Precipitation is the main control on the global distribution of soil clay minerals

The global distribution of clay minerals is intricately linked with climatic conditions and bedrock composition, which interact to control the weathering intensity of silicate and other minerals. However, the quantitative relationship between clay minerals and climate is not well understood, partly due to the lack of quantitative assessments of the role of the various factors responsible for clay mineral formation. Here we examine the quantitative relationship between soil clay minerals and climatic factors on the global scale, by extrapolating clay mineral abundances to a large scale. We found that mean annual precipitation (MAP) is the first order control on the global clay mineral distribution, while mean annual temperature (MAT), elevation, slope, lithology, upland hillslope soil thickness (UHST), and upland hillslope regolith thickness (UHRT) have secondary roles or localized impacts on clay mineral distributions. We found that precipitation thresholds exist in weathering intensity and clay mineral formation: (1) Illite decreases monotonically and kaolinite increases monotonically with increasing MAP below the threshold of 1800-2000 mm, whereas gibbsite increases monotonically with increasing MAP below the threshold of 2200-2400 mm. (2) Smectite and Fe oxides increase with increasing MAP below the threshold of 600-800 mm, and then decrease with increasing MAP between 600 and 800 mm and 1800-2000 mm. (3) All clay minerals are insensitive to extremely high MAP, above 1800-2000 mm. We suggest that paleoprecipitation can be semi-quantitatively reconstructed based on variations in the relative contents of clay minerals, which can be used as a proxy indicator of wet and dry variations for paleoclimate studies.