1. Geosciences Research Division; Scripps Institution of Oceanography; University of California San Diego; 9500 Gilman Drive La Jolla California 92093-0220 USA
2. Institute of Geophysics and Planetary Physics; Scripps Institution of Oceanography; University of California San Diego; La Jolla California USA
3. College of Earth; Ocean and Atmospheric Sciences; Oregon State University; Corvallis Oregon USA
4. Department of Earth Science; Centre for Geobiology; University of Bergen; Bergen Norway