3. Investigations of seashore deposits
Coastal topography and seashore deposits are particularly essential basic environmental components, and the grain size of sediment is a predominant factor for the natural life forms inhabiting the coastal area. Accordingly, it is of enormous importance to estimate the process of topographical formation in the coastal areas and to understand the properties of deposits.
The Coast Division conducts research on estimating the grain size, sedimentation process (sedimentation velocity), and sediment sources by collecting and analyzing core samples of sediment in the coastal areas. As an example, the results of investigations and analysis conducted at Banzu Tidal Flat in Tokyo Bay are shown below.
|Figure 1 Satellite image of Tokyo Bay
(courtesy of JAXA)
|Figure 2 View of Banzu Tidal Flat
Figure 3 Stratum sample
(Banzu Tidal Flat in Tokyo Bay)
Figure 4 Example of the results of analyzing the grain size and sedimentation ages
(results of analyzing the stratum sample shown in Figure 3)
*Analysis by Cs-137 (cesium-137) radiometric age determination
Cs-137 is an artificial radionuclide emitted by atmospheric nuclear tests, which did not exist before the commencement of nuclear testing (before 1954). In Japan, the amount of fallout reached the peak in 1963, whereas the influence of the Chernobyl nuclear reactor accident in 1984 can be taken into consideration. Therefore, the sedimentation ages of strata can be estimated based on Cs-137.
Nevertheless, sedimentation ages should be carefully determined in reference to the analysis results of other methods (e.g., the Pb-210 method).