average ∼ 2250 ppm) for global ocean seamount Fe-Mn crusts. In spite of general similarity in the range of major, minor, and strictly trivalent rare earth element composition, the dissimilarity between the present Fe-Mn crusts and the Pacific seamount Fe-Mn crusts in Co and Ce associations with major mineral phases indicates inter-oceanic heterogeneity and region-specific conditions responsible for their enrichment. The decrease in Ce-anomaly (from ∼ 8 to ∼ 1.5) with increasing water depth (from ∼ 1.7 km to ∼ 3.2 km) might suggest that the modern intermediate depth low oxygen layer was shifted and sustained at a deeper depth for a long period in the past.

The effect of plume processes on the Fe isotope composition of hydrothermally derived Fe in the deep ocean as inferred from the Rainbow vent site, Mid-Atlantic Ridge, 36°14′N

The Rainbow hydrothermal vent site, which is the largest known point source for dissolved Fe delivered to the deep North Atlantic ocean, has remained invariant in its Fe isotope composition over at least the past 16,000 years, based on analysis of metalliferous sediments beneath the plume. Because of the conservative behavior of Fe in the Rainbow plume, δ 56Fe values of particles in the neutrally buoyant plume (−0.18±0.05‰) and underlying sediments (−0.19±0.05‰) are indistinguishable from the δ 56Fe values of the high-temperature fluid sources (−0.23±0.04‰). Particles from the near-vent, buoyant stage of the plume, however, have higher δ 56Fe values (+0.15‰ to +1.20‰) relative to the original vent fluid, consistent with fractionation during oxidation of Fe(II) aq to Fe(III) aq. Isotope compositions become invariant in the plume once all Fe(II) aq is fully oxidized, preserving the original composition of the vent fluid. The constant Fe isotope compositions of the vent fluids over time implies that changes in seawater Fe isotope composition of the North Atlantic ocean, as they are recorded in Fe–Mn crusts, requires changes in the relative fluxes of Fe to the ocean.

단계별 추출법을 이용한 망간각 구성 원소의 존재 형태

Sequential extraction was carried out on twenty two subsamples of three ferromanganese crusts from three seamounts (Lemkein, Lomilik, and Litakpooki) near the Marshall Islands in the western Pacific. The extraction was designed to fractionate Fe-Mn crust forming elements into low defined groups: (1) exchangeable and carbornate, (2) Mn-oxide, (3) Fe-oxyhyd.oxide, and (4) residual fraction. X-ray diffraction result shows that target material were well removed by each extraction step except for CFA in phosphatized crusts generation. According to chemical analysis of each leachate, most of elements in the Fe-Mn crusts are bound with two major phases. Mn, Ba, Co, Ni, Zn, (Fe, Sr, Cu, and V) are strongly bounded with Mn-oxide <TEX>$({\delta}-MnO_2)$</TEX> phase, whereas Fe, Ti, Zr, Mo, Pb, Al, Cu,(V, P, and Zn) show chemical affinity with Fe-oxyhydroxide phase. This result indicates that significant amount of Al, Ti, and Zr can not be explained by detrital origin. Ca, Mg, K, and Sr mainly occur as exchangeable elements and/or carbonate phase. Outermost layer 1 and inner layer 2 which are both young crusts generations are similar in chemical speciations of elements. However, some of Fe-oxyhydroxide bounded elements (Pb, Y, Mo, Ba, Al, and V) in phosphatized innermost layer 3 are released during phosphatization and incorporated into phosphate (Pb, Y, Mo, and Ba) or Mn-oxide phase (Al and V). Our sequential extraction results reveal that chemical speciations of elements in the hydrogenetic crusts are more or less different from interelemental relationship calculated by statistical method based on bulk chemistry.

Cenozoic marine geochemistry of thallium deduced from isotopic studies of ferromanganese crusts and pelagic sediments

Cenozoic records of Tl isotope compositions recorded by ferromanganese (Fe–Mn) crusts have been obtained. Such records are of interest because recent growth surfaces of Fe–Mn crusts display a nearly constant Tl isotope fractionation relative to seawater. The time-series data are complemented by results for bulk samples and leachates of various marine sediments. Oxic pelagic sediments and anoxic marine deposits can be distinguished by their Tl isotope compositions. Both pelagic clays and biogenic oozes are typically characterized by ϵ 205Tl greater than +2.5, whereas anoxic sediments have ϵ 205Tl of less than −1.5 ( ϵ 205Tl is the deviation of the 205Tl/ 203Tl isotope ratio of a sample from NIST SRM 997 Tl in parts per 10 4). Leaching experiments indicate that the high ϵ 205Tl values of oxic sediments probably reflect authigenic Fe–Mn oxyhydroxides. Time-resolved Tl isotope compositions were obtained from six Fe–Mn crusts from the Atlantic, Indian, and Pacific oceans and a number of observations indicate that these records were not biased by diagenetic alteration. Over the last 25 Myr, the data do not show isotopic variations that significantly exceed the range of Tl isotope compositions observed for surface layers of Fe–Mn crusts distributed globally ( ϵ 205Tl=+12.8±1.2). This indicates that variations in deep-ocean temperature were not recorded by Tl isotopes. The results most likely reflect a constant Tl isotope composition for seawater. The growth layers of three Fe–Mn crusts that are older than 25 Ma show a systematic increase of ϵ 205Tl with decreasing age, from about +6 at 60–50 Ma to about +12 at 25 Ma. These trends are thought to be due to variations in the Tl isotope composition of seawater, which requires that the oceans of the early Cenozoic either had smaller output fluxes or received larger input fluxes of Tl with low ϵ 205Tl. Larger inputs of isotopically light Tl may have been supplied by benthic fluxes from reducing sediments, rivers, and/or volcanic emanations. Alternatively, the Tl isotope trends may reflect the increasing importance of Tl fluxes to altered ocean crust through time.

Molybdenum isotope records as a potential new proxy for paleoceanography

New high-precision isotope ratios of dissolved Mo in seawater from different ocean basins and depths show a homogeneous isotope composition (‘mean ocean water 98Mo/95Mo’ (MOMO)), as expected from its long ocean residence time (800 kyr). This composition appears to have been constant for the past 60 Myr at a 1–3 Myr time resolution as indicated from thick sections of Fe–Mn crusts from the Atlantic and Pacific. These records yield a constant offset from MOMO (average of −3.1 and −2.9‰). They are similar to our new data on recent oxic Mo sinks: pelagic sediments and six Fe–Mn crust surface layers range from −2.7 to −2.9‰ and −2.7 to −3.1‰, respectively. Recent suboxic Mo sinks from open ocean basins display heavier and more variable isotope ratios (−0.7 to −1.6‰ relative to MOMO). Crustal Mo sources were characterized by measuring two granites (and a mild acid leach of one granite), seven volcanic rocks and two clastic sediments. All show a narrow range of compositions (−2.0 to −2.3‰). These data indicate that isotope fractionation by chemical weathering and magmatic processes is insignificant on a global scale. They therefore represent good estimates of the composition of dissolved Mo input to the oceans and that of the average continental crust. Thus, the Mo input into the oceans appears to be distributed into lighter oxic sinks and heavier reducing sinks. This is consistent with steady-state conditions in the modern ocean. The constant isotope offset between oxic sediments and seawater suggests that the relative amounts of oxic and reducing Mo removal fluxes have not varied by more than 10% over the last 60 Myr. An equilibrium fractionation process is proposed assuming that Mo isotope fractionation occurs between (dominant) MoO42− and (minor) Mo(OH)6 species in solution, of which the latter is preferentially scavenged.

Global occurrence of tellurium-rich ferromanganese crusts and a model for the enrichment of tellurium

Hydrogenetic ferromanganese oxyhydroxide crusts (Fe-Mn crusts) precipitate out of cold ambient ocean water onto hard-rock surfaces (seamounts, plateaus, ridges) at water depths of about 400 to 4000 m throughout the ocean basins. The slow-growing (mm/Ma) Fe-Mn crusts concentrate most elements above their mean concentration in the Earth’s crust. Tellurium is enriched more than any other element (up to about 50,000 times) relative to its Earth’s crustal mean of about 1 ppb, compared with 250 times for the next most enriched element. We analyzed the Te contents for a suite of 105 bulk hydrogenetic crusts and 140 individual crust layers from the global ocean. For comparison, we analyzed 10 hydrothermal stratabound Mn-oxide samples collected from a variety of tectonic environments in the Pacific. In the Fe-Mn crust samples, Te varies from 3 to 205 ppm, with mean contents for Pacific and Atlantic samples of about 50 ppm and a mean of 39 ppm for Indian crust samples. Hydrothermal Mn samples have Te contents that range from 0.06 to 1 ppm. Continental margin Fe-Mn crusts have lower Te contents than open-ocean crusts, which is the result of dilution by detrital phases and differences in growth rates of the hydrogenetic phases. Correlation coefficient matrices show that for hydrothermal deposits, Te has positive correlations with elements characteristic of detrital minerals. In contrast, Te in open-ocean Fe-Mn crusts usually correlates with elements characteristic of the MnO 2, carbonate fluorapatite, and residual biogenic phases. In continental margin crusts, Te also correlates with FeOOH associated elements. In addition, Te is negatively correlated with water depth of occurrence and positively correlated with crust thickness. Q-mode factor analyses support these relationships. However, sequential leaching results show that most of the Te is associated with FeOOH in Fe-Mn crusts and ≤10% is leached with the MnO 2. Thermodynamic calculations indicate that Te occurs predominantly as H 5TeO 6 − in ocean water. The speciation of Te in ocean water and charge balance considerations indicate that Te should be scavenged by FeOOH, which is in agreement with our leaching results. The thermodynamically more stable Te(IV) is less abundant by factors of 2 to 3.5 than Te(VI) in ocean water. This can be explained by preferential (not exclusive) scavenging of Te(IV) by FeOOH at the Fe-Mn crust surface and by Fe-Mn colloids in the water column. We propose a model in which the extreme enrichment of Te in Fe-Mn crusts is likely the result of an oxidation reaction on the surface of FeOOH. A similar oxidation process has been confirmed for Co, Ce, and Tl at the surface of MnO 2 in crusts, but has not been suggested previously to occur in association with FeOOH in Fe-Mn crusts. Mass-balance considerations indicate that ocean floor Fe-Mn deposits are the major sink for Te in the oceans. The concentration and redox chemistry of Te in the global ocean are likely controlled by scavenging on Fe-Mn colloids in the water column and Fe-Mn deposits on the ocean floor, as is also the case for Ce.

Thallium isotope variations in seawater and hydrogenetic, diagenetic, and hydrothermal ferromanganese deposits

Results are presented for the first in-depth investigation of Tl isotope variations in marine materials. The Tl isotopic measurements were conducted by multiple collector-inductively coupled plasma mass spectrometry for a comprehensive suite of hydrogenetic ferromanganese crusts, diagenetic Fe–Mn nodules, hydrothermal manganese deposits and seawater samples. The natural variability of Tl isotope compositions in these samples exceeds the analytical reproducibility (±0.05‰) by more than a factor of 40. Hydrogenetic Fe–Mn crusts have ϵ 205Tl of +10 to +14, whereas seawater is characterized by values as low as −8 ( ϵ 205Tl represents the deviation of the 205Tl/ 203Tl ratio of a sample from the NIST SRM 997 Tl isotope standard in parts per 10 4). This ∼2‰ difference in isotope composition is thought to result from the isotope fractionation that accompanies the adsorption of Tl onto ferromanganese particles. An equilibrium fractionation factor of α∼1.0021 is calculated for this process. Ferromanganese nodules and hydrothermal manganese deposits have variable Tl isotope compositions that range between the values obtained for seawater and hydrogenetic Fe–Mn crusts. The variability in ϵ 205Tl in diagenetic nodules appears to be caused by the adsorption of Tl from pore fluids, which act as a closed-system reservoir with a Tl isotope composition that is inferred to be similar to seawater. Nodules with ϵ 205Tl values similar to seawater are found if the scavenging of Tl is nearly quantitative. Hydrothermal manganese deposits display a positive correlation between ϵ 205Tl and Mn/Fe. This trend is thought to be due to the derivation of Tl from distinct hydrothermal sources. Deposits with low Mn/Fe ratios and low ϵ 205Tl are produced by the adsorption of Tl from fluids that are sampled close to hydrothermal sources. Such fluids have low Mn/Fe ratios and relatively high temperatures, such that only minor isotope fractionation occurs during adsorption. Hydrothermal manganese deposits with high Mn/Fe and high ϵ 205Tl are generated by scavenging of Tl from colder, more distal hydrothermal fluids. Under such conditions, adsorption is associated with significant isotope fractionation, and this produces deposits with higher ϵ 205Tl values coupled with high Mn/Fe.

Temporal changes in the lead isotopic composition of red clays: comparison with ferromanganese crust records

A record of changes in Pb and Sr isotopic composition of two cores (DSDP 86 576A and LL44 GPC3) from the red clay region of the central North Pacific has been determined for the past 60–65 million years. The isotope records of the eolian silicate fraction of the red clays reflect the change in source area as the core sites migrated under different wind systems. The Sr isotope compositions of eolian silicate material are consistent with Asian loess and North American arc volcanism that has been recognized from mineralogical studies. The silicate-bound eolian Pb isotopic compositions similarly reflect Asian loess and arc volcanism. The isotope records of three ferromanganese crusts from similar locations in the central Pacific are similar to the eolian component of red clays, but offset to less radiogenic values. This may be due to two mechanisms: (1) Pb that can be removed from eolian material by seawater is much less radiogenic, or less likely (2) hydrothermal Pb can be transported further away from venting sites through particle exchange with seawater, despite hydrothermal venting acting as a net sink of oceanic Pb. The temporal changes in Pb isotopes in the ferromanganese crusts, bulk red clays and eolian silicates are similar although offset from each other suggesting that eolian deposition is an important source of Pb to seawater and to ferromanganese crusts. This contrasts with the Atlantic and Southern Ocean where more intense deep water flow leads to isotopic gradients in FeMn crusts that do not reflect surface water conditions immediately above the crust. A mechanism is proposed which accounts for Pacific deepwater Pb being isotopically influenced by eolian deposition.

Geographic control on Pb isotope distribution and sources in Indian Ocean Fe-Mn deposits

High-precision Pb isotope data have been obtained on Indian Ocean Fe-Mn deposits with a large geographic coverage. These data reveal a strong geographic control on the distribution of Pb isotopes as well as sources of Pb within this basin. The provinciality of Pb isotopes at the scale of the whole Indian Ocean, as well as that of individual basins, broadly matches the pattern of deepwater flow.The existence of several sources of Pb is best illustrated by the presence of three well-defined Pb isotopic arrays, each of which is confined to clear-cut geographic domains. These arrays imply a dominance of binary mixing of Pb sources within each of these domains. The domains consist of the North-Indian (N-Indian; north of 20°S), Southwest-Indian (SW-Indian; 20°S to 50°S, west of 45°E), South-Indian (S-Indian; 20°S to 50°S, east of 45°E) and Antarctic-Indian (A-Indian; south of 50°S), and clearly exhibit a strong control by latitude. The S-Indian domain and Mozambique Channel samples form a cluster, suggesting that there are more than just two sources contributing in these regions.We show that the N-Indian is dominated by sources of Pb derived from the High Himalayas and the Trans-Himalayan Complex, and most likely inherited from interaction at the water interface with Bengal and Indus Fan sediments. The SW-Indian and A-Indian domains share a common unradiogenic component, represented by circumpolar waters derived from the Pacific Ocean and flowing through the Drake Passage into the Atlantic and Indian Oceans. However, the radiogenic sources of Pb in these two domains are clearly distinct. In the SW-Indian, the radiogenic Pb signal reflects the tongue of North Atlantic Deep Water flowing around the tip of South Africa, while in the A-Indian, the source of radiogenic Pb is less certain, but quite probably originates in the Weddell Sea region.These data illustrate the complexity of the spatial Pb isotopic distribution, and Pb sources, within a single ocean basin at the present day, which should be borne in mind when interpreting long-term radiogenic isotope paleorecords from Fe-Mn crusts.

The Hf isotope composition of global seawater and the evolution of Hf isotopes in the deep Pacific Ocean from Fe–Mn crusts

This paper presents a revised chemical purification method of Hf for the measurement of Hf isotope ratios of Fe–Mn crusts on a new generation of double focusing multiple collector plasma source mass spectrometer (MC-ICP-MS). By measuring surface scrapings of hydrogenetic Fe–Mn crusts distributed over the three major ocean basins, the present-day Hf isotope distribution of oceanic deep water is assessed in detail. The results show an ε Hf provinciality correlated with those of other radiogenic isotope tracers such as Nd and Pb in agreement with previous studies. This supports the use of Hf isotopes as tracer of element source provenance and water masses. Fe–Mn crusts display the same present-day Hf–Nd isotope array as given before for Mn nodules. The smaller isotopic variability of Hf compared with Nd may either be caused by a more efficient mixing of Hf than Nd in the ocean due to a longer residence time or may be a consequence of a systematically more radiogenic Hf than Nd isotope signature delivered to the oceans by weathering of continental crust. A Hf isotope time series was measured on crust VA13/2 to assess the Hf isotope compositions of the Central Pacific deep water over the past 26 Ma. No consistency is observed between the Hf and Pb isotope time series. In contrast, Hf and Nd isotope time series display similar patterns which are, however, apparently offset by 2 Ma prior to 14 Ma. Differential diffusion of Hf and Nd does not explain this offset. The smaller amplitude in the Hf isotope variations compared with the Nd isotopes rather argues for more efficient mixing of Hf in the ocean. We suggest that both isotope systems have responded in a similar way to the processes affecting the dissolved radiogenic isotope composition of Pacific deep water during this time interval. The parallel increase in ε Hf and ε Nd observed between 14 and 3 Ma may probably be attributed to the increased inputs of Hf and Nd into the Central Pacific Ocean derived from the weathering of the Pacific Islands Arcs. Over the past 3 Ma an increased aeolian continental input derived from Asia most likely caused negative shifts in ε Nd and ε Hf recorded by VA13/2. The fact that Hf and Nd isotope compositions plot along the present-day array for Fe–Mn crusts and Mn nodules over the entire past 26 Ma suggests that aeolian supply of Hf to the Pacific Ocean has been a long-term important feature.

Genesis of ferromanganese crusts from the TAG hydrothermal field

The rare earth element (REE) geochemistry and Nd isotopic composition of marine ferromanganese crusts allow insight into their environment of deposition. We present REE along with major and trace element data from a suite of samples collected near an active hydrothermal site from the Mid-Atlantic Ridge and demonstrate extreme REE variability over short spatial scales. Both positive and negative Ce anomalies are observed, along with varied fractionation of the REEs. Some of the Fe–Mn crusts are formed directly from low-temperature hydrothermal fluids, channelled over 3 km from any high-temperature vent site. A significant hydrothermal component is observed in one Fe–Mn crust sampled. Other samples, collected from the same area, are predominantly hydrogenous and form at a wide range of accumulation rates depending on the hydrothermal plume input. The identification of extreme heterogeneity in source and depositional environment within one hydrothermal field suggests that (1) this variability can be resolved both spatially and temporally within a fossil hydrothermal deposit and (2) the integrated record provided by bulk analysis should be interpreted with caution.