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.
Abstract. Subsurface fluid pathways and the climate-dependent infiltration of fluids into the subsurface jointly control the intensity and depth of mineral weathering reactions. The products of these weathering reactions (secondary minerals), such as Fe(III) oxyhydroxides and clay minerals, in turn exert a control on the subsurface fluid flow and hence on the development of weathering profiles. We explored the dependence of mineral transformations on climate during the weathering of granitic rocks in two 6 m deep weathering profiles in Mediterranean and humid climate zones along the Chilean Coastal Cordillera. We used geochemical and mineralogical methods such as (micro-) X-ray fluorescence (μ-XRF and XRF), oxalate and dithionite extractions, X-ray diffraction (XRD), and electron microprobe (EMP) mapping to elucidate the transformations involved during weathering. In the profile of the Mediterranean climate zone, we found a low weathering intensity affecting the profile down to 6 m depth. In the profile of the humid climate zone, we found a high weathering intensity. Based on our results, we propose mechanisms that can intensify the progression of weathering to depth. The most important is weathering-induced fracturing (WIF) by Fe(II) oxidation in biotite and precipitation of Fe(III) oxyhydroxides and by the swelling of interstratified smectitic clay minerals that promotes the formation of fluid pathways. We also propose mechanisms that mitigate the development of a deep weathering zone, like the precipitation of secondary minerals (e.g., clay minerals) and amorphous phases that can impede the subsurface fluid flow. We conclude that the depth and intensity of primary mineral weathering in the profile of the Mediterranean climate zone is significantly controlled by WIF. It generates a surface–subsurface connectivity that allows fluid infiltration to great depth and hence promotes a deep weathering zone. Moreover, the water supply to the subsurface is limited in the Mediterranean climate, and thus, most of the weathering profile is generally characterized by a low weathering intensity. The depth and intensity of weathering processes in the profile of the humid climate zone, on the other hand, are controlled by an intense formation of secondary minerals in the upper section of the weathering profile. This intense formation arises from pronounced dissolution of primary minerals due to the high water infiltration (high precipitation rate) into the subsurface. The secondary minerals, in turn, impede the infiltration of fluids to great depth and thus mitigate the intensity of primary mineral weathering at depth. These two settings illustrate that the depth and intensity of primary mineral weathering in the upper regolith are controlled by positive and negative feedbacks between the formation of secondary minerals and the infiltration of fluids.
<p>Christopher Schwerdhelm<sup>1</sup>, Ferdinand Hampl<sup>2</sup>, Carolina Merino<sup>3,4</sup>, Francisco Matus<sup>4,5</sup>, Thomas Neumann<sup>2</sup>, Andreas Kappler<sup>1</sup>, Casey Bryce<sup>1</sup></p><p>&#160;</p><p><sup>1</sup> Geomicrobiology, Center for Applied Geoscience (ZAG), Eberhard-Karls-University Tuebingen, Sigwartstrasse 10, 72076 Tuebingen, Germany</p><p><sup>2</sup> Technische Universit&#228;t Berlin, Institute of Applied Geosciences, Department of Applied Geochemistry, Office BH 9-3, Ernst-Reuter-Platz 1, 10587 Berlin, Germany</p><p><sup>3</sup> Center of Plant, Soil Interaction and Natural Resources Biotechnology Scientific and Technological Bioresource Nucleus (BIOREN), Temuco, Chile</p><p><sup>4</sup> Network for Extreme Environmental Research, Universidad de la Frontera, Temuco, Chile</p><p><sup>5</sup> Department of Chemical Sciences and Natural Resources, Universidad de La Frontera, Avenida Francisco Salazar, 01145 Temuco, Chile</p><p>&#160;</p><p>Mineral weathering shapes Earth&#8217;s surface by transforming bedrock to soil in the &#8216;critical zone&#8217;. Among these transformation processes, microbial weathering plays an important role, as it contributes to all stages of rock-soil transformation such as primary rock colonization, rock breakdown, saprolite formation, and element cycling. Fe-metabolizing microorganisms, i.e. Fe(II)-oxidizing and Fe(III)-reducing microorganisms, are key players in weathering as they can directly attack minerals via their metabolism. However, most direct evidence for the role of these microbes in critical zone processes comes from shallow and humid tropical soils and saprolite, or from transects across corestones. Much less is understood about the direct role of these microorganisms in critical zone processes in more arid climates.&#160;&#160;</p><p>In this study we have obtained drill cores from the critical zone of a semi-arid region of the Chilean Coastal Cordillera (Santa Gracia Reserve). Despite receiving only 66 mm of rain per year, the weathering profile is very deep (>80 m). The rock material of the drill core is a Cretaceous quartz monzodiorite rich in hornblende, biotite and chlorite with ca. 1-2 wt.-% Fe(III) oxyhydroxides and very low TOC content. Using cultivation-based methods we found microaerophilic Fe(II)-oxidizing bacteria in zones of weathered saprolite (up to ca. 25 m depth) and at the weathering front (70-76 m), while Fe(III)-reducing bacteria, grown either with dihydrogen or organic carbon, were successfully enriched from samples across the whole 87 m profile. A robust contamination control confirmed that cultivated microbes were from the in-situ community and not related to drill fluid contamination. &#160;</p><p>These findings suggest there is potential for Fe-metabolizing microbes to contribute to mineral-weathering processes even in deep weathering profiles in semi-arid environments. The occurrence of cultivatable Fe(II)-oxidizing bacteria is controlled by the presence of highly fractured zones functioning as fluid and oxygen transport pathways. It is notable that despite the fact that much of the silicate minerals contain Fe(II), Fe(III)-reducing bacteria are more common. The co-occurrence of Fe(II)-oxidizing and Fe(III)-reducing bacteria in some isolated parts of the profile could represent a self-sustaining cycle of iron redox reactions.</p>
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