Description: Two Nd and Pb isotope time series of hydrogenous ferromanganese crusts, one from the Tehuantepec Ridge in the deep eastern equatorial Pacific and the other from Blake Plateau in the shallow northwestern Atlantic, which cover the past 7–8 m.y., show no variations coincident with the final closure of the Panama gateway, estimated as ca. 3.5 Ma. The record of the Atlantic crust located in the present-day Gulf Stream shows a shift in isotope composition from ca. 8 to 5 Ma that is explained by a diminishing supply of Pacific water. It is argued that the major restriction of water-mass exchange through the Panama gateway occurred before 5 Ma and thus cannot serve as a direct cause of the onset of Northern Hemisphere glaciation. The absence of a significant signature in the isotope records from the Pacific crust suggests that the volume of water exchanged with the Atlantic through shallow archipelagic straits of the gateway during the 3–4 m.y. prior to closure was too small to influence the radiogenic isotope composition of Pacific deep water.

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: 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.

Description: High-resolution Nd- and Pb-isotope time series for the last 8 Myr are reported for four Atlantic ferromanganese crusts, dated by 10Be/9Be chronology. These are compared to new high-resolution and high-precision Pb-isotope time series and recently published Nd-isotope time series for two previously studied crusts from the NW Atlantic Ocean. These records allow a more detailed examination of Atlantic deepwater variability over the time period of intensification of Northern Hemisphere Glaciation (NHG). Changes in the Pb-isotope time series started after 3 Ma, but were most dramatic over the last 1.8 Myr, coinciding with changes of Pb-isotopes in the Arctic Ocean. This latter change post-dates the intensification of NHG at 3.1 to 2.5 Ma and reflects an increase in the input of material eroded from the Archean Shield of Canada and Greenland. Shifts in Nd- and Pb-isotope compositions in a crust from the Blake Plateau occur before ∼5 Ma and most prominent at ∼8 Ma suggest that water masses from either the Pacific or Southern Ocean influenced the isotope composition of this crust. The relatively high εNd values around 8 Ma recorded by the Blake Plateau crust are explained by a contribution of eastward flowing Pacific water through the Panama Gateway into the Caribbean Sea. This high εNd signal decreased between 8 and 5 Ma suggesting that the supply of Pacific water into the Caribbean became restricted. This is earlier than the Caribbean seawater salinity increase at 4.2 Ma deduced from δ18O data, and may indicate that there was only a surface water connection between the Caribbean and Pacific between ∼5 and 4.2 Ma. The closure of the Panama Gateway to intermediate and deep water exchange (〉200 m depth) apparently occurred much earlier than the intensification of Northern Hemisphere Glaciation at 3.1–2.5 Ma, and cannot therefore have been a direct cause of this climatic change, but may, as recently argued, only have been a necessary precondition