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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.

We examined the impact of the earthquake and tsunami following the 2011 off the Pacific coast of Tohoku earthquake on the deep-sea benthic ecosystems based on radionuclide and benthic foraminiferal analysis of core sediments, collected from 3200 and 3600 m water depths 5 and 17 months after the earthquake. Radionuclide analysis of the excess 210Pb, 134Cs, and 137Cs indicated that some of the analyzed sediment core recorded deposits before the earthquake, event deposits just after the earthquake, and deposits after the Fukushima Daiichi Nuclear Power Plant accident, which caused the release of a large amount of radioactive material 4 days after the earthquake. Uvigerina senticosa, Chilostomella oolina, and Elphidium batialis were the dominant species in the study area prior to the earthquake. In core 4W-2012, the original or pre-earthquake assemblage layer was covered by 5-cm-thick event deposits following the earthquake that contained a high diversity allochthonous foraminiferal assemblage. Following the episodic deposition, foraminiferal density drastically decreased and many species disappeared, resulting in a decrease in species diversity. Above 10 cm depth in the sediment, living specimens of opportunistic and competitive species gradually increased toward the sediment surface and became dominant in the top 1 cm of the core. Thus, the episodic deposition resulting from the earthquake caused a drastic decrease in the original benthic foraminifera and colonization of opportunistic species with a low diversity within 17 months. Although there were differences in vertical change in the radionuclides and benthic foraminifera between sites, faunal change may have already occurred 5 months after the earthquake.

Sedimentary organic matter and porewater geochemistry at upper bathyal depths off Tohoku, Japan, were investigated 1 year after the 2011 off the Pacific coast of Tohoku Earthquake. Total organic carbon (TOC) and total nitrogen (TN) concentrations, organic carbon and nitrogen isotopic compositions, dissolved oxygen (DO) concentrations, and bottom water and porewater nutrient concentrations were examined in sediment core samples collected from eight upper bathyal stations. Event deposits 1 to 7 cm thick were observed at all sampling stations, indicating that a vast area off Tohoku has been influenced by sedimentological events caused by the earthquake or subsequent tsunami. Both TOC and TN concentrations were lower in coarse-grained event deposits. Carbon and nitrogen isotopic composition, together with the C/N ratio, suggested that the event deposits, including turbidites, were composed of local sediments that had been transported relatively short distances. Nutrient fluxes across the sediment–water interface correlated with surface TOC and bottom water DO concentrations. However, event deposit thickness showed no correlation with oxygen or nutrient fluxes. The organic matter content of the event deposits was similar to that in the underlying sediments at each site, and thus no apparent effect of event deposits on nutrient fluxes was observed. Our results suggest that biogeochemical cycles in sediments of disturbed areas are affected not only by event deposit thickness, but also by the source of the event deposits.

We examined the effects of mass sedimentation events caused by the 2011 off the Pacific coast of Tohoku Earthquake on abundances and vertical distributions of prokaryotes and metazoan meiofauna in sediments, using sediment cores collected from eight bathyal stations off Tohoku 1 year after the M9.0 earthquake. Event deposits 1–7 cm thick were observed at the topmost part of the sediment cores at all sampling stations. At some stations, prokaryotic cell abundances were lower in the surface event-deposit layers compared to those in deeper sediments. These variations were explained by environmental parameters such as a dimensionless sorting factor and mean grain size, suggesting that turbidite sedimentation affected prokaryotic cell abundances. Nematodes had anomalously higher subsurface abundances at the stations where subsurface peak prokaryotic cell numbers were observed, whereas copepods always showed peak densities in the sediment surface layer. Although there are no available data for prokaryotic cell abundances and meiofaunal densities before the earthquake from the same sites, it is likely that the subsurface peaks in prokaryotic cell numbers and nematode densities resulted from the sedimentation events. The effects of sedimentation events on the organisms were observed 1 year after the earthquake, indicating that episodic sedimentation events on scales of several centimeters have a large effect on small organisms inhabiting sediments.

We show the first real-time record of a turbidity current associated with a great earthquake, the Mw 9.0, 2011 Tohoku-Oki event offshore Japan. Turbidity current deposits (turbidites) have been used to estimate earthquake recurrence intervals from geologic records. Until now, however, there has been no direct evidence for large-scale earthquakes in subduction plate margins. After the 2011 Tohoku-Oki earthquake and tsunami, an anomalous event on the seafloor consistent with a turbidity current was recorded by ocean-bottom pressure recorders and seismometers deployed off Sendai, Japan. Freshly emplaced turbidites were collected from a wide area of seafloor off the Tohoku coastal region. We analyzed these measurements and sedimentary records to determine conditions of the modern tsunamigenic turbidity current. We anticipate our discovery to be a starting point for more detailed characterization of modern tsunamigenic turbidites, and for the identification of tsunamigenic turbidites in geologic records.

The March 11, 2011 Tohoku-Oki tsunami triggered by an earthquake off the east coast of northeastern Honshu Island (Tohoku region), Japan, deposited large amounts of sediment on land, including the Sendai Plain and Sanriku Coast. This study reports on the characteristics of the tsunami deposits in Rikuzentakata City, southeastern Iwate Prefecture, northeastern Japan. A field survey identified the inundation pattern of the tsunami in this region and the facies model of the tsunami deposits at the bay-head deltas of estuarine systems. The tsunami deposits in Rikuzentakata City generally consist of one to four units that represent a discrete runup or backwash flow. Each unit is characterized by initial inverse grading and successive normal grading that correspond to the accelerating and decelerating stages of the flow, respectively. An internal erosional surface often developed between the inverse-graded and normal-graded units. It corresponds to the maximum shear velocity of the flow and truncates the underlying inverse-graded unit. In the case of the runup unit, silty fine-grained drapes overlay the graded sandy interval. A correlation of the sedimentary structures and grain fabric analysis revealed that the Tohoku-Oki tsunami inundated Rikuzentakata City at least twice and that the flow velocity exceeded 2.4 m/s. Paleontological analysis of the sediment and kriging estimation of the total volume of the tsunami deposit implied that the sediments were sourced not only from eroded beach sands but also from the seafloor of Hirota Bay or more offshore regions.

[1] In Rikuzentakata City, Ostracode assemblages in sediment deposited by the Tohoku-Oki earthquake and tsunami of 11 March 2011 revealed that the sediment was derived from the seafloor from at least 9 m water depth, and was transported inland more than 1 km. The tsunami wave height at this location was higher than 10 m. Four hundred fifty seven modern ostracode assemblages were used in the modern analogue technique to estimate the depth source of the tsunami deposited assemblages. The application of this method to paleo-tsunami deposits may provide insight into past tsunami wave height and potentially earthquake slip and magnitude.