| Summary: | 碩士 === 國立臺灣大學 === 海洋研究所 === 101 === Small rivers in Taiwan export large amounts of substances to the ocean each year due to an uneven distribution of rainfall in time and space, and a high degree of weathering in rock of the river basin. As a result, chemical weathering rate of rivers in Taiwan are higher than other rivers in the world, and make Taiwan the ideal place to study properties of chemical weathering. The objectives of this study are to understand the characteristics of chemical weathering in Taiwan rivers and the extent of their impacts on the ocean by studying the major ions dissolved in 11 major Taiwan rivers.
Significant regional and seasonal changes were found in major ions dissolved in the Taiwan rivers. Most of the dissolved ions showed a dilution effect due to the rise of discharge by rainfall. Therefore, the concentrations of the dissolved major ions were high in the dry season and low in the wet season, especially in the southern region. The difference in the ion concentrations is up to 7 fold. The three main mechanisms that affect the concentration of ions in rivers are precipitation, evaporation and rock compositions. The major mechanism influencing the concentration of dissolved major ions in Taiwan rivers is rock composition. The rock composition in Taiwan is mainly silicate rock and contributes to more than 80% of the weathering.
Typhoons are very important for the major dissolved ions in Taiwan rivers. The three important mechanisms that influence the ions in the rivers during typhoon events are the dilution effect, enhanced hydrologic access and sea spray effect. TDS load and chemical weathering rate during typhoon events are both higher than normal. This chemical weathering rate is up to 70 times higher when compared with other rivers in the world. This indicates that typhoon events are a key factor in determining the dissolved ion carried by the rivers into the ocean.
Chemical weathering rate in Taiwan small rivers is estimated to be 711 tons/km2．yr. This chemical weathering rate is up to 30 times higher when compared to other rivers in the world. This result shows that effect of chemical weathering is very powerful in Taiwan rivers and very important to the ocean.
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