摘要：化學風化(chemical weathering rate, CWR)除了關係到全球的元素循環(Garrels and Mackenzie, 1971)與維繫生態系運作(Milligan and Morel, 2002)，更是碳循環中重要的影響因素(Berner et al., 1983)。因此，調查並量化化學風化速率，對於了解長時間尺度(百萬年)的地形演變與氣候變遷回饋作用至為重要。直至目前為止，侵蝕作用在台灣一直是國際上熱烈討論的議題(Dadson et al., 2003; Hilton et al., 2008)；然而化學風化的部分卻一直沒有比較完整的調查與討論，除了Li (1976)提供早期台灣河川的總溶解質資料卻沒有詳細的風化速率資料與種類，或是僅有局部河川的研究(Cheng and You, 2010; Chu and You, 2007)。除此之外，近來對於化學風化速率在物理侵蝕相當劇烈的條件下，學界仍存在相當大的爭議，理論上來說，快速的物理侵蝕與破碎的地表地形，能提供大量新鮮的岩石礦物以利風化的進行(Gaillardet et al., 1999)。然而，Gabet and Mudd (2009)利用模式驗證化學風化將會隨著物理侵蝕上升，逐漸達到一個定值，而可能存在著風化極限(Dixon and von Blanckenburg, 2012)；但另一方面，Larsen et al. (2014)研究紐西蘭現地原生土壤發現化學風化的極限可能超過原本的估計與想像，在崩塌主導的高侵蝕區（通常是造山帶）會呈現良好的線性關係。Emberson et al. (2016)，量測了紐西蘭造山帶的集水區河水與崩塌地的地表逕流驗證了這樣的結果，河川中風化後的總溶解質含量與該集水區的崩塌地面積呈現良好的線性關係。台灣所在的環境，由熱帶風暴與活躍的構造活動創造了絕佳研究化學風化的材料，因此本研究將透過全台的河川採樣：(1)詳細報導全台的化學風化種類與風化速率與其相關的二氧化碳消耗速率；(2)釐清高溫多雨與快速物理侵蝕環境下，化學風化速率與控制因子之間的關係。
結果顯示：以全台平均來看，最主要的陽離子是Na+與Ca2+；最主的陰離子為SO42¬-與HCO3 ，平均的TDS濃度為197 mg L-1，平均的pH為8.09。北部地區的TDS濃度平均最低，約105 mg L-1；但西部地區的河流TDS濃度則可達北部地區的兩倍以上，為264 mg L-1，同時pH 8.13也比其他區域高。河水TDS通常是集水區內化學風化強度的指標(Emberson et al., 2016)，例如在紐西蘭北島以及南島，TDS分別為194 mg L-1和64 mg L-1(Lyons et al., 2005)，在菲律賓以火山島弧為主的集水區中，平均的TDS濃度介於184.8-312.1 mg L-1之間 (Schopka et al., 2011)。這樣可能意涵著西部地區有較強的岩石風化作用，供應大量的溶解質進入到河水中，並且提高河水中的pH。而根據不同離子的組成比例可以看出大致上不同區域受到不同端源混合的情形，在陽離子中，北部與西部地區的河川主要是碳酸鹽礦物與長石類礦物混和的結果；但在東部河川，則明顯受到鐵鎂類礦物風化的影響，例如綠泥石與輝石、角閃石類礦物，應與背後的岩性組成有關。另一方面，在陰離子的組成比例中，西部與東部河川明顯受到SO42¬-與HCO3 所主控，高濃度的SO42¬-則與這兩個區域內旺盛的黃鐵礦風化有關(Das et al., 2012)。
經過混和模型與流量的計算，全台平均的化學風化速率為226 t km-2 yr-1，相當於世界平均值24 t km-2 yr-1(Gaillardet et al., 1999)的10倍，凸顯高溫多雨，構造活躍地區的亦具備風化快速且強烈之特質，同時也比岩性組成與構造環境相類似的紐西蘭南島157 t km-2 yr-1高出許多(Lyons et al., 2005)。台灣的化學風化由碳酸鹽風化所主控，平均為179 t km-2 yr-1，相較於平均的矽酸鹽風化速率為47 t km-2 yr-1，多出3倍以上。在北部地區的風化速率較其他區域低，為155 t km-2 yr-1，但矽酸鹽風化速率則較其他區域高，為51 t km-2 yr-1。儘管如此，矽酸鹽風化速率較低的東部低區仍有43 t km-2 yr-1，但碳酸鹽風化速率可達到236 t km-2 yr-1。與碳酸鹽風化相比較，矽酸鹽風化在全島較無明顯的區域差異。而透過化學風化所造成的CO2消耗速率，經過硫酸的修正之後，仍有28.8×105mol km-2 yr-，亦高出世界平均2.46×105mol km-2的10倍有餘。
而化學風化與主要控制因子之間的關係顯示，即使可能受到供應來源的限制，逕流主導了全台化學風化的輸出(R2 > 0.8)。儘管如此，SiO2輸出量與逕流之間的關係顯示台灣的化學風化仍然位在理論的風化極限之下。至於在化學風化與溫度、物理侵蝕的關係上，皆顯現低度的正相關，(溫度R2 = 0.12，物理侵蝕R2 = 0.19)，即便溫度差距小的因素造成與化學風化之間較微弱的關係，但仍可在矽酸鹽風化看到溫度產生的效果。若是進一步考慮抬升速率的影響，則發現化學風化占總剝蝕速率的比例，會隨著抬升速率上升而降低，說明快速的抬升與構造活動同時促進岩石破碎與物理侵蝕，所造成的大量新鮮岩石碎屑，雖然能加速化學風化速率，但其增加率卻減少，加上與化學風化與物理侵蝕之間的低度正相關係，顯現風化能力可能無法與物理侵蝕同步增揚，並達到飽和。化學風化與物理侵蝕間微弱的關係，表示大量新鮮的岩石被帶到下游堆積，雖然上游地區風化能力似乎有達到飽和的現象，但可能也在下游地區創造出另一個絕佳的風化環境，不但矽酸鹽風化速率與坡度呈現中度負相關，比對其他火山岩集水區的資料，這些下游集水區的矽酸鹽HCO3-濃度甚至可以高過玄武岩集水區，值得更進一步研究。
Abstract: Chemical weathering associated with landscape evolution, forestry ecosystem maintenance and carbon sink plays an important role in global geochemical cycle (Garrels and Mackenzie, 1971), nutrient supply to ecosystem (Milligan and Morel, 2002) and the consumption of atmospheric CO2 (Berner et al., 1983). Therefore, qualitative and quantitative understanding of chemical weathering is crucial for deciphering the mystery of long-term evolution of landscape- and weathering-driven climate feedbacks. Taiwan has been remarked as an arc-continental collisional island with extremely high physical erosion (Dadson et al., 2003). Unfortunately, very few researches have been conducted on chemical weathering in Taiwan. Only Li (1976) reported the total dissolved solid (TDS, an index of weathering) and sediment flux around Taiwan. In addition, recently, whether chemical weathering rates (CWR) could follow with increasing erosion rates is still under debate on the condition of an environment with extremely high erosion rates. Physical erosion (PER) representing the removal of surface materials downstream is known to enhance rock exposure and hence accelerate the mineral weathering processes rapidly (Gaillardet et al., 1999). However, Gabet and Mudd (2009) demonstrated that at high denudation rate, chemical weathering rate would reach a maximum and eventually decline, indicating a possible weathering speed limit (Dixon and von Blanckenburg, 2012). On the other hand, Larsen et al. (2014) found that in a plate-margin island with rapid erosion, South island of New Zealand, the weathering limit does not exist. Also, Emberson et al. (2016) found that the weathering rate had a positive correlation with landslide area in South island. Located on a plate margin and effected by typhoon events, Taiwan provides excellent materials, therefore, this study would apply the riverine chemistry for (1) To estimate the sources of the riverine total dissolved solid and then quantify the rate of silicate and carbonate weathering and the associated CO2 consumption. (2) To identify the controlling factors which regulate the chemical weathering rate in Taiwan.
Results showed that the main cations are Na+ and Ca2+ while the main anions are SO42-¬ and HCO3 . The average TDS concentration is 197 mg L-1 with average pH of 8.09. Western Taiwan has 2.5 times TDS higher than Northern Taiwan with pH of 8.13. Note that riverine TDS is widely used for characterizing the chemical weathering (Emberson et al., 2016). For example, in North Island and South Island of New Zealand, TDS were 194 mg L-1and 64 mg L-1 , respectively(Lyons et al., 2005). As a volcanic arc, TDS in Philippe islands ranged from 185 to 312 mg L-1 (Schopka et al., 2011). High TDS concentration and pH in Western Taiwan could represent as strong rock weathering which contributed amounts of dissolved solid to rivers. According to composition of major ions, most rivers in Eastern Taiwan close mafic and carbonate mineral endmembers whereas Northern and Eastern rivers are mainly mixed by felsic and carbonate mineral. Especially SO42¬- and HCO3 are major anions. High HCO3 may imply high rock weathering because both carbonate dissolution and silicate weathering can contribute HCO3 to rivers. High SO42¬- may be related to pyrite weathering (Das et al., 2012).
According to results of the mixing model and flux calculations, the average weathering rate in Taiwan is 226 t km-2 yr-1, which is 10 times higher than the global average of 24 t km-2 yr-1(Gaillardet et al., 1999). It reflects remarkable weathering rates on a condition of active tectonic with warm and moist climate. In addition, it is much higher than weathering rates of New Zealand with similar geologic background (Lyons et al., 2005). The average of carbonate weathering rate is 179 t km-2 yr-1 and dominates chemical weathering in Taiwan. The average silicate weathering rate is 47 t km-2 yr-1 with a mirror range from 43 to 51 t km-2 yr-1. After the correction of sulfate, the average CO2 consumption rate is 28.8×105mol km-2 yr- which is still 10 times higher than the global average of 2.46×105mol km-2.
For controlling factors, temperature is only slightly correlated with silicate weathering (R2 = 0.12) while total chemical weathering reveals the significant hydrological control (R2>0.8). However, the relationship between silica exports and runoff is still a line of thermodynamic calculated according to Maher and Chamberlain (2014). For the controlling factor of physical erosion, although CWR increases with PER (R2 = 0.19), the increasing rate of CWR could not follow with PER. The ratio of CWR to total denudation (CWR/CWR+PER) also decreases with increasing uplift rate, which is related to tectonic activity. It implies that the sediments from eroded bedrock spend little time going to deposits. Most sediments may be unweathered in the downstream areas and flood plain. With appropriate condition of rock-water reaction, the downstream areas in high standing islands could be another “hot spot” of chemical weathering that merits further researches.
