Changes in deep-sea benthic foraminiferal fauna caused by turbidites deposited after the 2011 Tohoku-oki earthquake

Abstract The impact of the 2011 Tohoku-oki earthquake and subsequent turbidites on the deep-sea environment was investigated by conducting sedimentological, radionuclide, and benthic foraminiferal analyses of core sediments collected from water depths of 3230 and ~3570 m, 5 and 17 months after the earthquake. Sedimentological profiles and patterns of excess 210Pb, 134Cs, and 137Cs indicate that the analyzed sediment core records deposits before the earthquake, turbidite deposits just after the earthquake, and deposits after the Fukushima Daiichi Nuclear Power Plant (FNPP1) accident, which caused the release of a large amount of radioactive material four days after the earthquake. Core 4W-2012 F-RN was collected at 3585 m water depth 17 months after the earthquake. Within this core, an event deposit layer (6.5 cm thick) consisting of turbidites contains highly diverse and possibly reworked foraminiferal assemblages. Overlying these turbidites are sediments with high 134Cs concentrations that were deposited after the FNPP1 accident. Overall foraminiferal and species abundances show a marked decrease in the sediments above the deposited turbidites. The abundance of opportunistic and competitive species shows a gradual upward increase in the uppermost sediment layers, and these species are dominant in the top 1 cm of the core. Our findings demonstrate that although foraminiferal assemblages in the sediment overlying the turbidites related to the 2011 earthquake might include allochthonous assemblages, turbidites deposited after the earthquake caused a marked decrease in benthic foraminiferal diversity and allowed opportunistic species to colonize in a low-diversity assemblage within 17 months. These foraminiferal faunal changes, together with analysis of sedimentological structure, provide valuable information for identifying these types of sedimentation events in core sediments from the deep-sea floor.