HR: 08:15h
AN: OS31C-02    [Abstracts]
TI: Hafnium and Neodymium Isotopes in Atlantic Ocean Waters
AU: * Rickli, J
EM: rickli@erdw.ethz.ch
AF: IGMR, Deptartment of Earth Sciences, ETH Zurich, Clausiusstrasse 25, Zurich, 8092, Switzerland
AU: Frank, M
EM: mfrank@ifm-geomar.de
AF: Leibniz Institute for Marine Sciences at the University of Kiel, Wischhofstrasse 1-3, Kiel, 24148, Germany
AU: Halliday, A
EM: alexh@earth.ox.ac.uk
AF: Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX1 3PR, United Kingdom
AB: Neodymium isotopic compositions (ICs) have been established as a tracer of water masses in the present and past oceans since the late 1970s. Hafnium isotopes share the capability of tracing water masses and in combination with Nd isotopes provide information on continental weathering regimes. Whereas Nd released during weathering reflects the bulk Nd IC of the weathered lithology, the released Hf is more radiogenic than the weathered lithology. This effect is due to highly variable Lu/Hf--ratios in rock--forming minerals ("zircon effect") and as a consequence physical weathering apparently leads to more congruent weathering of Hf than chemical weathering does. Our understanding of the Hf IC of seawater to date has been derived (with the exception of some as yet unpublished data from the Arctic and Pacific oceans (Zimmermann et al., in prep.)) from ferromanganese crusts and nodules, since Hf concentrations in seawater are low and have until recently hampered direct measurements of Hf IC of seawater. We present IC for the dissolved Hf and Nd in Atlantic seawater. Samples were taken mainly on a transect from the Bay of Biscay to Cape Town (RV Polarstern cruise ANT XXIII/1 in 2005). A few additional samples are from the Labrador Sea and the Drake Passage. Hafnium and Nd were pre--concentrated by iron co--precipitation from 60 to 140 liters of filtered (0.45 μm) seawater. Separation of Hf and Nd followed previously established ion chromatographic procedures. Hafnium and Nd ICs were measured by MC--ICPMS (Nu Plasma) with a 2σ external reproducibility of 0.65 and 0.3 \varepsilon--units, respectively. Sample sizes varied but were in most cases larger than 3ng of Hf. Surface seawater as well as deep water samples extending to ~5,000 m, plot on the "seawater array" defined previously from measurements of ferromanganese crusts and nodules. Surface seawater ICs are quite uniform for Hf ranging from \varepsilon_Hf = 0 to +2 at most sampling sites on the Atlantic transect. In the area of the Canary Islands, however, a strong shift with a maximum \varepsilon_Hf = +10.5 is observed. Neodymium ICs are more variable and can be divided into four distinct zones: the Bay of Biscay (\varepsilon_Nd ~ -10.9), the region of the Canary Islands (\varepsilon_Nd = -8.4), off North Africa (\varepsilon_Nd ~ -11.9) and the Angola and Cape Basins where \varepsilon_Nd values as low as -16.5 occur. Hafnium deep water ICs range from -2.1 in the Labrador Sea to +4.1 \varepsilon_Hf in the Drake Passage. Distinct ICs are observed for the Mediterranean Outflow Water (MOW, \varepsilon_Hf ~ +0.7) and the Antarctic Intermediate Water (AAIW, \varepsilon_Hf ~ +2.8). Deep and Bottom waters (North Atlantic Deep Water, NADW, Antarctic Bottom Water, AABW, and Circumpolar Deep Water, CDW) are quite uniform ranging from +3 to +4.2. In general, Hf and Nd ICs trace the deep water masses well. However, Nd ICs in the Angola Basin are shifted to unradiogenic signatures, requiring an additional external source of Nd to the seawater of this basin (Congo River, Namibian desert dust). Similarly, Hf ICs in North Atlantic Deep water close to the Mediterranean are shifted to less radiogenic isotopic compositions, probably reflecting contributions from MOW.
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 4875 Trace elements (0489)
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting