台灣東部地區和平溪與新武呂溪流域的風化作用在時空上的分佈關係

Abstract

本研究主要藉由和平溪和新武呂溪兩集水區流域於2012年至2016年之河水化性，以探討風化作用在時間和空間上之分佈關係，並且說明其可能影響的外部環境因子。台灣位處於亞洲季風氣候區，乾濕季節分明，造成不同程度上的化學風化作用，透過量測河水當中之溶解態離子，包括Na+、K+、Mg2+、Ca2+、SO42-和Cl-，經過分析之後發現，兩集水區流域具有明顯的差異性。而由溶解態離子進一步探討兩集水區流域當中，矽酸鹽類和碳酸鹽類等兩大主要鹽類之風化速率，在乾季時，兩流域之溶解態離子主要由矽酸鹽類所貢獻，濕季時碳酸鹽類貢獻比例會明顯增高。在和平溪流域方面，矽酸鹽類於濕季時平均風化速率為32.88 t/km2/yr；碳酸鹽類於濕季時平均風化速率為86.33 t/km2/yr。在新武呂溪流域方面，矽酸鹽類於濕季時平均風化速率為69.53 t/km2/yr；碳酸鹽類於濕季時平均風化速率為273.68 t/km2/yr。雖然兩流域之矽酸鹽類和碳酸鹽類的風化速率，濕季皆較乾季時高，但是在新武呂溪流域之差距明顯較和平溪流域大。 另外，在矽酸鹽類和碳酸鹽類的化學風化過程當中，會消耗掉大氣當中的二氧化碳，和平溪流域所消耗之平均二氧化碳率約為23.9 mole/km2/yr，新武呂溪流域則約為77.7 mole/km2/yr，兩者於濕季時皆較乾季時高，主要為濕季時之高流量所導致。在外部環境因子方面，主要包含輸砂量和輸砂濃度，除了矽酸鹽類以及碳酸鹽類之風化速率與輸砂濃度變化呈現正相關之外，由輸砂濃度所評估之物理侵蝕速率也影響著兩大主要鹽類之風化速率，在和平溪流域的部分，物理侵蝕速率直接或間接增強了化學風化速率，但在新武呂溪流域的部分，在物理侵蝕速率超過10000 t/km2/yr的情形之下，可能會抑制化學風化作用的進行。透過上述之分析結果，能讓我們更了解風化作用在自然界中的進行程度以及思考有其他影響因子的可能。

This study conducted a comprehensive investigation on weathering along two catchments in Taiwan during 2012 to 2016 for the estimation of spatial diversity and the relation to environmental factors. In Taiwan, the Asian monsoon is a major climate system which could cause a disproportionate amount of chemical weathering. The water hydro-chemical experiments were carried out along Heping and Sinwulyu catchments in the north and south part of eastern Taiwan, respectively. Water chemistry showed different patterns between two catchments with Na+, K+, Mg2+, Ca2+, SO42- and Cl- determining the major ion composition. Besides, based on regional lithological distribution, chemical weathering sources were dominated by the decomposition of silicates and carbonates. Along Heping catchment, the weathering rates of silicate and carbonate were 32.88 t/km2/yr and 86.33 t/km2/yr. In contrast, the weathering rates of silicate and carbonate were 69.53 t/km2/yr and 273.68 t/km2/yr along Sinwulyu catchment. Although the weathering rates during dry seasons were lower than which during the wet seasons, the gap was still higher along Sinwulyu catchment. The CO2 consumption rates by chemical weathering along Heping and Sinwulyu catchment were higher during the wet seasons with average of 23.9 mole/km2/yr and 77.7 mole/km2/yr, implying that the discharge played as a significant role. On the other hand, variations in the rates of chemical weathering show clear correlation with hydrological property including sediment yield and total suspended solid. The weathering rates of silicate and carbonate along two catchments were proportional with the total suspended solid. In addition, clearly physical erosion dominated the chemical weathering along the catchments in two different ways. Along Heping catchment, the findings suggested that physical erosion did indeed increase with chemical weathering. However, along Sinwulyu catchment, the chemical weathering rate would gradually decline when the physical erosion rate was up to 10000 t/km2/yr. All information let us understand the magnitude of weathering and realize that further factors will have to take into account between physical erosion and chemical weathering process.