How minerals govern the advance of weathering: comparison of a shallow and a deep weathering profile in different climatic zones (Chilean Coastal Cordillera)

Weathering is the fundamental precondition for erosion and soil formation which sculpture Earth´s surface. It is a complex interplay of minerals, rock fabric, tectonical fractures, climate, and organic activity.

To explore the dependences between these factors two weathering profiles on magmatic bedrock were compared using six-meter-deep soil pits and drill cores in both a humid and a Mediterranean climate regime of Chile. Detailed mineralogical and geochemical investigations of soil and saprolite were combined with spatially highly resolved geochemical analyses of fracture-related rock weathering.

The maximum saprolite depth in the humid climate turned out to be much shallower (approx. 6 m) than in the Mediterranean climate (almost 30 m). However, the entire soil-pit profile in the humid climate is characterized by distinct chemical depletion and intense mineral weathering (predominantly chemical weathering), whereas the Mediterranean profile only shows weak chemical and mineral weathering but high degrees of fracturing (predominantly physical weathering). This study suggests that surface inputs (water, O₂) initially enter the subsurface via tectonical fractures and trigger reactions such as iron-oxidation in Fe(II)-bearing silicates which induces fracturing or the transformation of feldspars which can hamper the weathering advance by porosity reduction. The higher content of Fe(II)-bearing silicates in the bedrock of the Mediterranean climate is thus considered the critical factor for the different developments of the two profiles.

This study stresses the magnitude of control the mineralogical composition has on weathering processes and that surface processes like erosion cannot be fully understood without a thorough investigation of the subsurface.