A SLICE THROUGH TIME: AN ANALYSIS OF SOIL PARENT MATERIALS IN HYALITE CANYON MONTANA

The Langohr Campground lithosequence compares soils on Archean Gneiss, Flathead Sandstone, Wolsey Shale, Meagher Limestone, and Absaroka Volcanic rocks within a 3.2 km transect ~16 km south of Bozeman, Montana. In this short distance, vegetation fluctuates between grassland meadows and lodgepole pine (Pinus contorta) forests. A state factor approach (Jenny 1941) was used to gain insights into soil processes: all other state factors (mean annual precipitation 76 cm, mean annual air temperature -1.1⁰C, west to northwest aspects, slopes of 15 to 20%, est. soil age ~20 ky) are held constant except lithology. Field observations and laboratory analyses for elemental and oxide concentrations as well as derivative calculations (e.g. chemical depletion fractions [CDF; the fraction of total mass loss due to chemical weathering], mass losses, europium anomalies, and lanthanum ytterbium ratios) of 6 rock, 28 soil, and 4 dust samples were used to compare these soils. Soil pit depths averaged 72 cm. Soil textures and coarse fragment content varied most: soils over gneiss and sandstone showed coarser textures and higher coarse fragments (up to 80%) compared to the fewer coarse fragments (<5%) and clayey textures of the other soil profiles. Geochemical differences were also apparent: CDF ranged from ~0.2 for the volcanic soil to ~0.9 for the limestone and depth weighted average mass losses for silica ranged from 4 kg Si cm^-2 on shale to 252 kg Si m^-2 on limestone. We conclude that underlying rock strongly influences soil properties, but in variable ways across this lithosequence. This influence appears to be both direct and indirect: while chemical weathering of rock leads to compositional changes in overlying soil horizons, it also produces coarse fragments that appear to greatly influence soil hydrology.