台灣由於受到強烈的構造運動以及頻繁的颱風侵襲,不論是抬升速率、降雨量、剝蝕速率乃至於沉積速率皆十分快速而且獨特。颱風所帶來的強降雨使陸域河流系統中的山崩、洪水和土石流等自然災變屢見不鮮,不但造成自然環境的演變,也深深影響著這片土地上的人類活動。2009年的莫拉克風災即為一大型自然災變的現代類比,提供我們地質歷史中的災害線索。 淇武蘭遺址為蘭陽平原近年來發現的重要鐵器時代遺址,介於上、下文化層中間的文化空白,顯示此居住地曾經在西元1200-1500年左右被遺棄,到底是甚麼原因造成此文化空白實是一個有趣的問題。 本研究整合了蘭陽溪上游的階地定年、蘭陽平原上湖泊岩芯的有機物指標以及海洋岩芯的木屑豐度計數,重建過去千年來台灣東北部蘭陽溪流域系統的洪水事件。蘭陽溪上游分佈了一系列的沖積扇階地,反映了過去的土石流事件,整理其碳十四定年結果發現階地發育期集中在西元850-1100以及1400-1600年。而由蘭陽平原上大湖和梅花湖的總有機碳和碳氮比資料,以及由Itrax-XRF岩芯掃描儀所得到的梅花湖有機物指標、光學及X光攝像,顯示陸源物質在西元900-950以及1400-1500年這兩個時段大量進入湖泊沉積物中。位於和平海盆內的OR1-801-7A岩芯也在西元950-1050以及1450-1550年這兩個時段記錄到木屑豐度的極大值,與湖泊岩芯的陸源物質指示之洪水訊號近乎同時。 總而言之,由蘭陽溪流域系統的階地、湖芯和海洋岩芯所重建出的證據顯示,蘭陽溪流域系統在西元900-950以及1400-1500這兩個時期,洪水頻率和規模有顯著升高的狀況,使整個蘭陽溪流域系統從源到匯皆能記錄到洪水事件的斷簡殘篇。其中西元1400-1500年的洪水事件與淇武蘭遺址的文化空白近乎同時,本研究認為遺址的文化空白是因為受到洪水事件的影響,生活條件惡化使淇武蘭先民搬離原先的居住地。
Active tectonic activities and frequent typhoon visits make Taiwan unique in having very high rates of uplift, precipitation, denudation and sedimentation. Particularly, intense rainfall associated with typhoons often causes natural disasters in river systems, such as flooding, large-scale landslides and debris flows, affecting human activities at the present time and in the past. Typhoon Morakot in 2009 may serve as a modern analog of such events in the geological past. Site Kiwulan is a newly discovered archaeological site of the Iron Age in Lan-Yang Plain. A cultural hiatus, found around 1200-1500 cal. yr AD, suggests that the settlement was once abandoned. It remains a mystery what caused this abandonment. This study assembles radiocarbon dates of upland river terraces, organic proxies in flood plain lake sediments and content of wood shreds in nearby marine sediment core. These records are synthesized to infer the frequency and magnitude of ancient floods over the past millennium in the Lan-Yang River system in northeastern Taiwan. Alluvial fan terraces distributed along the banks of upper Lan-Yang River are considered to be remains of ancient debris flow events, and their radiocarbon dates fall in two time ranges: 850-1100 and 1400-1600 cal. yr AD. Organic proxies such as TOC and C/N ratio representing terrestrial plant input were measured from bulk sediments of Lake Dahu and Lake Meihwa in the Lan-Yang Plain. Peak values of TOC, C/N ratio and organic indicator plus optical and radiographic images from ITRAX-XRF core scanner are conspicuous during 900-950, and 1400-1500 cal. yr AD, implying frequent flood events. Abundance peaks of wood shreds in marine box core ORI-801-7A occurred during 950-1050 and 1450-1550 cal. yr AD, coinciding with those terrestrial input events recorded in lake sediments. In summary, different lines of evidence collected from the Lan-Yang River system suggest that flood events were more frequent during two particular periods: 900-950 cal. yr AD and 1400-1500 cal. yr AD. The later period corresponds to the cultural hiatus at Site Kiwulan, suggesting that the lost civilization may be related to severe and frequent flooding of the Lan-Yang Plain during that period.