How deep is the critical zone?

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.