碩士 / 國立中山大學 / 海洋地質及化學研究所 / 87 / In order to delineate the particulate leaching and adsorption processes in the ocean, Pb-210, U- and Th-isotopes were analyzed on three grain-sized fractions of settling particulates collected by sediment traps and those of cores taken from offshore northeast Taiwan and Taiwan Strait. These two processes may be controlled by the chemical characteristics of the elements, the mineral composition and surface area. Thererfore, if the particulate compositions are homogeneous, both leaching and adsorption are more effective with smaller particules due to larger surface area to volumne ratios, and so particulale-reactive nuclides should have higher activities while easily leached out nuclides should have lower activities in fine particles.
Pb-210 acctivities of the particulates increase with decreasing grain size and show largest Pb-210 excess in the fine-grained fraction. This indicates that the effectiveness of adsorption is dependent on the grain-size of the particulates. There is a positive linear correlation between Pb-210 and L.O.I..(loss on ignition), suggesting that the organic matters may enhance the adsorption of Pb-210 by particulates.
Activities of thorium isotopes also increase with decreasing particle size. The fine-size fraction(<32痠) of the settling particles and the cores has Th-232 activities at 2.95 and 3.27 dpm./g, respectively. These values are comparable to that of shale, slate and black schist observed in Taiwan. The coarse size fraction(>63痠) has Th-232 activity at about 2.04 and 1.69 dpm/g, respectively, for the trapped particulates and cores. These values are quite similar to those of the sandstone from Taiwan. Thus different size particles are probably derived from weathered rocks of different mineral composition. Th-228/Th-232 activity ratios are close to 1 from the fraction greater than 32痠, but the ratios are greater than 1(average 1.22) for the fraction smaller than 32痠. Thus, Th-228 produced by Ra-228 decay in the water column is easily scavenged by the fine-grained particules. The particulate Th-228 must have included the portion scavenged from water column in addition to the portion produced by Ra-228 decay within the particles.
Activities of uranium isotopes increase slightly with decreasing grain size. Judging from uanium and thorium isotopic compositions and activity ratios, the trapped particles and sediments are derived from the detrital materials of land origin. If these particles and sediments are derived from the weathered rocks and soils in Taiwan, and if uranium in the particles is of detrital origin, then about 30% of the uranium may have been lost or dissolved into the aqueous phase in transit to the site of deposition. Uranium activities in fine grained sediments are higher than those of medium- and coase-grained sediments as well as those of fine-grained settling particuilates, suggesting that pore water uranium may have been produced and deposited in fine-grained sediments.
The U-238/Th-232 activity ratios are about 0.6 for the shale, slate and black schist from Taiwan. These rocks are sufficiently old so that the Th-230/Th-232 activity ratios should also be about 0.6 since secular equilibrium obtained from this study must have been established. The Th-230/Th-232 activity ratios fall between 0.4 and 0.65, suggesting that Th-230 have been retained in the detrital particles without contributions from scavenging, or even with some loss by leaching. Thus the 涆h-230 excess?is fictitious, being created by dissolution of uranium and retention of thorium in the detrial sediment. The dissolution or leaching of uranium is most effective in the fine-grained particules.
| Identifer | oai:union.ndltd.org:TW/087NSYSU271005 |
| Date | January 1999 |
| Creators | Yating Tsai, 蔡雅婷 |
| Contributors | Y.Chung, 鍾玉嘉 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 62 |
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