Geochemical and micropaleontological impacts caused by the 2011 Tohoku-oki tsunami in Matsushima Bay, northeastern Japan

Abstract

We present stratigraphic, geochemical (CNS elemental analysis), and micropaleontological (ostracode assemblages) evidence for the 2011 Tohoku-oki tsunami impact in Matsushima Bay, northeastern Japan, and examine the recovery process after the impact to the bay environment using short sediment cores from two different topographic settings at a water depth of approximately 4 m. At both sites, tsunami deposits are composed of two sedimentary layers that recorded the first tsunami run-up and backwash. At the western site near the channel that connects the bay with the Pacific Ocean, the first tsunami wave eroded the bay floor sediments and left the tsunami deposits composed of medium sand rich in shell fragments, but with sparse numbers of meiobenthic ostracodes. By contrast, at the eastern site, which is surrounded by many small islands, the first wave eroded very little of the bay floor sediments and left tsunami deposits consisting of sandy mud, rich in exotic ostracodes, such as phytal species, shallow marine sand dwelling species, and brackish water species. Overlying post-tsunami deposits are composed mainly of organic-rich mud in which organic matter was derived primarily from marine plankton. Ostracode assemblages in the tsunami deposits lack the offshore species that live in water depths >50 m and are dominated by the species from Matsushima Bay and its adjacent nearshore, upper sublittoral areas, and brackish water environments. The distance from deep-water offshore areas to the core sites is too far to transport ostracode valves by tsunami waves.