Basalt weathering and pedogenesis across an environmental gradient in the southern Cascade Range, California, USA

Basalt rocks occupy substantial land area and play a significant role in global weathering patterns and biogeochemical cycling. The objective of this research was to quantify climatic controls of weathering and pedogenic processes on basalt-derived soils across an environmental gradient on the western slope of the Casacade Range of California, USA. We hypothesized that climate controls mineral neogenesis, with cool, moist conditions favoring formation of short-range-order (SRO) materials and warm, dry conditions favoring smectite, crystalline Fe-oxyhydroxides and kaolins. Four pedons were sampled across an elevation gradient (250-2500 m) having large variation in mean annual soil temperature (5-17 °C) and mean annual precipitation (750-1350 mm). The soil mineral assemblage was characterized by X-ray diffraction, selective dissolution, total elemental analysis and light microscopy. The degree of weathering and mineral assemblage exhibited a clear threshold at the permanent winter snowline (~ 1200 m). Maximum soil development was noted just below the snowline with soils dominated by kaolinite and dehydrated halloysite, crystalline Fe-oxyhydroxides (48 kg m^{- 2}), extensive loss of cations (chemical index of alteration, CIA > 95%) and clay accumulation (447 kg m^{- 2}). In contrast, the high elevation snow-dominated pedons displayed less intense weathering (e.g., CIA < 75% and clay < 25 kg m^{- 2}) and a mineral assemblage dominated by primary minerals and SRO materials. The cool, moist conditions of mid-altitude (~ 1600 m) soils appear optimum for the formation and preservation of SRO materials (allophane = 30 kg m^{- 2}). All pedons contained hydroxy-Al interlayered smectite that was either neogenic or derived from eolian minerals. With increasing elevation soil development followed Alfisols → Ultisols → Andisols → Entisols, in agreement with similar gradients in the California Sierra Nevada. Weathering, mineralogical transformations and soil development are limited by water availability at low elevations, whereas low soil temperature is the major limitation at high elevations.