11:00 AM - 1:00 PM
[MIS13-P02] Geochemical characteristics and eDNA analysis of the 2011 Tohoku-oki tsunami deposits at Misawa city, Aomori Prefecture, Japan
Keywords:Tsunami deposit, Geochemical analysis, environmental DNA
In order to understand nature of paleo-earthquake and tsunami from the geological evidence, it is necessary to establish an effective methodology to identify tsunami deposits appropriately. In addition to sedimentological information such as upward-grading and landward thinning trends, geochemical and biological signatures of marine indicators (Ca, Sr, Cl, S, etc.) in the tsunami-affected sediments have been considered to provide important insights for the identification of tsunami deposits. However, these indicators are known to disappear from the coarser tsunami deposits within few months after the deposition or to penetrate down into finer soil below the tsunami deposits due to rain water seepage. It is also reported that the sediment sources of the 2011 Tohoku-oki tsunami deposits were not only the marine sediments from the sea floor, but also the terrestrial sediments from sand dunes. Therefore, geochemical and biological signatures of the tsunami deposits may reflect the characteristics of the mineralogy and biology of source sediments.
In this study, we investigated the effect of sediment source on the geochemical traces of tsunami deposits by XRF core scanner (ITRAX) and mineralogical analyses for the 2011 Tohoku-oki tsunami deposits at Misawa City, Aomori Prefecture. In addition, the environmental DNA (eDNA) metabarcoding method was applied to propose a new reliable method of tsunami deposit identification. In this study, 18S rRNA gene amplification was performed using eDNA from the tsunami deposit.
The collected sample (May, 2021) is composed of plant debris, post-tsunami soil, sandy tsunami deposit, pre-tsunami soil, and beach ridge sand from top to bottom. As a result of ITRAX and subsequent principal component analysis (PCA), the tsunami deposit was characterized and grouped by Ca, Al, Si, K, and Sr; Ca is regarded as a marine indicator in the previous studies. XRD analysis revealed that the tsunami deposit contains not only feldspars with Al, Ca and Na as major elements but also Ca and Mg rich orthopyroxene. This result is consistent with the fact that the magnetic susceptibility values are higher in the tsunami deposit than in the soil layers and beach ridge sand. Therefore, the results of geochemical analysis of the tsunami deposit doesn’t necessarily indicate the presence of marine indicators, but rather it is strongly controlled by mineral composition of the deposit. As a result of eDNA analysis, the tsunami deposit is characterized by high abundance of fungi among samples. Moreover, marine species (Heterocapsa rotundata) and unclassified marine environmental samples are detected only at the post-tsunami soil/tsunami deposit boundary and pre-tsunami soil, while no marine species or environmental samples are detected in the sandy tsunami deposit in genus or species levels, suggesting that marine indicator has penetrated from coarser tsunami deposit to underneath finer pre-soil due to heavy precipitation or that the preservation of eDNA during the tsunami differs between the upper and lower fine-grained soil layers.
In this study, we investigated the effect of sediment source on the geochemical traces of tsunami deposits by XRF core scanner (ITRAX) and mineralogical analyses for the 2011 Tohoku-oki tsunami deposits at Misawa City, Aomori Prefecture. In addition, the environmental DNA (eDNA) metabarcoding method was applied to propose a new reliable method of tsunami deposit identification. In this study, 18S rRNA gene amplification was performed using eDNA from the tsunami deposit.
The collected sample (May, 2021) is composed of plant debris, post-tsunami soil, sandy tsunami deposit, pre-tsunami soil, and beach ridge sand from top to bottom. As a result of ITRAX and subsequent principal component analysis (PCA), the tsunami deposit was characterized and grouped by Ca, Al, Si, K, and Sr; Ca is regarded as a marine indicator in the previous studies. XRD analysis revealed that the tsunami deposit contains not only feldspars with Al, Ca and Na as major elements but also Ca and Mg rich orthopyroxene. This result is consistent with the fact that the magnetic susceptibility values are higher in the tsunami deposit than in the soil layers and beach ridge sand. Therefore, the results of geochemical analysis of the tsunami deposit doesn’t necessarily indicate the presence of marine indicators, but rather it is strongly controlled by mineral composition of the deposit. As a result of eDNA analysis, the tsunami deposit is characterized by high abundance of fungi among samples. Moreover, marine species (Heterocapsa rotundata) and unclassified marine environmental samples are detected only at the post-tsunami soil/tsunami deposit boundary and pre-tsunami soil, while no marine species or environmental samples are detected in the sandy tsunami deposit in genus or species levels, suggesting that marine indicator has penetrated from coarser tsunami deposit to underneath finer pre-soil due to heavy precipitation or that the preservation of eDNA during the tsunami differs between the upper and lower fine-grained soil layers.