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Title :洞窟内ラドン濃度の季節変動要因と娘核種の鍾乳管年代測定への応用に関する研究
Title alternative :A Study of Seasonal Variations in Radon Concentration in Cave Air and the Application of Daughter Nuclides to the Dating of Straw Stalactites
Authors :棚原, 朗
Authors alternative :Tanahara, Akira
Issue Date :Mar-2010
Abstract :Due to karst topography, the southwest islands including Okinawa are characterized by limestone caves. Much of our ability to understand the cave ecosystem depends on knowledge of the spatial and temporal distribution of the meteorological aspects of cave climates. The micrometeorology of the Okinawa limestone caves was examined by assessment of long-term continuous measurements of ^<222>Rn, along with cave internal and external air temperatures, and moisture content. Attempts were made to date young speleothems (<1000y) using the ^<210>Pb produced from ^<222>Rn in cave air. ^<222>Rn concentrations measured using the activated charcoal method from July 1990 to January 1993 at the Gyokusen-do limestone cave on Okinawa varied from 10 to 3000 Bq/m^3 and showed significant and consistent seasonal variations: higher in summer and lower in winter. Concentration variations are controlled mainly by the exchange of air between external and internal cave air. During the winter, high density external cold air is blown in, exchanges with the cave air, and dilutes the radon. In the summer, when the external air is warmer than the cave air, the cold cave air stagnates and ^<222>Rn accumulates. To evaluate the variation details, measurements were taken at one hour intervals using a CsI(Tl) detector connected to a data logger at both the Gyokusen-dou cave and the Fere-dou cave for a period in excess of one year. Mean ^<222>Rn concentrations in the two caves were 1,330 Bq/m^3 and 2,280 Bq/m^3, respectively. In the Gyokusen-dou cave, the ^<222>Rn concentration is clearly associated with seasonal differences in air density between the outside and the inside of the cave. The same effect seems to occur over shorter periods as well. However, the changing radon concentration pattern does not synchronize with air density difference patterns at the site. The results of statistical analysis show that outside air takes about 8~18 hours to reach the measuring point of the radon in the cave. The average air flow velocity from the site to the exit was estimated to be 0.52~ 0.23 m/min. In the Fere-dou cave, the ^<222>Rn concentration in the long run varied from 264 Bq/m^3 to 15,280 Bq/m^3, showing the same seasonal variation as the Gyokusen-dou cave. The dominant factors influencing the ^<222>Rn variation in this cave were air density differences, wind direction and precipitation (rain). The excess ^<210>Pb dating method was applied to two young straw stalactites. The ^<210>Pb profiles in the matrix of the straws exhibited an exponential decrease with distance from the tip and gave longitudinal growth rate values of 2.2 and 5.9 mm/y. The excess ^<210>Pb is distributed uniformly over the outer and inner surfaces of the straws. Such an excess of ^<210>Pb is presumably produced from the airborne ^<222>Rn which exists at high concentrations in the cave air and the ground water running through the central channel of the straws. During the growing process of the straws, a fraction of the ^<210>Pb will be adsorbed on the tip of the straw, and be incorporated into the matrix during the re-precipitation of CaCO_3.
Type Local :学位論文
Publisher :棚原朗
URI :http://hdl.handle.net/20.500.12000/18549
Grant id :18001理工論第16号
Date of granted :2010-03-23
Degree name :博士(理学)
Grantor :琉球大学
Appears in Collections:Doctoral Dissertation (Graduate School of Engineering and Science)

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