Description: 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 εHf 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 εHf and εNd 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 εNd and εHf 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.

Description: The iron (Fe) isotope compositions of 37 hydrogenetic ferromanganese deposits from various oceans have been analysed by MC-ICPMS; they permit the construction of a global map of Fe isotopic values. The isotopic compositions range between −1.2 and −0.1‰ in δ57FeIRMM14. Averages for the Atlantic and the Pacific are −0.41 and −0.88‰, but their standard deviations are identical (0.27, 1σ) and the data very largely overlap. No correlation is found with Pb or Nd isotope compositions and there is no evidence that the observed oceanic Fe isotopic heterogeneity is directly controlled by variations in continental sources. The small quantities of Fe that can be introduced from hydrothermal sources render as unlikely the possibility that the isotopic variations reflect variable proportions of continental and hydrothermal Fe, as recently proposed. The more likely explanation is that the variations are induced locally within the ocean. The exact sources of fractionation remain unclear. Likely possibilities are the dissolution and reprecipitation processes that liberate Fe from sediments during anoxic events, dissolution in surface waters or processes occurring during growth of the crusts

Description: 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.

Description: A sediment core, collected from the western part of the continental slope of the Ross Sea at 2380 m water depth, records events of the last two climatic cycles (250 kyr). A Full-size image (〈1 K)-based chronology was obtained and boundaries of the isotope stages were set assuming that biological productivity was enhanced during periods of less ice cover. Then, Full-size image (〈1 K), organic carbon, biogenic silica and biogenic Ba distributions were compared to the glacial–interglacial stage boundaries and corresponding ages of the Full-size image (〈1 K) record of Martinson et al. [Martinson, D.G., Pisias, N.G., Hays, J.D., Imbrie, J., Moore, T.C., Jr., Shackleton, N.J, 1987. Age dating and the orbital theory of the ice ages: development of a high-resolution 0 to 300,000-year chronostratigraphy. Quaternary Research, 27: 1–29]. Sediment accumulation rates ranged between 1.2 cm kyr−1 in the isotope stage 6 and 3.8 cm kyr−1 during the Holocene. Variations in the concentrations and fluxes of organic carbon, biogenic Ba, biogenic silica and Mn gave information on palaeoclimate changes. Processes of sediment redistribution in the Ross Sea margin were enlightened from a comparison of the measured and expected fluxes of Full-size image (〈1 K). Calculation of the focusing-corrected accumulation rates of biogenic Ba enabled us to evaluate the export palaeoproductivity. Corrected accumulation rates of biogenic components and calculated palaeoproductivities were low, compared to the Antarctic Polar Front in the Atlantic sector, throughout the last two climatic cycles. Glacial–interglacial changes of sea ice cover and ventilation of the Ross Sea were probably major causes of variations in biogenic particle flux and distribution of redox-sensitive elements within the sediment column.

Description: 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.

Description: The reconstruction of geomagnetic field intensity variations during the last 200 kyr from paleomagnetic data is at present the subject of numerous studies and major debate. There is currently no generally accepted record. Here we present a global stacked record of (230Thex-normalized)10Be deposition in marine sediments representing relative variations in 10Be production rate which are translated into field intensity variations. The record shows major periods during which the field intensity was between 10% and 40% of the present day value; namely 30–42, 60–75, 85–110 and 180–192 kyr B.P. Our results are compared to independently derived paleomagnetic studies and Th/U calibrations of 14C dates on corals. During most of the observed period the geomagnetic field intensity was weaker than today, resulting in an overall 30% reduced value for the last 200 kyr.

Description: Surface exposure dating requires the knowledge of cosmogenic nuclide production rates. When determining time-integrated production rates the exposure ages of the calibration samples need to be accurately known. The landslide of Köfels (Austria) is very well suited for this purpose. It is the largest landslide in the crystalline Alps of Austria dating back to 7800±100 years BC (AMS 14C dating of buried wood), which is well within the 14C dendro calibration curve. Exposed quartz veins were sampled from the tops of large boulders from the toe of the landslide for analysis of 10Be and 26Al. To calculate sea level, high geomagnetic latitude (≥60°), open sky radionuclide production rates, corrections were applied for altitude and latitude, for shielding by surrounding mountains, for sample geometry, vegetation and snow cover, and for sample thickness. The production rates for an exposure age of 10,000 years are 5.75±0.24 10Be atoms/yr g SiO2 and 37.4±1.9 26Al atoms/yr g SiO2. A 26Al/10Be ratio of 6.52±0.43 can be calculated. The influence of the geomagnetic field on these production rates has been estimated using two different geomagnetic field records. Our production rates should be a good approximation for the use of surface exposure dating between about 5000 and 13,000 years BP.

Description: The sources of non-anthropogenic Pb in seawater have been the subject of debate. Here we present Pb isotope time-series that indicate that the non-anthropogenic Pb budget of the northernmost Pacific Ocean has been governed by ocean circulation and riverine inputs, which in turn have ultimately been controlled by tectonic processes. Despite the fact that the investigated locations are situated within the Asian dust plume, and proximal to extensive arc volcanism, eolian contributions have had little impact. We have obtained the first high-resolution and high-precision Pb isotope time-series of North Pacific deep water from two ferromanganese crusts from the Gulf of Alaska in the NE Pacific Ocean, and from the Detroit Seamount in the NW Pacific Ocean. Both crusts were dated applying 10Be/9Be ratios and yield continuous time-series for the past 13.5 and 9.6 Myr, respectively. Lead isotopes show a monotonic evolution in 206Pb/204Pb from low values in the Miocene (≤18.57) to high values at present day (≥18.84) in both crusts, even though they are separated by more than 3000 km along the Aleutian Arc. The variation exceeds the amplitude found in Equatorial Pacific deep water records by about three-fold. There also is a striking similarity in 207Pb/204Pb and 208Pb/204Pb ratios of the two crusts, indicating the existence of a local circulation cell in the sub-polar North Pacific, where efficient lateral mixing has taken place but only limited exchange (in terms of Pb) with deep water from the Equatorial Pacific has occurred. Both crusts display well-defined trends with age in Pb–Pb isotope mixing plots, which require the involvement of at least four distinct Pb sources for North Pacific deep water. The Pb isotope time-series reveal that eolian supplies (volcanic ash and continent-derived loess) have only been of minor importance for the dissolved Pb budget of marginal sites in the deep North Pacific over the past 6 Myr. The two predominant sources have been young volcanic arcs, one located in the northeastern part and one located in the northwestern part of the Pacific margin, from where material has been eroded and delivered to the ocean, most likely via riverine pathways.

Description: A method for direct measurement of the natural 10Be/9Be ratio in a sample by AMS is presented. Samples of 100 ng of Be with 10Be concentrations orders of magnitude higher than in conventional 10Be AMS samples were produced without the addition of 9Be carrier and analysed using the accelerator secondary ion mass spectrometry (Accelerator SIMS) source of the PSI/ETH AMS facility. Special mounting techniques and the analysis procedure using a focussed Cs+ beam of up to 600 nA and beam spot diameter of 100 μm are described. The results of measurements on two test samples from Pacific ferromanganese crust D11–1 are presented and compared with similar measurements by conventional secondary ion mass spectrometry (SIMS). Background measurements at a level of ∼5 × 10−11 indicate that ratios of a few times 10−10 could be measured to a precision of ∼10%. PACS 29.17.+w; 29.30.−h; 82.80.Ms; 89.60.+x

Description: Paleoproductivity changes in the western equatorial Atlantic have been estimated from carbonate and marine organic carbon accumulation for the last 180,000 yr from a Ceará Rise sediment core GeoB 1523-1. Accumulation rates were calculated by normalizing to excess 230Th activity. Paleoproductivity from carbonate accumulation was estimated by a correlation between carbonate and organic carbon fluxes derived from sediment traps deployed in oligotrophic waters. Results indicate minor productivity changes varying around 30 gC m−2 yr−1 for organic carbon-derived estimations and around 45 gC m−2 yr−1 for carbonate-derived estimations, suggesting that the study area remained a low productivity region throughout the time period examined. Maxima in western Atlantic paleoproductivity occurred during the warm substages of glacials and interglacials, in contrast to the eastern tropical Atlantic where maxima are recorded in the cold substages. This contrast might be caused by a deepening of the thermocline and nutricline in the west connected with a synchronous shallowing in the east. The carbonate preservation record was examined for the last 380,000 yr based on variations in percent planktonic foraminiferal fragments and percent coarse fraction. Only minor carbonate dissolution above 3300 m water depth is observed, except for three main dissolution events at isotope stages 4, 8.4 and 10. Considerable carbonate loss during these intervals is attributed to a decreased production of North Atlantic Deep Water which is associated with vertical expansion of Southern-Source Deep Water.