仙台平野における歴史津波堆積物の放射性炭素年代測定

Abstract

Ancient tsunami deposits are distributed in paddy fields throughout the Sendai plain, on the Pacific coast of the Tohoku area in northeastern Japan. These deposits contain gravel, mud, shells, and microfossils originating from the seafloor, sandy beaches, and coastal soils. Paleotsunami deposits offer valuable clues for identifying past tsunami inundation areas, but previous studies had difficulty dating tsunami deposits for correlation with historic disaster events. This study collects age data from continuous soil profiles that include tsunami deposits. Nine continuous soil sediment samples are collected using a Handy Geoslicer (Fukken Co. Ltd.) near the coast of Sendai Bay in northeastern Japan. The soil slices (HS1–9) are 84–183 cm long and consist of cultivated surface soils, peaty clay, silt, and fine to medium sands. Slices HS2–7 are subsampled at 1-cm intervals and water contents of 689 discrete samples are measured. Plant residues are extracted from the samples for radiocarbon (14C) dating with a Tandetron accelerator mass spectrometry system (Model 4130 AMS, HVEE) at the Center for Chronological Research, Nagoya University. Water contents range from 6.2 to 83.0 wt.%, and the lowest values are observed in layers of well-rounded medium sand. Variations in water content agree well with changes in sedimentary facies. Total organic carbon contents of plant residues range from 45.9 to 54.5 wt.% (50.4 wt.% on average) and their stable carbon isotope ratios range from -25.1 to -30.1 permil (vs. PDB), which are consistent with the ratios of modern terrestrial C3 plants. The calibrated ages of plant residues from these tsunami deposits are about 1000–1300 cal BP, which agree well with the date of the Jogan Earthquake and tsunami on the Sendai plain (869 AD).