- 學位論文
臺灣山區冰期堆積物之碳十四年代及其意義
The impact of glacial/interglacial climate changes on fluvial and mass-wasting processes in the Taiwan's mountains
摘要
冰期與後冰期氣候狀態有相當大的差異，而在地形上是否有所差異？本研究根據碳十四定年資料庫中的冰期定年資料進行地形差異之比較。發現三仙臺之冰期沖積扇規模與厚度明顯大於全新世沖積扇，還原冰期沖積扇面積，應大於現在觀察到的數倍到數十倍之多。小林附近支流之數個分水嶺為緩坡階地，應為冰期沖積扇所遺留，根據階地形態，當時楠梓仙溪主流位置比現生主流高。荖濃溪多筆冰期沖積扇與主流階地定年資料產出，時間包含末次冰盛期（LGM）和新仙女木事件（YD）。小林的五里埔階地根據上游坡地的定年，推斷應生成於冰期，並且在楠梓仙溪主流大量堆積。阿里山的坡地由堆積到稜脊的大量崩積物組成，根據定年資料確認為冰期產物，且當時的堆積規模應相當龐大。陳有蘭溪沿岸多處坡地具有冰期之碳十四年代資料，坡地堆積規模龐大且無法觀察到沉積物來源區。利用研究資料中基盤高度推算冰期以來基盤下切速率，發現變動幅度與後冰期以來之數據相比較為穩定（約0 – 9 mm/year）。本研究推論，冰期氣候乾燥，偶有因突發事件（颱風、地震），而造成山崩土石流事件，供應沉積物；但因整體氣候乾燥，河流搬運能力不足，使崩落的岩屑大多堆積在坡地上與主流沿岸。進入後冰期時，氣候較潮濕，頻繁的豪大雨增加河流搬運能力，使冰期的崩積物再次崩移，山崩土石流事件發生，岩屑與強大的水力對河流的快速下切，又使坡地變陡，進而造成更多的山崩土石流事件發生。根據以上資料，本研究認為：（1）無論氣候條件如何，冰期依然會發生山崩土石流事件，供應沉積物。（2）氣候變遷對於河道之搬運能力影響較顯著。（3）探討臺灣山脈的侵蝕除了考慮地殼隆升之外，也必須考慮加入氣候變遷的影響，特別是長時間尺度之下。
並列摘要
The Taiwan orogenic belt, located in Southeastern Asia, is under monsoon climate, frequently attacked by tropical typhoons, and characterized by rapid tectonic uplift with high seismicity. Researchers have been linking the Taiwan’s landscapes to active tectonic uplift. In this study, we show the significance of glacial/interglacial climate changes in shaping the landscapes. We focus on the mountain areas that have never been glaciated. Based on >400 radiocarbon dates (70 of which >12 ka), we find that both the slope and fluvial activities were generally low during the glacial time. Still, extensive alluviation had occurred at certain time periods, forming large debris slopes or alluvial fans (typically along mountain fronts), and causing significant aggradation along some major rivers. In contrast, with numerous landslides and debris flows, river incision has dominated during the postglacial time. Episodic river aggradation with alluvial-terrace development (typically at tributary mouths) also occurred during this time period, but was less extensive than previously. Some huge postglacial alluvial terraces have been proved sourced from the colluviums deposited in the glacial time. We attribute the low landscape activities of the glacial period to the dryness during the period. However, even in this time rare but severe rainfall events must have occurred to trigger some extensive alluviation. In contrast, the increase in both rainfall and typhoon frequency during the postglacial time drastically increased the slope instability and sediment yield. The great stream power, along with the sufficient coarse debris acting as erosion tools, ensured the rapid river incision during this time.