大屯火山群地熱區中土壤氣體之二氧化碳通量

Abstract

二氧化碳是除了水蒸氣以外溶解於岩漿中最大量的氣體，且在岩漿上升的過程中會率先釋放至大氣中，因此我們可以將其逸氣通量的大量增加視為岩漿上升的指標。在火山地區，不論火山處於活躍或休眠狀態，只要有岩漿活動，就會有大量的二氧化碳釋放到大氣中；不只從噴氣口釋放至大氣、也藉由擴散作用從周圍的土壤逸散至大氣中。因此可以利用觀測火山地區土壤二氧化碳的釋放量，來監測當地可能的火山活動，如此更可讓科學家在較安全的距離內監測火山活動。 本研究以密閉式氣罩法在大屯火山群地熱活躍區進行土壤二氧化碳逸氣通量的量測、並估算其總排放量。測量結果分別為：焿子坪地區為537 g m-2 day-1、大油坑地區為122 g m-2 day-1、四磺坪地區為425 g m-2 day-1、以及在大屯自然公園地區為24.6 g m-2 day-1。以上結果顯示，地熱活動較為活躍的大油坑地熱區之土壤逸氣通量卻較其他地熱區低，可能的解釋為大部分的火山氣體已經由孔隙度較高的噴氣孔逸散至大氣，而造成土壤逸氣通量與噴氣孔逸氣通量為互補的觀察結果。進一步估算大屯火山地熱區之二氧化碳總排放量約為113 t day-1 ，相較於世界上其他火山地區，本區的總排放量較低；不過本區的總逸氣通量卻是與其他活火山有相同的數量等級，造成此一現象初步推測是因本地區地熱活動面積較小之故。 本研究亦設置了火山土壤氣體連續監測站，利用土壤氣體中二氧化碳含量變化，配合其它觀測結果來監測大屯山地區可能之岩漿活動。經過有系統之土壤氣地球化學探勘後，在七星山亞群火山噴氣活動較為活躍區域的小油坑地熱區，選擇適當地點設置了土壤氣二氧化碳連續監測站。小油坑監測站自2008年06月20日運作至今，在這期間二氧化碳通量介於0.012 至2130 g m-2 day-1。結果顯示，土壤中二氧化碳通量明顯受到降雨量影響，在降大雨初期逸氣通量會突增，隨後因降雨改變土壤的滲透率造成逸氣通道不順而使逸氣通量下降。排除降雨影響外，逸氣通量亦有明顯變化，此大幅之通量變化有可能與當地之地熱活動有關。為了能更清楚了解大屯火山的地熱活動情形，於2009年底在焿子坪地區增設了另一座土壤氣體監測站，以利後續同步監測、及資料之解釋。

Carbon dioxide is, after water vapor, the most abundant gas dissolved in magma. CO2 was discharged into atmosphere from not only craters but also volcanic area soil; moreover, CO2 and volcanic activity are in close correlation. Thus, the soil CO2 flux variation in volcanic area has been considered as a useful tool to investigate the volcanic activity in a safe distance. In this work, we have measured the soil CO2 flux in representative geothermal areas of Tatun Volcano Group (TVG) and further discuss its spatial and temporal variations. In this study we measured the soil CO2 flux in the representative areas of TVG by closed-chamber method. Soil CO2 flux can be obtained ca. 537 g m-2 day-1 at Geng-tze-ping (GTP), ca. 122 g m-2 day-1 at Da-you-keng (DYK), ca. 425 g m-2 day-1 at She-haung-ping and ca. 24.6 g m-2 day-1 at Tatun Natural Park, respectively. The results show that the emission flux of soil CO2 at DYK is much lower than the value of GTP and LHK. It could be explained that most CO2 gas can be released to the surface through the highly permeable conduit/pathway (fumaroles) at DYK and hence, less emission flux of soil CO2 can be observed. Furthermore, we can estimate the total amount of 113 t day-1 of soil CO2 in the geothermal area of TVG. It is close to the values from other active geothermal areas in the world. The Hsiao-You-Keng (SYK) area was chosen for continuous monitoring of soil CO2 flux. The station, located about 50 meters away from the major fumaroles, is equipped with two CO2 sensors covered and protected with plastic box, and then can be used for long term monitoring under corrosive environments. The system has continuously worked from 2008/6/20 until recent; during the monitoring period, the soil CO2 flux is from ca. 0.012 to 2130 g m-2 day-1. The results of soil CO2 flux showed significant variations and also closely related to local rainfall. The flux apparently reduced after heavy rainfall. This may be explained by the decrease of the soil permeability, and consequently blocked the degassing pathways, due to the heavy rain. However, there is no clear relationship between the flux variation and earthquake activities observed from our data so far. For better monitoring the potential magmatic activity of TVG, another monitoring station was just setup at GTP in 2009.