1. Department of Physics University of Toronto Toronto Ontario Canada
2. Department of Earth and Planetary Sciences Macquarie University Sydney New South Wales Australia
3. Now at Research School of Earth Science The Australian National University Canberra ACT Australia
4. Department of Earth and Planetary Sciences Harvard University Cambridge MA USA
5. Cooperative Institute for Research in Environmental Sciences and Department of Geological Sciences University of Colorado Boulder Boulder CO USA
6. Department of Earth Sciences University of Toronto Toronto Ontario Canada