| 摘要: | 本研究對鑽採自宜蘭梅花湖湖積物岩芯MHL5-A進行環境磁學分析，其目的在探討台灣東北部宜蘭地區古氣候和古環境變遷情形。並盼對東亞季風系統之消長有進一步之了解。此項研究工作為中央研究院地球科學研究所主題計畫「亞洲古環境變遷」工作之一部份，該計畫目的在對東亞地區晚更新世以來古氣候及環境變遷作系統性的研究。 台灣位於東亞地區，氣候受來自西伯利亞高氣壓冬季季風和中國南方印度洋之低氣壓夏季季風交互影響，宜蘭在台灣東北部，因中央山脈阻擋西南季風，同時因喇叭狀的地形，地處迎風面，因此降雨現象顯著，主要為秋冬季因東北季風引起之豪大雨以及夏季颱風帶來大量降雨。本研究希望藉由分析包括磁感率、自然殘磁、逆磁滯殘磁、等溫殘磁等磁學參數，探討研究地區之古氣候和古環境變遷情形。 岩芯年代模式之建立係根據所選擇之七個C-14定年點，結果指出28.63公尺長之岩芯涵蓋過去5200多年來之訊息。而磁學參數(包括ARM、S-ratio及ARM/SIRM等)量測分析所得之結果指出，過去5200多年來之氣候變遷可劃分為20個時期，而氣候變化型態可歸納為四大類，分別如下：(1)約5300 ~ 5000、4850 ~ 4500、4300 ~ 4100、3700 ~ 2950、2500 ~ 2300 1950 ~ 1850、1700 ~ 1400、1100 ~ 700、500 ~ 400 yr BP: S-ratio變化較小，大部分介於0.9 ~ 1，以低氧化性磁鐵礦為主，氣候可能較穩定，乾濕變化不大，整體來說較潮濕；(2)約5000 ~ 4950、3850 ~ 3700、2950 ~ 2500、1850 ~ 1700 yr BP: S-ratio可低至小於0.8，高氧化性赤鐵礦也較多，磁性礦物粒度較粗，氣候可能較乾燥，一旦暴雨發生，會帶來較粗顆粒的磁性礦物和較多的赤鐵礦;(3)約4950 ~ 4850、4500 ~ 4300、4100 ~ 3850、2300 ~ 1950、1400 ~ 1100、700 ~ 500 yr BP: S-ratio值在0.8 ~ 1之間變化快速，磁性礦物的豐度也變化較其他時期快速，顯示當時氣候可能很不穩定，乾濕變化較其他時期快速，但乾燥的程度沒上一種氣候嚴重，當時可能降雨頻繁，降雨結束氣候又逐漸乾燥;(4)到近期400年來，乾濕變化大，但比上一種氣候的變化慢，而且似乎有100 ~ 150年之週期變動，與早期不同的地方是，細顆粒磁鐵礦較多，以及當具殘磁的磁性礦物較多，磁鐵礦粒度卻較細，此情形發生四次，或許是鄰近羅東溪水氾濫帶來細顆粒磁鐵礦;反之，較粗，而且順磁性物質較多，可能是乾燥時周圍岩屑崩落帶來順磁性物質。總而言之，宜蘭地區於晚全新世的氣候環境變化劇烈。 The goal of this research is to investigate the characteristics of the paleo-climatic and paleo-environmental changes in Ilan, NE Taiwan by applying environmental magnetic analyses to a lacustrine core MHL5-A, drilled from Mei-Hwa lake in Ilan, and expect further to understand the evolution of the East Asia monsoon system. This study is a part of an integrated project, named as 「Asia Paleo-environmental Changes」, held by Institute of Earth Sciences, Academia Sinica. The purpose of this project is to systematically study the paleo-climatic and paleo-environmental changes since late Pleistocene in East Asia. Taiwan is located at the intersection of the East Asia. Winter monsoon generated from Siberia high atmospheric pressure and the summer monsoon originated from the Indian Ocean. Nonetheless, the heavy rainfalls were generally brought mainly by the NE winter monsoon and the typhoons during summer and autumn, but not the summer monsoon. This is due to the high Central Range stems the SW summer monsoon and horn shape topography faces the NE winter monsoon in Ilan. By measuring magnetic proxies, such as magnetic susceptibility (χ), natural remanent magnetization (NRM), anhysteretic remanent magnetization (ARM), and isothermal remanent magnetization (IRM) etc., we hope to probe into the patterns of the paleo-climatic and paleo-environmental changes in the study area, Ilan. The age model of the core was established by 7 C-14 dating data. The results lead to totally of about 5200 yrs covered by this 28.63 meters core. And the climatic condition could be concluded into 4 types by the variation trends of the used magnetic proxies, major by S-ratio, ARM/χ, ARM/SIRM, etc. Twenty episodes then could be compartmentalized for the last 5200 yrs in Ilan. The appearance and characteristics of these 4 climate patterns are summarized as follows: (1) During 5300~5000, 4850~4500, 4300~4100, 3700~2950, 2500~2300, 1950~1850, 1700~1400, 1100~700, and 500~400 yr BP: The values of S-ratio varied small and almost ranged in 0.9~1. This signifies low-oxidized magnetite is the major magnetic mineral contained in the sediments. The climate then could be steady and wet. (2) During 5000~4950, 3850~ 3700, 2950~2500, and 1850~1700 yr BP: The values of S-ratio were almost lower than 0.9, even some were lower than 0.8. Besides the grain size of magnetite is relatively coarser. This indicates that high-oxidized hematite is the major magnetic mineral in the sediments. The climate then could be dry. Surface soils may have highly weathered and oxidized to form much more high-oxidized hematite. As the torrential rain happened, hematite with coarser grain-sized magnetite could be transported from the upstream into the lake. (3) During 4950~4850, 4500~4300, 4100~3850, 2300 ~1950, 1400~1100, and 700~500 yr BP: The values of S-ratio changed frequently between 0.8 and 1. The abundance of magnetic minerals also altered frequently. The climate in these episodes could be very unstable and rainfall happened frequently. (4) Till the last 400 yrs, the climate also changed frequently, but not so fast as the climate condition in (3). And it seems had 100~150 years’ period variation. Moreover, the most different thing from previous episodes is that relatively much finer grain-sized magnetite were conducted into the lake and it has happened four times. The delivered mechanism could be due to the floods of nearby Lo-tung River, which induced much fine grain- sized magnetite overflow into the lake. Between the occurrences of each flood, the climate might gradually become dry. So there were less magnetic minerals inputted into the lake. But still some paramagnetic minerals were eroded into the lake. In conclusion, the paleo-climatic and paleo-environmental changes severely in Ilan since the late middle Holocene. |
