USE OF MAGNETIC-SUSCEPTIBILITY MEASUREMENTS IN ASSESSING SOIL UNIFORMITY IN CHRONOSEQUENCE STUDIES

Recent observations show that soils with vertical morphological differentiation often exhibit a distinct ferrimagnetic profile with the magnetic susceptibility (MS) of eluvial horizons enhanced with respect to that of illuvial, Cr, or R horizons. In addition, pedogenic ferrimagnetic material is mostly superparamagnetic (SP) and single domain (SD) (<30 nm), whereas inherited titanomagnetites are predominantly multidomain (MD) grains. We suggest that the continuous pedogenic development of a soil profile in homogeneous parent material results in enhancement of MS and in a gradual shift from MD to SD and SP magnetic states, and that deviations from this pattern arise from discontinuities in the soil or from climatic or topographic changes. This hypothesis was tested on 18 northern California pedons, formed in four marine and river terrace systems and in volcanic parent material. The ages of the terraces range from 600 yr to 1.6 million yr and the MS of the parent materials range from 10 to > 1000 x 10(-8) m3 kg- 1. Established chronosequence members were used to determine that enhancement of MS and shifts in magnetic domain state are correlated with age. Exceptionally large or small magnetic susceptibilities throughout the soil profile, lack of magnetic enhancement, and discontinuous magnetic profiles in other soils could each be related to morphologically or chemically determined discontinuities. Thus, magnetic susceptibility, an easily measured soil attribute, can be used to evaluate the suitability of pedons for chronosequence studies.