Original Article M. Kimura, Corresponding Author kimura.makoto@nipr.ac.jp orcid.org/0000-0003-3352-4989 National Institute of Polar Research, Tokyo, 190-8518 Japan Ibaraki University, Mito, 310-8512 Japan Corresponding author. E-mail: kimura.makoto@nipr.ac.jp Search for more papers by this author R. C. Greenwood, The Open University, Milton Keynes, MK7 6AA UK Search for more papers by this author M. Komatsu, SOKENDAI, Hayama, Kanagawa, 240-0193 Japan Search for more papers by this author N. Imae, A.