Pedogenesis of a chronosequence in the Sacramento Valley, California, U.S.A., II. Elemental chemistry of silt fractions

Abstract

A chronosequence of six soils from alluvium along Honcut Creek in central California has been analyzed to document pedogenic changes in the fine silt (2–20 μm) and coarse silt (20–50 μm) fractions with increasing soil age. The soils range in age from 600 yr to 1.6 million yr and have been formed in a xeric-thermic soil climate. Parent materials are moderately stratified loams, silt loams, and sandy loams derived from basic metavolcanic and granitic rocks. Parent material uniformity was quantified using particle-size distribution of non-clay fractions and X-ray fluorescence (XRF) elemental analysis of silt fractions. Pedogenic changes were quantified using XRF elemental data and a stable index calculation with Zr as the index element. Parent sediments of four of the six profiles fine upward. Parent elemental composition of the silt fractions was less variable than was texture: for example, clay-free silt/sand ratio changes by a factor of 5 down the horizons of the 600-yr-old soil whereas Ca/Zr ratio in fine silt changes by a factor of only 1.1. Comparison of C and deep BC horizons among soils demonstrate that the parent elemental composition was not constant for all chronosequence members. Silicon, K, Ti and Zr become more abundant and Na, Mg, Al, Ca and Fe less abundant in silt fractions of A and B horizons with increasing soil age due to differential weathering rates of primary minerals. All elements decrease in absolute abundance in silt fractions when Zr is used as a stable index element. About 40–50% of the Ti has been lost from the silt fractions of the 1.6-million-yr-old soil and proportional amounts from younger soils, clearly indicating that Ti is mobile in these soils. Magnesium loss can be detected over the effects of stratification in the 10,000-yr-old soil and Mg-bearing minerals are virtually absent from the silt fractions of the upper 1.5 m of soils that are 130–250,000 yr in age. Calcium, Na and Mg are almost completely weathered from the silts in the upper 4 m of the oldest soil, whereas half to more than three-fourths of K is retained. Age trends of element depletion are systematic for those elements and indices that are not influenced by stratification or change in other soil-forming factors. Age trends of depletion are linear when regressed on the log of soil age and have correlation coefficients ( r) that range from 0.57 to 0.93. A mobility sequence based on the degree of depletion of elements from fine silt of the 130,000- and 250,000-yr-old soils is: Mg ⪢ Na > Ca ≅ Fe > Al > Si > Ti > K > Zr.