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

Abstract

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.