稀土元素在時間序列土壤中的分布

Distribution of rare earth elements in the soils along a chronosequence

廖玟銨(Wen-An Liao)

指導教授 ： 許正一

國立臺灣大學/生物資源暨農學院/農業化學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202102806

摘要

稀土元素（rare earth elements）包含鈧、釔和鑭系 （lanthanide series）等共17個元素，具有相似的物化特性， 依原子序大小可分為輕稀土元素 （鑭至銪）和重稀土元素 （釓至鎦），而隨著鑭系收縮 （lanthanide contraction）現象。輕、重稀土元素間的化學行為不同，導致在土壤剖面會產生分化 (fractionation)，通常以(La/Yb)N值表示，其他稀土元素指標值還有(La/Sm)N，其值愈高表示輕稀土元素較為富集。至於(Gd/Yb)N值，愈高表示重稀土元素愈富集，因此稀土元素的分布可應用於土壤化育的解釋上。本論文利用已知定年資料的八卦台地時間序列土壤為材料，其成土年代分別是橫山剖面 (400 ka)、埔中剖面 (350 ka)、松柏坑剖面 (322 ka)、弓鞋剖面 (200 ka)、赤水剖面 (105 ka)、大莊剖面 (28 ka)，將土壤進行稀土元素全量分析，以探討稀土元素在不同年代土壤中的分化及其與土壤性質之關係。結果顯示，總稀土元素含量在大莊剖面 (184-232 mg/kg)最高，埔中剖面 (155-170 mg/kg)含量最低。在所有剖面中，(La/Yb)N值隨著深度愈深而下降，代表隨著化育作用程度提高時，輕、重稀土元素分化程度會愈大。鐵結晶度 ((Fed-Feo)/Fet)與(La/Sm)N值之間有顯著的直線負相關 (n = 47, p < 0.01)，然而稀土元素各指標值與八卦台地土壤的年代並無顯著關係。不過，利用鐵結晶度和鐵活度 (Feo/Fed)，對同為第四紀洪積母質的平鎮系、鹿野系、陳厝寮系的年代推估，結果顯示平鎮系的年代約為35萬年左右，而鹿野系和陳厝寮系的年代約為2萬8千年至10萬年間。

關鍵字

化育作用 ； 稀土元素 ； 土壤時間序列 ； 鑭系收縮 ； 稀土元素分化

並列摘要

Rare earth elements (REEs) are a group of seventeen elements in the elemental period table, including fifteen metallic elements of the lanthanide series and scandium (Sc) and yttrium (Y). However, they are commonly divided into two sub-groups: those from lanthanum (La) to europium (Eu) which are lower atomic numbers and masses being referred to as the light rare earth elements (LREEs) and those from gadolinium (Gd) to lutetium (Lu) which are higher atomic numbers and masses as the heavy rare earth elements (HREEs). The REEs display decreasing atom radii with the increase of atomic numbers. This phenomenon is known as lanthanide contraction, making chemical behaviors of LREEs and HREEs different to be fractionated in the environment. The fractionations between LREEs and HREEs are generally quantified by (La/Yb)N, (La/Sm)N and (Gd/Yb)N. The (La/Sm)N ratio means the fractionation in LREEs and the (Gd/Yb)N ratio means the fractionation in HREEs. Therefore, the different processes of fractionating REEs can be used as tracers of pedogenetic processes. Six study soil profiles were collected from a chronosequence with known dating on the Pakua Tableland. The first to sixth soil profile in descending order are Hen Shan profile (400 ka), Pu Jhong (350 ka), Song Bai Keng (322ka), Gong Sieh (200 ka), Chih Shuei (105 ka), Da Jhuang (28 ka). The REEs were tried to invinvestigate the relationships between their fractionation and soil properties. The highest total REEs content, ranging from 184 to 232 mg/kg, was found in Da Jhuang profile. However, the lowest total REEs content was in Pu jhong profile (155-170 mg/kg). The (La/Yb)N value increased as the depth decreased, indicating the clear fractionation between LREE and HREE during pedogenesis. Furthermore, a significant and linear correlation (n = 47, p < 0.01) exhibited between ratio of iron crystallinity and (La/Sm)N. However, no significant correlation was found between all REEs indices and the age along the chronosequence. Nevertheless, the consideration of (La/Sm) N with the iron crystallinity ratio and iron activity ratio are potential to interpolate the soil age. The additional soil profiles including Pinchen, Luyeh and Chentsoliao series from the Quaternary fluvial materials out of the chronosequence were involed to estimate the relationship between REEs and soil properties. After the estimation based on (La/Sm) N and ratio of iron crystallinity, Pinchen profile was approximate 350 ka and Luyeh and Chentsoliao profiles were 28 ka-100 ka.

並列關鍵字

pedogenesis ； rare earth elements ； soil chronosequence ； lanthanide contraction ； fractionation of REEs

參考文獻

何春蓀。1986。臺灣地質概論·臺灣地質圖說明書增訂第二版：104-105頁。臺北市：經濟部中央地調所。

沈姿岑。2007。林口台地鋯石核飛跡定年。國立臺灣大學地質科學研究所碩士論文，台北市。

林朝棨。1957。壹灣地形。豪灣省通志稿卷一：土地志地理篇第一冊地形 (282-284頁)。臺北市：臺灣省文獻委員會。

陳尊賢、陳慎德、管永愷、林威州、許正一、蔡呈奇、洪英修、賴允傑、李家興、楊慧庭、林文嵐、沈穎、余政剛、葉佳珉、李文成。2017。全國土壤性質調查與管理計畫。行政院環境保護署委託計畫。

陳正祥。1993。臺灣地誌中冊：804-805頁。臺北市:南天書局。