Pedogenesis along a thermal gradient in a geothermal region of the southern Cascades, California

Hydrothermal alteration is an important soil forming factor in the vicinity of active volcanic centers, yet we are aware of no studies that have addressed the role of active fumaroles on soil development. This paper examines a soil developmental sequence of five pedons established across a thermal gradient (∼ 100 m) induced by an active fumarole in Lassen Volcanic National Park in the southern Cascades of California. The soil temperature gradient at the time of sampling ranged from 100 to 15 °C with a corresponding change in vegetation from barren areas near the fumarole, to pygmy conifer, conifer, and meadow (grasses and herbs) with increasing distance from the fumarole. The primary objectives of this study were to characterize morphological, chemical, and mineralogical soil properties across the thermal gradient, and document pedogenic processes within this exotic soil forming environment. Field morphological properties, standard soil characterization, soil solution chemistry and detailed mineralogical investigations were conducted. The hydrothermally altered parent material from which the soils formed was relatively clay-rich (18–48% clay) with a mineralogical composition dominated by mica–smectite, chlorite, and montmorillonite. Soils near the fumarole were strongly acidified (pH < 4) by sulfuric acid and pH values increased with distance from the fumarole (conifer pH = 4.5–5.5; meadow pH > 6). High soil temperatures near the fumarole resulted in iron (hydr)oxides being primarily crystalline, while non-crystalline iron (hydr)oxides were generally dominant in the upper horizons of the non-thermal soils. Steam-heated alteration in the vicinity of the fumarole resulted in the transformation of the mica–smectite and chlorite to smectite, kaolinite, and alunite (KAl(SO4)2). A similar, but less intense alteration appeared in the conifer pedons resulting in both smectite and kaolinite as weathering products. The meadow pedon displayed the least alteration with appreciable mica–smectite and/or chlorite–smectite still present. There was no evidence of poorly crystalline aluminosilicates materials (e.g., allophane, imogolite) in any of the soils. The lack of vegetation establishment on soils near the fumarole appears to result primarily from the high soil temperatures (> 59 °C at 50 cm), with strong soil acidity (pH < 4), high exchangeable Al3+ (generally > 10 cmolc kg− 1), low base saturation, and low nutrients further impairing soil fertility.