菲律賓中部樹輪及塔奧湖岩心AMS 14C 定年及地球化學研究

Abstract

塔奧火山(Taal Volcano)位於菲律賓中部，為世上十六個最為活躍的火山之一，自1572年以來超過30次的噴發紀錄。近年來塔奧火山仍處在活躍狀態，引發的地震、地表破裂及新的熱泉不斷生成，加上在湖岸周圍密集人口活動，監測其活動實為重要的議題。本研究藉由分析水樣、沉積物及樹木樣品全面性地了解塔奧湖系統地球化學特徵，並從中探討可用於監測塔奧火山活動的地球化學指標。 在柚木(Tectona grandis)及湖泊沉積物的AMS 14C定年結果中觀察到：(1)菲律賓地區確實記錄了1950至1970年陸上核子試爆之趨勢；(2)在全纖維素與原木結果中我們觀察到，在核爆峰值年前原木結果略高於全纖維素，反之則情況相反，此一現象為樹木在生長過程中吸收並利用大氣CO2形成自身結構時的先後順序造成些微的差異，且柚木生長為連續性生長，並未如溫帶樹種有其明顯休眠期而使此現象較為明顯；(3)從核爆曲線對比中發現目視數輪上未判識的生長輪邊界及偽輪，因此藉由碳十四定年法提升了樹輪年代學之精確度；(4)由沉積物中植物殘體測年看到核爆曲線之紀錄，並與柚木結果比對，將其深度與年代對應，推得此120 cm長之岩心約有60年之沉積，推估其線性沉積為2.04 cm/yr，獲得此岩心之年代序列。從Taal湖水樣及表層沉積物分析結果觀察到：(1)元素分析中Fe、Mn、Cu、Zn、Ba或能作為指示火山活動的指標；(2)同位素分析結果觀察到塔奧湖系統為地熱流體、天水及海水三者的混合結果。最後分析重力岩心之稀酸可溶相中元素特徵並搭配其年代資料，對比1940至2006間已知且規模較大的歷史噴發紀錄，對比結果顯示，稀酸可溶相中Fe、Mn、Cu、K、Ba這五項元素在歷史事件中有較為明顯的異常紀錄，或能作為火山活動之指標。 在AMS碳十四定年分析中提升了樹木年代學之精確度，並填補了菲律賓地區樹輪年代學之空白，提供了高解析度的年代資料做後續發展。在利用大氣核爆14C作為近代湖泊沉積岩心定年的方法尚屬首次嘗試，能夠給予較為可信的岩心年代。現階段成果中觀察到五種元素或能作為塔奧火山活動之地球化學指標，然而對於上述指標在反映火山活動的靈敏度並未著墨，因此仍需更進一步的研究作探討。

Taal Volcano is an active volcano in the Philippines and is one of the 16 monitored volcanoes by the Global Volcanism Network in the world. Historic documents show that the volcanic eruption of Taal has been more than 30 times since 1572 C.E.. In this study, we attempt to find out geochemical proxies of volcanic eruption and to reconstruct volcanic activities recorded in the Taal Lake sediments during the past 60 years. In order to demonstrate the nuclear bomb 14C curve in the studying area, we attempt to reconstruct the nuclear bomb 14C curve in nearby tree rings and Taal Lake sediments. The AMS 14C dating on tree rings of Tectona grandis taken near the Taal Lake in 2011 and a 120-cm long gravity core (Core TLS-2) retrieved from 15-m water depth of Taal Lake indicates that: 1) The nuclear bomb 14C pulse recorded in the Tectona grandis growth rings is the same as other published tree-ring records in the tropical regions. 2) The 14C values of the holocellulose were higher than these of whole wood from the same rings when the atmospheric bomb 14C was rising (1956-65 C.E.); but the situation was reversed when the atmospheric bomb 14C was dropping quickly after 1965 C.E.. This is because the holocellulose in the tree was formed in the early season and the whole wood was developed in the entire year. 3) By comparing the 14C curve of the tree ring with the atmospheric bomb 14C curve, we found a fake ring and three missing rings in the tree ring counting, which helped the tree ring chronology. 4) AMS 14C dates on plant remains at different layers of the core show that the nuclear bomb 14C pulse was recorded in the sediments of Core TLS-2, but the magnitude of 14C values is lower than that of the tree-ring record due to mixing and dilution effects. 5) The comparison of the bomb 14C curves between the tree ring record and a Taal Lake sediment core TLS-2 allows us to build up the chronology of the 120-cm long Core TLS-2 which has an average sedimentation rate of 2.04 cm/yr. Investigations on the water and modern sediment samples of Taal Lake show that: 1) Fe, Mn, Cu and Zn may be used as indicators of volcanic activity. 2) Isotopic composition of Taal Lake bias from MWL indicates mixing of isotopically heavy geothermal fluid and isotopically light surface input. 3) For Core TLS-2, the δ18O and δ13C of Total Inorganic Carbon (TIC) co-vary in general. 4) The anomalies of high concentrations of Fe, Mn, Cu, K, Ba in the acid-leachable phase around 1991-1994 may correspond to high activity of Taal Volcano during this interval. These proxies may be considered as indicators of geochemically monitoring volcanic activity for long-term prediction.