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: 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: 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.

Description: Two ferromanganese crusts from the Indian Ocean and one from the Atlantic Ocean have been analysed for 10Be/9Be, 143Nd/144Nd and 208,207,206Pb/204Pb ratios as a function of depth beneath their growth surfaces. 10Be/9Be ratios provide growth rate estimates for these crusts between 1.55 and 2.82 mm Ma−1 and further suggest that 87Sr/86Sr in crusts do not in any case examined so far provide reliable estimates for growth rates. A crust ALV-539 from 35°N in the western N. Atlantic has ϵNd and Pb-isotope variations indistinguishable from crust BM-1969.05 from 39°N in the N. Atlantic [K.W. Burton, H.-F. Ling, R.K. O'Nions, Closure of the central American isthmus and its impact on North Atlantic deepwater circulation, Nature (London) 386 (1997) 382–385] when 10Be/10Be ratios are used to estimate growth rates. Both crusts provide evidence for a marked change in deepwater composition in the western N. Atlantic with a reduction in ϵNd and an increase in 206Pb/204Pb from ∼8 Ma ago towards the present day. The two crusts from the Indian Ocean show comparatively small variations in ϵNd between −8.0 and −7.0 over the last 20 Ma and do not show the large shift in ϵNd seen in the Atlantic crusts. Comparison of ϵNd in the crusts analysed here with those published previously [H.-F. Ling, K.W. Burton, R.K. O'Nions, B.S. Kamber, F. von Blanckenburg, A.J. Gibb, J.R. Hein, Evolution of Nd and Pb isotopes in central Pacific seawater from ferromanganese crusts, Earth Planet. Sci. Lett. 146 (1997) 1–12; K.W. Burton, H-F. Ling, R.K. O'Nions, Closure of the central American isthmus and its impact on North Atlantic deepwater circulation, Nature (London) 386 (1997) 382–385] shows that provinciality in the present-day ϵNd structure of the Pacific, Atlantic and Indian Oceans has been maintained over ∼20 Ma or more despite the palaeogeographic changes that have occurred within this period. These include the closure of the Panama gateway and the uplift of the Himalayas. Superimposed on this broad inter-ocean structure are changes in ϵNd of the western N. Atlantic which may relate to the Panama gateway closure and shifts in the ϵNd of equatorial Pacific deepwater from 3–5 Ma ago. The absence of any such structure in ϵNd of the southwest and central Indian Ocean suggests that Himalayan erosion products such as preserved in the Bengal Fan sediments have not contributed significantly to Indian Ocean deepwater over the last 20 Ma. There is no straightforward relationship between records of 87Sr/86Sr in the global ocean and ϵNd in ocean deepwater as would be expected if inputs of radiogenic Sr and unradiogenic Nd were coupled.

Description: High resolution 230Thex and 10Be and biogenic barium profiles were measured at three sediment gravity cores (length 605–850 cm) from the Weddell Sea continental margin. Applying the 230Thex dating method, average sedimentation rates of 3 cm/kyr for the two cores from the South Orkney Slope and of 2.4 cm/kyr for the core from the eastern Weddell Sea were determined and compared to δ18O and lithostratigraphic results. Strong variations in the radionuclide concentrations in the sediments resembling the glacial/interglacial pattern of the δ18O stratigraphy and the 10Be stratigraphy of high northern latitudes were used for establishing a chronostratigraphy. Biogenic Ba shows a pattern similar to the radionuclide profiles, suggesting that both records were influenced by increased paleoproductivity at the beginning of the interglacials. However, 230Thex0 fluxes (0 stands for initial) exceeding production by up to a factor of 4 suggest that sediment redistribution processes, linked to variations in bottom water current velocity, played the major role in controlling the radionuclide and biogenic barium deposition during isotope stages 5e and 1. The correction for sediment focusing makes the ‘true’ vertical paleoproductivity rates, deduced from the fluxes of proxy tracers like biogenic barium, much lower than previously estimated. Very low 230Thex0 concentrations and fluxes during isotope stage 6 were probably caused by rapid deposition of older, resedimented material, delivered to the Weddell Sea continental slopes by the grounded ice shelves and contemporaneous erosion of particles originating from the water column.

Description: The sediments recovered on ODP Leg 104 have been reported to be characterized by hiatuses. The hiatuses were defined by biostratigraphy and were believed to be caused by erosion related to temporary changes of bottom current composition and velocity. They have been associated with major paleoenvironmental changes, reorganization of global deep water production, and increased bottom water flows. Because of the importance of hiatuses for ongoing research, we decided to closely investigate the sedimentation history for the most significant Pliocene and Miocene biostratigraphic hiatuses by sedimentologic and geochemical means. The sedimentologic studies include clay mineral distributions, grain size data, and organic carbon concentrations. The geochemical studies include determination of Full-size image (〈1 K)Sr ratios, 10Be and Ir concentrations. The results of the sedimentologic studies suggest either that paleoenvironmental changes associated with hiatuses are not represented in the preserved sediments, or that the hiatuses are an artifact of interpretation of the biostratigraphic data. Strontium isotopes indicate continuous sedimentation for the interval investigated at Site 642, an interpretation confirmed by the steady decline in 10Be. Full-size image (〈1 K)Sr ratios in the interval from above and below proposed hiatuses H Full-size image (〈1 K) and Full-size image (〈1 K) at Site 643 display stronger changes with depth than expected by steady sedimentation. Ir data for this same interval indicate reduced sedimentation rates. Combining both, sedimentologic and geochemical evidence, the proposed hiatuses could not be confirmed and may represent preservation artifacts.

Description: 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

Description: A new generation multiple collector plasma source mass spectrometer (PSMS) produced by Nu Instruments Ltd is evaluated. The instrument has a double-focusing Nier–Johnson analyser with laminated magnet and a novel variable dispersion ion optical arrangement, enabling all masses to be located in the centre of the Faraday collectors of a fixed static array. mThe performance of the instrument has been assessed through the analysis of NBS-981 Pb using a Tl doping technique with Faraday collector efficiencies and amplifier gains determined independently. A second method of analysis involves comparison of interspersed standard and sample Pb measurements with effective gains for each collector determined from the standards. In both cases the repetition of Pb isotope measurement is competetive with the thermal ionization mass spectrometry (TIMS) double-spike method

Description: Variations of intensity and composition of biogenic particle flux at the northern boundary of the present PolarFrontal Zone in the Atlantic sector of the Southern Ocean are indicators of major changes of paleoenvironmentalconditions on glacial/interglacial time scales during the Late Quaternary. In order to estimate those pastchanges, sediment accumulation patterns of two piston cores, one from just north and one just south of thepresent day position of the Subantarctic Front were reconstructed. Using the 230Thex method large contributionsof laterally supplied material were quantified and used to correct sediment accumulation rates. During the lastglacial focussing of biogenic opal-dominated material exceeded the original contribution from the surfacewater above by a maximum factor of 8.7. The initial activity ratio of 231Paex/230Thex was used as tracer forbiogenic particle flux and composition and indicates that during the glacial stages 2 and 4 the area of high opalproductivity was situated above the location of the southern core whereas the northern core has not beenreached by this northward shift during the last 130 kyr as shown by the pattern of focussing-corrected bulkaccumulation rates. If the position of the Antarctic Polar Front has remained at the northern boundary of thehigh opal productivity area during the last 130 kyr, the results suggest that was located exactly between thetwo core sites during glacial stages 2 and 4. A two-box modeling approach involving particle flux and boundaryscavenging intensity of 231Pa was applied to estimate the possible range of the 231Paex/230Thex ratio recordedin Southern Ocean sediments. Previous estimates on the export of 231Pa from the Atlantic into the SouthernOcean are corroborated but the model suggests a low sensitivity of the 231Paex/230Thex ratio in Southern Oceansediments to variations of the residence time of North Atlantic Deep Water in the Atlantic Ocean.