Friday, 9 September 2005 - 10:50 AM

This presentation is part of: New Developments in Terrestrial Cosmogenic Nuclide Research

Cosmogenic Nuclides in Underwater Rocks as Indicators of Pre-irradiation Record in Low Sea-Level Periods

Mineo Imamura, National Institutes for the Humanities and Graduate University for Advanced Studies, National Museum of Japanese History, 117 Jonai, Sakura, Chiba, 285-8502, Japan, Kyeong J. Kim, Dept. Physics, Univ. Arizona, Tucson, AZ 85721, A. J. Timothy Jull, NSF Arizona AMS Laboratory, Univ of Arizona, 1118 East Fourth St, Tucson, AZ 85721, Hiroyuki MATSUZAKI, Department of Nuclear Engineering and Managemant, School of Engineering, The University of Tokyo, Tokyo, Japan, Kazuhiro Kato, Department of Nuclear Engineering and Management, The University of Tokyo, Bunkyo-ku Yayoi 2-11-16, Tokyo, Japan, and Kazuho Horiuchi, Departments of Earth and Environmental Science, Hirosaki University, Bunkyo-cho, Hirosaki, 036-8561, Japan.

Sea-level change during ice ages has been of great concern, since it is closely related to the formation of land bridges between islands and continents and is associated with key issues in understanding movement of human being and animals in the world. In the case of north-west part of the Pacific, migration of people from the Eurasian continent to Japanese main islands and vice versa is thought to have occurred through the straits at current water depth of -130m (Korea and Japan), -60m (Far East Russia and Japan) and -140m (Honshu and Hokkaido islands). In the case of Japanese islands arcs, estimation of sea-level during ice ages is complicated by tectonic and hydro-isostatic movements, which may cause differences of more than 30 meters among localities.

This study was designed to determine the feasibility of using cosmogenic nuclides as a dating tool in the investigation of land bridges between Eurasian continents and Japan during the past glacial ages. In our previous study, cosmogenic nuclide production of 10Be and 26Al was investigated using underwater rock samples obtained from the Tsugaru Strait between Honshu and Hokkaido, the two main islands of Japan. Four sedimentary rock samples obtained from depth (water + rock) range between 3,760 g cm-2 and 6,140 g cm-2 showed 10Be concentrations, (8.1±0.92) x 103 – (8.6±1.5) x 104 atoms/g SiO2, indicating the effect of pre-exposure by neutron spallation during the past glacial period [1]. However, low contents of quartz in these sedimentary rocks hampered further detailed research.

We recently have collected bedrock samples, which are rich in quartz, for 11 depths from the present sea level down to a depth of 35 m in the Tsugaru Strait (Aomori area) to unravel the pre-exposure history of underwater rocks. From these samples (mostly granites), we have prepared quartz samples for multiple cosmogenic nuclide analysis such as, 10Be, 26Al, 14C, and 21Ne. We will show a detailed procedure of collecting and processing underwater rock samples and some preliminary results.

This study was supported in part by Grant-in-Aid (#12874064) for Scientific Research from the Japan Society for the Promotion of Science.

[1] Kim and Imamura, NIM B 223-224 (2004) 608-612.


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