Mineralogy of loess and volcanic ash eolian mantles in Pacific Northwest (USA) landscapes

Abstract

Eolian mantles consisting of Quaternary loess, Holocene volcanic ash, and mixtures of the two are prominent features of most landscapes in the inland Pacific Northwest region of the USA. Soils of the loess-mantled Palouse region of eastern Washington and northern Idaho exhibit regional mineralogical trends related to mean annual precipitation and age. Clay content increases and CaCO 3 decreases from areas of lower to higher precipitation. Clay mica is the dominant clay mineral in soils formed in the youngest loess (ca. < 15 ka). In the next older loess paleosol (ca. 40 ka), vermiculite is dominant, indicating a time-dependent mica-to-vermiculite weathering sequence. Smectite and kaolin also occur to a lesser extent in soils of the region. This mineralogical signature provides a useful tool in identifying paleosols and interpreting the complex pedogenesis occurring in local Palouse landscapes. More recently, Holocene volcanic ash has influenced many mid- and high-elevation forested soils. Relatively thick mantles of volcanic ash from the cataclysmic eruption of Mount Mazama 7600 years ago have undergone slight-to-moderate weathering, giving rise to Andisols (Andosols). Environmental conditions generally promote development of allophanic properties across the region, with allophane and ferrihydrite being dominant mineral components. Non-allophanic mineralogy is much less extensive in ash mantles of the region, but is favored in two pedogenic environments — E horizons of podzolized soils and horizons supporting vigorous growth of bracken fern.