Benthic foraminiferal evidence of deep-sea sediment transport by the 2011 Tohoku-oki earthquake and tsunami

Benthic foraminiferal assemblages within turbidites recovered from two upper slope stations off Sendai Bay, northeast Japan, have been analyzed to estimate the source area of the turbidites resulting from the 2011 Tohoku-oki earthquake and tsunami. The turbidite at one station (St. 5; 893 m water depth) is composed of a surface layer that contains foraminiferal species living on the outer shelf or inner bay, and a lower part with bathyal species. In the turbidite at the second station (St. 6; 1446 m), the assemblage consists of only bathyal species (living in 800-1000 m). The composition of the St. 6 assemblage gradually changes from the base to the top of the turbidite, consistent with the upward-fining graded structure (i.e. thin-walled, tiny species are increased in the upper layer). The lower part of the turbidites at both sites, St. 5 and St. 6, are considered to have originated from turbidity currents triggered by earthquake shaking. In contrast, the most probable generation mechanism for the turbidity current that formed the surface layer at St. 5 is the tsunami-related agitation of sediment in shallow water. The relatively good preservation of benthic foraminiferal tests in the turbidites suggests transport processes with minimal internal friction and/or collision of grains within the turbidity current. These results show that benthic foraminifera are useful in turbidite paleoseismology.