Climatic control of the assemblages of secondary clay minerals in the A‐horizon of United States soils

Abstract

The concentrations (per cent) of secondary clay minerals (mica, vermiculite, montmorillonite, chlorite and kaolinite) in the A-horizon of 99 soil sample sites were entered into a cluster program to determine the common clay mineral assemblages found in U S . soils. Five major assemblages were identified with the most common assemblage consisting of mica, montmorillonite and kaolinite. A suite of climatic variables was determined for each of the 99 sites, including the water balance variables and Arkley's leaching index. A discriminant analysis was then used to predict the occurrence of a clay assemblage from the climate data. Sixty per cent of the sites could be correctly predicted, although not all assemblages could be predicted equally well; two assemblages could be predicted with 88 and 82 per cent accuracy. The precipitation of the wettest month and Arkley's leaching index accounted for most of the predictive power. Thus this study supports theoretical models which suggest that seasonal moisture supply is an important control of clay genesis and persistence. A residuals analysis indicated that many of the palaeosols in the data set and many of the soils from glacial parent materials possessed clay mineral assemblages not expected on the basis of modem climate.

Climatic Control of the Geography of Clay Minerals Genesis

“…635-650 0 Copyright 1987 by Association of American Geographers mochemical theory, and experimental studies (Garrels and Christ 1965;Rai and Lindsay 1975;Kittrick 1977;Folkoff 1983). 635-650 0 Copyright 1987 by Association of American Geographers mochemical theory, and experimental studies (Garrels and Christ 1965;Rai and Lindsay 1975;Kittrick 1977;Folkoff 1983).…”

“…When weather stations were more than 16 km from a sample site, data from the three nearest weather stations were used in a triangulation procedure to determine the climate at the site (Folkoff, Meentemeyer, and Box 1981;Folkoff 1983; Folkoff and Meentemeyer 1985). For sample sites within 16 km of a weather station, the data for that station were used.…”

“…Geochemists have developed models of clay mineral formation based on observations, ther-AMak of the Associohon of American Gecgmphefs, 77(4), 1987, pp. 635-650 0 Copyright 1987 by Association of American Geographers mochemical theory, and experimental studies (Garrels and Christ 1965;Rai and Lindsay 1975;Kittrick 1977;Folkoff 1983). According to these models, individual clay minerals are formed in equilibrium with the chemical potential or ionic activities of the soil system.…”

“…Folkoff and Meenterneyer (1985) found that the occurrence and abundance (i.e., assemblages) of secondary clay minerals in the A horizon of soils, when analyzed at the scale of the entire 48 conterminous United States, are controlled by climate, especially seasonal variations in climate. Often the size of the area studied, the minerals investigated, the parent materials present, and the range of climates examined differ markedly.…”

“…It seems that in general, the broader the geographic scale, the more likely climate will be found to be the dominant control. Folkoff and Meenterneyer (1985) found that the occurrence and abundance (i.e., assemblages) of secondary clay minerals in the A horizon of soils, when analyzed at the scale of the entire 48 conterminous United States, are controlled by climate, especially seasonal variations in climate.…”