澎湖玄武岩土壤之風化作用與化育過程

Abstract

澎湖群島主要由玄武岩構成，過去較少研究探討澎湖玄武岩土壤的化育作用，並以風化指標描述澎湖玄武岩土壤之風化程度。本研究以既有的岩石定年資料，採取涵蓋不同地層年代的五個剖面，分別為土壤樣體1701與1702 (12.2 Ma)、土壤樣體1703 (10.6 Ma)、土壤樣體1704與1705 (8.2-8.5 Ma)。實驗結果顯示，土壤樣體1701、1702、1703的B層符合美國土壤分類系統中黏聚層之定義，而長年蒸發量大於降雨量造成各剖面之鹽基飽和度均大於35 %，故土壤樣體1701、1702及1703為典型簡育乾潤淋溶土 (Typic Haplustalfs)，土壤樣體1704及1705為典型乾潤正常新成土 (Typic Ustorthents)。以元素全量分析結果選擇六個風化指標，包含Chemical index of weathering (CIW)、Chemical index of alteration (CIA)、Modified weathering potential index (MWPI)、Ruxton ratio、Bases/Alumina (B/A) 和 Bases/R2O3 (B/R)。以土壤形態特徵、黏粒含量、鐵活度與結晶度、岩層相對年代大小，檢視各加權平均之風化指標值，顯示各風化指標均能將岩層相對年代較老的土壤樣體1701、1702以及岩層相對年代較年輕的土壤樣體1704, 1705區分開來，土壤樣體1703則由於排水不良在CIW、CIA、MWPI、B/A、B/R等風化指標中偏差較大。各風化指標值在各剖面垂直深度中以計算過程不含鎂之CIA較具有系統性的變化，反映玄武岩風化過程中鋁的富集與鉀、鈉、鈣隨著時間遞移連續性的離子淋洗流失現象。由形態特徵、黏粒含量、鐵結晶度與活度以及風化指標判斷，風化程度以土壤樣體1701, 1702相對較高，而土壤樣體1704, 1705相對較低，符合岩層年紀之相對關係。

Penghu Islands are mainly composed of basalts. Basalt weathering is relatively fast when compared with other lithologies. In the previous studies, less research focused on investigating in the Penghu basaltic soils of different weathering degree and characterizing the weathering condition of Penghu basaltic soils via weathering indices. In this study, we took the dating information published by Central Geological Survey (CGS) as reference to sample five soil pedons covering older stratigraphic age (Pedon 1701, 1702), middle stratigraphic age (Pedon 1703) and younger stratigraphic age (Pedon 1704, 1705). The result of particle distribution analysis indicated that the clay illuviation of Pedon 1701, 1702 and 1703 meet the requirements of argillic horizon (Bt) of USDA Soil Taxonomy. Years of evaporation overwhelming precipitation in Penghu Islands made the base saturation percentage (BSP) of all pedons over 35 %. Based on the USDA soil taxonomy, Pedon 1701, 1702 and 1703 were classified as Typic Haplustalfs, and Pedon 1704 and 1705 were classified as Typic Ustorthents. The result of total elemental analysis indicated the high aluminum content of Penghu pedons and the loss of cations under the subsurface during the weathering. Based on the result of total elemental analysis, six weathering indices were selected to characterizing the weathering condition of Penghu pedons, including chemical index of weathering (CIW), chemical index of alteration (CIA), modified weathering potential index (MWPI), Ruxton ratio, bases/alumina (B/A) and bases/R2O3 (B/R). The result indicated that all weathering indices showed the conformity with the morphological characteristics, clay content, activity and crystallinity ratios of free iron oxides, and stratigraphic age, discriminating the highly weathering degree of Pedon 1701, 1702 and relatively lower weathering degree of Pedon 1704, 1705. Because of the poor drainage condition, Pedon 1703 bore too much bases, which led to the outlying outcome in the weathering indices related to base elements. Among all of the selected weathering indices, changes in the CIA value with depth showed the most systematic and steadiest tendency, indicating the enrichment of aluminum and the continuously loss of K, Na, Ca during the process of basalt weathering.