[MIS11-P10] Use of microfossils analyses in tsunami deposit survey
Keywords:Tsunami deposit, Microfossils, Diatom, Coccolith, Silicoflagellate, Sponge spicule
In Numanohama marsh in Taro Otobeno, Miyako City, Iwate Prefecture, we identified 17 event layers with sedimentary characteristics for tsunami deposits during the past 2,000 years (Goto et al., 2015, 2017, submitted). In order to estimate changes in the sedimentary environment, we counted diatom and coccolith. Diatom assemblages indicate that the sedimentary environment (diatom zones) changed after the deposition of event layers which can be correlated with historical large tsunamis. High appearance of coccoliths in the event layer showed that the seawater inflowed landward such as tsunami/storm event.
In Idagawa lowland in Minami Soma City, Fukushima Prefecture, we identified 7 event deposits during the past 3,800 years (Kusumoto et al., submitted). Autochthonous deposit can be clearly distinguished from event deposit by using three types of microfossils/organic remains; diatoms, coccoliths, and sponge spicules from the geological column samples with several meters. Among them, abundant coccoliths well corresponds to event layers. Then the appearance of silicoflagellates from autochthonous deposit indicates that the survey area was connected to the open sea at that time.