A Geochemical approach for identifying marine incursions; Implications for tsunami geology on the Pacific coast of northeast Japan

Watanabe, Takahiro   ; Tsuchiya, Noriyoshi*; Yamasaki, Shinichi*; Sawai, Yuki*; Hosoda, Norihiro*; Nara, Fumiko*; Nakamura, Toshio*; Komai, Takeshi*

Possible tsunami inundation areas can be predicted from the distribution of paleo tsunami deposits, which are mainly composed of marine-derived sands and muds on land. Such exotic sandy and muddy layers have been identified by multiple approaches. However, there still remain uncertainties and other useful proxies need to be investigated for the identification of tsunami deposits. Here we show the geochemical signatures of modern tsunami deposits from the Pacific coast of Northeast Japan (2011 Tohoku tsunami), as well as those paleo tsunami deposits, which were taken from the Tohoku District (Jogan, ca. 1080 calibrated ages before present [cal BP] and Yayoi, ca. 2000 cal BP). A geochemical ternary diagram (Seawater-Rock-(As+Metals)) enables the weathering trend of tsunami deposits over ca. 2000 years in the Sendai Plain in the Tohoku to be shown. In the paleo tsunami layers from the Tohoku, the Na/Ti atomic ratios markedly increased to 23.4, and the average values were 19.33.0 (Jogan and Yayoi tsunami deposits), which were clearly higher than those of other layers (soil deposits, 10.33.5 on average). These results show that the Na/Ti ratio is a useful indicator of marine incursions in our case. Our rapid and simple method using the Na/Ti ratio can be easily applied to the Tohoku, and it may contribute to the detection of unrecorded muddy tsunami deposits.