臺灣與菲律賓蛇綠岩母質土壤中主要、微量與稀土元素的化育特徵

Abstract

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Ophiolitic soils are derived from mafic and ultramafic parent materials and exhibit distinctive geochemical properties commonly characterized by elevated concentrations of trace metals such as Cr, Ni, and Co, as well as REEs. These soils, however, are often nutrient-deficient and present environmental challenges due to the mobility and bioavailability of potentially toxic metals. This study investigates the pedogenesis, geochemistry, and elemental mobility of ophiolitic soils in Taiwan and the Philippines, focusing on major, trace, and REE distributions across different landscape positions. Soil samples were collected from two ophiolite complexes: one in eastern Taiwan (mudstone-derived soils) and another in Zambales, Philippines (serpentine-derived soils). Comprehensive analyses, including XRD, soil micromorphology, total elemental quantification via ICP-OES/MS, and spectroscopic techniques, were employed to assess mineralogical composition, trace metal distribution, and REE fractionation. Statistical approaches, including Pearson’s correlation and PCA, were applied to elucidate geochemical relationships. The results revealed distinct textural and chemical differences between the two sites. In Taiwan (CP1 to CP4), soils were clay-rich (45.0%–62.5%), with OC levels ranging from 5.0% to 5.4% in surface horizons, decreasing to 1.3%–3.4% in subsurface layers. In contrast, soil in Zambales (ZB1 and ZB2) exhibited low amounts of clay (31%-50%) but comparable OC levels in surface and subsurface horizons. The concentrations of the trace metals in pedons CP1 to CP4 (Cr: 71.2–105 mg kg⁻¹; Ni: 26.6–43.2 mg kg⁻¹; Co: 20.2–27.9 mg kg⁻¹) were ten times lower than in pedons ZB1 and ZB2 (Cr: 2,437– 3,410 mg kg⁻¹; Ni: 2,919– 4,309 mg kg⁻¹; Co: 209– 300 mg kg⁻¹), consistent with their mafic and ultramafic parent material. Results also indicate significant differences in soil properties, with Taiwan soils displaying higher SiO₂, Al₂O₃, and CaO, indicative of sedimentary influence, whereas Zambales soils exhibit enrichment in Fe₂O₃ and MgO, characteristic of ultramafic origins. Weathering indices suggest that soils in Taiwan have undergone moderate weathering (CIA: 70.6-79.5; RR: 3.28-4.32; VRI: 2.92-5.38), resulting in elemental redistribution, whereas soils in Zambales exhibit lower degrees of weathering (RR: 7.22-16.9), reflecting the less clay contents. The Ca and Mg differed significantly, with the Taiwan soils showing higher Ca/Mg (0.31–1.13), while the Zambales soils had lower Ca/Mg levels (0.00–0.03). Results revealed that Cr, Ni, and Co concentrations in pedons CP1 to CP4 were highly correlated with clay content (p < 0.001) and Fe oxides (p < 0.01, p < 0.05), indicating that trace metals were immobilized through co-precipitation and adsorption. REE fractionation patterns showed significant enrichment of LREEs relative to HREEs, with ΣREEs ranging from 95.7 to 229.5 mg kg⁻¹ in Taiwan and 17.7 to 51.7 mg kg⁻¹ in Zambales, indicating the preferential adsorption of LREEs and greater solubility and weaker affinity of HREEs onto secondary minerals. Environmental risk assessments indicated that while Cr and Ni levels in Taiwan soils were below contamination thresholds (Cr: 175 mg kg⁻¹; Ni: 130 mg kg⁻¹), Zambales soils exceeded global averages (Cr: 100 mg kg⁻¹; Ni: 35 mg kg⁻¹), posing potential toxicity risks for vegetation and groundwater. These findings highlight the impact of parent material, topography, and pedogenic processes on elemental mobility and soil development in the ophiolite complex.