Sediments of Matsushima Bay, Northeastern Japan: Insights Gained From 5 Years of Sedimentological Analysis Following the 2011 Tohoku Earthquake-Tsunami

Matsushima Bay was less affected by the 2011 Tohoku Earthquake Tsunami than other Pacific coastal areas of northeast Japan because of the sheltering effects of islands in the bay mouth. To understand the recovery from the 2011 tsunami of benthic environments in Matsushima Bay, we conducted surveys of the geochemical properties of surface sediments over 5 years following the earthquake and compared them to sedimentary data before the earthquake. Before the tsunami, the mud fraction ('63-μm size) proportion of the surface sediments varied over a wide range, whereas after the tsunami, the mud content range was relatively narrow. During 2012–2015, the mud content was linearly correlated with the total organic carbon (TOC) content, but the slope of the relationship differed from that before the tsunami. The tightly coupled mud-TOC relationship and the almost constant C/N ratios in the surface sediments suggest that the bay sediments were resuspended and transported by the tsunami. In addition, loss of some organic matter sources in the bay may partly account for the uniform C/N ratios. By 2016, the slope of the mud-TOC relationship was almost the same as the pretsunami value, but the C/N ratios remained constant. These results suggest that ecosystems and biogeochemical cycles in the bay had not yet fully recovered their pretsunami state. Trace element compositions of core samples indicated that sediment sources were little changed by the tsunami, probably because islands in the mouth of the bay reduced sediment transport into the bay from distant sources.