Identifying tsunami traces beyond sandy tsunami deposits using terrigenous biomarkers: a case study of the 2011 Tohoku-oki tsunami in a coastal pine forest, northern Japan

Abstract The distributions of sandy tsunami deposits do not reflect the true extents of tsunami inundation areas, leading to underestimates of inundation by past tsunamis and thus the magnitudes of their associated tsunamigenic earthquakes. To archive the sedimentological and geochemical features of the 2011 Tohoku-oki tsunami deposit, we performed visual observations and computed tomography, grain-size, water content, and organic geochemical analyses of sediments from a coastal forest at Oirase Town, northern Japan. Stratigraphic observations revealed the 2011 tsunami deposit to be a landward-thinning interbedded sand and soil layer that became ambiguous in landward locations. The sediment samples from the inundated area did not contain marine-sourced biomarkers; instead, peak concentrations of isolongifolene, an organic compound derived from Pinus in the forest, were observed within or just above the sandy tsunami deposits in sediment sections. Peak isolongifolene concentrations were also detected in landward soils inundated by the tsunami in which no sand layer was observable, but were not observed beyond the inundation limit. Although this characteristic biomarker is unique to this and similar depositional environments, these results suggest that lateral changes of the concentrations of environment-specific biological proxies in the sedimentary column may record tsunami inundation.