Clay minerals in the critical zone and links to climate change

This contribution uses a recently published global clay mineral inventory of the critical zone to assess how well clay mineral assemblages reflect climate. The relative abundance of the main clay mineral groups (the 1:1, 2:1 and 2:1:1 hydrous phyllosilicates are used to evaluate a Clay Mineral Alteration Index (CMAI), which is compared with current latitudes and the Köppen-Geiger climatic zones. This CMAI relationship is defined as:

CMAI = (2:1LE + 2:1:1LE) – (1:1LE + 2:1HE)

(physically weathered – newly formed)

where, LE = Low to no expandability, HE = high expandability and CMAI values range between -100 (warm tropical) and +100 (cold polar).

For selected soil types, such as the alfisols, some general correlations exist between CMAI values and distance from the equator. However, the database indicates that lithological controls on clay mineral assemblages introduce a large degree of heterogeneity to the system. This makes a direct interpretation based on numerical indices difficult to implement. Improved correlations can be achieved by selecting consistent soil types developed on the same host lithologies (e.g. soils on Silurian shales). Following this procedure, very good correlations can be attained as long as the climatic parameters of temperatures and rainfall are both considered. A refined correlation between the CMAI and current climatic conditions is put to the test on palaeosols and shales from the geological record.