Geochemical characteristics preserved in the 2011 Tohoku-oki tsunami deposits

Correct understanding of tsunami event is important for predicting their magnitude, frequency and timing of occurrence. Since tsunami occurs with low frequency, observation data and historical records only are not sufficient for estimating tsunami. Thus it is required to analyze tsunami back in prehistoric age. Although, sand deposit formed by tsunami has been generally used for identification of paleotsunami and estimation of seawater inundation limit, sand deposits does not always form around inundation limit. On the other hand, sea-derived elements, such as Cl, Na and Ca, were detected within seawater inundation area. This means that using geochemical approaches may enable to identify less-visible paleotsunami deposits and to reconstruct precise inundation limits. However, very few attempts have been made to use geochemical analysis for tsunami research. Comprehension of geochemical characteristics preserved in tsunami deposits formed by the 2011 Tohoku-oki tsunami over the Pacific coast of East Japan, which provides an opportunity for evaluating the potential of geochemical analysis in paleotsunami identification. We analyzed sand layers formed by the 2011 Tohoku-oki tsunami by using biomarker, such as n-alkane and alkenone, elemental analysis, stable carbon and nitrogen isotope ratio, and C/N ratio to check whether these ocean-derived biomarkers can be used for identifying tsunami events. Samples were collected at 2 sites in East Japan. One study site is the Sendai coastal plain, Northeast Japan, where a 1.3-m-long sediment core was obtained c. 1.5 km inland from the shoreline. A c. 3-cm-thick 2011 tsunami layer was found at the top of the core. In addition, a 7-cm-thick sand layer occurs at 61-68 cm deep, and this was probably formed by the 869 Jogan tsunami. Another site is Hasunuma at the Kujukuri coast, Southeast Japan, c. 300 km south from the Sendai coastal plain. Samples of 2011 tsunami deposits were collected just below the ground surface in an area within 400 m from the shoreline. The collected 2011 tsunami deposits were 12-31 cm thick at Hasunuma. We will discuss the geochemical characteristics of the sampled 2011 Tohoku-oki tsunami deposits.