Methods for the study of paleosols

Just as soil individuals (pedons) are studied as soil columns in soil pits, paleosols are studied in columnar stratigraphic sections of the sort also used in sedimen-tology and stratigraphy (O Figure 13.3). Grain size is emphasized because it is important to soil formation, as weathering transforms sand and silt grains to clay. A graphical representation of grain size profiles conveys important information on the abruptness of horizon transitions. Color from a Munsell chart should also be represented, as redness denotes the degree of chemical oxidation and drainage of soils and paleosols. Calcareousness determined by relative effervescence with dilute hydrochloric acid also is important as a guide to chemical leaching and soil nutrient status (Retallack 1997).

Laboratory studies of paleosols do not employ all the same techniques used in soil science because some important soil measures, such as base saturation, are altered upon burial of soils (Retallack 1991b). Petrographic thin sections are especially useful for revealing soil microfabrics, and the point counting of thin sections furnishes estimates of changes in grain size and mineral composition of paleosols. For example, increased subsurface clayeyness can be used to recognize diagnostic horizons (argillic horizon) for forest soils (Alfisols and Ultisols), whereas traces of nutrient-rich minerals, such as calcite and feldspar, distinguish fertile forest soils (Alfisols) from infertile forest soils (Ultisols: Retallack 1997).

Chemical analyses also are useful in characterizing and classifying paleosols, especially molar ratios designed to gauge the progress of common soil-forming chemical reactions. The hydrolysis reaction common in silicate weathering leaches cationic bases (Ca2+, Mg2+, K+, Na+) from host minerals, such as feldspar, to create clay (Al rich) and is thus indicated by high ratios of alumina/ bases. An alumina/base ratio higher than 2 is a good proxy for the transition from fertile forest soils (Alfisols) to infertile forest soils (Ultisols). Soda/potash molar ratios in excess of 1 indicate unusually salty soils. Ferrous/ferric molar ratios in

O Figure 13.3

Pétrographie and chemical data for the Tek paleosol from the 18 Ma Hiwegi Formation of Rusinga Island, Kenya [data from Retallack et al. (1995)]

field observations

Munsell color pornl count data percent Percent r ■ compos-grain size ¡tj0n

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