NH24A-04
Inorganic and organic geochemical signatures of tsunami deposits

Tuesday, 15 December 2015: 16:45
309 (Moscone South)
Tetsuya Shinozaki, University of Tsukuba, Graduate School of Life and Environmental Sciences, Tsukuba, Japan; National Institute of Advanced Industrial Science and Technology (AIST), Geological Survey of Japan, Tsukuba, Japan and Shigehiro Fujino, University of Tsukuba, Faculty of Life and Environmental Sciences, Tsukuba, Japan
Abstract:
Previous studies have revealed that the geochemical evidence left behind following tsunami inundation is often preserved (Chagué-Goff, 2010). Geochemical signatures can allow us to identify tsunami deposits, but also to accurately estimate the inundation area. However, there is still little known about the characteristics; what kind of chemical component is transported, and how long such a component is preserved.

High concentrations of water-leachable ions attributed to seawater have been reported from paleo-tsunami deposits (e.g., Minoura and Nakaya, 1991). They were also reported in the 2011 Tohoku-oki tsunami deposit (e.g., Chagué-Goff et al., 2012). Water-leachable ions derived from the tsunami were concentrated in the muddy tsunami deposit or the soil below the sandy tsunami deposit. However, the concentrations decreased with time (a few months to years) (e.g., Chagué-Goff et al., 2014; Shinozaki et al., submitted), probably as a result of leaching by meteoric water. While the evidence of marine inundation is sometimes preserved for a long time (e.g., Minoura et al., 1994), we still need to carry further research of modern and paleo-tsunamis.

Biomarker analysis is a new study area in tsunami research. Biomarkers are molecular fossils originating from living organisms, which play a role as tracers of particular sources. If we could find marine-origin biomarker from terrestrial sediment, it would be a strong evidence of seawater incursion. Shinozaki et al. (2015, submitted) revealed the characteristics of biomarker in the 2011 tsunami deposit. Marine-origin biomarkers were found in the soil below the sandy tsunami deposit or in the tsunamigenic mud, while they were absent in the sandy tsunami deposit and sandy mud. Biomarkers can adsorb to fine particles and organic matter but not to sand-sized particles. They are expected to be preserved for a long time because marine-origin biomarkers were detected from samples collected more than two years after the tsunami.

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