Ferromanganese Encrustations Research Articles

Geochemical, mineralogical, and Sr–Nd isotopic compositions of ferromanganese encrustations from Central Indian Ridge at 06°38.5′S

Mineralogical, geochemical, and isotopic (Sr and Nd) studies of 14 ferromanganese encrustations on basaltic substrates from Central Indian Ridge at 06°38.5′S indicate that except for two (NC3 and NC4), the samples are hydrogenous type. NC3 and NC4 have slightly higher growth rates, lower bulk trace metal contents, and low ∑REE. They have a lower content of high field strength (e.g., Hf, Ta, Zr, and Nb), incompatible (e.g., Pb, Th, and U), and lithophilic elements (e.g., Sr and Ba) than others. In addition, NC3 has a lower 87Sr/86Sr ratio (0.708825) than the remaining samples (range 0.709169–0.709253) but higher 143Nd/144Nd ratio (0.512296) than others (range 0.512258–0.512279). Both NC3 and NC4 have a higher radiogenic component indicated by their low εNd value (−6.67) than the remaining samples (range −7.00 to −7.41). These evidences together indicate a possible contribution of trace metals by a local mixed source of hydrothermal and hydrogenous origin during their growth history.

Instrumental neutron activation analysis of ferromanganese oxide encrustations of Indian Ocean by the k0 NAA method

Three ferromanganese crusts from different locations of the Indian Ocean were analysed by neutron activation analysis using thek0 method (k0 NAA method) for studying the distribution of some trace elements along with Fe and Mn. Another crust from the Lau basin of the Pacific Ocean was investigated for comparison of the influence of different oceanic conditions on the trace element distributions in ferromanganese crusts. Variation in Co concentration along with the Mn/Fe ratio were discussed in terms of the hydrogenous/hydrothermal nature of the crusts. The normalised REE content was used to identify possible anomalies. The observed Ce anomaly is discussed in the light of the depositional environment. The precision and accuracy of the method were confirmed by measuring the elemental concentrations in a USGS nodule standard NOD A-1.

Quantitative PIXE analyses of ferromanganese oxide deposits from different locations of the Indian Ocean and a deposit from the Pacific Ocean

The present studies on ferromanganese nodules and encrustations from the Indian Ocean and a sediment from the Pacific ocean, were carried out to investigate the role of Mn and Fe in the uptaking of various trace elements from seawater. The Indian Ocean samples comprised eight iron rich ferromanganese nodules and four ferromanganese encrustations, out of which two are from Mid Indian Ocean Ridge (MIOR) and one each from Afanasiy Nikitin seamount and topographically elevated region. The Pacific Ocean sample was a metalliferrous sediment from Lau Basin. The Proton Induced X-ray Emission (PIXE) measurements were carried out on all the samples using 3 MeV proton beam from a 3 MV Pelletron accelerator for quantitative multielemental analysis. The spectral data were analysed using GUPIX software. Quantitative estimate of nearly 21 elements, e.g., K, Ca, Ti, V, Mn, Fe, Co, Ni, Cu, Zn, Ga, As, Sr, Y, Zr, Nb, Mo, Ba, Tl, Pb, and Bi have been made. The classification of the deposits into hydrogenous and hydrothermal types was carried out on the basis of their Mn/Fe ratios and subsequently, the distribution of various minor and trace elements were studied. The growth rate of these deposits are estimated. The findings show that, the concentrations of various elements in hydrogenous deposits are higher compared to that in hydrothermal ones. The reason for this behaviour is attributed to suitable physico-chemical oceanic conditions prevailing at the depositional sites and the growth rate of these deposits. The variation in the concentrations of Co, Ni and Cu is shown to be dependent on ocean depth. It was found that the Co content is significantly high for the deposit from the Afanasiy Nikitin seamount. It has also been shown and discussed that the hydrothermal processes seem to be more controlling in the uptake of V and As than the hydrogenous processes.

Geochemical constraints on the hydrothermal origin of ferromanganese encrustations from the Rodriguez Triple Junction, Indian Ocean

Abstract Ferromanganese crusts recovered from a spreading ridge north of the Rodriguez Triple Junction in the Indian Ocean were studied for their mineral and geochemical compositions. The crusts are highly porous, have maximum thickness of 2 cm, and occur on basaltic substrates. The composition of the crusts resembles that of hydrothermal ferromanganese crusts from other oceanic spreading centres. Compared to hydrogenous ferromanganese crusts and nodules, the hydrothermal crusts exhibit very low Mn/Fe ratios, lower transitional metal contents, low ΣREE contents and negaive Ce anomalies, which are all characteristics of hydrothermal origin. Geochemical discriminant plots of crust data show similarities to data for metalliferous sediment and hydrothermal deposits recovered from other areas. Shale-normalized REE patterns indicate that the hydrothermal contribution to the crusts is between 60 and 92%. Crusts have rarely been reported to occur at Indian Ocean spreading centers, and those described here are consistent with the earlier findings on water column temperature and chemical anomalies indicative of hydrothermal activity in the region. Some aspects of the crusts, such as δ -MnO 2 mineralogy and middle REE enrichment, are not typical of hydrothermal Fe-Mn oxides. Microprobe studies show that some layers within the crusts have Mn/Fe ratios close to unity, suggesting a significant input from normal seawater. Growth rates, estimated from an algorithm based on Mn, Fe and Co contents, are orders of magnitude faster than rates determined for hydrogenous Fe-Mn oxides. The outer layers and some internal layers of the crust had relatively faster growth rates indicating that the crusts precipitated from distinct hydrothermal episodes.

Cenozoic accumulation history of a Pacific ferromanganese crust

To investigate the growth rates and absolute time stratigraphy of marine hydrogenetic ferromanganese encrustations, we performed 10Be profiling and ‘Co chronometry’ of crustal layers, as well as 87 Sr 86 Sr and δ 18O analysis of phosphatised limestone (francolite) within a ∼ 9.5 cm thick ferromanganese crust from Schumann Seamount in the Hawaiian Archipelago. Together with microfossil stratigraphy, our results indicate that some seamount crusts greatly exceed the commonly accepted Miocene maximum age, in this case probably approaching the Cretaceous age of the seamount. In addition to the unconformity at the crust-substrate boundary, at least eight major disconformities are indicated in the Schumann Seamount crust which probably represent depositional hiatuses or episodes of crust erosion. Three of the six upper disconformities can be placed at the Plio-Pleistocene, Middle Miocene and Paleocene-Eocene based on 10Be, microfossil and Co chronometer evidence. 87 Sr 86 Sr and δ 18O values of purified francolite from an inclusion-rich layer between the depths of 44 and 49 mm suggest apparent ages that approach those of Eocene-Late Paleocene microfossils reported in overlying layers, whereas francolite vein infillings in the lower part of the crust and in the basaltic substrate yield values that, if interpreted as ages of phosphatization, suggest a minimum Oligocene age. Paleotracking suggests the phosphogenesis observed here and on other Central Pacific seamounts could not have resulted from upwelling enhanced productivity associated with equatorial divergence if the Oligocene and Middle Miocene isotopic ages reported here and elsewhere are correct; however, a maximum Late Paleocene age for the phosphogenesis, consistent with the stratigraphy, would place these seamounts within 10°N of the equator. Paleotracking also suggests northeast tradewind transport of aluminosilicates in the Cenozoic, in agreement with other evidence for the antiquity of this ferromanganese crust.

Growth rates of ferromanganese encrustations on rocks from the Romanche Fracture Zone, Equatorial Atlantic

Based on radiochemical techniques, ferromanganese oxide crusts from the Romanche Fracture Zone (RFZ) have accretion rates ranging between 1.4 and 2.8 mm/106 years. In comparison with other ferromanganese deposits, the accretion rates for the RFZ crusts are close to the growth rates for typical hydrogenetic deposits formed on elevations and exposed rock surfaces in the Pacific Ocean. This would suggest that the RFZ crusts are formed by slow accumulation of metal hydroxides from the water column, probably in a manner analogous to the Pacific seamount crusts.

World map of Nd isotopes in sea-floor ferromanganese deposits

The Nd isotopic compositions of hydrogenous ferromanganese encrustations and nodules largely reflect those of the overlying seawater, integrated over the few millions of years over which they grow. Available Nd isotope data on ocean-floor hydrogenous deposits suggest control by Nd from particulates blown from the continents and released by volcanic activity associated with subduction zones and midocean ridges. The regional Nd isotope variations display a marked similarity to the broad patterns of present-day deep-ocean circulation, suggesting that the particulates that are the sources of oceanic Nd travel along with water masses. Despite rapid (10-100 ka) perturbations during glacial periods, the long-term patterns of water-mass movements and oceanic fluxes of particulates have remained essentially stable over the last few million years.

Depth profiles of 230Th excess, transition metals and mineralogy of ferromanganese crusts of the Central Indian basin and implications for palaeoceanographic influence on crust genesis

Two ferromanganese encrustations of hydrogenetic origin from water depths well below the CCD (carbonate compensation depth ) from a seamount zone in the Central Indian basin were studied for growth rates using U-Th series isotopes and transition-metal fluxes. The 230Th excess inventories and transition-metal fluxes in depth profiles separated by a few cm on one crust (F-380) show insignificant variations. In another crust (SS-663X) this variation is within a factor of 2 as compared to variations by a factor of 5 in the Pacific crusts. The minor variations in growth rates, 230Th and transition-metal fluxes within a single crust as well as two adjacent crusts suggest that they are due to the contact of the accreting crust surface with a dynamic veneer of sediment in space and time. Nearly uniform Mn/Fe ratios (1–1.6), δ-MnO 2 as the main Mn mineral phase and a smooth exponential decay pattern of 230Th excess and 230Th excess/ 232Th activities with depth indicate that these crusts have not recorded any palaeoceanographic events of the past 0.4 Ma. The interpolated age of these crusts is between 10 and 13 Ma which is comparable to the period of increased carbonate dissolution due to enhanced activity of AABW (Antarctic Bottom Water) currents during the middle Miocene. Probably the middle Miocene oceanographic conditions, which were associated with increased carbonate dissolution, might have initiated the formation of crusts in the Central Indian basin.

Depth profiles of 230Th excess, transition metals and mineralogy of ferromanganese crusts of the Central Indian basin and implications for palaeoceanographic influence on crust genesis

Two ferromanganese encrustations of hydrogenetic origin from water depths well below the CCD (carbonate compensation depth) from a seamount zone in the Central Indian basin were studied for growth rates using UTh series isotopes and transition-metal fluxes. The 230Th excess inventories and transition-metal fluxes in depth profiles separated by a few cm on one crust (F- 380) show insignificant variations. In another crust (SS- 663X) this variation is within a factor of 2 as compared to variations by a factor of 5 in the Pacific crusts. The minor variations in growth rates, 230Th and transition-metal fluxes within a single crust as well as two adjacent crusts suggest that they are due to the contact of the accreting crust surface with a dynamic veneer of sediment in space and time. Nearly uniform Mn/Fe ratios (1–1.6), δ-Mn0 2 as the main Mn mineral phase and a smooth exponential decay pattern of 230Th excess and 230Th excess/ 232Th activities with depth indicate that these crusts have not recorded any palaeoceanographic events of the past ∼ 0.4 Ma. The interpolated age of these crusts is between 10 and 13 Ma which is comparable to the period of increased carbonate dissolution due to enhanced activity of AABW (Antarctic Bottom Water) currents during the middle Miocene. Probably the middle Miocene oceanographic conditions, which were associated with increased carbonate dissolution, might have initiated the formation of crusts in the Central Indian basin.

New Ichthyoliths from Ferromanganese Crusts and Nodules from the Central Indian Ocean Basin

Ferromanganese encrusted hardgrounds, their intraclasts and the nuclei of manganese nodules collected from the central Indian Ocean basin have yielded plentiful numbers of ichthyoliths. Forty well-known ichthyoliths, one new type and 35 new subtypes are encountered. To accommodate these newly recovered ichthyoliths, the coded descriptive system for the taxonomy of ichthyoliths (Doyle and Riedel 1979a and its later modifications) is updated herein. New ichthyoliths are described and illustrated. A majority of them are believed to be of Paleogene age. INTRODUCTION SYSTEMATICS The distribution of microscopic skeletal fragments of fishes or ichthyoliths has been studied in various DSDP sites from the world's oceans. Recent works of Edgerton et al. (1977), Kaneps et al. (1981), Doyle and Riedel (1979b, 1985a and 1985b), Gottfried et al. (1985) and Tway et al. (1985) have established biostratigraphic ranges of the ichthyolith subtypes, which can be used for correlation in otherwise unfossiliferous pelagic Tertiary sediments. The ferromanganese encrustations and the nodules collected during various cruises for the survey of polymetallic nodules in the central Indian Ocean basin (text-fig. 1, table 1) yielded many ichthyoliths in otherwise unfossiliferous substrates and nuclei of encrustations and nodules. The nuclei and substrates of ferromanganese nodules and encrustations are made up of burrowed and bored pelagic clays. The earlier described ichthyoliths are recognized and many new ichthyoliths are encountered. In this paper, the author has attempted to describe new ichthyoliths by modifying the coded descriptive system of ichthyoliths. MATERIAL AND METHODS The nuclei and substrates of the ferromanganese encrustations and nodules are chiefly of two types; altered basalts and pelagic claystones and clayclasts. Tie clayclasts are the intraclasts of burrowed and bored pelagic hardgrounds (Gupta, in prep.). The nuclei material was scraped with a sharp knife and broken into small pieces (0.5cm), dried, weighed and dispersed in sodium hexa-meta-phosphate then boiled in H202 for 4 hours. The material was sieved on a 62gm mesh screen. The fragmentary fish micro-remains were picked using a fine camel hair brush under a binocular microscope, placed on glass slides and then mounted in Canada balsam. These fish micro-remains were studied under a transmitted light microscope and identified by decoding the coded descriptors. The type material and subtypes of the newly described ichthyoliths are housed at the Geological Oceanography Division of the National Institute of Oceanography, Dona Paula, Goa 403 004, India. The taxonomy of ichthyoliths is determined by decoding the coded descriptive system given by Doyle et al. (1974), Dunsworth et al. (1975), Ramsey et al. (1976), Doyle and Riedel (1979a, 1980, 1985b), Gottfried et al. (1984), Tway et al. (1985) and Winfrey et al. (1987) and are described using the same 70' E 80' IO N \\I LEiAi890 I

Ferromanganese encrustations on the seabed west of Misool, eastern Indonesia

The chemical analyses of ferromanganese encrustations found on the seabed west of Misool, eastern Indonesia, indicate that these deposits formed in a way different from that of world-wide occurring manganese nodules. Ferromanganese coated pebbles and fragments that were found in the deeper parts of the study area probably originate from nearby ridges. The ferromanganese crust on the upper part of a dolomite fragment of ≈30 kg is likely to be formed by hydrogenous processes, whereas that from the lower part seems to be formed by diagenetic processes mainly. These assumptions are supported by pore-water data from two box cores taken in the same area. The manganese and iron profiles versus depth in these cores indicate a high flux of these metals to the uppermost sediment layer, and possibly into the overlying bottom water. Factor analysis for the principal components of the microprobe analytical results of the mainly hydrogenous ferromanganese crust demonstrates a strong correlation of manganese with the trace metals, of iron with phosphorus and an antipathetic relationship between iron and manganese. Similar results have also been reported for abyssal manganese nodules in the world oceans. Factor analysis for the principal components of the analytical data obtained for the diagenetic ferromanganese crust results in a clear dolomite (Ca/Mg) dilution factor only.

Morphology of an uncharted seamount from central Indian Basin

Abstract The morphology of a seamount at 12°35'S and 76°16'E is studied. This conical seamount has a maximum height of 1275 m (water depth 4135 m) and covers an area of about 189 sq km. The maximum basal extension is about 24 km. The mount is gentler and lengthier on the western flank and steeper and shorter on the eastern flank. A conspicuous flat steplike feature is present on the western flank. Slope angles are high (up to 35°). The trend of the seamount is approximately NNE‐SSW. Dredging yielded basalts containing ferromanganese encrustations indicating its volcanic origin.

Palaeogene Ichthyoliths from the substrates of ferromanganese encrustations and nuclei of manganese nodules from the central Indian Ocean basin.

Palaeogene Ichthyoliths from the substrates of ferromanganese encrustations and nuclei of manganese nodules from the central Indian Ocean basin.

The chemistry and mineralogy of ferromanganese encrustations on rocks from the Sierra Leone Rise, equatorial Mid-Atlantic Ridge and New England Seamount Chain

Chemical and mineralogical compositions of ferromanganese oxide coatings on rocks dredged from the New England Seamounts, the Sierra Leone Rise and the Mid-Atlantic Ridge near the Equator have been determined in an investigation of regional differences in Atlantic ferromanganese deposits. Most encrustations are clearly of hydrogenous origin, consisting mainly of todorokite and δMnO 2, but several recovered from the equatorial fracture zones may be hydrothermal accumulations. Mn Fe ratios are similar to those observed in nodules and encrustations elsewhere in the Atlantic, generally falling below unity except for samples from the New England Seamounts. Cu (0.02-0.2%) is lower than in other Atlantic deposits. Ni contents vary from 0.064 to 0.478%, the latter value being found near the St. Paul's Fracture Zone. A large difference in Co is observed between the Northwest and Equatorial Atlantic (0.048-0.135%, New England Seamounts; 0.355-1.010%, near the Equator), ferromanganese coatings on phosphatized limestones from the Sierra Leone Rise being richest in Co. Differences in the chemistry of the water column and in growth rates of the ferromanganese coatings may be important in producing this regional contrast in composition. Fine-scale changes in element abundances within the encrustations indicate that the nature of the substrate has little influence on compositional variations.

143Nd/ 144Nd in Pacific ferromanganese encrustations and nodules

143Nd/ 144Nd ratios have been measured in Fe-Mn encrustations and nodules taken from a wide area of the Pacific and over a large depth range. Over much of the Pacific,ε Nd(0) values fall between −2.5 and −5.0, similar to values for Nd presently dissolved at mid-depth in the Pacific. Deposits forming within the realm of Antarctic Bottom Water (AABW) are distinctly less radiogenic (ε Nd(0) = −6.0 to −7.5) and appear to have sampled Nd both from ambient AABW (ε Nd(0) = −8.5) and from higher in the water column (ε Nd(0) = +0.3 to −4.5). Samples drilled from 50 to 65 mm within one crust are chemically and isotopically distinct from surface layers of the same crust. The isotope values suggest that the mid-depth Pacific may have previously exhibited a less radiogenic Nd isotope signal than that presently observed.

Submarine hydrothermal manganese deposits from the Ogasawara (Bonin) Arc, off the Japan Islands

Numerous submarine hydrothermal sulfide and oxide deposits have been found in the mid-ocean ridge systems, but until now, no activity has been found in an island arc system. The Geological Survey of Japan (GSJ) has started a geological, geophysical, and geochemical investigation program in the Bonin-Mariana Arc since 1984, aiming at evaluation of submarine hydrothermal metal deposits and understanding of their geological background. During the reconnaissance survey by R/V “Hakurei-maru” in June 1984, thick hydrothermal manganese crusts were recovered from three seamounts of the Bonin Arc at a depth of ca. 1000 m. The chemical and mineralogical features and characteristic structure, and occurrence of associated calcareous fossils and nontronite claystones suggest the Quaternary low-temperature hydrothermal activity forming well-crystallized todorokite, pyrolusite, goethite, and nontronite deposits. Thin hydrogenous ferromanganese encrustations on the hydrothermal crusts from a guyot of the northern arc indicate that the recent very active hydrothermal activities around this area may be declining or already finished; whereas the fresh hydrothermal manganese crusts from the Kaikata Seamount suggest recent hydrothermal activity.

Ferromanganese oxide deposits from the Central Pacific Ocean, I. Encrustations from the Line Islands Archipelago

Chemical and mineralogical analyses of a well-controlled suite of ferromanganese encrustations from the Line Islands Archipelago (Central Pacific) suggest that they represent purely hydrogenous deposits— i.e. they have formed through the slow accumulation of trace metal-enriched oxides directly from the water-column. Mineralogically they consist predominantly of δMnO 2 and amorphous FeOOH xH 2O. Compositionally, they are similar to δMnO 2 nodules from adjoining basinal areas but are enriched in both Mn (mean = 20.4%, max = 29.3%) and Co (mean = 0.55%, max = 1.57%). δMnO 2 is the most important trace metal bearing phase; strong associations are noted between it and Co, Mo, Ni, Zn, and Cd, whilst only Be is associated specifically with FeOOH. V, Sr and Pb are partitioned between the authigenic oxide phases, whilst Ti most probably occurs as TiO 2 xH 2O. Cu is contained in both aluminosilicate contaminant phases and Fe oxide phases. These relations are considered to reflect the differing scavenging behaviour of Mn and Fe oxides in the water column. Crusts from ~1–2 km are enriched in Mn and the Mn-related elements and exhibit higher Mn Fe ratios than deeper crusts, which are compositionally constant. The higher Mn Fe ratios may result from a supply of Mn from continental borderland sediments at these depths, which is transported horizontally by advective-diffusive processes. Since manganophile elements are enriched relative to Mn in the 1–2 km crusts, it is considered that the supply of Mn is scavenged by existing oxides, is oxidised and effectively occludes them. A higher proportion of oxide particles thus exhibit Mn oxide scavenging properties in the 1–2 km depth zone. The increased vertical flux of Mn resulting from the supply at ~1–2 km is not reflected by higher Mn Fe ratios in deeper crusts, so that the vertical flux of oxides is not simply related to the standing crop. The Mn Fe ratios of the crusts thus reflect the composition of suspended oxides at similar depths.

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, δMnO 2-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. δMnO 2 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.

Rare earth element geochemistry of Central Pacific ferromanganese encrustations

Analyses have been made of the REE contents of a suite of hydrogenous δMnO 2-rich ferromanganese encrustations obtained from a variety of depths (1000–4700 m) on the Line Islands Archipelago. The crusts form a coherent sample group, the REE contents of which are distinctly higher than those of diagenetic nodules. Crusts from 1 to 2 km depth exhibit higher Mn/Fe ratios and are approximately 50% REE depleted with respect to crusts below ∼ 2 km. Furthermore, the shale-normalised patterns of the REE fall into the same depth-related categories. Crusts from depths greater than 2000–2500 m exhibit slight heavy REE depletion relative to intermediate REE whilst crusts from above that level exhibit more fractionated, heavy-REE-enriched patterns. To explain these depth-related processes, it is proposed that Mn and Fe oxides exhibit distinct behaviour with respect to REE scavenging. Between ∼ 1 and 2 km, higher Mn/Fe ratios in the crusts are considered to be caused by an enhanced supply of Mn 2+ —by diffusion and advection from nearshore sediments—which is adsorbed by and occludes existing oxide flocs. At these depths, oxides therefore tend to exhibit predominantly Mn oxide adsorptive properties. Consideration of the REE patterns shows that Fe oxides must be enriched overall in REE relative to Mn oxides and exhibit a flat shale-normalised REE pattern, whilst Mn oxide exhibits a heavy-REE-enriched pattern. Comparison of dissolved REE depth profiles with the crust data enables a qualitative appraisal of the application of scavenging models to the oxide-REE system. The data are not simply described by either equilibrium or irreversible adsorption models alone. It is concluded that whilst oxides may play a general role in creating the LREE depleted seawater pattern, they have little effect on individual vertical profiles, except close to points where fresh oxides precipitate.

Spatial variation of U-Th series radionuclides and trace metals in deep-sea manganese encrustations

A series of measurements to assess the spatial variability on a scale of a few centimeters of U Th series nuclides and trace metals in surface layers of deep-sea ferromanganese encrustations are reported here. Two samples, one from an Atlantic seamount and another from MANOP site S in the Pacific are reported. The inventories of 230Th exc cm −2 and 231Pa exc cm −2 as well as the ratios of the inventories are seen to vary by large amounts in samples only a few centimeters apart. These observations infer that the fluxes of these radionuclides to small areas on the ferromanganese crust may also vary with time. No strong correlation between the radioisotope inventories and concentrations of various elements has been observed.