J. R. Hein

Evolution of Nd and Pb isotopes in Central Pacific seawater from ferromanganese crusts

Hydrogenetic ferromanganese crusts incorporate elements from ambient seawater during their growth on seamounts. By analysing Nd, Pb and Be isotope profiles within crusts it is possible to reconstruct seawater tracer histories. Depth profiles of 10Be/9Be ratios in three Pacific ferromanganese crusts have been used to obtain growth rates which are between 1.4 and 3.8 mm/Ma. Nd and Pb isotopes provide intact records of isotopic variations in Pacific seawater over the last 20 Ma or more. There were only small changes in Pb isotope composition in the last 20 Ma. This indicates a constant Pb composition for the erosional sources and suggests further that erosional Nd inputs may have been uniform too. eNd values vary considerably with time and most probably reflect changes in ocean circulation. The eNd values of the crusts not only vary as a function of age but also as a function of water depth. From 25 to 0 Ma, crust VA13/2 from 4.8 km water depth has a similar pattern of eNd variation to the two shallower crusts from 1.8 and 2.3 km, but about 1.0 to 1.5 units more negative. This suggests that eNd stratification in Pacific seawater, as demonstrated for the present day, has been maintained for at least 20 Ma. Each crust shows a decrease in eNd from 3–5 Ma to the present, which is interpreted in terms of an increase in the NADW component present in the Pacific. From 10 to 3–5 Ma ago the crusts show an increase in eNd. This suggests a decreasing role for a deep water source with eNd less than circum-Pacific sources. In this regard the Panamanian gateway restriction from ∼ 10 Ma with final closure at 3–5 Ma may have played an important role in reducing access of Atlantic-derived Nd to the Pacific.

60 Myr records of major elements and Pb–Nd isotopes from hydrogenous ferromanganese crusts: reconstruction of seawater paleochemistry

We compare the time series of major element geochemical and Pb- and Nd-isotopic composition obtained for seven hydrogenous ferromanganese crusts from the Atlantic, Indian, and Pacific Oceans which cover the last 60 Myr. Average crust growth rates and age-depth relationships were determined directly for the last about 10 Myr using Be-10/Be-9 profiles. In the absence of other information these were extrapolated to the base of the crusts assuming constant growth rates and constant initial Be-10/Be-9 ratios due to the lack of additional information. Co contents have also been used previously to estimate growth rates in Co-rich Pacific and Atlantic seamount crusts (Puteanus and Halbach, 1988). A comparison of Be-10/Be-9- and Co-based dating of three Co-rich crusts supports the validity of this approach and confirms the earlier chronologies derived from extrapolated Be-10/Be-9-based growth rates back to 60 Ma. Our data show that the flux of Co into Co-poor crusts has been considerably lower. The relationship between growth rate and Co content for the Co-poor crusts developed from these data is in good agreement with a previous study of a wider range of marine deposits (Manheim, 1986). The results suggest that the Co content provides detailed information on the growth history of ferromanganese crusts, particularly prior to 10-12 Ma where the Be-10-based method is not applicable. The distributions of Pb and Nd isotopes in the deep oceans over the last 60 Myr are expected to be controlled by two main factors: (a) variations of oceanic mixing patterns and flow paths of water masses with distinct isotopic signatures related to major paleogeographic changes and (b) variability of supply rates or provenance of detrital material delivered to the ocean, linked to climate change (glaciations) or major tectonic uplift. The major element profiles of crusts in this study show neither systematic features which are common to crusts with similar isotope records nor do they generally show coherent relationships to the isotope records within a single crust. Consequently, any interpretation of time series of major element concentrations of a single crust in terms of paleoceanograghic variations must be considered with caution. This is because local processes appear to have dominated over more basin wide paleoceanographic effects. In this study Co is the only element which shows a relationship to Pb and Nd isotopes in Pacific crusts. A possible link to changes of Pacific deep water properties associated with an enhanced northward advection of Antarctic bottom water from about 14 Ma is consistent with the Pb but not with the Nd isotopic results. The self-consistent profiles of the Pb and Nd isotopes suggest that postdepositional diagenetic processes in hydrogenous crusts, including phosphatization events, have been insignificant for particle reactive elements such as Pb, Be, and Nd. Isotope time series of Pb and Nd show no systematic relationships with major element contents of the crusts, which supports their use as tracers of paleo-seawater isotopic composition

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 ϵ205Tl of +10 to +14, whereas seawater is characterized by values as low as −8 (ϵ205Tl represents the deviation of the 205Tl/203Tl ratio of a sample from the NIST SRM 997 Tl isotope standard in parts per 104). 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 ϵ205Tl 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 ϵ205Tl values similar to seawater are found if the scavenging of Tl is nearly quantitative. Hydrothermal manganese deposits display a positive correlation between ϵ205Tl 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 ϵ205Tl 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 ϵ205Tl 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 ϵ205Tl values coupled with high Mn/Fe.

Global distribution of beryllium isotopes in deep ocean water as derived from FeMn crusts

The direct measurement of the ratio of cosmogenic 10Be (T12 = 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 10Be9Be ratios of the outermost millimetre of hydrogenetic ferromanganese crusts from the worlds oceans. 10Be9Be ratios, time-corrected for radioactive decay of cosmogenic 10Be using 234U238U, 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 (τ10Be) in deep water is constant globally, then the observed variations in 10Be9Be 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 τ10Be ≌ 600 a. An alternative possibility is that the Be isotope abundances are controlled by local scavenging. For this scenario τ10Be would vary according to local particle concentration and would ≌ 600 a in the central Pacific, but τ10Be ≌ 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 10Be9Be, and establishment of a relatively constant 9Be concentration. As τ10Be (∼ 600 a) is less than the apparent age of deep water in the Pacific (∼ 1500 a), the Pacific record of 10Be9Be is not expected to show secular variations due to changes in deep-water flow, despite the large variations in 10Be9Be between different water masses. Because of this insensitivity to deep-water flow, however, it is suggested that the 10Be9Be 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.

Deep and bottom water export from the Southern Ocean to the Pacific over the past 38 million years

The application of radiogenic isotopes to the study of Cenozoic circulation patterns in the South Pacific Ocean has been hampered by the fact that records from only equatorial Pacific deep water have been available. We present new Pb and Nd isotope time series for two ferromanganese crusts that grew from equatorial Pacific bottom water (D137-01, “Nova,” 7219 m water depth) and southwest Pacific deep water (63KD, “Tasman,” 1700 m water depth). The crusts were dated using 10Be/9Be ratios combined with constant Co-flux dating and yield time series for the past 38 and 23 Myr, respectively. The surface Nd and Pb isotope distributions are consistent with the present-day circulation pattern, and therefore the new records are considered suitable to reconstruct Eocene through Miocene paleoceanography for the South Pacific. The isotope time series of crusts Nova and Tasman suggest that equatorial Pacific deep water and waters from the Southern Ocean supplied the dissolved trace metals to both sites over the past 38 Myr. Changes in the isotopic composition of crust Nova are interpreted to reflect development of the Antarctic Circumpolar Current and changes in Pacific deep water circulation caused by the build up of the East Antarctic Ice Sheet. The Nd isotopic composition of the shallower water site in the southwest Pacific appears to have been more sensitive to circulation changes resulting from closure of the Indonesian seaway.

THE HF ISOTOPIC COMPOSITION OF FERROMANGANESE NODULES AND CRUSTS AND HYDROTHERMAL MANGANESE DEPOSITS : IMPLICATIONS FOR SEAWATER HF

We present Hf and Pb isotopic data, and chemical compositions of the outermost layers of marine ferromanganese deposits of different types (hydrogenous and hydrothermal) with a worldwide distribution. The Hf isotopic compositions display a broad range and refine previously reported regional differences as follows: Atlantic Ocean ɛHf = −4to+2, Indian Ocean ɛHf = +2to+4, Pacific Ocean ɛHf = +3to+10. The most radiogenic Hf isotopic compositions in the Pacific samples are for hydrothermal manganese deposits that also have low 207Pb204Pb, demonstrating that this signature reflects a contribution from hydrothermal venting of Hf leached from oceanic volcanic rocks rather than from riverine inputs, volcanic ash, or eolian dust. Hafnium concentrations in the deposits increase from 20 ppb to 20 ppm with decreasing ɛHf, The Hf and Pb isotopic compositions for ferromanganese crusts define an apparent mixing trend between literature values of average continental crust and MORB. The range in ɛHf for ferromanganese crusts is narrower than it is for 206Pb204Pb compared to the differences in isotopic composition of the sources of Hf and Pb. This is consistent with Hf having a longer residence time than Pb. The concentration of Hf in ferromanganese crusts has been found to co-vary with growth rate, and inversely correlates with Hf isotopic compositions. Hf isotope ratios may be used to determine not only the source of Hf, but possibly the source of Fe and Mn. Measurements of ɛHf and Hf concentrations in nodule tops, bottoms and associated sediments show that the ɛHf of nodules is sensitive to sedimentary oxic and sub-oxic diagenetic processes and thus most nodules may not reliably reflect the isotopic composition of Hf in seawater.

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 ϵ205Tl greater than +2.5, whereas anoxic sediments have ϵ205Tl of less than −1.5 (ϵ205Tl is the deviation of the 205Tl/203Tl isotope ratio of a sample from NIST SRM 997 Tl in parts per 104). Leaching experiments indicate that the high ϵ205Tl 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 (ϵ205Tl=+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 ϵ205Tl 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 ϵ205Tl. 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.

Corrigendum to “The Hf isotope composition of global seawater and the evolution of Hf isotopes in the deep Pacific Ocean from Fe–Mn crusts” [Chem. Geol. 178 (2001) 23–42]

0009-2541/02/

The global variation in the iron isotope composition of marine hydrogenetic ferromanganese deposits: implications for seawater chemistry?

see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S0009 -2541 (02 )00012 -8

The Line Islands revisited: New 40Ar/39Ar geochronologic evidence for episodes of volcanism due to lithospheric extension

PII of original article S0009-2541(00)00427-7. * Corresponding author. Present address: Laboratoire Magmas et Volcans, CNRS-UMR 6524, Universite Blaise Pascal, 5 rue Kessler, 63038 Clermont-Ferrand, France. Fax: +33-473-346-744. E-mail addresses:K.David@opgc.univ-bpclesmont.fr (K. David), frank@erdw.ethz.ch (M. Frank). www.elsevier.com/locate/chemgeo