Mn Crusts Research Articles

Pb and Nd isotopes in NE Atlantic Fe–Mn crusts: Proxies for trace metal paleosources and paleocean circulation

We report high precision Pb isotopic data (2σ ≤ 100 ppm) together with Nd isotopes on depth profiles from two Fe–Mn crusts from the eastern Atlantic basin. The profiles provide a 13 Ma record of changes in eastern North Atlantic Deep Water (ENADW), and over the past 8 Ma for Mediterranean Outflow Water (MOW). Pb isotope ratios in the two records display gradual and systematic changes, tracking each other through time. The highly precise Pb isotopic data also resolve, for the first time, changes in all three Pb isotope ratios in a single Fe–Mn crust, and exhibit well-defined binary mixing lines in Pb isotope space. The Pb isotopic record of the Tropic Seamount crust shows that eastern Atlantic Pb has been dominated by binary mixing throughout the last 13 Ma. The Pb binary mixing lines further demonstrate that a change in Pb provenance to the eastern Atlantic occurred at 8 and 4 Ma. This is shown by the distinctive Pb isotopic compositions of the mixing components in the time intervals 4–0 Ma and 13–8 Ma. Consideration of Pb and Nd isotope systematics show that from 4 Ma to the present, the two endmember components correspond to NADW and Southern Component Water (SCW). The quasi-cyclic character of 206Pb/ 204Pb variations, as well as age progressive changes in the Pb mixing proportions of NADW and SCW endmembers, appear to reflect changes in deep water circulation. In particular, strengthening of the Pb and Nd isotopic signal associated with NADW since 3–4 Ma in both the eastern and western Atlantic basins implies that NADW advection from the western into the eastern Atlantic has been in operation over the past 4 Ma. During the period from 13 to 8 Ma, two entirely different sources of Pb and Nd existed in the eastern Atlantic. The radiogenic Pb and unradiogenic Nd component may have originated in the Norwegian–Greenland seas and the low 206Pb/ 204Pb—high ε Nd endmember component in the Southern ocean or Tethys. This suggestion is supported by evidence from paleoceanographic studies indicating the importance of Greenland–Scotland ridge activity in Miocene deep water production, and also the presence of a distinctive high δ 13C water mass in the Southern Ocean, derived either locally or flowing in via Tethys. The 8 Ma record of the Lion Seamount crust (65GTV) demonstrates that Pb and Nd sources in MOW switched from predominantly internal, Mediterranean (European) sources prior to 4 Ma, to mainly external (Saharan) sources after 4 Ma. The gradual increase of Pb isotope ratios seen following the end of the Messinian reflects enhanced input of Saharan dust into the water column in the eastern Atlantic. The strengthening of the “Saharan” isotopic signal from about 4 Ma also matches the documented increase in aridity and dustiness in the Saharan and sub-Saharan regions and coincides with the re-establishment of water exchange between the Atlantic and Mediterranean after the Messinian. Although Pb and, to a lesser extent, Nd isotope ratios are distinct during the Messinian (6.5–5 Ma), there is no clear evidence for either a shutdown of MOW or a stronger North Atlantic signal during this period. The fact that a similar isotopic signal is observed, with an even higher amplitude in the ENADW record, shows that this signal is a feature of the whole eastern Atlantic. From 3 to 4 Ma ago, a source of radiogenic Pb and unradiogenic Nd appears to have dominated not only the eastern Atlantic but the world oceans, since it is seen ubiquitously in other Fe–Mn crusts at this time. This isotopic signal must be conveyed around the globe via the ocean circulation or the atmosphere. While the closure of the Panama gateway may have played an important role in these global changes in source(s) and/or fluxes of Pb and Nd to the oceans 3–4 Ma ago other events, in conjunction, such as the emergence of the Greenland–Scotland ridge, were probably also as influential.

Response of beryllium and radiogenic isotope ratios in Northern Atlantic Deep Water to the onset of northern hemisphere glaciation

We present time-corrected ratios of cosmogenic 10Be to continent-sourced 9Be from two Fe–Mn crusts located in the northwest Atlantic in the vicinity of the origins of Northern Atlantic Deep Water (NADW). These initial 10Be/ 9Be ratios represent those of the ambient seawater at the time of crust deposition and are constant through the last 7 Ma. Previously published isotope records of radiogenic Nd and Pb isotopes in these crusts show a decrease in ε Nd and an increase in 206Pb/ 204Pb over the last 5 Ma. These changes are consistent with a gradual increase in the strength of NADW production, an increase in the Labrador Sea component of NADW, or an increased erosional input into the Labrador Sea. Since the residence time and thus the advective length scales of dissolved Be in deep water lie between those of Nd and Pb, records of 10Be/ 9Be should, contrary to what has been observed, vary in phase with those of Nd and Pb if these variations are accounted for by changes in paleo-circulation. The observed variations in radiogenic Nd and Pb isotopes are therefore most likely to be explained with an increase in the flux of terrigenous matter in the area of the Labrador Sea, which was possibly associated with ice rafting and selective leaching of Pb isotopes. An increase of the Labrador Sea component that contributes to NADW is also a possibility, but based on the 10Be/ 9Be data presented here large-scale changes in the thermohaline circulation can be discounted as cause for the radiogenic tracer variations.

Origin of negative Ce anomalies in mixed hydrothermal–hydrogenetic Fe–Mn crusts from the Central Indian Ridge

Layered Fe–Mn crusts from the off-axis region of the first segment of the Central Indian Ridge north of the Rodrigues Triple Junction were studied geochemically and mineralogically. Vernadite (δ-MnO 2) is the main mineral oxide phase. 230Th xs and Co concentrations suggest high growth rates of up to 29 mm/Myr and a maximum age of the basal crust layer of 1 Ma. Whereas most of the major and minor elements show concentrations which are typical of hydrogenetic formation, Co, Pb, Ni and Ti concentrations are strikingly lower. Concentrations and distribution of the strictly trivalent rare-earths and yttrium (REY) are typical of hydrogenetic ferromanganese oxide precipitates, but in marked contrast, the crusts are characterized by negative Ce SN (shale normalized) anomalies and (Ce/Pr) SN ratios less than unity. Profiles through the crusts reveal only minor variations of the REY distribution and (Ce/Pr) SN ratios range from 0.45 to 0.68 (compared to ratios of up to 2 for typical hydrogenetic crusts from the Central Indian Basin). The apparent bulk partition coefficients between the crusts and seawater suggest that for the strictly trivalent REY the adsorption–desorption equilibrium has been reached. Positive Ce anomalies in the partition coefficient patterns reveal preferential uptake of Ce, but to a lesser extent than in normal hydrogenetic crusts. A new parameter (excess Ce, Ce xs) to quantify the degree of decoupling of Ce from REY(III) is established on the basis of partition coefficients. Ce xs/Ce bulk ratios suggest that the CIR crusts formed by precipitation of Fe–Mn oxides from a hydrothermal plume and that in hydrothermal plumes and normal seawater the enrichment of Ce results from the same oxidative sorption process. The growth rates, calculated with 230Th xs data as well as with the Co formula, are inversely related to Ce xs.

Hf-Nd Isotopic Covariance in the Crust and Mantle and Constraints on the Evolution of the Depleted Mantle

It has long been recognized that Lu-Hf and Sm-Nd isotope systems behave analogously during most magmatic processes but the details of Hf-Nd isotopic behaviour in the major crust and mantle reservoirs have not been fully demonstrated. How Hf and Nd isotopes behave in the depleted mantle through time, for example, has been uncertain because of the apparent different Hf-Nd variations in MORBs and OIBs and the potential for decoupling these systems in a number of important mantle (e.g. Salters and Hart, 1991) and crustal (e.g. Patchett et al., 1984; Vervoort and Patchett, 1996) environments. New HfNd results in combination with existing data, allow us to make some first-order observations of Hf-Nd behaviour in some of the Earth's major reservoirs. Oceanic Basalts. Hf-Nd data for plot along an array defined by the line eHr = 1.3 eNd + 3.1. The OIB array is indistinguishable from the array for all oceanic basalts (including MORBs and IABs) of eHf = 1.3 eNd + 3.6. Crustal Samples. Muds and shales, recent to Archaean, plot along an array enf = 1.4 end + 2.9, similar to that for OIBs and all ocean basalts. Sands and sandstones are more scattered and plot along a steeper array (eHf = 1.8 eNd + 2.5) due to the less radiogenic Hf character in some sands. This 'zircon effect' occurs in some sands derived from older crustal sources where the Hf in the sands is dominated by zircons less radiogenic than the SmNd in the bulk rock. All whole-rock data. All terrestrial whole-rock Hf and Nd data define a remarkably coherent array of eHf = 1.4 aN~ + 2.9 (Fig. 1). Mn crusts and pelagic red clays, some peridotite xenoliths, and a few sandstones with extreme zircon effects plot outside of the main array and are not included in this regression. This array contains diverse samples of wide-ranging compositions and ages. The close similarity of the Hf-Nd arrays for continental crust (as defined by muds) and the mantle (as defined by all oceanic basalt data) indicates no large-scale decoupling of Hf and Nd between crust and mantle. This tight Hf-Nd covariation may also argue for the efficacy of crust to mantle recycling in modulating Hf-Nd behaviour in the silicate Earth. With this background, we report new whole-rock Hf-Nd isotope data for 1) mantle-derived rocks, midArchaean to Mesozoic in age and 2) early Archaean gneisses from West Greenland. Hf and Nd isotopic compositions are well correlated in the juvenile rocks, corresponding to eHf = 1.4 end +2. l, and plot well within the collective Hf-Nd array of terrestrial samples (eHf = 1.4 r +3.1). The early Archaean Greenland gneisses, in contrast, have an extreme range in eNd values (--4.4 to +4.2; Bennett et at., 1993) while corresponding eHf values are more restricted and entirely positive (0 to +3.4) resulting in a Hf-Nd array for these rocks (eHf = --0.1 eNd +2.3) quite different from that defined by other terrestrial samples. This Nd isotopic heterogeneity is not a result of extreme depletions and enrichments in the early Earth, but from disturbances in the Sm-Nd isotope system of these rocks.

Factors controlling the genesis of FeMn crusts in stratigraphic breaks of the eastern Betic Cordillera (SE Spain) deduced from numerical analysis of geological data

FeMn crusts occur in four stratigraphic breaks of Jurassic pelagic swell sequences in the eastern Betic Cordillera. Crusts were characterized by quantitative and qualitative variables deduced from their stratigraphic position (under- and overlying lithologies), texture (thin or banded), mineralogical and chemical composition. Quantitative chemical variables were summarized in two new variables using principal component analysis. Component A groups detrital elements, and Fe with chalcophile elements, which suggests a seawater enrichment by detrital and hydrothermal contributions. Component B associates elements of hydrogenous precipitation and adsorption during exposure of Fe-oxide to the seawater. Relations between quantitative and qualitative variables were established by the use of correspondence analysis. Variables form two groups: TX1 (variables representing thin crusts, type 1) and TX2 (banded crusts, type II). Thin crusts are related with chemical variables denoting high detrital element contents (Al), low Mn, REE, and transition metal element contents (B3), presence of detrital minerals (DET) and MnCo-poor goethite (GT). Banded crusts are associated with variables representing low detrital element contents (A2, A3), high Mn, REE, and transition metal element contents (Bl, B2), presence of MnCo-rich goethite (MNG), Mn-oxyhydroxides (MNO), and REE-phosphates (PH). These associations indicate that, in the type II crusts, adsorption processes have a greater importance than in the type I crusts. Correspondence analysis helps to clarify the geological variables determining the extent of the adsorption processes, since variables related with textural type I are associated to lithological variables (OCL: overlying cherty limestones; UML3: underlying thick rhythmite of marls and limestones) indicating a geological context of pelagic swell slightly deeper than those associated with the textural type II (UCC: underlying crinoidal calcarenite; UML1: thin underlying marl-limestone rhythmite).

A model for the formation of hydrothermal manganese crusts from the Pitcairn Island hotspot

The distribution of elements in the hydrothermal Mn crusts from the Pitcairn Island hotspot can be explained by considering processes occurring within the hotspot volcano. These include the role of boiling of the ascending hydrothermal fluid which leads to sulphide deposition and seawater intrusion into the upper part of the volcano. This in turn results in the development of an oxycline above which FeOOH can precipitate. The high Mn/Fe ratios and low contents of Ni and Zn in the massive crusts are thought to reflect the rapid incorporation of the ore elements into sulphide minerals such that only a small proportion of these elements are available for incorporation in the crusts. The small positive Eu anomaly in the crusts on a NASC-normalized basis reflects the lower temperatures in the hydrothermal fluids at the Pitcairn Island hotspot (<25dgC) compared to those at mid-ocean ridges (c.350°C). The oxyanions (CrO 4 2−, VO 4 3− AsO 4 3− and PO 4 3−), the rare earth elements (REE), and Th are scavenged by FeOOH particles within the upper part of the seamount and can, therefore, be incorporated into the crusts associated with the Fe-rich phase. Molybdenum and uranium are scavenged in only negligible amounts by the FeOOH. Molybdenum is, therefore, most probably scavenged from seawater by manganese oxides during the actual formation of the Mn crusts. The alkali metals, Li, Rb, and Cs, remain in solution in the hydrothermal fluids and are selectively incorporated into the buserite structure of the Mn crusts in the absence of strong competition from the divalent transition metal ions. The compositional characteristics of the hydrothermal Mn crusts can, therefore, be explained in terms of a simple model of element scavenging and uptake occurring mainly within the seamount. A laser-ablation ICPMS profile in one of the hydrothermal crusts (DS 63-4) revealed varying REE concentrations and patterns in the different layers of the crust. These data indicate that the hydrothermal component was variable during the formation of the crust. It was calculated to be almost 100% in the upper 18.0 mm of this crust (apart from the samples at 6.0–8.0 mm) and about 80% in the lower layers.

Effects of phosphatization on the geochemical and mineralogical composition of marine ferromanganese crusts

Most hydrogenetic ferromanganese crusts in the central Pacific consist of two growth generations: a phosphatized older growth generation that is impregnated with carbonate fluorapatite (CFA), and a younger non-phosphatized growth generation. Geochemical and mineralogical investigations of non-phosphatized and phosphatized crust layers revealed significant differences that are not simply dilution effects by CFA. Certain elements are depleted in old crusts compared to young crusts in the order Si > Fe > Al > Ti ≥ Co > Mn ≥ Pb; others like Ni, Zn, Cu, Y, and REEs (rare earth elements) are mostly enriched. Suboxic conditions in the phosphate-rich water of the oxygen-minimum-zone that infiltrated the crusts caused a partial redissolution of crust phases, resulting in mobilization of associated elements, a partial recrystallization of more stable phases, and a secondary input of nutrient-type elements. The frequent occurrence of todorokite in phosphatized crust layers indicates that Mn mobilized from the vernadite crust phase was partially recrystallized as todorokite, which is more stable under suboxic conditions, incorporating Ni, Cu, and Zn into the crystal lattice, but rejecting Co. Other metals like Pb, Y, and REEs probably form stable phosphate phases in phosphatized crusts. Our investigations emphasize that under the influence of phosphatization, diagenetic remobilization and reorganization in FeMn crusts took place. In contrast to non-phosphatized crusts, the older phosphatized crust generation no longer represents a complete primary crust precipitate. Therefore, young non-phosphatized and old phosphatized crust generations should be considered as two different sample types with respect to geochemical composition, mineralogy, and evolution. We also suggest that most methods for age determination like the 87 Sr 86 Sr isotopic dating method or Co chronometry cannot be used for phosphatized crusts without caution.

Secular changes of lead and neodymium in central Pacific seawater recorded by a FeMn crust

Oceanic and atmospheric circulation patterns varied considerably during the Tertiary and Quaternary and influenced the geochemical cycles of elements in seawater. We report the first resolution lead and neodymium isotopic record of such changes at a high time resolution in two depths profiles from a hydrogenous FeMn crust. The crust, Va13-2, is located in the central Pacific (146°W, 9°25′N, 4830 m) and has previously been dated by 230Th and 10Be. The first profile was drilled with a sample time resolution of ∼3 kyr and allows evaluation of short-term changes to lead and neodymium sources to central Pacific seawater over the last 400 kyr (marine δ 18O stages 2 to 11). Longer-term changes were monitored at lower time resolution in a second profile to an age of 10 Ma. Short-term variations in lead and neodymium isotope ratios are resolved in the high resolution profile (0 to 400 kyr). Superimposed on the short-term variations is a secular decrease in 206 Pb 204 Pb ratios beginning at ∼130 kyr in marine δ 18O stage 5, implying a change in the lead sources to the central Pacific. Lead and neodymium isotopic compositions indicate an increased influence from Central American eolian sources to Pacific seawater at this time. Lead isotopes are found to be statistically more variable during interglacial than glacial periods. These observations are supported by the greater eolian dust fluxes found in sediment cores from the equatorial Pacific during interglacial stages. The most important paleoceanographic event of the last 10 Ma to affect Pacific seawater was the closure of the Panama gateway. Changes in lead and neodymium isotopes in Val3-2 during the last 10 Ma occurred along with gradual closure of the Panama straits. However, these changes did not occur in tandem: while neodymium isotope ratios increase between 10 and 8 Ma, lead isotope ratios remain constant. In contrast, the period 7 to 1 Ma is marked by a secular increase in lead isotope ratios but nearly constant neodymium. These changes are consistent with a source of radiogenic lead and neodymium conveyed by the Circumpolar Current into the Pacific, rather than by the Panama gateway, and involve 20 to 40% Southern Component Water (SCW) input of lead and neodymium. Modelling of lead and neodymium isotopic mixing between the different water masses involved in generating Pacific deep waters lead us to the following conclusions: (1) Small variations in the strength and composition of North Atlantic Deep Water (NADW) have a relatively minor effect on the amounts of lead and neodymium from SCW contributing to the Pacific and (2) an enhanced SCW flow with an open isthmus of Panama, as suggested by General Circulation Models (GCM), requires a corresponding reduction in NADW Pb and Nd contributions to SCW. The general agreement between the isotopic compositions of surface layers of Mn nodules, integrated over such long time intervals, and those of present-day bottom waters at their respective locations show that the present-day ocean circulation pattern has dominated through the Pleistocene. Our study of Mn crust Va13-2 shows that shorter-term changes in lead and neodymium isotope ratios can be resolved, provided that such crusts are sampled at an appropriate time resolution.

238U [sbnd]234U [sbnd]230Th disequilibrium in hydrogenous oceanic Fe [sbnd]Mn crusts: Palaeoceanographic record or diagenetic alteration?

A detailed TIMS study of ( 234U exc/ 238U), ( 230Th/ 232Th), and Th/U ratios have been performed on the outermost margin of ten hydrogenous Fe Mn crusts from the equatorial Pacific Ocean and west-central Indian Ocean.Th/U concentration ratios generally decrease from the crust's surface down to 0.5-1 mm depth and growth rates estimated by uranium and thorium isotope ratios are significantly different in Fe Mn crusts from the Peru Basin and the west-central Indian Ocean. Fe Mn crusts from the same geographical area define a single trend in plots of Ln( 234U exc/ 238U) vs. Ln( 230Th/ 232Th) and Th/U ratios vs. age of the analysed fractions. Results suggest that (l) hydrogenous Fe Mn crusts remain closed-systems after formation, and consequently (2) the discrepancy observed between the 230Th and 234U chronometers in Fe Mn crusts, and the variations of the Th/U ratios through the margin of FeMn crusts, are not due to redistribution of uranium and thorium isotopes after oxyhydroxide precipitation, but rather to temporal variations of both Th/U and initial thorium activity ratios recorded by the Fe Mn layers. Implications of these observations for determination of Fe Mn crust growth-rates are discussed. Variations of both Th/U and initial Th activity ratios in Fe Mn crusts might be related to changes in particle input to seawater and/or changes in ocean circulation during the last 150 ka.

Volcanic‐hosted sulfide‐barite deposit from the Kita‐Bayonnaise submarine caldera, Izu‐Ogasawara arc, northwestern pacific

Volcanic‐hosted sulfide and barite mineralization with high Au content (19.3 ppm) was discovered by submersible study (Shinkai 2000) on the eastern floor of the Kita‐Bayonnaise submarine caldera on the Shichito‐Iwojima Ridge, Izu‐Ogasawara Arc, northwestern Pacific. The hydrothermally mineralized rhyolitic tuff is partly coated by Mn oxyhydrox‐ide, and associated with hydrothermal Mn crusts and debris of mussel shells in the vicinity of the hydrothermally altered tuff‐breccias. The minearlized rhyolitic tuff occurs as a fragment of a large block (about 2 ×1 ×1 m) on talus, while the crusts occur on the caldera floor as a few thin patches several meters across or smaller. Ore‐mineral composition of the mineralized rhyolitic tuff consists mainly of sulfides and barite. Randomly interstratified seriate and montmorillonite is the only hydrothermal clay mineral from glass shards in the mineralized rhyolitic tuff. The mineralized tuff is significantly enriched in Zn, Fe, Cu, Ba, Au, Ag, and As, and has high Au /Ag ratio (0.036 to 0.071) compared with other marine volcanogenic sulfides. Au is probably present as submicrometer‐sized particles within sulfide minerals or is contained within the crystal lattice of the sulfide minerals. The δ34S values of sulfides of subsamples from the mineralized rhyolitic tuff indicate a fairly narrow range between +3.4 and +4.8% but are characterized by lighter isotopes in a drusy cavity compared with vein and disseminated sulfides. The sulfide sulfur is mainly from volcanic rocks of the caldera basement. A single barite sulfur isotopic value (+20.7%o) and salinity of fluid inclusions in barite are close to modern seawater values, suggesting a major contribution of seawater sulfate to the barite precipitation.

Distribution and sources of pre-anthropogenic lead isotopes in deep ocean water from Fe[sbnd]Mn crusts

The lead isotope composition of ocean water is not well constrained due to contamination by anthropogenic lead. Here the global distribution of lead isotopes in deep ocean water is presented as derived from dated (ca. 100 ka) surface layers of hydrogenetic Fe-Mn crusts. The results indicate that the radiogenic lead in North Atlantic deep water is probably supplied from the continents by river particulates, and that lead in Pacific deep water is similar to that characteristic of island and continental volcanic arcs. Despite a short residence time in deep water (80–100 a), the isotopes of lead appear to be exceedingly well mixed in the Pacific basin. There is no evidence for the import of North Atlantic deep water-derived lead into the Pacific ocean, nor into the North Indian Ocean. This implies that the short residence time of lead in deep water prohibits advection over such long distances. Consequently, any climate-induced changes in deep-water flow are not expected to result in major changes in the seawater Pb-isotope record of the Pacific Ocean.

Global distribution of beryllium isotopes in deep ocean water as derived from FeMn crusts

The direct measurement of the ratio of cosmogenic 10Be ( T 1 2 = 1.5 Ma ) to stable terrigenously sourced 9Be in deep seawater or marine deposits can be used to trace water mass movements and to quantify the incorporation of trace metals into the deep sea. In this study a SIMS-based technique has been used to determine the 10Be 9Be ratios of the outermost millimetre of hydrogenetic ferromanganese crusts from the worlds oceans. 10Be 9Be ratios, time-corrected for radioactive decay of cosmogenic 10Be using 234U 238U , are in good agreement with AMS measurements of modern deep seawater. Ratios are relatively low in the North and equatorial Atlantic samples (0.4–0.5 × 10 −7). In the Southwest Atlantic ratios increase up to 1 × 10 −7, they vary between 0.7 and 1.0 × 10 −7 in Indian Ocean samples, and have a near constant value of 1.1 ± 0.2 × 10 −7 for all Pacific samples. If the residence time of 10Be ( τ10 Be) in deep water is constant globally, then the observed variations in 10Be 9Be ratios could be caused by accumulation of 10Be in deep water as it flows and ages along the conveyor, following a transient depletion upon its formation in the Northern Atlantic. In this view both 10Be and 9Be reach local steady-state concentration in Pacific deep water and the global τ10 Be ≌ 600 a . An alternative possibility is that the Be isotope abundances are controlled by local scavenging. For this scenario τ10 Be would vary according to local particle concentration and would ≌ 600 a in the central Pacific, but τ10 Be ≌ 230 a in the Atlantic. Mass balance considerations indicate that hydrothermal additions of 9Be to the oceans are negligible and that the dissolved riverine source is also small. Furthermore, aeolian dust input of 9Be appears insufficient to provide the dissolved 9Be inventory. The dissolution of only a small proportion (2%) of river-derived particulates could in principle supply the observed seawater 9Be content. If true, ocean margins would be the sites for 9Be addition. Due to the particle-reactive nature of Be, these would also be the primary sites of Be removal. A possible net result of horizontal water masses passing through these marginal areas might be a decrease in seawater 10Be 9Be , and establishment of a relatively constant 9Be concentration. As τ10 Be (∼ 600 a) is less than the apparent age of deep water in the Pacific (∼ 1500 a), the Pacific record of 10Be 9Be is not expected to show secular variations due to changes in deep-water flow, despite the large variations in 10Be 9Be between different water masses. Because of this insensitivity to deep-water flow, however, it is suggested that the 10Be 9Be ratio, determined in the authigenic phase of marine sediments or hydrogenetic precipitates, should be a suitable tool for monitoring changes in continental input or cosmic ray intensity on longer time scales.

A simplified procedure for the measurement of 231Pa in Mn encrustations

A simple and efficient method for the measurement of 231Pa was developed. It consists of one ion exchange step followed by electrodeposition of Pa (chemical yield approx. 50%). The concentrations of 231Pa (α-decay) and of the spike 233Pa (β-decay) may be measured simultaneously with a semiconductor detector. The new procedure was applied to the measurement of high resolution profiles in Mn crusts.

238U 234U 230Th chronometry of FeMn crusts: Growth processes and recovery of thorium isotopic ratios of seawater

Comparison of ( 234 U)excess ( 238 U) and ( 230 Th 232 Th activity ratios in oceanic Fe-Mn deposits provides a method for assessing the closed-system behaviour of 238U 234U 230Th, as well as variations in the initial uranium and thorium isotopic ratios of the precipitated metal oxides. This approach is illustrated using a FeMn crust from Lotab seamount (Marshall Islands, west equatorial Pacific). Here we report uranium and thorium isotopic compositions in five subsamples from the surface of one large 5 cm diameter botryoid of this crust, and from two depth profiles of the outermost rim of the same botryoid. The decrease of ( 234 U)excess ( 238 U) and ( 230 Th 232 Th activity ratio with depth in the two profiles gives mean growth rates, for the last (238U) 212Th 150 ka, of 7.8 ± 2 mm/Ma and 6.6 ± 1 mm/Ma, respectively. All data points (surface and core samples) but one, define a linear correlation in the Ln ( 230 Th 232 Th − Ln ( 234 U)excess ( 238 U) diagram. This correlation indicates that for all points the UTh system remained closed after the FeMn layer precipitated, and that the different samples possessed the same initial Uranium and thorium isotope ratios. Furthermore, these results show that the preserved surface of this FeMn crust may not be the present-day growth surface, and that the thorium and uranium isotopic ratios of seawater in west equatorial Pacific have not changed during the past 150 ka. The initial thorium activity ratio is estimated from the correlation obtained between Ln ( 230 Th 232 Th − Ln ( 234 U)excess ( 238 U) .

Mineralogy and composition of manganese crusts and nodules and sediments from the Manihiki plateau and adjacent areas: Results of HMNZS Tui Cruises

Forty‐four sediment samples and 23 Mn crusts and Mn nodules have been analyzed for several elements in order to characterize their geochemical nature. Most of the Mn crusts and Mn nodules are of hydrogenous origin, although one crust (U375) from the flanks of the Rarotonga volcano is thought to be of hydrothermal origin. δ‐MnO2 is the principal Mn mineral in the crusts, whereas todorokite and birnessite occur in addition to δ‐MnO2 in the nodules. The sediments can be divided into carbonates, with calcite as the dominant phase, and deep‐sea clay, consisting mainly of illite and smectite.

Geochemistry of hydrothermal manganese deposits from the Pitcairn Island hotspot, southeastern Pacific

Hydrothermal Mn crusts from a submarine volcano in the Pitcairn Island region have been studied mineralogically, radiometrically, and by bulk and partition chemical techniques. The crusts contain layers that comprise three morphological types. A mechanism of formation by downward growth from a Mn-cemented surface sand hardpan is proposed. Age dating of the crusts suggests relatively recent, and rapid, formation. Mineralogically, the crusts exhibit variable amounts of 10Å and 7Å manganite. Chemical data, however, indicate that the crusts do not show element associations characteristic of marine birnessite, suggesting that the 7Å manganite phase may have transformed from structurally unstable 10Å manganite as a result of air drying. Trace element concentrations in massive, finely laminated types 2 and 3 crust layers are significantly lower than those in similar crusts from island arc and back-arc settings and, in general, are comparable with, or lower than, those of the most trace element poor crusts from mid-oceanic ridge environments. REE data suggest that these crusts are almost pure hydrothermal deposits with little or no hydrogenous component. In the types 2 and 3 crusts, only Mo shows a significant enrichment that is likely to be hydrothermal in origin, although it is enriched to lower levels than in hydrothermal crusts from most other geological settings. Friable, Mn-cemented sand type 1 crusts contain elevated trace element concentrations, in part as a result of incorporation of biogenic and volcaniclastic phases, but also as a result of hydrogenous inputs consistent with their greater age and closer proximity to seawater. In contrast to types 2 and 3 crusts which show no Li enrichment, type 1 crusts are enriched in Li to levels above those that can be accounted for by hydrogenous input and a hydrothermal source for it is proposed. The observed variability in the composition of hydrothermal Mn crusts between midplate hotspot, mid-oceanic ridge, island arc, and back-arc environments may be influenced by variability in basement composition and sea water/rock ratio in the hydrothermal systems. As both parameters are directly related to tectonic setting, this may play an important role in controlling the chemistry of hydrothermal deposits.

Mineralogical genetic groups of fine-psammitic and coarse-aleuritic fractions and some indicators of the submarine hydrothermal activity history (East Pacific Rise, 21°S)

The mineral composition of fine-psammitic (0.10-0.50 mm) and coarse-aleuritic (0.05-0.10 mm) fractions of metalliferous sediments from the East Pacific Rise axial zone (20°30'-22°00' S) is investigated in details by a complex of analytical methods. The mineral composition of the sediments of this region is controlled by the follow factors: remoteness from the landmasses; ocean depth; appurtenance of investigated area to the South arid zone and the axial part of the fast spreading centre with high hydrothermal activity. The sedimentary formation of the investigated spreading segment is polygenetic. Seven mineralogical genetic groups can be distinguished: biogenous, terrigenous, hydrothermal, basaltic, diagenetic, hydrogenetic and cosmogenic. The pelagic background is formed mainly by biogenous CaCO3 and detrital and clay minerals with eolic origin. The diagenetic and hydrogenetic mineral formation have subordinate significance in that environment. The tectonovolcanic processes in the region supply a specific "local" material to the sediments - basaltic and hydrothermal. Some of the minerals accumulating in the metalliferous sediments of the spreading sedimentary formation can be recommended as indicators for sulphide ore prospecting. The sulphides of Fe, Cu, Zn; barite; Fe umbers; compact black-brown Fe crusts; black Mn crusts; colloform semitransparent aggregates of Fe oxyhydroxydes and amorphous SiO2 are the best indicators for sulphide mineralization in the investigated area. Their contents increase regularly towards the hydrothermal fields. The spatial distribution of the minerals related to the hydrothermal activity in the rift is controlled by the rate and direction of the seabottom currents and by the hydrothermal system locality and power. A regular variation of the mineral composition of the investigated fractions is established during cyclic tectono-volcanic development of the spreading centre. The content of basaltic minerals increases during the volcanic cycles (before and after the hydrothermal cycles). The concentration of hydrothermal-sedimentary minerals in the sediments increases during the hydrothermal cycles. Low-temperature minerals are formed during their initial and final phases.

Climatic influences on the growth rates of Mn crusts during the Late Quaternary

The concentration of 230Th in Mn crust VA13-2 (146°W, 9°25′S, 4830 m) from the Pacific Ocean was measured with high depth resolution (0.02 mm) to a depth of 1.4 mm corresponding to an age of ∼226 ka B.P. The mean growth rate estimated from these results is 6.2 ± 0.4 mm/Ma. However, the exponential curve obtained shows two drastic events in the 230Th concentration record with durations of 41 ± 4 ka and 37 ± 6 ka which are interpreted as growth standstills. The decay-corrected 230Th concentration is inversely correlated with those of Mn, Fe and Co indicating that the 230Th flux into the crust has been constant throughout time and that the observed fluctuations in 230Th are due to dilution by the main components. By applying a constant 230Th flux model, high-resolution growth rates were derived from every sample investigated. A detailed comparison of the growth pattern with the SPECMAP time table reveals that the VA13-2 growth rates tend to be higher during interglacial than during glacial periods. In particular, the observed growth standstills are associated with glacial stages 6 and 8. Further investigation of Mn crust 72 DK 9 taken from shallower water (170°35′W, 20°13′N, 1550 m) tend to confirm that there is a link between the growth rates of hydrogenetic crusts and the climate of the Late Quaternary.

Paleoceanographic implications of rare earth element variability within a Fe [sbnd]Mn crust from the central Pacific Ocean

Results of detailed geochemical analyses of thick Fe Mn oxide crust accumulations from the central Pacific Ocean lead to the suggestion that variations in their chemical composition and microstructure reflect the influence of prevailing environmental conditions at the time of formation of the deposit. These findings imply that well-preserved Fe Mn crusts represent a geologic record that may extend back to the onset of Fe Mn oxide precipitation on the subsrtate material. Current crust growth rate estimates of 1–3 mm/m.y. imply that 6–10 cm thick Fe Mn oxide accumulations represent a growth record of 20 to a maximum of 100 m.y. Detailed chemical and mineralogical analyses were performed on successive hand-picked layers of a 95-mm-thick Fe Mn crust recovered from Schumann Seamount in the Hawaiian Archipelago. The analyses reveal significant variations in the elemental distribution and abundance of major constituents and REEs as a function of depth within the crust. The trivalent REEs generally exhibit sympathetic concentration changes in pure Fe Mn oxides but the fractionation observed between the light and heavy REEs differs in layers enriched in phosphatic matter. The compositional variability and the extent of REE fractionation can be explained in large part by mineralogical control but can also be inferred to reflect differing depositional conditions in the seawater from which the deposits formed. Variations in the Ce anomaly and REE fractionation patterns within the individual layers of the crust are related to their major element composition and mineralogy and to the anomalies observed by other workers. These are used to attempt to reconstruct the growth history of the crust.

Response of manganese in the ocean to the climatic cycles in the Quaternary

Profiles of the 230Th concentration in Mn crusts from the central Pacific Ocean measured at extremely high depth resolution reveal that the growth rates of Mn crusts are influenced by climate. Based on a “constant flux model” the sections of maximum 230Th concentration correspond to periods of slow growth during glacial stages. Fast growth occurred during interglacial stages 1, 5, and 7, probably due to a larger supply of Mn oxides from the water column. High‐resolution profiles of 230Th and 10Be in sediment cores from high biological productivity areas display radioisotope maxima in the interglacial stages and minima during glacial periods, the ratio of the fluxes of 10Be/230Th being ≥4. The only exceptions are observed at 135 and 270 kyr B.P., where the ratio of the fluxes is as low as 0.1 to 0.3. We presume that this “230Th anomaly” reflects short periods of time at the end of glacials when precipitation of MnO2 occurred. This hypothesis is confirmed by peaks of Mn observed in sediment cores mainly at the transitions from glacial to interglacial stages. The standing crops of Mn in these layers suggest release of Mn2+ from the sediments during glacial stages and buildup of Mn in the water column to concentrations of up to 10 µmol/L.