Source of Organic Matter in 2011 Tohoku-oki Tsunami Deposits Determined by C/N Ratios and δ13C

Abstract:

For the future mitigation of tsunami hazard in coastal areas, assessments of ancient tsunami deposits are crucially important. In order to identify tsunami deposits, several indicators have been proposed to be useful including multiple proxies using sand units, grain size and/or microfossils such as diatoms. However, tsunamis do not always leave such visible evidence in their deposits. Therefore, to characterize the tsunami inundation area for the cases without such evidence, geochemical compositions have also been applied recently. In this study, C/N and isotopic (δ¹³C) analyses were used to determine sources of organic matter in 2011 Tohoku-oki tsunami deposits collected from 19 coastal areas which range about 500km from north to south. We find that these parameters can be alternatively used to distinguish marine organic matter from freshwater and land plants; we confirm that a marine signature is found in most beach samples, while a signature of freshwater or land plant is recognized in soil samples obtained from unaffected inland areas. Marine signature is found in some of tsunami sand deposits close to the coastline, while the signature of freshwater or land plant is recognized in sand deposits far from the coastline. This finding suggests that marine-affected sediment was not transported to the proximity of the limit of the inundation. Marine signature is typically found in paddy field and marshy areas but not in forest or grassland, which would have supplied a large amount of land plant that affects the source of the organic matter in a tsunami deposit.