[S-EM20] [JJ] 地磁気・古地磁気・岩石磁気

[SEM20-14] Flooding history in Lake Nakaumi of western Japan inferred from sediment records during the last 700 years

*太田 雄貴^1,2,3、川幡 穂高^1,2,3、佐藤 高晴⁴、瀬戸 浩二⁵ (1.東京大学大学院理学系研究科地球惑星科学専攻、2.東京大学大気海洋研究所、3.独立行政法人産業技術総合研究所地質情報研究部門、4.広島大学総合科学研究科、5.島根大学汽水域研究センター)

キーワード：中海、洪水、堆積物

Management of flood risk to local communities located near river mouths can benefit from a knowledge-based approach to flooding history. On some rivers in western Japan, flood events since the 16th century have been well documented in the literature. However, the data of those events are insufficient for an objective evaluation of flooding such as its intensity. Therefore, we analyzed a ¹⁴C-dated sediment core recovered from near the Iinashi River mouth in Lake Nakaumi, western Japan, and reconstructed extreme flooding records in the past 700 years from rock magnetism, grain-size distribution and sediment geochemistry. The sediment core contained three black layers, characterized by high magnetic susceptibility (MS), a low anhysteretic remanent magnetization to saturation isothermal remanent magnetization, a high total organic carbon (TOC) to total nitrogen ratio, and coarse mean grain size, that we identified as possible flooding event deposits. In addition, the chemical index of alterlation was low and the TOC to total sulfur ratio was high in these layers, indicating intensified erosion sediments associated with more freshwater inflow due to the flood events. We tentatively correlated the three event deposits with historically documented flooding events along the Iinashi River in AD 1596, 1666, and 1826. The magnitudes of the variations in sedimentary properties relating to rock magnesium and geochemistry suggest that the smallest was the AD 1666 event.