1Environmental Magnetism Laboratory, Department of Geography, University of Liverpool, Liverpool L69 7ZT, UK
2School of Environmental Science, University College Northampton, Northampton NN2 7AL, UK
3Sheffield Centre for International Drylands Research, University of Sheffield, Winter St., Sheffield S10 2TN, UK
4Department of Geography, University of Reading, Whiteknights, Reading, RG6 6AB, UK