1. Department of Physical Sciences; The Open University; Walton Hall Milton Keynes MK7 6AA UK
2. Department of the Geophysical Sciences; University of Chicago; 5734 S. Ellis Ave. Chicago Illinois 60637 USA
3. Department of Environment, Earth and Ecosystems; The Open University; Walton Hall Milton Keynes MK7 6AA UK
4. Department of Geological Sciences; University of Oregon; Eugene Oregon 97403 USA