The expression “Rock+Acid+Water -> Clays+Ions+Bicarbonate” may seem simple, but it encompasses many complex processes working in the Earth’s Critical Zone (CZ). The SE United States Calhoun CZ Observatory (O) has been observing the CZ for decades, leading to new understanding of how geological and human factors impact landscape evolution and management. The distribution of clays in the CZO is linked to the response of land to various factors that influence soil formation. These factors include vegetative cover (such as crops, regenerative pines, and old-growth hardwoods), interfluve order (old ridge-crests versus young legacy sediments), underlying geology (felsic versus mafic rocks), climate change (wetter versus dryer seasons), and denudation rates (erosion rates ranging from 1 to 1000 m/Ma).

Through observations, we have found that plant cover and rooting depth affect soil gas (CO₂ and O₂) distributions, which in turn affect clay mineral hydrolysis and redox reactions. We have also found that clay mineral signatures show increasing indices of chemical alteration along increasing interfluve orders. Additionally, felsic rocks weather more deeply into the CZ than mafic rocks under similar biota, climate, relief, and time conditions. Furthermore, we have found that the chemical signatures captured in clays may not reflect average conditions, but rather specific points in soil moisture states. Finally, we have observed that rates of material transport within the CZ vary greatly depending on the chronometer used. The CZ constitutes a small volume of the Earth’s clay budget however it is an important clay factory in exogenic cycles.

Deeply weathered soils are iconic components of the tropical critical zone. Being short of exchangeable nutrients, physical features beyond sheer depth determine more than elsewhere the provision of life-sustaining resources. Biogeochemical models can hardly capture tropical fluxes, due to them being mainly developed and validated for temperate zones, focusing more on topsoil processes. To overcome this systematic bias, pedotransfer functions are regionally adapted for modeling soil water movements and then used for tropical biogeochemical modeling. However, soil texture based on single grain size distribution neglects the impact of actual soil structures in the field and leads to pronounced discrepancies between field measurements and model predictions for tropical soils. A prominent example of this mismatch is overestimated N₂O emissions.

Now, scientific efforts are being made to correct this systematic bias in predicting soil functioning. A well-known characteristic of tropical soils, potentially responsible for the systematic error, are water-stable aggregates called pseudo-sands. In the field, they are perceived as sand, but in the lab measured as clay and silt. The simple assumption that pseudo-sands act just like sands in the field seems to work satisfactorily for certain hydrological predictions. We pursue the hypothesis that, biogeochemically, pseudo-sands do not simply act like sands. We provide evidences why pseudo-sands cannot be simply treated neither like “regular sand” nor like the sum of its units. The long-term goal is to develop tropical biogeochemical model versions related to the properties of pseudo-sands that will lead to improved models of the critical zone of the tropics.

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.

The depth of the critical zone, the zone that spans from the canopy to the weathering front in the subsurface, is mostly unknown due to its inaccessible nature. To identify the depth of the critical zone and infer its assumed control by water flow we conducted four drilling campaigns in granitoid rock along a climate gradient in the Chilean Coastal Cordillera. The drilled cores differ in the depth of the weathering front between arid, semi-arid, mediterranean, and humid climate. The arid study site is located at the southern end of the Atacama Desert and the drill core is hydrothermally overprinted. No chemical top-down weathering or physical disintegration of the granitic rock is found. The semi-arid drill core reveals multiple weathering fronts along fractures and shows top-down chemical weathering in the uppermost 10 m. In the mediterranean study site, we found the deepest weathering front with saprolite to a depth of ~45 m, followed by bedrock with wide fractures. The physical disintegration is stronger than chemical mass losses due to weathering. The humid study site is characterised by a shallow weathering front at ~10 m depth. Even though sufficient water is available to form a deep weathering zone, the formation of clay and other secondary minerals inhibits further advance of the weathering front by clogging pore space thus preventing water flow. We conclude while the of degree of (chemical) weathering is set by climate, the depth of the weathering front depends on the abundance and width of tectonic fractures.