從日月潭井樣物化性質論萬餘年來之環境變遷

Abstract

本研究的取樣對象為台灣中海拔地區的湖泊日月潭。以藉由分析沉積物的物理與化學性質並配合現有研究結果，說明自更新世末期以來的環境變遷，以及可能的地表作用及其影響規模。 湖泊沉積物研究的基本概念為自湖底以垂直向下鑽探的方式取得沉積柱之後，再利用一系列的物理、化學等實驗方法，對於沉積物當中的特定物質之有無，以及組成比例進行量化分析。本研究協同日本金澤大學、大阪市立大學以及神戶大學之研究團隊，以活塞式取樣器於日月潭底泥鑽探出一長約3.8米之沉積柱，以2.5公分為一單位進行水平切割，共計切割出152個樣本。 根據沉積物樣本進行碳-14定年的結果，本研究得以推算不同深度區間每個樣本所代表的年代長度。自更新世末期(14ka)至全新世極盛期(6ka)之間，每個2.5公分的樣本所代表的年代長度約為300年，此後朝向現代呈現指數性下降，直至現代的沉積物樣本每個2.5公分的樣本則代表約為10年不等的期間。本研究共計進行粒徑、顆粒密度等物理性質分析以及有機質、生物矽、碳酸鹽、礦物等化學性質分析以辨明沉積物當中的主要組成物質及其比例。並藉由個別種類的含量多寡及其背後所代表的環境條件，歸納出更新世以來環境變遷的具體概況，並與其他相關研究進行比對以進行討論。 本研究的研究結果為利用物理、化學、定年等分析方法，將沉積物當中各種不同性質，以及各種性質的組成比例進行量化。根據台灣現有前人研究之成果，統合自更新世末期以來的環境變遷概況。將不同變異趨勢以及沉積作用的可能作用年代作為比對的依據。本研究的結果將更新世末期以來，至今共計一萬四千餘年的時間帶，依據沉積物組成比例的差異，劃分出五個不同的階段。以及前人的研究成果，可將自14ka以來的五個主要階段中氣候重建的結果與沉積物分析的結果相互對應。更新世末期(16ka - 11.7ka)、全新世前期(11.7ka – 6ka)、全新世極盛期(6ka – 4ka)、作用於3ka之特殊事件(3.5ka – 2.9ka)、全新世晚期至現代(2.9ka – 1930AD)。 這些階段的劃分除了作為個別不同年代期沉積環境差異的質性說明以外，更可藉由個別參數組成成分差異與氣候條件彼此間的相關性進行歸納。最後再將本研究的判定結果，配合孢粉學、沉積學等相關研究的成果進行整合。 依據本研究的分析結果，除了千年尺度的長時間性，變遷趨勢較為穩定的階段之外，本研究亦自部分樣本的分析結果中，發現沉積物的組成成分，在短時間內有顯著的改變。本研究的研究成果指出這些遽變型事件的發生年代為9.7ka、3ka、2ka、1ka這幾個年代中皆可以找到沉積物組成迅速改變的現象。藉由碳-14定年對於沉積作用發生年代的推算，可以得知其中又以距今3ka所出現的階段為一明確反應短時距環境變遷的案例。推測應為劇烈事件改變地表沖蝕特性所引發的結果。

This study focused on the lakes in mid-altitude area of Taiwan. In particular, we use several different proxies including both of physical and chemical characteristics to assessment possible environmental change and its distribution since late Pleistocene. In addition, we compared with archive data of Paleao-Environmental studies to enhance reliability of the interpretations. Furthermore, this study had supplemented with landforms and surface process as possible genesis and reactions to different environmental change trends. According to the methodology of limnology, this study sampling a core of lacustrine sediment and use physical and chemical analysis to detect characteristics of proxies and quantified for further interpolation. In 2012, this study had collected a 3.6 meter long sediment core vertically to the bottom of Sun-Moon Lake by piston corer which cooperated with Kobe Univ., Osaka City Univ., Kanazawa Univ. and cut into 2.5 cm for further analysis. This study collected a total of 152 samples. According to Carbon-14 dating, this study extrapolated different deposition rate in different intervals of the sediment core. In particular, this study aimed within the interval during 16ka until present. Time resolution of samples between the End of late Pleistocene and Holocene Optimum represents 300 years per sample, and thereafter decreased exponentially toward modern. In the interval of late Holocene to modern, each sample represents only 10 years. Laboratory analysis had been finished by physical and chemical analysis including grain size, grain density, dating, and chemical compositions for further interpretation in the hydrological laboratory in Kanazawa Univ. Our result showed a quantified data set of sediment characteristics to represents the environmental conditions and its changing trends. This study based on the data set for interpretations and discussion with archived data to generalized possible scenes since late Pleistocene. The results of this study had proposed different sub-stages by occurrence of parameters we had analysed. In particular, there are 5 stages since late Pleistocene to present. These stages include 1. the End of Pleistocene (16ka – 11.7ka), 2. Early Holocene (11.7ka – 6ka), 3. Holocene Optimum (6ka - 4ka), 4. 3ka event (3.5ka – 2.9ka) and 5. Late Holocene (2.9ka – 1930AD). In addition, these stages not only as a division of ages, but also defined differences in depositional environment and possible genesis and process acted on the scene. In addition, the further outcome of this study is related with palynology, sedimentology, and other related research results for integration. Preliminary results showed at least different scene of environmental change between each stage. In particular, some stages like long-term scale as millennial stages showed stable trends of environmental change. However, in some special case we can find significant rapid environmental change events are recorded by the quantity change of chemical components in the sediment. This study had proposed several catastrophic events, according to the result of radiocarbon dating, these events are shown in 9.7ka, 3ka, 2ka and 1ka. The most special case had revealed that according to radio-carbon dating, there had occurred a severe event around 3ka that brought massive sediments into the lake. It seems that Sun-Moon Lake’s sedimentation environment is affected during these stages and rapid events.