1. Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA.
2. Département Géologie et Océanographie, UMR 5805 EPOC, Université Bordeaux I, 33405 Talence, France.
3. Department of Geophysics, University of Copenhagen, DK-2100 Copenhagen OE, Denmark.
4. Lamont-Doherty Earth Observatory of Columbia University