Lithologic and Climatic Controls on Regolith Transformation in the Southern Sierra Nevada, California.

Climosequence studies of soils in the Sierra Nevada, California have revealed a zone of intense weathering at mid-elevations (from 1000 to 1600 m) just below the rain-snow transition line. We hypothesize that regolith (soil and weathered bedrock) evolution is regulated by climate plus subtle variations in lithology. Variable regolith thickness, from 1.4 to 10.7 m, was found at two catchments of 1100 m and 2000 m elevations of the Southern Sierra Nevada. Regolith sampling in both sites showed that thicknesses of weathered bedrock were quite variable regardless of degree of soil weathering. Biotite content and mineral grain size were measured in sand fractions to assess lithologic controls on regolith thickness. Biotite content in parent material was positively correlated with regolith thickness at both sites. At the drier site (1100 m elevation), mineral grain size was negatively correlated with regolith thickness. Meanwhile, evidence of weathering based on clay and secondary Fe oxide concentrations was intense in soils, but weak in weathered bedrock at both sites. The degree of regolith development was controlled by temperature, which is increasingly buffered by depth. Heat energy estimated from surface to deep layers show a significant correlation with both clay and secondary Fe oxide concentrations. As the same amount of heat energy increases, more weathering occurred at the higher temperature site (1100 m elevation) than that at the lower temperature site (2000 m elevation). Overall, differences in lithology and temperature regulate regolith evolution, yet the effect of temperature was dampened with increasing regolith thickness as heat was absorbed by overlying material.