1. Massachusetts Institute of Technology–Woods Hole Oceanographic Institution Joint Program in Oceanography/Applied Ocean Science and Engineering, Cambridge, MA 02139, USA.
2. Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA.
3. Department of Terrestrial Magnetism, Carnegie Institute for Science, 5241 Broad Branch Road, NW, Washington, DC 20015, USA.