Gamma-Ray Spectral Survey and ¹⁴C Measurements on the Biological Communities at the Subduction Zone Sagami Trough Using the Submersible "SHINKAI 2000"

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Gamma-ray surveys at the giant clam colonies on the deep-sea floor in Sagami Bay in Japan were carried out using an NaI(T1) detector fixed at the side of "SHINKAI 2000." The gamma-ray spectrum was obtained with a multichannel pulse height analyzer. The giant clams get the biological energy from the upwelling fluid coming from the bottom of the sea. Therefore, relatively strong venting of fluid is expected through the colony. The upwelling rate is affected by the crustal activity (e.g., crustal movement, earthquake, and volcanism). The colony is a suitable place for monitoring the crustal activity. A gamma-ray method was useful on the colony area to detect the upwelling rate of fluid.
Relatively strong intensity of the gamma-rays of ²¹⁴Bi, an offshoot of inert gas ²²²Rn, was observed at living clam colony sites as expected. A large difference in the intensity among the colonies was also observed. This was due to the difference in the composition of rocks at the deep part of the site.
¹⁴C concentration on 34 shells collected from the colonies was measured. It was discovered that the data of ¹⁴C concentration in the shell were scattered. It could be understood from the phenomena that ¹⁴C concentration did not correspond to the age of the shell because upwelling carbon compounds, which are the source of the shell, should have no ¹⁴C. Therefore, the ¹⁴C concentration also indicated the activity of fluid upwelling.
The data of gamma-ray survey and ¹⁴C concentration in shells strongly indicated the existence of fluid transportation from the deep part of the seafloor in the colonies. It was proven that the gamma-ray method and the measurement on ¹⁴C concentration in the shell at the colony were useful for estimating the crustal activity.