[1] Keele Univ, Inst Sci & Technol Med, Stoke On Trent ST4 7QB, Staffs, England
[2] Univ Nottingham, Sch Biol, Mol Toxicol Div, Nottingham NG7 2RD, England
[3] Univ Lancaster, Ctr Environm Magnetism & Palaeomagnetism, Lancaster LA1 4YW, England
[4] Univ Cambridge, Dept Mat Sci & Met, Cambridge CB2 3QZ, England