UNIQUE 210-PB AND 137-CS PROFILES IN MARINE SEDIMENT CORES CONTAINING RECENT EVENT DEPOSITS OFF KUMANO AND SANRIKU, 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.