1. Department of Earth and Planetary Science, University of California, Berkeley, 307 McCone Hall, Berkeley, CA 94720-4767, USA.
2. Geomagnetism Laboratory, Department of Earth, Ocean and Ecological Sciences, University of Liverpool, Liverpool L69 7ZE, UK.
3. Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA.
4. Department of Biodiversity, Earth and Environmental Science, Drexel University, 3245 Chestnut Street, PISB 123, Philadelphia, PA 19104, USA.
5. Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India.