Fig. 1. Map of sample locations. Circum-Antarctic area south of 40° S with locations of investigated glacio-marine core-top sediments (yellow stars) and previously published ferromanganese nodules (yellow circles; van de Flierdt et al., 2006). White lines with arrows indicate the main eastward flow in the Antarctic Circumpolar Current (ACC) associated with the Subtropical Front and the Subpolar Front, the Weddell Gyre, and the Ross Gyre. White arrows in the Indian Ocean sector reflect westward flowing currents south of the ACC.

Fig. 2. Observed global Nd–Hf isotope systematics. Silicate rocks (gray triangles: oceanic rocks; gray squares: sediments and continental rocks) form the terrestrial array, and ferromanganese crusts and nodules (gray circles) define the seawater array. Black diamonds and hollow triangles highlight circum-Antarctic sediments and ferromanganese nodules respectively. MORB compositions for the Indian Ocean (red), the Pacific Ocean (green), and the Atlantic Ocean (blue) are highlighted in order to show the potential composition of hydrothermal Hf. The black stippled line shows the average Hf isotopic composition of global MORB (ε_Hf  + 15), which is significantly more radiogenic than any data point observed in the seawater array. Ferromanganese crusts and nodules from the three different ocean basins are circled in the same colors as MORB, revealing that there exists a clear provinciality among the different ocean basins, although MORB data for all three ocean basins are overlapping. Both of these observations are in contrast to the hypothesis that most Hf in the ocean is of hydrothermal origin. A full list of references for the background dataset and the different MORB compilations can be obtained from the authors upon request (see also PETDB database and GEOROC database). Date of compilation: December 2005.

Fig. 3. Schematic and modeled Nd–Hf isotope systematics. Different scenarios for creating the seawater array using a simple mass balance between the Sm–Nd and Lu–Hf isotope systems in zircons on the one hand, and a zircon-free upper continental crust residue on the other hand. (a) Schematic trends of hypothetical seawater values in the Nd–Hf isotope correlation diagram for different end member scenarios. (1) If all Nd were continental in origin, and all Hf were hydrothermal, the seawater isotopic composition should plot along a horizontal line with the Hf isotopic composition of MORB. (3) On the contrary, if Nd and Hf are both continental in origin, and the continental crust is weathered and eroded congruently, the seawater isotopic composition should plot along the terrestrial array. What is observed in nature is scenario (2), in between the two extremes. The trend labeled (2) can be generated either by mixing hydrothermal and continental end members ( 50:50 by Hf mass) or by Hf solely derived from continental weathering with fractionation of REE and Hf (incongruent weathering). (b) and (c) Model results for a simple mass balance between a zircon-component and a zircon-free upper continental crust. The colored lines are calculated mixing curves between zircons and zircon-free upper continental crust of different ages (2.7 Ga to 0.7 Ga). The large squares are calculated upper continental crust (UCC) compositions, which match the terrestrial array very well. The small squares represent 10% steps on the mixing lines. In case (b) the calculation was carried out under the assumption that 65% of the Hf in the UCC is sequestered in zircons, and that none of this Hf is available for weathering. A line similar to the seawater array can be derived by weathering the non-zircon fraction of the UCC (35%). In case (c) the assumption has been made that nearly all of the Hf contained in the continental crust is hosted in zircons (99%). In this case, some zircon weathering has to be included to create the seawater array as indicated by the intercept of the modeled lines with the seawater array at  25% zircon contribution (red stippled line). In summary, as long as 65–70% or more of the Hf in the UCC is sequestered in zircons, the seawater array can be created by incongruent weathering from continental crust only. If less than 65–70% Hf is hosted in zircons, some radiogenic Hf contributions are required, which could be hydrothermal. However, even in this case we would consider island-arc weathering as the most plausible source of radiogenic Hf, as argued in van de Flierdt et al. (2004a).