更新世上次冰期(32-23ka)越南北部石筍中同位素記錄的降水變遷史

Abstract

亞洲夏季季風（Asian Summer Monsoon, ASM）影響亞洲地區數十億人的日常生活，因此，了解ASM還有它連帶造成的影響，有助於各國政府應對未來的氣候變化。近二十年來，鐘乳石成為重建軌道尺度到百年尺度古氣候的重要材料之一，因為其內含的氧同位素（δ18O）可提供高（達年）解析度的陸地降雨訊息，本研究即使用越南北部的鐘乳石來重建ASM在32000年到23000年前的變化。 此研究有三個主要發現，首先，30000年至23000年間的δ18O紀錄（解析度為10年）中有三個千年尺度的事件，其可對比於格陵蘭（Greenland）冰芯中的D–O 2, 3, 4事件（Dansgaard–Oeschoger），而且綜合此區域所有的古氣候記錄，可發現整個東亞及東南亞地區的氣候，大致同步反應了北大西洋的D–O事件；第二，32000年至30000年的高解析紀錄可完整涵蓋北大西洋Heinrich Event 3（HE-3）的完整過程，δ18O值（降雨）於30800年前開始上升（減少），在30300年前達到最大（最少），並維持了約100年，接著，δ18O值開始降低，這代表降雨增加，於29600年前回到HE-3開始前的狀態，同時，雨量減少與增加的這兩個時段的變化是“漸變”，不同於北極冰芯中的“快速增溫然後逐漸降溫“，暗示ASM的變化可能是受海洋改變所致，這樣的結論即支持了以下的學說：在HE事件開始時，北大西洋經向翻轉環流（Atlantic Meridional Ocean Circulation, AMOC）會減弱甚至停止，使得南大洋（Southern Ocean）累積熱能而變暖，當HE事件停止後，AMOC會恢復並將南半球累積的熱量，經由低緯度地區傳回北半球高緯度。 最後，在HE-3時期，碳同位素（δ13C）開始變化的時間晚了δ18O約100年，但結束卻早了約300年，它指示了較耐乾的C4植物，在此時期取代原本的C3植物，成為越南北部的優勢種屬，此外，不論是在δ18O或δ13C的紀錄中皆存在200年的週期，這個數字恰巧等同於太陽活動中的Suess Cycle，因此，暗示了太陽本身的活動會影響短時間尺度的ASM。 以上的發現都使得我們更加了解亞洲夏季季風的變化，也證明了熱帶地區與全球氣候的緊密關聯，這可幫助我們在全球暖化的大框架下，進一步預測此地區未來的降雨變化，以對人類社會做出實際貢獻。

The Asian Summer Monsoon (ASM) and its variations affect the living of billions of people in the Asia region. Understanding ASM variability and its effect are crucial for governments to prepare for climate change. For nearly twenty decades, speleothems have been used as one of a vital nature archives to reconstruct paleo-hydroclimate variation from orbital to centennial timescale levels. The climatic interpretation based on stalagmite stable isotope proxies, particularly the oxygen isotope ratio (δ18O), can provide trustworthy, high-resolution terrestrial records in pair with ice core records on monthly-to-decadal resolutions. In my dissertation, I used speleothems from northern Vietnam to reconstruct the variability of the ASM during 32-23 thousand years (ka, relative to AD 1950) and its behavior which is essential to Asia's economic and cultural development. Three important key findings have been reached in this dissertation. Firstly, a decadal-resolved stalagmite oxygen isotope record retrieved from a cave in Northern Vietnam, corresponding to three millennial Dansgaard–Oeschoger (D-O) events registered in the Greenland ice cores. The oxygen isotope record suggests that the monsoon has responded synchronously across the Asia region to D–O events. Secondly, I obtained a high-resolved speleothem δ18O record, covering the entire Heinrich Event (HE) 3. The δ18O result clearly displays a drying trend during HE-3, which started from 30.8 ka, stayed at a maximum of over 100 years at around 30.3 to 30.2 ka, and fully recovered at 29.6 ka. The onset and recovery periods are characterized by gradually stepping change, depicting a V shape on a millennium scale. It implies the moisture source of Northern Vietnam site, the Indian Summer Monsoon, underwent an intensity decrease during HE-3. The gradual V-shape record also suggests that this monsoonal decrease was probably provoked by the thermal behavior change of the source ocean. It is consistent with the hypothesis that the Southern Ocean was under relatively warm conditions during HE due to shutting down the Atlantic Meridional Ocean Circulation (AMOC), which drives the ITCZ to migrate to the south. Third, relative to the δ18O record, the δ13C excursion commenced 100 years later but ended 300 years earlier. The timing and spatial location of the carbon isotope excursion indicate a temporary decline in C3 plant coverage and expansion of C4 plants at the study site during HE-3. The δ13C record thus documents the resilience of plant communities and associated ecosystems in response to HE-3. Additionally, the author shows evidence of the solar cycle in late Pleistocene speleothem record from Thuong Thien (TT) Cave, Vietnam. The record demonstrated the existence of centennial-scale events, which possibly correlate to the Sues cycle of solar activity, suggesting an essential role of solar activity in controlling ASM. These findings in this dissertation contribute to our understanding of ASM variability and demonstrate the critical plays of the tropical region in the climate system, and therefore, help forecast hydro-climate change of the area in the future.