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

Abstract

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.

Design of a Prototype in-situ Gamma-ray Spectrometer for Deep Sea

“…On the other hand, although HPGe has a better energy resolution than NaI(Tl) scintillator, the disadvantages of the HPGe used in deep sea are high cost and the requirement of external cooling equipment. In these published experiments based on NaI(Tl) crystal in the depth from 110 meters to 6500 meters, the spectrometers usually have an energy range of about 100 keV to 1.8 ∼ 3.6 MeV, spectra of 240 to 2048 channels and a resolution of 7% to 10% [2,3,[7][8][9]. Moreover, only the gamma ray dose is counted without the energy spectrum in some experiments, such as an investigation in the Southern Mariana Trough [10].…”

“…The main radio-nuclides in the deep ocean are 214 Bi at 609 keV, 228 Ac at 696 keV, 137 Cs at 662 keV, 40 K at 1461 keV, 208 Tl at 2615 keV, etc. [2,4,5,7,8]. The sources for calibration are usually 137 Cs, 133 Ba, 60 Co and 22 Na.…”