Spatial soil variability in the Cajon Pass chronosequence: implications for the use of soils as a geochronological tool

Abstract

Spatial variation of soil properties on a single geomorphic surface has a significant influence on surface correlations and age determinations made on the basis of soil properties. Few chronosequence studies have been based on a sufficient number of soils to evaluate the influence of soil variability. We described and evaluate 35 soils on four terrace surfaces which constitute part of a previously described, well-dated soil chronosequence in Cajon Pass in southern California. Soils were described on both bar and swale sites, removed from the influence of deposition adjacent slopes. The influence of eolian silt on the rate of pedogenesis in Cajon Pass indicates that this sequence of soils cannot be considered a chronosequence in the strict sense of the term. Furthermore, the evidence for a pedogenic threshold in these soils indicates that the soils do not develop monotonically with time and that soil development cannot be defined by a continuous univariant chronofunction. The discrete nature of the data from a chronosequence and the complex nature of soil development are better analyzed by multivariate statistical procedures. Systematic variation in soil properties between soils from bar sites and soils from swale sites is Attributed to geomorphic processes which affect either the rate or duration of eolian dust accumulation. Soils from bar sites were less variable and generally less strongly developed that soils from swale sites from the same surface. The degree of variation of soil properties determined for a given landscape position on a surface is sufficient to raise questions concerning the usefulness of chronofunctions for determining the nature of time-dependent pedogenic changes and for providing numerical age determinations. Multivariate statistical methods were used to differentiate among soils developed on three surfaces which differ in age by only 1500 years.