Deep weathering in the semi-arid Coastal Cordillera, Chile

Abstract

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.

The story of a summit nucleus: Hillslope boulders and their effect on erosional patterns and landscape morphology in the Chilean Coastal Cordillera

“…As tectonically-induced faults and fractures are products of the same regional stresses, we assume that regional faults have orientations consistent with fractures in our field sites (c.f., Krone et al, 2021). We observe that stream channels in our field sites (Amin ≥ 10 5 m 2 ) generally follow fault orientations (Fig.…”

“…6), presumably because faults and fractures reduce grain size and allow easier transport of hillslope material and directing stream incision (Roy et al, 2016). This is especially clear in SG, where the north-south striking Atacama Fault System is reflected in the orientation of faults, streams, and also fractures measured in a nearby drill core (Krone et al, 2021;Fig. 6).…”

“…Although the differences in denudation rates between grain sizes is subtle in SG, soils have higher denudation rates than the boulders they directly surround, suggesting that, if the boulders are delineated by fractures at depth as we infer for NA, fractures could also have an effect on landscape morphology in SG. Additionally, Krone et al (2021) noted that the fractures in a drill core in SG were rimmed by halos of weathered material depleted in soluble elements, and concluded that the fractures act as pathways for fluid transport into the subsurface, which enhances chemical weathering. In summary, we suggest that fractures likely accelerate chemical weathering and erosion in SG, but the resultant differential denudation rates are subtle due to low MAP and low tectonic uplift rates.…”

Recently fixed carbon fuels microbial activity several meters below the soil surface

“…The deep soil (>1 meter) harbors not only a large part of the global soil microbial biomass (Pedersen, 1997;Krumholz, 2000;Akob and Küsel, 2011), but also large amounts of organic carbon (C) (Rumpel and Kögel-Knabner, 2011;Marin-Spiotta et al, 2014;Jackson et al, 2017;Moreland et al, 2021;Marín-Spiotta and Hobley, 2022). In topsoils (<1 m), microorganisms have been shown to rely primarily on young organic C, tying microbial activity belowground closely to photosynthetic activity aboveground (Trumbore, 2000;van Hees, 2005;Högberg and Read, 2006;Högberg et al, 2008).…”

“…To test the hypothesis that microbial activity in deep, non-permafrost soil is driven by young organic C, we explored C cycling in soils of the Coastal Cordillera of Chile using isotopes. The bedrock in this region is extraordinarily deeply weathered (Vázquez et al, 2016;Hayes et al, 2020;Krone et al, 2021), possibly because of the CO2 and organic acids produced by soil microorganisms (Berner, 1997;Berg and Banwart, 2000;Hoffland et al, 2004;Finlay et al, 2020;Uroz et al, 2022). We studied soils in the arid, mediterranean and humid climate zone that correspond to the vegetation zones arid shrubland, sclerophyllous forest, and humid temperate forest, respectively (Fig.…”