Graduate School of Environmental Earth Science, Hokkaido University, Kita10 Nishi5, Kita-ku, Sapporo, 0600810 Japan ; Institute of Low Temperature Science, Hokkaido University, Kita19 Nishi8, kita-ku, Sapporo, 0600819 Japan
Department of Physical Geography and Quaternary Geology, Stockholm University, S-106 91, Stockholm, 10691 Sweden