Effects the precipitation level and human-introduce eutrophication on microbenthos ecosystem presumed by lagoonal lamina deposit analysis

Lamina (annual varve) in the sediment is regarded as prominent tool for high resolution paleoenvironmental reconstruction. Development of laminated sediments requests the strong seasonal signals while preservation is effected by bottom water/sediment anoxia resulting from stratification, salinities or high sedimentation rates. Therefore, eutrophic lagoon is one of well laminated sediment developed area. "Mokoto-ko" and "Notoro-ko" are lagoons which locate along the coast of the Okhotsk Sea in northern Japan. During the last several decades, sediment inflow and eutrophication of these lagoons have become a social issue. Then, we have investigated this lagoon since 2009, and obtained several cores from the bottom of these lagoons. There was no lamina deposits before the half century ago in the lagoon Notoro-ko. On the other hand, the lamina layer has continuously deposited at least top 2 m depth, indicating the last 400 years. Based on the comparison of these cores, we confirm that black and gray lamina sets in these lagoons are annual varve, and these lamina sets include more fine structure which might be regarded as event layers related with heavy precipitation. To understand the relationship between climate event and microbenthos ecosystem, diatom and foraminifera assemblages in the laminated sediment layer of several sediment cores were analyzed. We show the periodic heavy precipitation events and microbenthos ecosystem change might be depended on the Pacific decadal oscillation and solar irradiance changes in this presentation. Additionally, the impact of human introduced eutrophication on the lagoon ecosystem and the initial compression process of lamina deposit are also discussed.