Secondary mineral formation in the White River tephra in grassland and forest soils in central Yukon Territory

Abstract

Selected surface horizons of grassland and forest soils formed under a cold, semi-arid climate were investigated to evaluate the formation of secondary minerals within the White River tephra, a Late Holocene rhyolitic tephra (~115014C yr BP) veneer that overlies the soil landscapes of central Yukon. Concentrations of extractable Fe (< 0.48%), Al (< 0.26%) and Si (< 0.082%) concentrations in surface tephra-contaning horizons of grassland and forest pedons are low. The high amount of exchangeable calcium in grassland soils is likely due to cycling by vegetation and perhaps, aeolian inputs of Ca and Mg carbonates. Al is incorporated into Al-humus complexes in forest pedons and allophane in grassland pedons. Allophane content is low (< 0.56%) in all soils as is ferrihydrite (< 0.34%). Mineral composition of the sand fraction from tephra horizons is dominated by volcanic glass, plagioclase feldspars, amphiboles, epidote, pyroxenes and very limited quantities of quartz and primary Fe oxides. Chlorite and an expanding phyllosilicate were also detected and are assumed to be of detrital origin. Clay mineralogy is dominated by volcanic glass, quartz, feldspars and minimal quantities of kaolinite and dehydrated halloysite in surficial horizons. Kaolinite is assumed to be of detrital origin while dehydrated halloysite is a product of a low leaching and dry environment where limited resilication occurs. Scanning electronmicroscopy (SEM) investigation indicates the presence of opaline silica in surface horizons from forest pedons which has likely formed due to freezing of the soil solution in combination with dehydration and resilication. Overall, the soil horizons formed within the veneer of White River tephra have experienced minimal weathering and very little silicate clay mineral development. Key words: Tephra, glass, Yukon, minerals (secondary), weathering