Evaluating Soil Dating by Amounts and Activity of Pedogenic Iron on Quaternary Terraces of Taiwan

利用成土鐵的量與活性推估臺灣第四紀階地的土壤年齡

陳尊賢(Zueng-Sang Chen) ； 蔡衡(Heng Tsai) ； 許正一(Zeng-Yei Hseu) ； 蔡呈奇(Chen-Chi Tsai)

《土壤與環境》 15卷1&2期 (2012/12) Pp. 13-31

https://doi.org/10.30058/SE.201212.0002

摘要

作為歐亞與菲律賓海板塊構造碰撞所創建的島弧群的一部分，臺灣有許多河流和海的階地。然而，在缺少絕對的定年與地質年代資訊的情況下，在這些已變形的地表面所得到的資料是有問題的。以前主要依據地貌及成土因子與階地之間的相關性來進行研究，但結果經常不一致與不令人滿意。其它土壤年齡的指標為鐵的比值，包括結晶性成土鐵氧化物與全鐵量的比值〔（Fed-Feo）/Fet〕，以及成土鐵氧化物的活性比（Feo/Fed）。本研究在臺灣地區具有相似土壤母質之四個不同地質環境之第四紀河階地， 採集1 0 個代表性土壤樣體，主要的目的在於描述不同河階地之土壤特性， 以及利用游離鐵氧化物的結晶性比推估這些土壤的年齡。本研究之土壤，依據美國新土壤分類系統可分類為具有濕潤土壤水份境況之氧化土（Oxisols）、極育土（Ultisols）與弱育土（Inceptisols）三種土綱，以及依據世界土壤參比系統（WRB, WorldReference Base for Soil Resources system）可分類為鐵鋁土（Ferralsols ）、強淋溶土（Acrisols）與始成土（Cambisols）；這些土壤常為紅色的、有明顯的黏粒包覆土塊表面的細質地土壤、強酸性以及含有低量有機碳的土壤，這些特性指出這些土壤經歷過強烈的化學風化作用，以及黏粒與鐵離子的明顯洗出與洗入作用。高含量的高嶺石與水鋁氧以及缺少蒙特石礦物，也指出這些土壤有高度的淋洗作用與脫矽作用，降低了可風化初級礦物的數量。在臺灣地區階地中， 裏土化育層（即氧化物層、黏聚層與變育層）的鐵活性比（Feo/Fed）與游離鐵氧化物的結晶性比〔（Fed-Feo）/Fet〕可作為土壤化育的良好指標。依據已知土壤年齡之鐵活性比與游離鐵氧化物的結晶性比所建立的相關性，本研究推估發育自八卦、斗六、竹山與老埤階地之土壤年齡分別為19-202、16、83-97與13 ka（千年），這個結果與臺灣目前對於成土過程與階地生成的瞭解相當一致。為了改進鐵活性比與游離鐵氧化物的結晶性比的相關性，未來的研究需要更多不同地景、母質與年齡的土壤資料。

關鍵字

結晶性成土鐵氧化物 ； 鐵活性 ； 第四紀階地 ； 土壤分類

並列摘要

As part of the island arc groups created by tectonic collision between the Eurasian and Philippine Sea Plates, Taiwan has many fluvial and marine terraces. However, the absence of absolute dating and geochronological information means that extracting information from deformed surfaces is problematic. Previous studies correlated terraces with date based primarily on geomorphic and pedogenic parameters, but the results were usually inconsistent and unsatisfactory. Other indices of soil age include the ratio of iron from crystalline pedogenic iron oxides to total iron [(Fed-Feo)/ Fet] and the activity of the pedogenic iron oxides (Feo/Fed). This study sampled ten representative soil pedons from four Quaternary fluvial terraces of different geological environments in Taiwan with similar parent materials. The objectives of this study are to characterize soil properties of different fluvial terraces and use the crystallinity ratio of free iron oxides to estimate the age of these soils. This study classifies soils as Oxisols, Ultisols, and Inceptisols with udic soil moisture regime based on Soil Taxonomy and as Ferralsols, Acrisols and Cambisols based on the WRB (World Reference Base for Soil Resources) system. These soils are generally red, fine textured with clear clay coatings on ped surface, strongly acidic, and contain low organic carbon levels. These characteristics indicate strong chemical weathering and significant eluvial and illuvial processes of clay and iron in these soils. High kaolinite and gibbsite contents and the absence of smectite indicate high levels of leaching and desilification processes in the soil samples, reducing the amount of weatherable primary minerals. The iron activity ratio (Feo/Fed) and crystallinity ratio of free iron oxides [(Fed-Feo)/Fet] of subsurface (i.e. Bo, Bt, Bw) horizons may serve as a good indicator of soil development in Taiwanese terraces. Based on the relationship of iron activity ratio and crystallinity ratio of free iron oxides established with known soil ages, this study estimates approximate ages of 19-202 ka, 16 ka, 83-97 ka, and 13 ka for soils developed in the Pakua, Touliu, Chushan, and Laopi terraces. These results agree with extant knowledge of pedogenic processes and terrace formation in Taiwan. The correlation between the iron activity ratio and the crystallinity ratio of free iron oxides can be improved if more databases become available for soils from different landscapes, parent materials, and ages.

並列關鍵字

Crystalline pedogenic iron oxides ； Fe activity ； Quaternary terraces ； soil taxonomy