1. Roke Manor Research, Roke Manor, Old Salisbury Lane, Romsey SO51 0ZN, United Kingdom, d.keir@soton.ac.uk
2. Department of Earth Science, University of Bergen, Allegaten 41, 5007 Bergen, Norway
3. School of Ocean and Earth Science, University of Southampton, European Way, Southampton SO14 3ZH, United Kingdom
4. School of Environmental Science, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, United Kingdom
5. Institute of Geophysics, Space Science and Astronomy, University of Addis Ababa, King George VI Street, Addis Ababa, Ethiopia