[1] Swedish Museum Nat Hist, Lab Isotope Geol, S-10405 Stockholm, Sweden
[2] Univ Oxford, Dept Earth Sci, Oxford OX1 3PR, England
[3] Univ Kiel, GEOMAR, IfM, D-24098 Kiel, Germany
[4] Gothenburg Univ, Dept Oceanog, S-41124 Gothenburg, Sweden
[5] Univ Stockholm, ITM, S-10691 Stockholm, Sweden