1. Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA.
2. Polar Science Center, Applied Physics Lab, University of Washington, 1013 NE 40th Street, Seattle, WA 98105–6698, USA.
3. School of Earth Sciences and Byrd Polar Research Center, The Ohio State University, 1090 Carmack Road, Columbus, OH 43210–1002, USA.
4. School of Civil Engineering and Geosciences, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK.
5. Department of Geology and Geophysics, Massachusetts Institute of Technology/Woods Hole Oceanographic Institution Joint Program, Woods Hole, MA 02543, USA.