Shallow-marine deposits associated with the 2011 Tohoku-oki tsunami in Sendai Bay, Japan

Abstract

Little is known about offshore tsunami deposits, although they have the potential to improve palaeotsunami reconstructions, especially in areas with limited preservation of onshore tsunami deposits. We analysed 44 sediment cores collected in 2012 at water depths of 14–30 m in Sendai Bay, north‐eastern Japan, to identify the deposits of the 2011 Tohoku‐oki tsunami with the aid of 134Cs released from the Fukushima‐Daiichi Nuclear Plant. The preserved tsunami deposits in the inner shelf are interbedded with very fine sand and mud, showing similar features to storm deposits. In the shoreface, a distinct layer of coarse sand overlies fine sand, appearing to have originated as beach sand transported offshore by the tsunami backwash. The coarse sand fades offshore around the boundary between the shoreface and inner shelf. Many of the core sites show evidence that surface sediment was reworked, probably by post‐tsunami storm waves, suggesting that further reworking would make the ultimate preservation of the tsunami deposits unlikely.

Palaeoenvironments and elemental geochemistry across the marine Permo‐Triassic boundary section, Guryul Ravine (Kashmir, India) and a comparison with other North Indian passive margin sections

“…Unit D is dominated by thick-bedded, parallel and crosslaminated, calcareous fine to medium-grained, well-sorted, often argillaceous, very sandy limestones, or calcareous quartz sandstones, interbedded with sporadic thin calcareous shales. Graded beds show consistent trends from parallel lamination to festoon (hummocky) cross-lamination typical of deposits of waning storms or tsunamis ( Figure 5B) (Brookfield et al, 2013;Tamura et al, 2015). Most of the quartz grains are angular to sub-rounded and first-cycle ( Figure 4B, 4), but a few are very well-rounded, indicating a desert source for those (Nakazawa et al, 1975).…”

New coring study in Augusta Bay expands understanding of offshore tsunami deposits (Eastern Sicily, Italy)

“…) collected within and at the base of L1 and L2, did not contain enough foraminifera specimens to be considered statistically representative of the study sample as confirmed by very low values in BFN. Indeed, this reduction of foraminifera can be interpreted as related to a major input of fine sediments, as also observed elsewhere in modern offshore tsunami samples collected close to the shelf‐break (Ikehara et al ., ; Tamura et al ., ).…”

“…After the 2004 Indian Ocean and 2011 Tohoku‐oki tsunamis, offshore surveys were conducted to collect material, mainly from the inner continental shelf [0 to 30 m below sea level (bsl)] and over the shelf‐break (>100 m bsl). These studies highlighted that in the marine realm the tsunami‐related units are subjected to the action of currents, waves and bioturbation that occur immediately after their deposition (Feldens et al ., ; Sakuna et al ., ; Ikehara et al ., ; Tamura et al ., ; Yoshikawa et al ., ; Seike et al ., ). Thus, it is often not easy to recognize offshore tsunami deposits even when they are related to recent events or with a precise age control (Toyofuku et al ., ) because of their potentially low preservation in shallow water due to reworking and transport by currents, waves (Weiss & Bahlburg, ) and gravity flows which disperse sediment on the continental shelves.…”

Lasting Impact of a Tsunami Event on Sediment‐Organism Interactions in the Ocean

“…They are well preserved as anomalous coarse layers in peat or mud deposits typical of low-energy terrestrial or shallow-seabed environments (Atwater, 1992;Bourgeois et al, 1988;Dawson & Stewart, 2007;Monecke et al, 2008;Sawai et al, 2015;Seike et al, 2016Seike et al, , 2017a. Although tsunami deposits might be modified or obliterated by subsequent physical (Shinozaki et al, 2015;Szczuci nski, 2012;Tamura et al, 2015) and/or biological processes (Seike et al, 2016), they can remain for long periods in calm coastal embayments (Seike et al, 2017a). This indicates that tsunamis have semipermanent effects on the seafloor topography and the grain-size composition of seafloor sediments; in other words, tsunami sedimentation is a short-term phenomenon that has a major effect on the benthic environment over the longer term.…”