National Institute of Polar Research, 1-9-10, Kaga, Itabashi-ku, Tokyo, 173-8515 Japan ; Research Fellow of the Japan Society for the Promotion of Science, 1-8, Chiyoda-ku, Tokyo, 102-8472 Japan
School of Earth Sciences, Ohio State University, 275 Mendenhall, 125 South Oval Mall, Columbus, OH 43210 United States ;
National Institute of Polar Research, 1-9-10, Kaga, Itabashi-ku, Tokyo, 173-8515 Japan ;
Frontier Collaborative Research Center, Tokyo Institute of Technology, G1-25, 4259 Nagatsuta, Midori-ku, Yokohama, 226-8502 Japan ;