Soil Mineralogy of a Vernal Pool Catena in Southern California

Abstract

Core Ideas Soil mineralogy, as with morphology, varies with slope position in the vernal pool watershed. Vernal pool soil mineralogy is determined by parent material, weathering, relative solubility of elements, and hydrology. Kaolin dominates on upper slope positions; smectite dominates on lower slope positions. Vernal pools are seasonal wetlands that are noted for their unique flora and fauna. Soils are critical to the hydrologic function of the vernal pools, but few studies have examined the mineralogy of vernal pool soils in detail. In this study, the soil mineralogy of a vernal pool catena on basalt in southern California was investigated to identify the mineral components, their origin, and their relationship to the soil hydrologic environment. Kaolin and smectite are found in the soil clay fraction throughout the catena. Kaolin, derived from plagioclase weathering, is the predominant clay mineral in the loamy upper slope soils. Excess silica and other soluble cations from the weathering of plagioclase and pyroxene are carried in throughflow to the lower slope positions where they precipitate primarily as smectite, producing clayey Vertisols. Minor amounts of quartz and biotite are present in the soils, apparently from eolian deposition. The biotite is interpreted to weather to vermiculite, and subsequently to a trioctahedral smectite, which is found in both upper and lower slope soils. Slope wash has transported soil material, including trioctahedral smectite and kaolin, from the upper slope to lower slope soils. Two dioctahedral smectite species are distinguished in the basin soils based on d(060) spacings and are interpreted to have neoformed and transformed from biotite or pyroxene. The mineralogy of the vernal pool catena is differentiated by pedogenic processes related to mineral weathering, relative solubility of released elements, slope wash, and soil hydrology.