1. Department of Earth Sciences, Uppsala University, Villavägen 16, 752 36 Uppsala, Sweden
2. Present address: Department of Mechanical Engineering, Nagaoka University of Technology, 1603-1 Kamitomioka-machi, Niigata 940-2188, Japan
3. National Institute of Polar Research, 10-3, Midori-machi, Tachikawa-shi, 190-8518 Tokyo, Japan