樹木樹幹内の<14>^C濃度

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The rapid increase of atmospheric ^<14>CO_2 was caused by the nuclear weapons tests since 1950. The correspond increase of ^<14>C concentration was detectable in tree stems. Because the nuclear weapons tests were mainly performed at mid- and high-latitudes in the Northern Hemisphere, the atmospheric ^<14>C concentrations in the Northern Hemisphere had latitudinal dependence between 1954-1967 (Nydal et al., 1983), but those in the Southern Hemisphere were independent of latitudes (Rafter, 1965). Nakamura et al. measured the ^<14>C concentrations in a Hinoki from Japan with annual rings and in a Seraya from Malaysia with no annual rings, and showed the process to presume the growth rate of the Seraya in accordance with the ^<14> C concentrations in the Hinoki. In the Seraya with a diameter of 90cm, the ^<14>C concentration of the wood section at a distance of 110mm from tangential surface of xylem showed the maximum value (Nakamura et al., 1987). In this study the ^<14>C concentrations in the tree stems grown in the Southern Hemisphere were shown. And the latitudinal dependence of the ^<14>C concentrations in trees at various latitudes was discussed. Using excess ^<14>C as a tracer, the process to presume the time when a section had been formed in a tree with no annual rings was investigated. Two Teaks (Tectona grandis) from Indonesia and a Lingue (Persea lingue) from Chile were used as samples. The ^<14>C concentrations in their annual rings were measured with the Tandetron Accelerator Mass Spectrometer, and the variations of their concentrations vs. years were investigated. The data by these measurements were compared with the ^<14>C concentrations in a Hinoki from Japan (Nakamura et al., 1987) and in the atmosphere (Nydal et al., 1983,Rafter, 1965). The results are as follows. (1) The ^<14>C concentrations in the trees which had grown south of Lat. 7°S. were independent of latitude and remained constant. (2) The ^<14>C concentration of a section in a tree on earth is nearly equal to that in the atmosphere at the latitude where the tree grows and at the time when the section is formed. (3) The time when the ^<14>C concentration in trees had shown the maximum values was different to the latitudes where the trees had grown. (4) It seems that the latitude where a tree had grown should be taken into consideration when the process to presume the growth rate in a tree with no annual rings is used. On the basis of this point, in a Seraya, the section at a distance of 110mm from tangential surface of xylem seems to have formed at the later term of 1964. The result showed the growth rate was about 5.6mm/yr. after 1964.
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樹木樹幹内の<14>^C濃度

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