[H-TT35_1AM1] 地球人間圏科学研究のための加速器質量分析技術の革新と応用

★ [HTT35-06] 連続土壌堆積物中の津波イベント層のC-14年代測定と津波堆積物の地球化学分析

*渡邊 隆広¹、細田 憲弘¹、土屋 範芳¹、中村 俊夫²、山崎 慎一¹、奈良 郁子¹ (1.東北大学大学院環境科学研究科、2.名古屋大学年代測定総合研究センター)

キーワード：Radiocarbon dating, Tsunami deposits, Jogan tsunami, Geochemistry, EDXRF

Past tsunami sediments preserved in continuous soil and lake sediments are crucial and unique clues to reconstruct the past tsunami invasion area. Generally, the tsunami sediments originated from sea floor, sandy beach and/or coastal land soils containing gravels, sands, muds, shells and microfossils. In particular, muddy tsunami sediments should be found to detect the limit of tsunami invasion areas, because relative small particles move to more inland area with tsunami in comparison with sand deposits. Additionally, dating of tsunami sediments is indispensable to refer for historical disaster events. Therefore, we have to make age models of continuous soil deposits with tsunami sediments and new techniques for detection of invisible muddy tsunami sediment in strata. In this study, we performed the nine radiocarbon measurements of plant residues in continuous soil deposits as well as geochemical characteristics of tsunami sediments. 2m-continuous soil deposits were taken by the handy geoslicer (Fukkenn co. ltd.) from the Pacific coast of Tohoku area in northeast Japan. The samples were composed of cultivated surface soils, peaty clay, silt and sub-rounded medium sands. The sandy deposits were found between the peaty clay layers. To show the sedimentary ages of sandy deposits, plant residues were taken from the sandy and peaty clay layers in the continuous soil sediments. The plant residues were washed with ultra pure water using ultra sonic cleaner to remove soil particles containing relative old carbon. Then, the samples were treated sequentially with 1.2M-HCl, 1.2M-NaOH and 1.2M-HCl at 60 degrees of Celsius for 3 hours. After neutralization and freeze-drying, the samples were combusted in evacuated quartz tubes. Then, the purified carbon dioxide was reduced to graphite using Hydrogen gas with iron catalysts. Radiocarbon measurements were performed by the Tandetron AMS system (Model-4130, HVEE) in Center for Chronological Research, Nagoya University. Total organic carbon contents of the plant residues were from 45.9 to 54.5 wt.% (50.4 wt.% in average) and stable carbon isotope ratios of the plant residues ranged between -26.7 and -30.1 permil (vs. PDB), which consist with those of modern terrestrial C3 plants. As a result of this study, part of the calibrated ages of plant residues taken from just above the sandy tsunami sediments was about 1000-1300 cal BP, and these ages were agreed well with those of the Jogan earthquake and tsunami in the Sendai plain.