Paper No. 15
Presentation Time: 9:00 AM-6:00 PM


IKEHARA, Ken1, IRINO, Tomohisa2, USAMI, Kazuko1, ASHI, Juichiro3 and JENKINS, Robert4, (1)Geological Survey of Japan, AIST, Institute of Geology and Geoinformation, Tsukuba Central 7, 1-1-1 Higashi, Tsukuba, 305-8567, Japan, (2)Hokkaido University, Sapporo, 060-0810, Japan, (3)Ocean Floor GeoScience, Ocean Rsch Institute, Univ of Tokyo, Minamidai 1-15-1, Nakano-ku, Tokyo, 164-8639, Japan, (4)Yokohama National University, Yokohama, 240-8501, Japan,

In 2011, Japan experienced two severe hazards; 2011 off the Pacific Coast of Tohoku earthquake and tsunami, and 2011 Kii Peninsula flood. After two events, we conducted marine survey to clarify the influence of earthquake/tsunami and flood to sea bottom environments. To understand the depositional ages of the cores, we measured the short-lived radioisotopes such as 210-Pb and 134- and 137-Cs. Clayey flood deposits were found at the top of two cores obtained from off Kii Peninsula. In flood layers, excess 210-Pb was lower than underlying normal hemipelagic mud. But 137-Cs was high in the flood deposits. Therefore, selective concentration of Cs and selective dilution of Pb in clayey flood deposits might occur.

We found the 2011 earthquake- and/or tsunami-induced turbidites from outer shelf to trench slope off Sanriku. Almost of excess 210-Pb profiles showed the simple decreasing trend with increasing depth and grain size and decreasing water contents in turbidite beds. This relation indicates the grain size control on excess 210-Pb concentration in the turbidite beds. On the other hand, 134- and 137-Cs was highest at almost of the core top sample. But in two cores, one from outer shelf (122 m in water depth) and one from lower slope (5522 m in water depth), sub-surface peak of 134- and 137-Cs was also found. 134- and 137-Cs concentration at sub-surface peak was higher than that at core top (sea floor surface) sample. X-ray radiographs indicated that the turbidite was two-storied at the cores. Lower turbidite did not contain 134-Cs, but upper one contained 134-Cs. This suggested that the lower formed before Fukushima accident, but the upper formed after the accident. Clear peak of 134- and 137-Cs in the upper turbidite suggests selective concentration of Cs. Low excess 210-Pb concentration in the underlying hemipelagic mud in the shelf core suggests that the tsunami-induced sea bottom erosion might occur before the turbidite deposition.

These results indicated that sedimentary processes controlling the grain size and mineral composition play important role on short-lived isotopes.