臺灣高山崩積層剖面特徵與焦炭年代指示的環境變遷意義

The Profiles and the Radiocarbon Ages of Charcoal in Colluvium as Indicators of Taiwan Alpine Environmental Change

張姍姍(Shan-Shan Chang) ； 齊士崢(Shyh-Jeng Chyi) ； 謝孟龍(Meng-Long Hsieh) ； 林建偉(Jian-Wei Lin)

《地理學報》 70期 (2013/09) Pp. 1-22

https://doi.org/10.6161/jgs.2013.70.01

摘要

臺灣海拔高度2,500公尺以上山區的崩積層剖面，常可見野火形成的焦炭夾於其中。本研究以土層中的35個焦炭樣本的碳十四年代，及崩積層的剖面特徵，討論高山可能的環境演化過程。研究結果顯示，所有樣本均分布在近距今4,500年以內，在距今4,500至3,500年間，樣本出現頻率最高，距今3,500以來，樣本出現頻率逐漸降低。大多數的採樣剖面則具有類似特徵，保存焦炭的是細粒層，下部多是含有多量角礫的粗礫層，其上或再覆蓋多量角礫的粗礫層，或直接覆蓋富含腐植質與植物根系的表土，這應是代表野火發生前、後，地表曾發生較大規模的崩積土加積、增添事件。所有的焦炭樣本年代分布指示著，野火的發生與全新世中期臺灣冰河消融後的氣候狀況無關，而樣本的年代和野外剖面特徵，則反映了高山地區冰河消融後的植被復育與地形作用情形。因此，根據研究結果，本研究將高山地形變遷過程，劃分為三個階段，分別是距今6,000－4,500年前的地形不穩定期、距今4,500－3,500年前的過渡期、距今3,500年前至今的地形穩定期。本研究推測，距今4,500年以前，臺灣高山區仍處於冰河消融而冰緣作用盛行的環境，地表植被覆蓋差，缺乏發生野火的條件，或者即使發生野火，也因地表侵蝕嚴重，無法累積細粒物質以保存焦碳。距今4,500至3,500年間，植被覆蓋程度已較前期佳，且地表化育較多細粒物質，頻率較高的野火事件產生的焦碳，就容易被保存於野火後裸露地表局部侵蝕與再堆積作用形成的細粒層中。距今3,500年以來，焦碳的數量減少，雖然植被生長更佳，但因為地表侵蝕與再堆積事件不明顯或其規模降低，以致於焦炭不容易保存，出現頻率逐漸降低，分布連續性也逐漸變差。也就是，高山崩積層中的剖面特徵和焦炭應視為整體性環境變遷指標。

關鍵字

高山崩積層剖面特徵 ； 焦炭 ； 炭十四年代 ； 環境變遷

並列摘要

The charcoals, which were the remnants of wildfires, were often found in the alpine colluvium of Taiwan. In this study, 14C dating of 35 charcoal samples and the characteristics of colluvium profiles at the sampling sites were used to discuss the possible evolution of alpine environment. Results showed that all samples were younger than 4,500 yr BP.; the highest frequency of dates was in the period from 4,500 to 3,500 yr BP., and the frequency was gradually decline in the last 3,500 years. The colluvium profiles had similar characteristics, the charcoal was always reserved in the fine-grained material layer which was often covered by coarse-grained material layer or the present topsoil, and had the coarse-grained material layer underneath. It revealed that the wildfires were always occurred in the stage of minor material accumulation, and there were always the major material accumulation events took place before and/or after that. All of the dating ages indicated that the wildfires and the climatic conditions were unrelated, and these ages and the characteristics of the profiles both can reflect vegetation recovering and the surface processes after deglaciation. Therefore, the suggested alpine environmental change model depending on geomorphological conditions can be divided into three stages: the first stage from 6,000 to 4,500 yr BP. is unstable and the second stage from 4,500 to 3,500 yr BP. is transition, and the latest stage from 3,500 yr BP. to preset is stable.This study inferred that the alpine environment had been just recovered from glacial environment before 4,500 yr BP., the land surface which covered with rock fragments and sparse vegetation was characterized by the intense periglacial processes. Even the wildfire occurred, the charcoals were easily eroded and hard to be reserved. From 4,500 to 3,500 yr BP., the wildfires increased because of the better vegetation cover and the charcoals were easily reserved by the fine material which removed by the processes of slope wash after wildfire. In the last 3,500 years, the quantity of charcoal was decreased, it was owing to the developed topsoil which reduced the erosion and the charcoals were hard to be reserved as well. Therefore, alpine colluvium profiles and the appearance of charcoal in the colluvium layers can regard as the result of overall environmental changes.

並列關鍵字

alpine colluvium profile ； charcoal ； radiocarbon date ； environmental change

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