Distribution of Radium Isotopes in the sea around Taiwan and northern SCS based on gamma spectrometr

• Main Authors: Feng-Chao Yu, 游豐兆
• Other Authors: Yu-chia Chung
• Format: Others
• Language: zh-TW
• Published: 2003
• Online Access: http://ndltd.ncl.edu.tw/handle/94350433600369105034

碩士 === 國立中山大學 === 海洋地質及化學研究所 === 91 === ABSTRACT This thesis was to develop a simplified technique for the analysis of 210Pb in sediment samples with gamma spectrometry. Compared with the results of the conventional beta method, we found that the gamma method yields values that are 1.5 times higher. This discrepancy awaits further investigation. However, the results determined on two sediment-trap samples and a 210Pb source by the gamma technique agree within 10 % to those determined by the same method at the Institute of Earth Sciences, Academia Sinica. Also, using the manganese-impregnated fiber extraction technique, the distributions of 228Ra and 226Ra in surface waters around Taiwan and in the northern South China Sea（SCS） were measured by a high-purity germanium（HPGe） detector coupled with gamma spectrometry. The distribution of 228Ra in the surface sea water off eastern Taiwan was controlled by its source function, horizontal mixing and dilution of the Kuroshio. It was higher in coastal or estuarine areas but much lower farther away from the coast line. The 228Ra/226Ra activity ratio（AR） varied in the same trend. Estuaries and coastal zones in eastern Taiwan were probably the main source for 228Ra in the adjacent sea. Concentrations of 226Ra on transect LY, CC and PN were almost constant. The AR values along the transect CC were lower than unity, because there was no estuarine input and it was entirely occupied by the Kuroshio water. The concentration of 228Ra along this transect was about 30 % higher in summer than in spring, probably because 228Ra was preferentially released from the coastal sediments by typhoon Nichole. Applying a steady-state one-dimensional diffusion model to the LY and PN lines extending from the estuarine coastal areas, we estimated a horizontal eddy diffusion coefficient of about 1.3 × 105 cm2/s for the sea off eastern Taiwan. This value is comparable to that of the Japan Sea determined by Nozaki et al.（1989）. Taiwan Strait（TS） is mainly continental shelf with water depth less than 50 m on the average. Ra isotopes in the surface water were probably supplied by diffusion from the bottom sediments; their distributions were affected by currents and tides, and so no clear trend could be recognized when sampling was conducted in different times. 228Ra was seen inversely correlated with salinity, showing effect of the fresh water. Concentrations of 228Ra and 226Ra along the transect CS decreased away from the estuary during the spring. The 228Ra values along the TS transect in the same season increased toward the northwest, suggesting a possible source from the coastal zone of the mainland. Distribution of 228Ra in the northern SCS was determined consecutively in 2000 and 2001. The 228Ra activities in the northern continental shelf and northwestern Luzon were higher than in the central region during the spring season. The NE monsoon prevails in fall and winter causing the longshore current to transport the shelf water of high 228Ra to the central region. The 228Ra activities of surface water decreased southward and so the central region away from land had low 228Ra values. The low 228Ra concentration observed at the sea off southern Taiwan was probably due to dilution by the Kuroshio. Based on the 228Ra distribution along a transect roughly perpendicular to the 200m bathymetric contour, we calculated a horizontal eddy diffusion coefficient, Kh of about 6.2 × 106 cm2/s, a value lower than that estimated by Huang et al.（1996）in the Nansha sea area. Compared to the Kh values estimated for the northwest Pacific by Yamada and Nozaki（1986）, this value falls in between those within and without 200 km away from land in the Kuroshio region.