Paleoenvironmental changes and a tsunami deposit inferred from fossil diatom assemblages at Ohki lowland in Tosashimizu City, Kochi Prefecture, western Japan

Abstract

Fossil diatom assemblages and stratigraphic features revealed the environmental history and an event deposit at a coastal lowland in Tosashimizu City, facing the Nankai Trough. The paleoenvironment of the study site changed during the last 600 years from a sandy tidal flat, a standing-water environment with submergent plants, to a wetland. A sandy event deposit identified by bare eyes and computed tomography (CT) images was interpreted to have formed under the standing-water environment. The event deposit (Sand B) was diagnosed as a tsunami deposit based on sedimentary structures, mixed assemblages of freshwater, brackish–marine diatoms, and the environmental change before and after the event. The depositional age of the Sand B was constrained by radiocarbon ages as 550–495 cal yr BP (1411–1453 CE; Common Era). This constrained event age indicated that the Sand B might have been formed by an unknown tsunami associated with an earthquake along the Nankai Trough, although either 1361 Shohei or 1498 Meio earthquake were the other possible origins of the Sand B regarding the error range of radiocarbon dating.