wAbstract
@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 Cascade
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)
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, CIA95)
and clay accumulation (447 kg m-2). In contrast, the
high elevation snow-dominated pedons displayed less intense weathering
(e.g., CIA75 and clay25 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.
Keywords: Basalt; Kaolin; Mineralogy; Pedogenesis; Smectite; Soil;
Weatheringx
1. Introduction
2. Methods
@2.1. Field sites
@2.2. Physical and chemical analyses
@2.3. Data analyses
@@2.3.1. Weathering indices and elemental concentrations
@@2.3.2. Statistical methods
3. Results
@3.1. General soil characterization
@3.2. Volcanic glass
@3.3. Selective dissolution
@3.4. X-ray diffraction
@@3.4.1. Lithic material
@@3.4.2. Very fine sands and silts
@@3.4.3. Clays
@3.5. Total elemental analysis
@3.6. Pedon variation with climate
@3.7. Soil taxonomy
4. Discussion
@4.1. Soil mineralogy and mineral transformation
@@4.1.1. Primary silicates and volcanic glass
@@4.1.2. allophanic materials
@@4.1.3. Phyllosilicates and oxyhydroxides
@4.2. Weathering intensity
5. Summary
Acknowledgements
References