[1] MIT, DEPT EARTH ATMOSPHER & PLANETARY SCI, CAMBRIDGE, MA 02139 USA
[2] UNIV CAMBRIDGE, DEPT EARTH SCI, BULLARD LABS, CAMBRIDGE, ENGLAND
[3] UNIV TEXAS, INST GEOPHYS, AUSTIN, TX 78712 USA
[4] WOODS HOLE OCEANOG INST, WOODS HOLE, MA 02543 USA
[5] DALHOUSIE UNIV, DEPT OCEANOG, HALIFAX B3H 4H2, NS, CANADA