With a minimum of three reported waves, the 2011 Tohoku-oki tsunami’s destructive force caused massive damage along the northern Japanese Aomori coast. At Misawa the coastal control area was inundated up to 550 m inland and sandy sediment remnants can be traced to c. 350 m (c. 61–63% of the maximum inundation) from the shoreline. Linking the discovery of floatable plastic objects within a woody and organic layer to our analytical data lead to the detection of a yet undocumented woody-organic tsunami deposit first appearing on top of the sandy deposit but then reaching even further inland (approx. 69–72% of the max. inundation). By this observation our understanding of the documented part of the tsunami inundation may be improved. As a consequence, sand sheets of historic and paleo-tsunamis represent minimum estimates for the coastal inundation and underestimation may be reduced by addressing the woody and organic fraction of a tsunami’s inundation.
Far‐field tsunami deposits observed in the Kahana Valley, O‘ahu, Hawai‘i (USA), were investigated for their organic‐geochemical content. During short high‐energy events, (tsunamis and storms) organic and chemical components are transported with sediment from marine to terrestrial areas. This study investigates the use of anthropogenic based organic geochemical compounds (such as polycyclic aromatic hydrocarbons, pesticides and organochlorides) as a means to identify tsunami deposits. Samples were processed by solid–liquid extraction and analyzed using gas chromatography–mass spectrometry. A total of 21 anthropogenic marker compounds were identified, of which 11 compounds were selected for detailed analysis. Although the tsunami deposits pre‐date industrial activity in Hawai‘i by several hundred years, distinct changes were found in the concentrations of anthropogenic marker compounds between sandy tsunami deposits and the surrounding mud/peat layers, which may help in identifying tsunami deposits within cores. As expected, low overall concentrations of anthropogenic markers and pollutants were observed due to the lack of industrial input‐sources and little anthropogenic environmental impact at the study site. This geochemical characterization of tsunami deposits shows that anthropogenic markers have significant potential as another high‐resolution, multi‐proxy method for identifying tsunamis in the sedimentary record.
<p>In AD&#160;1755 a strong earthquake-generated tsunami destroyed large parts of the southwest Iberian coastline. Data for the study of the sedimentological characteristics and palaeo-ecological effects of the backwash of this well-known AD&#160;1755 Lisbon tsunami and possible preceding events on the continental shelf was obtained during RV METEOR cruise M152 in November 2018, since the hydrodynamics of tsunami backwash currents are as yet poorly understood. Furthermore, the suitability of the shelf as a reliable sedimentary archive for tsunami deposits was investigated.</p><p>Along the Algarve coast, prominent AD&#160;1755 Lisbon tsunami deposits have been detected onshore for quite some time. Cruise M152 conducted a geophysical survey on the corresponding shelf area to obtain bathymetry and sub-bottom profiles for the recognition of depositional basins. Subsequently, 19 sediment cores were retrieved from the most suitable depositional basins by vibracoring at water depths from 65 to 114&#160;m. The cores were analysed in a multiproxy approach (granulometry, magnetic susceptibility, P-wave velocities, organic and inorganic geochemistry, micropalaeontology). Deposits of the AD&#160;1755 Lisbon tsunami were identified in most of the cores as a thin layer at ca. 20&#160;cm depth.</p><p>More surprisingly, a second event deposit dating to ca.&#160;3700&#160;years&#160;cal.&#160;BP was detected at core depths of 122 to 155&#160;cm. It is even traceable in the sub-bottom profiles and consists of a distinctive ca. 30&#160;cm thick well sorted medium-sized siliciclastic sand. Due to the thickness of the deposit an in-depth study of its characteristics was possible. It displays an erosive basal contact followed by a thin matrix-poor shell hash layer, a reversely graded fine sand layer and ultimately a massive, quite homogeneous medium sand resembling the T<sub>a</sub> division of the Bouma sequence or the S<sub>1</sub>, S<sub>2</sub> and S<sub>3</sub> divisions of the Lowe sequence. The deposit is distinguishable from the silt to silty sand-dominated background sedimentation not only due to the textural and compositional features, but also due to contrasting geophysical and geochemical properties. Terrestrial provenance for (at least parts of) the sediment is revealed by biomarker analysis. Based on these characteristics, the deposit is interpreted as the result of a high density hyperpycnal flow from the coast towards the offshore caused by tsunami backwash. This event layer may be correlated to onshore observations of tsunami deposits along the southwest coast of Spain but has never been identified in Portugal where the onshore record of tsunami deposits only covers the last three millennia.</p><p>The results of this multiproxy analysis strongly suggest the shallow offshore area below storm wave base to host reliable sedimentary archives for tsunami backwash deposits, which allow the discovery of as yet unknown events. Palaeotsunami research can benefit from the investigation of offshore archives, especially where onshore records are incomplete or sparse.</p>
The 2011 Tohoku-oki tsunami left a characteristic geochemical signature in the sediments of the Misawa harbor on the Aomori coastline (northern Japan), not only in vertical stratigraphy but also in lateral distribution. Suitable indicator compounds for the tsunami impact were used to identify and characterize the distribution of geochemical patterns within the harbor area. Specific compounds are illustrating the different emission sources and distribution during the 2011 tsunami. Petrogenic-derived markers, such as hopanes and polycyclic aromatic hydrocarbons, provide information about the tsunami-related destruction of facilities and technical material and the subsequent release of, for instance, oil and grease. Linear alkylbenzenes and diisopropylnaphthalene are used to identify sewage-derived contaminants released by the tsunami. Old burden markers such as dichlorodiphenyltrichloroethane and its metabolites or polychlorinated biphenyl signal erosion and rearrangement of contaminants present in the sediments prior to the tsunami. Distribution of the analyzed pollutant groups indicate the tsunami-related release through various emission sources and their potential origin. While petrogenic-derived pollutants revealed a significant local spread with hotspot formation near the release, sewage-derived compounds were widely distributed and originated from a diffuse source not necessarily located in the harbor area. In contrast to freshly released contaminants, old burden markers are characterized by erosion of contaminated pre-tsunami sediment, the remobilization of pollutants and subsequent deposition of these sediment-bound contaminants in the tsunami layer. The correlation between all pollutant groups by their preferred accumulation reveals that source-specific compounds show different emission sources but reveal also a topographical control of the pollutant distribution by the 2011 tsunami.
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