Organic Geochemistry of the Tohoku Tsunami Deposits of 2011 (Japan)

Abstract:

Geochemical investigations on paleotsunami deposits have mainly focused on inorganic proxies. Organic geochemistry has been used to distinguish between terrestrial and marine matter within the sediments, reflecting the mixture and transport of marine and terrestrial matter. The approach using organic substances with indicative properties (anthropogenic and xenobiotic compounds) for recent tsunami deposits is novel, but the approach of using specific bio- and anthropogenic markers indicators to determine (pre)historic and recent processes and impacts already exists.

The Tohoku-oki tsunami in March 2011 showed the huge threat that tsunamis pose to society and landscape, including flooding of coastal lowlands and erosion/deposition of sediments. The mainly sandy tsunamites reach more than 4.5 km inland as there were run-up heights of ca. 10 m in the Sendai plain near the Sendai airport. The destruction of infrastructure by wave action and flooding was accompanied by the release of environmental pollutants (e.g. fuels, fats, tarmac, plastics, heavy metals, etc.) contaminating the coastal areas and ocean over large areas. To detect and characterize this process, we analyzed several sedimentary archives from the Bay of Sendai area (by using the same sample material as Szczucinski et al., 2012 from rice paddies of the Sendai Plain, Japan). The layers representing the tsunami deposits have been compared with pre-tsunami samples (supposedly to be unaffected) by means of organic-geochemical analyses based on GC/MS.

Natural compounds and their diagenetic transformation products have been tested as marker compounds and proxies. The relative composition of fatty acids, n-alkanes, sesquiterpenes and further substances pointed to significant variations before and after the tsunami event. Additionally, anthropogenic marker compounds (such as soil derived pesticides, source specific PAHs, halogenated aromatics from industrial sources) have been detected and quantified. Concentration profiles of distinct terrestrial pollutants revealed shifts either to increasing but for selected compounds also to decreasing contamination levels. We will extend and test the approach in future on paleotsunami deposits of the 869 AD Jogan event and others.

Szczucinski et al. 2012, Sed. Geol., 282:40-56.