蛇紋岩土壤化育作用與鉻、鎳礦物特性的關係

Abstract

蛇綠岩套 (ophiolite)是由不同岩性比例組成的混同岩體 (mélange)，其出露地表風化後的土壤多統稱為蛇紋岩土壤 (serpentine soils)。蛇紋岩土壤特徵為鈣鎂比值低於一般土壤，鉻 (Cr)、鎳 (Ni)等重金屬濃度過高，使其土壤肥力及環境品質受限。其中，鉻與鎳的分布及釋出，可能受到母質特性差異而在礦物崩解及轉變上表現不同。蛇紋岩土壤中抗風化能力強的鉻尖晶石 (Cr-spinels)為鉻主要來源；鎳則主要存在於易風化的矽酸鹽礦物 (silicate minerals)。然而，鉻尖晶石的風化程度仍可能因元素取代及淋洗強度而有所不同。因此，本研究旨在解析臺灣東部蛇紋岩土壤中鉻和鎳礦物的來源及其在土壤化育過程的特徵，並透過土壤磁化率揭示這些元素的分布和行為模式。本研究從臺灣東部四個蛇綠岩套無毛山、銅安山、電光及石頭山採集共九個剖面，46個化育層土壤樣本，並透過供試土壤的形態、微形態特徵、元素組成、理化性質、礦物特性反映土壤化育作用及鉻、鎳分佈差異。接著，利用濕式及乾式磁分離土壤搭配電子微探、X光電子能譜及X光吸收光譜鑑定鉻和鎳來源礦物及其風化特徵，並以0.1N HCl萃取鉻和鎳加以驗證。進一步地，本研究以微波輔助酸消化法，得到稀土元素 (rare earth elements, REEs)的分佈及其分化指標，最後運用偏最小平方回歸 (partial least squares regression)以土壤磁化率作為綜合指標，量化供試土壤中鉻和鎳礦物特性與化育作用之關係。供試土壤被分類為多種不同風化作用影響和化育程度的土壤，包括深棕至黑色的淺層新成土 (Lithic Udorthents)和淺層弱育土 (Lithic Dystrudepts)、黃棕至黑色的弱育土 (Typic Humudepts)、具黏粒遷移或伴隨棕化作用的暗紅棕至黑色淋餘土 (Mollic Hapludalfs)，以及經強烈淋溶、黏粒移動與鐵鋁質化的黃紅至暗紅棕色極育土 (Typic Hapludults)。研究發現，鉻和鎳主要來自強磁性礦物，如磁鐵礦和鉻尖晶石，這些礦物隨著風化逐漸崩解，新的非磁性礦物生成積累，導致元素的釋放和遷移，從而使鉻和鎳在不同化育程度的剖面中呈現出各自的分布特徵。其中強磁性礦物中鐵和鉻能穩定地保留，鎳和鎂則相對容易流失。然而，鐵和鉻部分被氧化，加速礦物的風化。土壤磁化率反映了蛇紋岩土壤中礦物的風化和化育過程，特別是鉻和鎳的分布特性。初期土壤磁化率由初生強磁性礦物主導，隨著土壤化育，反磁性初生矽酸鹽類礦物崩解以及次生亞鐵磁性礦物生成，土壤磁性增強。然而，當風化作用加劇，初生強磁性礦物嚴重溶解，次生反磁性矽酸鹽和反鐵磁性鐵氧化物逐漸主導土壤的磁性，導致磁化率顯著下降。鉻、鎳和REEs在土壤中的分佈皆受到母質與化育作用影響。REEs來源廣泛，並在含鉻的鐵氧化物礦物中富集。土壤化育釋放大量的鐵和鎂等鹽基離子，促使大量的鐵氧化物及黏土礦物形成並吸持初生礦物釋出的鉻、鎳和REEs。因此，REEs的分佈進一步支持了鉻、鎳的礦物來源特性。總上所述，本研究表明，土壤磁化率是一個有效的綜合指標，能夠統合不同風化階段的礦物組成、化育作用以及鉻、鎳、REEs等元素的分布特性。這些研究結果對蛇紋岩土壤的環境風險評估和管理具有重要應用價值，為高鉻和鎳背景土壤的分類和風險控制提供了科學依據。然而，本研究僅使用臺灣東部九個蛇紋岩土壤剖面作為樣本，未來可加入更多氣候區的樣本，以更全面驗證研究結果。

The soils formed from ophiolites exposed to surface weathering are commonly referred to as serpentine soils. These soils are characterized by a low calcium-to-magnesium ratio and elevated concentrations of chromium (Cr) and nickel (Ni), limiting their fertility and environmental quality. The distribution and release of Cr and Ni varied due to differences in parent material properties. In serpentine soils, Cr is primarily derived from weathering-resistant Cr-spinels, while Ni is predominantly found in easily weathered silicate minerals. However, the degree of Cr-spinels weathering may vary depending on cation substitution and leaching intensity. Thus, this study aims to analyze the sources of Cr and Ni in serpentine soils from eastern Taiwan and their characteristics during pedogenesis. Nine soil profiles comprising 46 pedogenic horizons were collected from four ophiolite complexes in eastern Taiwan. The morphological, micromorphological, elemental, physicochemical, and mineralogical properties of the studied soils were analyzed. Magnetic separation, electron microprobe analysis, X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and 0.1 N HCl extraction were conducted. Additionally, rare earth elements (REEs) distribution were obtained via microwave-assisted acid digestion. Partial least squared regression (PLSR) was applied to quantify the relationship between pedogenic processes and Cr and Ni. The studied soils were classified into Lithic Udorthents, Lithic Dystrudepts, Typic Humudepts, Mollic Hapludalfs exhibiting brunification and/or clay translocation, and intensely weathered, Typic Hapludults, characterized by clay illuviation and ferralitization. The findings indicated that Cr and Ni primarily originate from strongly magnetic minerals such as magnetite and Cr-spinels. While Fe and Cr remained stable within strongly magnetic minerals, Ni and Mg were more susceptible to leaching during pedogenesis. However, evidences of Fe and Cr oxidation may accelerate the weathering of these minerals. In the studied soils, the magnetic susceptibility was initially dominated by primary ferrimagnetic minerals. As pedogenesis progressed, the breakdown of diamagnetic primary silicate minerals and the formation of secondary ferrimagnetic minerals enhanced soil magnetism. However, after intense weathering, strongly magnetic primary minerals dissolved significantly, and the dominance of secondary diamagnetic silicates and antiferromagnetic iron oxides reduced the magnetic susceptibility. REEs, which originate from various sources, were enriched in Cr-bearing iron oxide minerals. The above-mentioned iron oxides and clay minerals adsorbed Cr, Ni, and REEs released from primary minerals. In conclusion, this study demonstrated that soil magnetic susceptibility is an effective integrative indicator that links mineral composition, pedogenesis, and the distribution of Cr and Ni in serpentine soils at different weathering stages. These findings have significant implications for environmental risk assessment and management, providing a scientific basis for the classification and risk control of soils with high Cr and Ni background levels. However, this study is limited to nine serpentine soil profiles from eastern Taiwan. Future studies should incorporate samples from diverse climatic regions to validate these results comprehensively.