Mussel Shell Geochemical Analyses Reflect Coastal Environmental Changes Following the 2011 Tohoku Tsunami

Coastal
areas are socioeconomically important but are susceptible
to disturbances by both natural and man-made events. On March 11,
2011, the eastern coast of Japan was seriously inundated by a massive
tsunami following the 2011 Tohoku earthquake. The tsunami caused a
major disturbance around the coastal area. However, an understanding
of the consequences of such an event is often hampered by a lack of
knowledge of prior conditions. Furthermore, field observations during
and immediately after the event are often particularly difficult.
The present study demonstrates that environmental reconstruction by
geochemical and growth pattern analyses of mussel shells successfully
revealed transitional (daily) environmental changes caused by the
Tohoku tsunami. A pronounced surge in shell Mn/Ca ratios observed
immediately after the tsunami implies a drastic emission of pore water
following sediment disturbance as well as a large input of terrestrial
material through backwash. The subsequent decrease of the high Mn/Ca
peak indicates a prolonged tsunami disturbance effect over ca. 40
days, with the stabilized shell Mn/Ca ratios observed thereafter (being
higher than that prior to the tsunami) suggesting that the latter
had altered the coastal environment, allowing for greater susceptibility
to terrestrial input following ground subsidence and loss of coastal
levees. Shell Mn/Ca patterns provide evidence for the tsunami-generated
release of materials stored in sediments, such as organic, nutrient,
and pollutant materials, which are then suspended in the water column
for sufficient periods to allow for incorporation into geochemical
cycles. Although the greatest environmental disturbance occurred immediately
after the tsunami, the effects lasted for longer than several months
thereafter.