The weathering front is the boundary beneath Earth’s surface where pristine rock is converted into weathered rock. It is the base of the “critical zone”, in which the lithosphere, biosphere, and atmosphere interact. Typically, this front is located no more than 20 m deep in granitoid rock in humid climate zones. Its depth and the degree of rock weathering are commonly linked to oxygen transport and fluid flow. By drilling into fractured igneous rock in the semi-arid climate zone of the Coastal Cordillera in Chile we found multiple weathering fronts of which the deepest is 76 m beneath the surface. Rock is weathered to varying degrees, contains core stones, and strongly altered zones featuring intensive iron oxidation and high porosity. Geophysical borehole measurements and chemical weathering indicators reveal more intense weathering where fracturing is extensive, and porosity is higher than in bedrock. Only the top 10 m feature a continuous weathering gradient towards the surface. We suggest that tectonic preconditioning by fracturing provided transport pathways for oxygen to greater depths, inducing porosity by oxidation. Porosity was preserved throughout the weathering process, as secondary minerals were barely formed due to the low fluid flow.
<p>The weathering front, the interface beneath Earth&#8217;s surface where unweathered bedrock is converted into weathered rock, is a zone where chemical disequilibrium results in some of the most intense mineralogical transformations. These are focused into a narrow zone; yet its depth is poorly known due to its inaccessible nature deep beneath the Earth&#8217;s surface. Studies in humid and temperate climate suggest a maximum depth of 20 m for the weathering front in granitoid rock (Hayes et al., 2020).</p><p>To explore whether this depth is unique to humid climate we drilled into fractured rock in the semi-arid climate zone of the Coastal Cordillera of Chile. We found deep weathering down to 76 m below the surface which represents one of the deepest weathering fronts ever found. To characterise and quantify rock weathering, we investigated mineralogical and geochemical transformations. Iron (Fe) oxidation and related porosity formation is the first weathering process taking place and hence an indicator for the onset of weathering (Buss et al., 2008). Elemental (&#964;) and bulk loss (chemical depletion fraction, CDF) calculated from the chemical composition reveal multiple zones with more intense weathering compared to bedrock, and where the specific surface area also increases due to formation of secondary solids. Fracturing and the related increase in macro-porosity thus induce these mineralogical and chemical transformations. Below 76 m, bedrock is devoid of weathering features. We suggest that tectonic pre-fracturing in this geologically active region provided transport pathways for oxygen to greater depths, inducing porosity by oxidation. This porosity was preserved throughout the weathering process, as secondary minerals that might fill pores were not formed due to the low fluid flow.</p><p>Hayes, N. R., Buss, H. L., Moore, O. W., Kr&#225;m, P. and Pancost, R. D. (2020): Controls on granitic weathering fronts in contrasting climates. Chemical Geology, 535, 119450.</p><p>Buss, H.L., Sak, P. B., Webb, S. M. and Brantley, S. L. (2008): Weathering of the Rio Blanco quartz diorite, Luquillo Mountains, Puerto Rico: Coupling oxidation, dissolution, and fracturing. Geochimica et Cosmochimica Acta, 72 (18), 4488-4507.</p>
<p>The weathering front, the boundary beneath Earth&#8217;s surface where unweathered bedrock is converted into weathered rock, is the base of the critical zone. Typically, this front is located no more than 20 m deep in granitoid rock in humid climate zones and its depth is commonly linked to oxygen transport and fluid flow. To disclose the depth of the weathering front in dry climate, we conducted a drilling campaign in the semi-arid climate zone of the Chilean Coastal Cordillera to investigate a complete weathering profile by mineralogical and geochemical methods as well as geophysical borehole measurements.</p><p>We found multiple weathering fronts of which the deepest is located at 76 m beneath the surface. Dioritic rock is weathered to varying degrees, contains core stones, and strongly altered zones featuring intensive iron (Fe) oxidation and high porosity. We found more intense weathering where fracturing is extensive, and in these zones porosity is higher than in bedrock. Only the uppermost 10 m feature a continuous weathering gradient towards the surface. Porosity was preserved throughout the weathering process, as secondary aluminium-silicon minerals were barely formed due to the low fluid flow.</p><p>We suggest that tectonic fractures act as major pathways for oxygen to greater depth, generating porosity by oxidation of Fe-bearing minerals. The depletion of soluble elements is also concomitant with high fracture density and highest elemental loss is detected in the proximity of planar fractures or fractures zones. The orientation and dip angle of the fractures are consistent with the arrangement of tectonic faults in the area and the general strike and kinematics of the Atacama fault system. We interpret that most of these fractures have formed during the Late Mesozoic activity of the fault system. Further fractures in the study area may be related to the cooling of the diorite or may be modern and have formed either by stress relief during denudation or through Fe oxidation. We hypothesise that advection of fluids and gases through tectonic fractures sets deep weathering at multiple weathering fronts, since we found elevated degrees of chemical depletion close to larger fractures and no continuous weathering gradient exists. Although the fluid flow is minor, the slow turnover of the weathering zone provides sufficient time to form and preserve these deep weathering features. For the drill sites&#8217; denudation rate of 29.6 t km<sup>-2 </sup>year<sup>-1</sup> from cosmogenic nuclides, corresponding to about 11 m Myr<sup>-1</sup>, the entire weathering may get turned over about every 7 Myr, if steady state denudation is assumed.</p><p>This study is prerequisite to detailed investigation of the microbial processes involved at weathering at great depth.</p><p>&#160;</p><p>Krone, L.V., Hampl, F.J., Schwerdhelm, C. et al. Deep weathering in the semi-arid Coastal Cordillera, Chile. <em>Sci Rep</em> <strong>11</strong>, 13057 (2021). https://doi.org/10.1038/s41598-021-90267-7.</p>
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