1. Sterkfontein Research Unit and Climatology Research Group, University of the Witwatersrand, Private Bag 3, WITS 2050, Johannesburg, South Africa.
2. Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, IN 47907–1397, USA.
3. Department of Physics, Purdue University, West Lafayette, IN 47907–1396, USA.
4. Sterkfontein Research Unit, University of the Witwatersrand Medical School, 7 York Road, Parktown, Johannesburg 2193, South Africa.