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