以湖泊沉積物內花粉與矽藻重建台灣東部晚全新世氣候與環境變遷

Abstract

湖積物內的花粉與矽藻為重建古陸域環境變遷的良好指標。藉由花粉與矽藻類群的演替得以重建古時植群演替與水質狀態，並推論氣候變遷。為重建台灣東部全新世的自然史，本研究選用花蓮鯉魚潭、宜蘭翠峰湖與宜蘭大湖的湖積物進行分析。 花蓮鯉魚潭為位在花東縱谷無河流流入與流出的低海拔湖泊，其658公分長的岩芯LYHL-B約包含7020年以來的紀錄。湖相沉積物自2480 cal BP開始出現，在2480-2090 cal BP大量保存的水生植物花粉顯示溼地擴張，隨後在2090-1500 cal BP，河流指標聚藻花粉與溪流型矽藻Fragillaria construens 和Cymbella的穩定存在顯示長期存在的淺水環境。自1500-330 cal BP，河流指標花粉與矽藻的顯著降低，顯示此時沒有河流流入鯉魚潭。從850 cal BP開始，農耕禾本科與碳屑的共同存在，則反應農業活動增加。近330年來降雨指標蕨類孢子與優養指標矽藻Discostela stelligera的增加，指示較多的季風降雨。 宜蘭翠峰湖為中高海拔湖泊，其55.5公分的岩芯TF-2紀錄了1460 年來的環境變遷。最初氣候較為乾燥溫暖，湖水水質處於酸性寡養的狀態。自630 cal BP （AD 1320）鐵杉花粉增加與花粉類群改變顯示氣候變得較為涼爽，可能與小冰期的開始有關。在小冰期初期氣候維持乾燥，但自470 cal BP（AD 1480）嗜鹼矽藻類群、沉積物土壤輸入與侵蝕度的同時增加，顯示季風逐漸增強所帶來的大量降雨。近百年來花粉通量的下降則反應人類森林開發的事件，而嗜酸性矽藻類群的再度增加則反應酸雨事件。 宜蘭大湖岩芯DH-7B長達3486公分，共包含7750年來的紀錄。在7550-530 cal BP宜蘭區域處河流氾濫時期，沉積物內花粉與矽藻含量少。在沉積物內有較多的花粉或矽藻則反應乾旱事件，環境轉變為溼地狀態。在這段期間共有8次東亞季風減弱所造成的乾燥事件，分別發生在7300-7280、6630-6590、4940、4790- 4770、4140-3690、2040-2010、1000-950、530 cal BP。自2000 cal BP開始，低地植物種類增加與溼地植物棋盤腳的出現，顯示近代蘭陽平原的形成。在310-130 cal BP浮游性矽藻的增加事件，反應東亞季風增強造成降雨增加。穀類花粉與開闊地花粉到近200年才顯著增加，反應人類的農耕活動。 最後我們運用地理資訊系統技術將13個分佈於北部台灣的花粉樣點資料進行1500年來森林覆蓋、濕度與溫度變化的模擬。結果顯示台灣北部區域在1500-1000 cal BP氣候較為涼爽乾燥，在1000-500 cal BP（對應到中世紀暖期）則變得溫暖潮濕。在500-200 cal BP，鐵杉花粉分佈的增加則對應到小冰期氣溫下降的事件。而近200年來鐵杉花粉分佈的減少與蕨類孢子分佈的增加，則指示近代溫度上升與降雨增加的狀態。本結果指示花粉的GIS分析能反應出氣候變遷的狀況，而台灣古氣溫的變化與全球氣候變遷事件中世紀暖期與小冰期有良好對比。

The pollen and diatom preserved in lacustrine sediment are the good indicators for reconstructing the paleo-environment. The successions of pollen and diatom assemblages respectively infer to the vegetation history and aquatic condition as well as elaborate the climate changes. In order to reconstruct the nature history of East Taiwan, the sediments from tree lakes were used namely Liyu Lake of Hualien County, Tsuifong Lake of Ilan County and Dahu Lake of Ilan County. Liyu Lake is a lowland lake situated in Hualien valley, from which a 658 sediment core (LYHL-B) was taken. The sediments covered the record form 7020 cal BP to present. However, the lacustrine sediment appeared after 2480 cal BP. Over 2480-2090 cal BP, a considerable amount of the aquatic plants pollen was found, suggesting an expansion of the swamp. Then, during 2090-1500 cal BP, it was exhibited by stable occurrences of water plant, Myriophllum spicatum and fluviatile diatoms, Fragillaria construens and Cymbella spp., suggesting a persistant occurance of the shallow water environment. During 1500-330 cal BP, a significant destitution of shallow water habitat indicator pollen, in association with lowering in fluviatile diatoms and increase in planktonic species, suggested the possible formation of inland lake condition. After 850 years BP, the co-occurrence of the cultivated Poacese and the charcoal suggest increased agriculture activity. Over last 330 years, increased frequencies of fern spore and euplanktonic diatom, Discostela stelligera, suggested the elevated monsoon precipitation. Tsuifong Lake is a subalpine lake, from which a 55.5-cm sediment core (TF-2) was taken. It covered a time period of recent 1460 years. Over the early period, the climate was warm and dry and the lake was acidic and oligotrophic and remained little changes. A remarkable change to cooler climate was traced back to 630 cal BP (AD 1320), which was corresponding to the time of the Little Ice Age (LIA). It was indicated by the slight increase of Tsuga and changes in pollen assemblages. The climate remained dry in early Little Ice Age. However, the increases of alkaliphilous diatom assemblage, soil input and the erosions after ca. 470 cal BP (AD 1480) was possibly a result of elevated rainfall caused by the intense East Asia monsoon. Over last century, a decline in pollen influx was attributed to deforestation. This was associated with increases dominance of acidophilous diatoms, possibly as a consequence of acid rain. A 3486 cm sediment core of Dahu Lake, a lowland lake, covered a time period of recent 7750 years. Over 7550-530 cal BP, the higher percentages of pollen or diatoms in the sediments indicated 8 traceable drought events, occurred at 7300-7280, 6630-6590, 4940, 4790- 4770, 4140-3690, 2040-2010, 1000-950, 530 cal BP. In ca. 2000 cal BP, the increase of lowland plant pollen as well as the appearance of wetland arboreal pollen, Barringtonia, indicated the possible beginning of the formation of the recent floodplain. Over ca. 310-130 cal BP an increase of euplanktonic diatoms indicated the elevated precipitation event related to East Asia monsoon. Moreover, increases in the amount of cultivated Poaceae pollen and open land pollen in recent 200 year were the indication of agricultural activity over that time. Finally, we synthesized the fossil pollen data from 11 lakes and 2 land cores to integrate the alteration of forest covered areas in northern Taiwan with special respect to changes in humidity and temperature over the last two millennia. Our results suggested that the climate in northern Taiwan was stably cool and dry during 1500-1000 cal BP, but changed to wet and warm during 1000-500 cal BP, which corresponded to the Medieval Warm Period (MWP). In addition, an increased density and dispersal of Tsuga pollen corresponding to 500-200 cal BP was observed, which corresponded to the LIA. In recent decades, a decline in Tsuga pollen density and increased fern spore density indicated an elevation in temperature associated with increased precipitation. The results revealed that the temporal and spatial climate changes could be inferred from the palynological GIS mapping method, and that the fluctuations in temperature over time matched well with the global climate events, including MWP, LIA and recent warming.