Abstract
The purpose of this study is to clarify the transition of the sediment environment of the Seto Inland Sea. To estimate the sedimentation rate and deposition year in Harima-nada, Osaka Bay, and Suo-nada, the vertical distribution of 137Cs was determined for sediment cores. The carbon and nitrogen stable isotope ratios (δ13C, δ15N) of sediment core layers in Harima-nada and Osaka Bay were measured and plotted in a δ13C vs δ15N diagram. The plots were distributed in a very narrow range during 1865 to 1948, but an increase in δ15N and a slight increase in δ13C were simultaneously observed during 1954 to 2000. Since 1950, the frequency of occurrence of an ostracode and a foraminiferal species resistant to oxygen-deficient environments has increased. From 1950, hypoxia was occurred in the bottom layer, or phytoplankton in the surface layer thrived. Consequently, the δ15N of phytoplankton increased, thereby increasing the δ15N of sediment core samples. The plots for Suo-nada, from 1825 to 2008 were distributed only in a very narrow range of the δ13C vs δ15N diagram. Ostracode assemblages were significantly rich in diversity. It was estimated that δ15N of phytoplankton did not vary because denitrification did not occur and hypoxia did not occur in the bottom layer.
