In a semi-arid and a mediterranean study site in the granitoid Chilean Coastal Cordillera, we investigated drilled weathering profiles of 100 m depth and found significant differences in the depth of the weathering zone. Where rainfall is limited, fractures may be transport pathways for water to depth, inducing deep weathering. With more rainfall present, diffusive water transport through pore spaces allows surficial weathering in the profiles.
These distinct modes of water infiltration can be traced with the meteoric cosmogenic nuclide 10Be (T1/2 = 1.4 My) while stable 9Be is released during silicate weathering. By combining these isotopes, a 10Be/9Be ratio emerges in fluids that traces water infiltration and the element release by weathering. This isotope ratio is preserved in the “reactive” form as attached to amorphous Fe-oxides.
Concentrations of reactive Be ([Be]reac) were determined in soil, saprolite, bedrock, and fracture surface samples. Both study sites show an increase in [9Be]reac towards the surface. [9Be]reac is higher on fracture surfaces compared to soil and saprolite, indicating that fractures and pore spaces are conduits for water flow. [10Be]reac decrease from surface to depth in soil and saprolite, reaching detection limit at 2 m in the semi-arid, and at 25 m in the mediterranean study site. [10Be]reac in deep fracture samples are close to detection limit although high [9Be]reac indicate Be mobility by water flow. Decay and/or adsorption of 10Be during transport, or insufficiently low water infiltration, may explain why so little 10Be is found on fracture surface and at depth.