Feedbacks between the formation of secondary minerals and the infiltration of fluids into the regolith of granitic rocks in different climatic zones (Chilean Coastal Cordillera)

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.

Full text

Earth Surf. Dynam., 11, 511–528, 2023
https://doi.org/10.5194/esurf-11-511-2023
© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.
Feedbacks between the formation of secondary minerals
and the infiltration of fluids into the regolith of granitic
rocks in different climatic zones
(Chilean Coastal Cordillera)
Ferdinand J. Hampl1, Ferry Schiperski1, Christopher Schwerdhelm2, Nicole Stroncik3, Casey Bryce4,
Friedhelm von Blanckenburg3,5, and Thomas Neumann1
1Department of Applied Geochemistry, Technische Universität Berlin,
Ernst-Reuter-Platz 1, 10587 Berlin, Germany
2Geomicrobiology Group, Eberhard Karl University of Tübingen,
Schnarrenbergstrasse 94–96, 72076 Tübingen, Germany
3Earth Surface Geochemistry, GFZ German Research Centre for Geosciences,
Telegrafenberg, 14473 Potsdam, Germany
4School of Earth Sciences, University of Bristol, Wills Memorial Building,
Queens Road, Bristol BS8 1RJ, United Kingdom
5Institute of Geological Sciences, Freie Universität Berlin, Malteserstrasse 74–100, 12249 Berlin, Germany
Correspondence: Ferdinand J. Hampl (ferdinand.j.hampl@tu-berlin.de)
Received: 13 December 2022 – Discussion started: 17 January 2023
Revised: 11 May 2023 – Accepted: 16 May 2023 – Published: 22 June 2023
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 6m 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 gen-
erates 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
Published by Copernicus Publications on behalf of the European Geosciences Union.

512 F. J. Hampl et al.: Feedbacks between the formation of secondary minerals and the infiltration of fluids
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.
1 Introduction
The formation of weathered material (regolith) from un-
weathered rock (bedrock) is a key process for shaping Earth’s
surface. It is of major importance for making mineral-bound
nutrients accessible to the biosphere of the critical zone (e.g.,
Dawson et al., 2020) and to supply rocks and minerals to
the sediment cycle. In this process, the in situ disaggregation
and chemical depletion of weathered rock (saprock) to sapro-
lite plays an essential role. This transformation is a result of
fracturing and mineral dissolution (e.g., Navarre-Sitchler et
al., 2015). Both are associated with chemical, physical (e.g.,
Goodfellow et al., 2016), and biological weathering pro-
cesses (e.g., Drever, 1994; Lawrence et al., 2014; Napieralski
et al., 2019). These processes are linked to climate-related
parameters such as precipitation rate, fluid flow (water and
gases), and biological activity. Apart from that, the weather-
ing processes and hence the saprolite formation also depend
on primary fractures (e.g., Molnar et al., 2007; Hynek et al.,
2017; Kim et al., 2017; Holbrook et al., 2019; Hayes et al.,
2020; Krone et al., 2021; Hampl et al., 2022a), discontinu-
ity density and tortuosity (Israeli et al., 2021), thermoelastic
relaxation (e.g., Nadan and Engelder, 2009), and the topo-
graphic surface profile (e.g., Rempe and Dietrich, 2014; St.
Clair et al., 2015). However, one of the most fundamental
parameters for the regolith formation is the mineral content
of the bedrock. The weathering of some of these primary
minerals and the consequent formation of secondary miner-
als can lead to an amplification of the depth and intensity
(i.e., the parameter describing the elemental loss and relative
amount of secondary minerals) of primary mineral weather-
ing (e.g., Fletcher et al., 2006; Lebedeva et al., 2007; Buss
et al., 2008; Behrens et al., 2015; Hampl et al. 2022a). Such
mechanisms comprise (1) a forcing process like the forma-
tion of secondary minerals that is triggering (2) a responsive
process such as more intense infiltration of fluids to depth.
The latter process reinforces the initial forcing process of
secondary mineral formation. Such a mechanism is therefore
called positive feedback between (1) and (2). The formation
of secondary minerals can also have a weathering-impeding
effect (e.g., Lohse and Dietrich, 2005; Navarre-Sitchler et al.,
2015; Kim et al., 2017; Gerrits et al., 2021) causing a miti-
gation of the weathering depth and intensity. Such mecha-
nisms comprise (1) a forcing process like the formation of
secondary minerals and (2) a responsive process such as re-
duced infiltration of fluids to depth. The latter process damps
the initial forcing process of secondary mineral formation,
and the mechanism is therefore called negative feedback be-
tween (1) and (2).
Deciphering the relationship between the formation of
secondary minerals and the climatic conditions they were
formed under is a prerequisite for understanding the weath-
ering system. It allows us to determine whether feedbacks
between the formation of secondary minerals and the infiltra-
tion of fluids affect the intensity and depth of primary mineral
weathering. We hypothesize that a positive feedback loop re-
sults in a deep weathering depth as secondary minerals form
fluid pathways by fracturing due to volume increase. On the
other hand, we think that a negative feedback loop leads to
a shallow weathering depth as the precipitation of secondary
minerals seals fluid pathways.
To explore such connections and to elucidate the im-
pact of secondary minerals on the development of weath-
ering systems in different climatic zones, we investigated
two 6 m deep weathering profiles in the Chilean Coastal
Cordillera. One profile is located in a Mediterranean cli-
mate zone (mean annual temperature: 14.9 ◦C, mean annual
precipitation: 436 mm yr−1) and another in a humid climate
zone (mean annual temperature: 14.1 ◦C, mean annual pre-
cipitation: 1084 mm yr−1) (Scheibe et al., 2023), and both de-
veloped from the weathering of granitic rock. Both sites are
eroding and the surfaces in the locations are thus constantly
turned over (see compilation of rates and environmental pa-
rameters in Oeser and von Blanckenburg (2020) and refer-
ences therein). The profiles were sampled in soil pits and
complemented with rock samples obtained by deep wireline
rotary drilling close to the soil pits. Samples were investi-
gated by a combination of analytical techniques such as X-
ray fluorescence (XRF), micro-X-ray fluorescence (µ-XRF),
and oxalate and dithionite extraction, which are used to char-
acterize the geochemical composition, and X-ray diffraction
(XRD), magnetic susceptibility measurements, electron mi-
croprobe (EMP), and light microscopy, which are used to
identify the mineral assemblages. The combined results of
these techniques are used to derive weathering-intensifying
and -mitigating processes during subsurface weathering and
to elucidate how these processes influence the depth and in-
tensity of weathering in the different climate zones.
Earth Surf. Dynam., 11, 511–528, 2023 https://doi.org/10.5194/esurf-11-511-2023

F. J. Hampl et al.: Feedbacks between the formation of secondary minerals and the infiltration of fluids 513
2 Study sites
2.1 La Campana (LC)
The soil pit (−33.02833◦N, −71.04370◦E, 894 m) and
the drilling site some 15 m next to it (−33.02833◦N,
−71.04354◦E, 898 m) are located south of the La Campana
National Park approximately 60 kmNW of Santiago de Chile
(Fig. 1a). They are situated on a ridge with steep slope dip
angles of 20–30◦.
The vegetation can be characterized as Mediterranean
sclerophyllous forest with Cryptocarya alba and Lithraea
caustica as dominant plants (Luebert and Pliscoff, 2006;
Oeser et al., 2018; Fig. 1b, c). The annual precipitation rate
(measured from April 2016 to April 2020) is 346 mm yr−1
(Übernickel et al., 2020), and the Holocene net primary pro-
duction is 280±50 g C m−2yr−1(Werner et al., 2018; Oeser
and von Blanckenburg, 2020). Records of long-term meteo-
rological data (e.g., precipitation at ground level, soil water
content, air temperature, relative humidity) from a weather
station near the study site can be found in Übernickel et
al. (2020).
The regolith profile developed on top of Upper Creta-
ceous intrusions of mainly granodiorites and tonalites with
subordinate quartz monzodiorites (Gana et al., 1996). The
depths of the soil horizons are as follows: A – 0–30 cm, B
– 30–83 cm, and C (saprolite) – >83 cm (Fig. 1d). Uplift
rates for the north of Santiago de Chile vary between 0.01
and 0.23 mm yr−1with a general mean value of 0.13 ±0.04
(Melnick, 2016). The soil denudation rate in the nearby La
Campana National Park is 53.7±3.4 (S-facing slope) to
69.2±4.6 t km−2yr−1(N-facing slope) (Oeser et al., 2018),
or assuming a material density of 2.6 gcm−3, 0.024 mm yr−1
on average.
2.2 Nahuelbuta (NA)
The investigated soil pit (−37.79371◦N, −72.95065◦E,
1113 m) and the drilling site next to it (−37.79381◦N,
−72.95043◦E, 1114 m) are located approximately 20 km
west of Angol (Region IX (Araucanía), Malleco Province)
in southern Chile (Fig. 1a). The borehole was located on a
plateau-like ridge with gently dipping slopes (ca. 10◦).
The pre-land-use vegetation in the study area resembled
the recent vegetation found in the Nahuelbuta National Park
which can be characterized as temperate forest with Arau-
caria araucana as the dominant tree (Luebert and Pliscoff,
2006; Fig. 1e). However, extensive modern pastoral farm-
ing (cow grazing) and fires have converted the ecosystem
in the study area to a sparse forest of deciduous trees such
as Nothofagus obliqua (see Oeser et al., 2018; Fig. 1f). Nu-
merous signs of burning can be observed in the field and
charcoal is an integral component of the soil down to 25cm
(A horizon). The precipitation rate (measured from the end
of March 2016 to April 2020) is 1927 mm yr−1(Übernickel
et al., 2020) and the Holocene net primary production is
520 ±130 g C m−2yr−1(Werner et al., 2018; Oeser and von
Blanckenburg, 2020). Records of long-term meteorological
data (e.g., precipitation at ground level, soil water content, air
temperature, relative humidity) from a weather station near
the study site can be found in Übernickel et al. (2020).
The regolith profile developed on top of granitoid rocks
of the Nahuelbuta central pluton which contains heteroge-
nous lithological portions (Hervé, 1977; Ferraris, 1979). It
is part of the Nahuelbuta Batholith which in turn belongs to
the Late Carboniferous Chilean Coastal Batholith (Steenken
et al., 2016; Deckart et al., 2013). The depths of the soil
horizons are as follows: A – 0–25 cm, B – 25–90 cm, and C
(saprolite) – >90 cm (Fig. 1g). Today’s exhumation rates in
NA are high (>0.2 mm yr−1; Glodny et al., 2008b), whereas
the catchment-wide denudation rate is small (27.4±2.4 mm
kyr−1; van Dongen et al., 2019) compared to LC. The soil de-
nudation rate in the nearby Nahuelbuta National Park ranges
between 17.7±1.1 (N-facing slope) to 47.5±3.0 t km−2yr−1
(S-facing slope) (Oeser et al., 2018), or assuming a material
density of 2.6 g cm−3, 0.013 mm yr−1on average. Tectonic
fractures in NA can be related to the Lanalhue Fault Zone
(see Glodny et al., 2008a).
3 Materials and methods
3.1 Soil pit sampling, drilling, and sample preparation
The sampled 6 m deep soil profiles were located close to the
main boreholes at the respective sites. Bulk samples were
collected in 20 intervals in each soil pit and weighed around
3 kg. Corestones were not encountered in the soil pit profiles
of LC and NA. By using a rotary splitter (type PT, Retsch) the
bulk samples were separated into aliquots (see Hampl et al.,
2022b). During the drilling campaigns, up to 1.5 m long core
runs were recovered by wireline diamond drilling (∼80 mm
core diameter) using potable water as drilling fluid (see
Krone et al., 2021 for a detailed description of the drilling
technique). Rock samples were separated from the core by
mechanical methods (angle grinder, hammer, and chisel), cut
(diamond saw), impregnated with blue artificial resin filling
the porosity, and subsequently thin-sectioned. Representa-
tive bedrock samples were separated from the core (diamond
saw) and crushed (jaw crusher).
3.2 Analytical methods and calculations
A detailed description of the analytical methods can be found
in the accompanying data publication of this study (Hampl et
al., 2022b).
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514 F. J. Hampl et al.: Feedbacks between the formation of secondary minerals and the infiltration of fluids
Figure 1. Overview of the study sites and soil profiles. (a) Position of La Campana (LC) and Nahuelbuta (NA) in Chile. Modified map data
from OpenStreetMap (© OpenStreetMap contributors). (b) Original vegetation in LC (i.e., before human intervention; La Campana National
Park). (c) Vicinity of the soil pit and drilling site in LC and (d) the first 2m of the soil profile in LC with inscribed soil horizons (A–C). A
prominent discontinuity (dashed line) can be found in the depth interval of 120–140 cm.(e) The original vegetation in NA (i.e., before human
intervention; Nahuelbuta National Park) in comparison to (f) the recent vegetation in the vicinity of the soil pit and drilling site. (g) The first
2 m of the soil profile in NA with inscribed soil horizons (A–C).
3.2.1 X-ray fluorescence (XRF) and micro-X-ray
fluorescence (µ-XRF)
Soil pit samples were ground with an agate disk mill and an-
nealed (950 ◦C for 1 h) before adding a lithium borate flux
to produce glass beads in platinum crucibles. The element
composition of the glass beads was analyzed with a Thermo
Scientific ARL PERFORM’X X-ray fluorescence sequen-
tial spectrometer (WD-XRF; Thermo Fisher Scientific Inc.,
USA). Additional powder pellets were produced by mix-
ing the ground air-dried samples with wax. The mixtures
were pressed and analyzed with a SPECTRO XEPOS en-
ergy dispersive X-ray fluorescence spectrometer (ED-XRF,
SPECTRO Analytical Instruments GmbH, Germany). Pol-
ished sample slabs of bedrock (Fig. 2) were mapped for the
spatial distribution of elements with a µ-XRF spectrometer
M4 Tornado (Bruker, Germany).
Geochemical calculations
Zr contents obtained from the XRF element analyses on pow-
der pellets were used as an immobile element for the calcu-
lation of the chemical depletion fraction (CDF; Riebe et al.,
2003; Eq. 1) and the mass transfer coefficient (τ; Anderson
et al., 2002; Eq. 2).
CDF =1−Zrb
N
Zrw
N
,(1)
τ=Xw·Zrb
Xb·Zrw−1,(2)
where Xbis the concentration of element Xin the bedrock,
Xwis the concentration of element Xin the weathered sam-
ple, Zrbis the concentration of Zr in the bedrock, Zrb
Nis
the zirconium content of the bedrock normalized to a LOI-
free (loss-on-ignition-free) sum of 100 % (see Hampl et al.,
2022b), Zrwis the concentration of Zr in the weathered sam-
ple, and Zrw
Nis the zirconium content of the weathered sam-
ple normalized to a LOI-free sum of 100 % (see Hampl et al.,
2022b).
The chemical index of alteration (CIA; Nesbitt and Young,
1982) was modified to 1CIA (Eq. 3).
1CIA = Al2O3w
Al2O3w+CaOw+Na2Ow+K2Ow
− Al2O3b
Al2O3b+CaOb+Na2Ob+K2Ob!#·100,(3)
where superscript w means in the weathered sample and su-
perscript b means in the bedrock.
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F. J. Hampl et al.: Feedbacks between the formation of secondary minerals and the infiltration of fluids 515
3.2.2 Oxalate and dithionite extractions
Air-dried bulk samples of <2mm (dry sieved) were used
for oxalate and dithionite extractions. The solutions thus ob-
tained were measured with an ICP-OES iCAP 6300 DUO
(Thermo Fisher Scientific, USA) to determine the extractable
Fe, Al, and Si contents. The oxalate extraction employed tar-
gets the easily extractable, mainly X-ray amorphous Fe(III)
oxyhydroxides and (poorly) crystalline Al-containing min-
erals (see review by Rennert, 2019 and references therein).
The dithionite extraction dissolves crystalline and amor-
phous iron oxides (McKeague and Day, 1966). In doing so,
it can (partly) attack Al-bearing (mineral) phases (see review
by Rennert, 2019 and references therein).
The oxalate extractions were performed after Schwert-
mann (1964) with an oxalic acid/oxalate extraction solution
(0.2 M, pH 3.0). After the addition of the solution to the sam-
ple and shaking for 2 h in the dark (overhead shaker), the so-
lution was filtered in a darkened room and immediately mea-
sured. The cold dithionite extractions were performed based
on Holmgren (1967) with an extraction solution (mixture of
0.2 M NaHCO3and 0.24 M trisodium citrate) and sodium
dithionite under oxic conditions. The resulting mixture of
chemicals and sample was shaken for 16 h and centrifuged
before the supernatant was filtered and immediately mea-
sured. Additional reference samples, blanks, and calibration
solutions were also prepared and measured like the soil pit
samples. The results of the samples presented here are the
mean of duplicate measurements performed on two individ-
ually extracted sample aliquots.
3.2.3 Grain size determination
Sample aliquots were suspended in deionized water
(<10 µS m−1) and dispersed in a rotating overhead shaker
(approximately 15 h) and a subsequent ultrasonic bath before
vibrational wet sieving. The >63 µm sieving fractions were
dried (50 ◦C, approximately 24 h) and their weight percent-
ages were measured. The clay and silt contents were deter-
mined using the <63 µm suspension and a pipette method.
Organic-rich samples were treated with H2O2to decompose
organic matter, and sodium pyrophosphate was used as a dis-
persion agent to prevent coagulation. Clay (<2 µm) was sep-
arated from the <63 µm fraction slurry via centrifugation.
3.2.4 X-ray diffraction (XRD)
Untreated air-dried aliquots of bulk samples were crushed
in a porcelain mortar and then processed with a microniz-
ing XRD-mill McCrone (Retsch, Germany) to obtain a fi-
nal powder of <10 µm. These powders were mounted to
XRD sample holders by backloading, and X-ray diffrac-
tion measurements were performed with a Rigaku SmartLab
equipped with a 9 kW rotating Cu anode and a HyPix-3000
detector in Bragg–Brentano geometry (3–80◦2θ, scan step:
0.01◦, scan speed: 1◦min−1, and 60 rpm sample rotation).
For the identification and semiquantitative analyses, the soft-
ware SmartLab Studio II and the mineral database PDF-4
Minerals 2021 including reference intensity ratio (RIR) fac-
tors were used. Image processing (imageJ, version 1.53a;
Schneider et al., 2012) performed on the µ-XRF element dis-
tribution maps in Fig. 2 was used to get rough compositional
information of the mineral content in the sampled bedrock.
These analyses were used as a supporting basis for the semi-
quantitative XRD analyses with RIR factors.
Clay mineral contents in the samples were quantitatively
estimated by combining the results of the grain size determi-
nation with the semiquantitative results of the XRD analy-
ses. The clay-size fraction (<2 µm) of which the mass was
determined by sieving/pipetting, was assumed to represent
the entire clay mineral content of the sample, while the other
size fractions were considered to be free of clay minerals.
This assumed clay mineral content (in wt %) was combined
with the XRD-semiquantitative weight percentages of the
primary minerals in the same sample to approximate the min-
eral composition of the whole soil pit sample (summarized to
100 wt%). Despite the assumption that only the <2 µm grain
size fraction contains clay minerals, this estimate appears to
be the most accurate because there are no matching files in
the mineral database used here that would accurately semi-
quantify the identified interstratified clay minerals.
The separated clay-size fractions were measured as ori-
ented clay films (texture preparation). A D2 Phaser XRD de-
vice (Bruker) equipped with a Cu anode was utilized for the
measurements. The diffractograms were recorded in Bragg–
Brentano geometry in the range of 3–35◦2θ(step width:
0.01◦2θ, 0.5 s per step). The samples were measured after
air-drying, during ethylene glycol saturation, and after a ther-
mal treatment at 550 ◦C for 1 h. Selected samples were also
treated with glycerol and KCl (1 M) to characterize the clay
minerals in more detail. The identification was supported by
a clay mineral identification chart (Starkey et al., 1984).
3.2.5 Magnetic susceptibility measurements
The magnetic susceptibility was measured on all 21
McCrone-milled bulk samples of the LC profile with a KLY-
3 Kappabridge (AGICO, Czechia). Measurements were per-
formed in triplicates at room temperature, at a frequency of
875 Hz, and a peak magnetic field of 300 Am−1.
To obtain the magnetite content of the bedrock, a repre-
sentative 60×60 mm sample slab (Fig. 2a) was mapped with
the µ-XRF spectrometer M4 Tornado. The µ-XRF map that
depicts only the maximum Fe content was used as an approx-
imation of the magnetite content since magnetite is the min-
eral with the highest Fe concentration in the rock. Finally,
the map was analyzed with the image-processing program
imageJ (version 1.53a; Schneider et al., 2012) to quantify
the magnetite content. The obtained value was equalled to
the measured magnetic susceptibility of the same sample and
used to convert the magnetic susceptibility results of the LC
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516 F. J. Hampl et al.: Feedbacks between the formation of secondary minerals and the infiltration of fluids
soil pit samples into approximated magnetite contents by the
rule of three. The investigated bedrock of NA contains no
magnetite.
3.2.6 Light microscopy and electron microprobe
analysis (EMPA)
Thin sections were investigated with the light microscope
DM750P (Leica, Wetzlar, Germany) equipped with a micro-
scope camera (Euromex, The Netherlands). Electron micro-
probe element distribution maps of selected areas were ob-
tained for Al, Ca, Fe, K, and Mg by using standard wave-
length dispersive techniques on a JEOL Superprobe JXA-
8230 fitted with a W-emitter electron gun (accelerating volt-
age: 15 kV, beam current: 20nA, beam diameter and step
width: 1 µm).
4 Results
The data tables (cited as Tables S1–S5) are included in the
accompanying data publication (Hampl et al., 2022b).
4.1 Bedrock
According to the Streckeisen nomenclature, the bedrock of
LC can be described as granodiorite, and the investigated
bedrock of NA can be described as granite. However, the
drill core revealed that the bedrock of NA occasionally con-
tains more mafic sections. The most abundant minerals in
the fine-grained bedrock of LC are plagioclase, quartz, mi-
crocline, hornblende, biotite, and chlorite (Fig. 2a, b). The
latter occurs solely and abundantly along with (former) bi-
otite crystals as their hydrothermal transformation products
(i.e., chloritization; e.g., Kogure and Banfield, 2000). Mag-
netite is a ubiquitous accessory mineral (Fig. 2c; <1 vol. %)
in LC and shows no signs of alteration to hematite (marti-
tization). Pyrite and chalcopyrite are also observed in much
smaller abundance than magnetite. Mafic xenoliths can fre-
quently be found in the granodiorite of LC.
In the coarse-grained Nahuelbuta granite, quartz, plagio-
clase, microcline, biotite, and chlorite are the main con-
stituents (Fig. 2d, e). In contrast to LC, amphiboles can only
be found as an accessory mineral (<1 vol. %) in the investi-
gated bedrock of NA. Like in LC, biotite is often chloritized.
Magnetite and sulfides could not be identified in the investi-
gated rock samples of NA. Variations in the biotite content,
the occurrence of amphibole crystals, differences in fabric
(microcline of a few centimeters), the alternation with mafic
portions, and the presence of pegmatites in the core make the
overall lithology of NA far more heterogenous compared to
LC.
Figure 2. Bedrock of the investigated profiles. (a) Bedrock from
La Campana (IGSN: GFFJH0095) with (b) a corresponding µ-XRF
map reflecting the spatial mineral distribution. (c) Aµ-XRF map of
the maximum Fe content (black dots) representing the magnetite
crystals in the bedrock sample slab of La Campana. (d) Typical un-
weathered granite from Nahuelbuta (IGSN: GFFJH00H0) and (e) a
µ-XRF map reflecting the mineral content of the same.
4.2 Regolith
4.2.1 Incipient weathering in rock
Weathered rock from the borehole of LC shows abundant
indications of weathering-induced fracturing (WIF) due to
Fe(II) oxidation in biotite, like fanned-out edges or open-
ing due to dilatation (Fig. 3a, b). Secondary minerals like
Fe(III) oxyhydroxides are subordinate and are mostly asso-
ciated with biotite. They are detectable as Fe enrichments at
the edge of biotite crystals and within the cracks encompass-
ing biotite (Fig. 3c, d). To a minor degree, Fe(III) oxyhydrox-
ides are also associated with hornblende. Nevertheless, most
microfractures in feldspar and quartz of the investigated thin
sections are solely filled with blue resin and are bare of any
secondary minerals.
Indications of WIF around biotite are also present in
weathered rock of NA (Fig. 4a, b). However, the cracks
are often filled and covered with Fe(III) oxyhydroxides and
clay minerals as observed with light microscopy (Fig. 4c)
and electron microprobe investigations. Unlike LC, weath-
ered rock in NA is characterized by distinct Ca-depletion and
Alenrichment in plagioclase which indicates partial dissolu-
tion (Fig. 4d–f). These alteration sites host secondary miner-
als covering the newly formed surfaces which were formed
by the dissolution of the plagioclase.
4.2.2 Saprolite and soil
Chemical alteration
The mass transfer coefficient τindicates moderate depletion
below 80 cm (not smaller than −0.2) in the LC soil pit profile
but clear depletion in the uppermost few decimeters where
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F. J. Hampl et al.: Feedbacks between the formation of secondary minerals and the infiltration of fluids 517
Figure 3. Rock weathering in La Campana (LC; porosity is rep-
resented by blue-dyed resin). (a) Thin section image (transmitted
light) of a weathered rock sample obtained from a depth of approxi-
mately 27 m in the LC drill core (IGSN: GFFJH00HY). (b) A detail
image of biotite showing signs of dilatation (dashed lines indicate
cleavage planes). (c) Secondary minerals in cracks around biotite.
(d) The electron microprobe map of the contact zone between bi-
otite and quartz/feldspar displays Fe enrichments at the interface.
Bt denotes biotite.
Na, K, Mg, Ca, Si, and P can reach up to τ= −0.5 and −0.6
(Fig. 5; Table S1). A pronounced P depletion can be detected
down to a depth of 1.4 m in LC. The chemical depletion frac-
tion (CDF) of LC and the bedrock-normalized chemical in-
dex of alteration (1CIA) indicate a weak chemical weather-
ing degree below ca. 0.5–1m, but minor chemical depletion
was analyzed down to the bottom of the 6m deep profile of
LC (see 1CIA; Fig. 5).
In contrast, Nahuelbuta is characterized by distinct chem-
ical depletion of Ca and Na (up to τ= −0.9; Fig. 5). K
is depleted to a depth of approximately 5 m, Si to a depth
of ∼6 m, and Mg shows moderate depletion (τ≥ −0.3)
throughout the profile. P is strongly depleted between ca. 2–
6 m (τ∼ −0.6) but the P content gradually increases from
a depth of approximately 3 m towards the surface and is
enriched in the uppermost ∼20 cm of the soil (A horizon;
Fig. 5). The CDF values of NA indicate depletion down to
the bottom of the profile at a depth of 6 m. The 1CIA of the
profile underpins strong chemical alteration compared to the
bedrock (Fig. 5). However, overall chemical depletion de-
creases towards the bottom of the soil profile and according
to the τvalues in 550–600 cm, only Na, Ca, and P seem to
be significantly depleted at a depth >6 m.
Since many secondary minerals are formed via a
metastable or amorphous precursor (e.g., Steefel and van
Cappellen, 1990; Hellmann et al., 2012; Behrens et al.,
2021), we assume that the extractable Fe, Si, and Al con-
tents are indicative of recent weathering of primary minerals
Figure 4. Rock weathering in Nahuelbuta (NA). (a) Thin sec-
tion image (transmitted light) of weathered rock obtained from ap-
proximately 6 m depth in the NA drill core (note that the porosity
(blue) is largely associated with weathered plagioclase; IGSN: GF-
FJH00HX). (b) Indications of WIF in quartz (backscattered electron
image, EMP). (c) Thin section image (transmitted light) of a crack
covered with brown Fe(III) oxyhydroxides from a depth of approx-
imately 12 m (IGSN: GFFJH00J2). (d) Backscattered electron im-
age (EMP) of partly dissolved plagioclase and (e) the respective Ca
and (f) Al map of the section (IGSN: GFFJH00HX). Qz denotes
quartz, Bt denotes biotite, Pl denotes plagioclase, and Kfs denotes
potassium feldspar.
(Fig. 6; see Sect. 3.2.2 for an assignment of the extractable
elements to the minerals they likely originate from).
Extractable contents of Fe in LC are moderately elevated
in the uppermost meter of the profile (up to Fedit /Fetot ∼
14 %) compared to the other depth intervals which show low
contents (Fedit /Fetot<1 m: ∼4 %–5 %; Table S2; Fig. 6a,
b). The extractable Si contents show no clear pattern (Fig. 6c,
d), whereas oxalate- and dithionite-extractable Al contents
are variable in the profile of LC (Fig. 6e, f). The elevated
Aldid /Altot value in the depth interval of 120–140cm in LC
(∼0.5 %; Fig. 6f) coincides with a discontinuity in the sapro-
lite (Fig. 1d) and may indicate more secondary crystalline
and amorphous Al-bearing phases in this section. The pro-
file in NA is characterized by high amounts of extractable
Fe, Si, and Al contents which are especially elevated in the
uppermost meter of the profile (Fedit /Fetot up to ∼40 %,
Sidit /Sitot up to ∼0.14 %, Aldit /Altot up to ∼12 %). The
extractable contents rapidly decrease from the surface to-
wards the bottom of the NA profile and starting at approx-
imately 2 m, they are similar down to 6m (Fig. 6).
The Fe2O3content in the investigated bedrock of LC is
more than twice as high as that of the NA bedrock, but the
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518 F. J. Hampl et al.: Feedbacks between the formation of secondary minerals and the infiltration of fluids
Figure 5. τvalues of Na, K, Mg, Ca, Si, and P as well as the CDF (all based on Zr) and 1CIA values of the soil pit profiles in La Campana
(LC) and Nahuelbuta (NA; Table S1). Note that the scales are equal for the individual indices of LC and NA; dep. denotes depletion and enr.
denotes enrichment.
oxalate- and dithionite-extractable Fe contents (and hence
the amount of the respective secondary minerals) are far
higher in NA (Fig. 6a, b). The difference between LC and
NA is even more pronounced for the extractable Al contents
as values in NA can be 10 times higher than in LC (Fig. 6e,
f). The extractable contents in the profiles of both study sites
are generally within the range of previous investigations on
soil samples from the La Campana and Nahuelbuta National
Parks, but the Fedit /Fetot contents in the upper profile sec-
tion of NA in this study are much higher (up to 40%) than
those measured in the Nahuelbuta National Park (<25 %;
Oeser et al., 2018).
Mineral content and grain sizes
The sieving results of LC show a gradual decrease in particle
size from the bottom of the profile towards the surface and a
relatively constant sand-size content ranging from 65wt %–
80 wt % with similar portions of the individual sand-size
fractions (Fig. 7a). The small geochemical depletion below
the uppermost ∼2 m of the LC profile (Fig. 5) is also re-
flected in the little changing mineral composition of the
profile (Fig. 7b). Only the plagioclase (Ca–albite) content
slightly decreases from a depth of approximately 1 m towards
the surface. A small decrease of biotite in the depth interval
of 120–140 cm coincides with the mentioned discontinuity
of this profile section (Fig. 1d). The abundant chlorite of the
investigated bedrock in LC (∼5wt%) is completely weath-
ered and absent from the soil pit samples (Fig. 7b).
Significant alteration of magnetite (e.g., martitization)
could not be observed in ore microscopic investigations of
the magnetic particles in soil pit samples of LC. Thus, the
magnetic susceptibility directly reflects the magnetite con-
tent of the samples (e.g., Ferré et al., 2012). A relative mag-
netite enrichment was detected in the uppermost 40 cm of
the LC profile (1–1.6 vol. %), whereas the rest of the pro-
file shows approximately constant magnetite contents (mean
∼0.9 vol. %) close to the value of the investigated bedrock
(0.94 vol. %; Fig. 7c). This almost consistent magnetite con-
tent underlines the homogeneity of the bedrock that was
weathered in the 6 m deep soil pit (i.e., no mafic dikes, peg-
matites, or major xenoliths).
The soil pit profile of NA is characterized by a much
higher gravel-, silt-, and clay-size content compared to LC
(Fig. 7d). This reflects the more heterogeneous grain size
distribution of the investigated bedrock in NA compared to
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F. J. Hampl et al.: Feedbacks between the formation of secondary minerals and the infiltration of fluids 519
Figure 6. Oxalate- and dithionite-extractable Fe, Si, and Al contents divided by the respective total element contents of the bulk soil pit
samples of La Campana (LC) and Nahuelbuta (NA; Table S2). The elevated ratio at 120–140 cm in (f) (arrow) coincides with the position of
a discontinuity in the profile (Fig. 1d). Note that the scales for LC and NA are equal in (a–d). The scale in (e) and (f) is 1 order of magnitude
larger for NA compared to LC.
the bedrock of LC (see Fig. 2). High clay contents can be
detected in the uppermost meter of the NA profile (partly
>20 wt %), and the identified mineral content of the soil pit
samples differs significantly from the mineral content of the
investigated bedrock (Fig. 7e). The plagioclase (Ca–albite)
content distinctly decreases from the bottom of the profile
towards the surface and the bedrock content of ∼28 wt %
partly decreases down to 1 wt % in the soil pit. The micro-
cline content on the other hand is relatively uniform. Just as
in LC, the chlorite of the bedrock analyzed here (∼1 wt %) is
completely weathered in the NA soil pit profile and is absent
from the samples.
The mineral content of the clay-size fraction in LC dif-
fers significantly from that in NA (Fig. 8). La Campana is
characterized by abundant expandable clay minerals (inter-
stratified chlorite–smectite and interstratified mica–smectite)
which can largely be traced back to the weathering of chlo-
rite and biotite (Fig. 8a). Kaolinite can be found through-
out the LC profile, whereas interstratified mica–vermiculite
only occurs in the depth interval of the discontinuity (120–
140 cm; see Fig. 1d). The expandable portion of the inter-
stratified minerals gradually decreases from the profile bot-
tom towards the surface and cannot be detected in the upper-
most centimeters of the LC profile. Only mica and kaolinite
constitute the clay-size fraction of the uppermost part of the
profile in LC. The mineral content in the clay-size fraction
of NA is characterized by small amounts of interstratified
mica–vermiculite below a depth of 1m and ubiquitous kaoli-
nite, which shows small expandable portions below a depth
of 2 m. Hydroxy-interlayered vermiculite (HIV) and gibbsite
can first be detected in 400–450 cm depth and the content
increases towards the surface. The main minerals of the clay-
size fraction in the uppermost part of the profile are HIV,
kaolinite, and gibbsite (Fig. 8b).
5 Discussion
5.1 Climate-dependent mineral transformations
Chemical depletion and mineral transformations are far more
pronounced in the profile of NA compared to the profile of
LC, even though the bedrock of LC contains more minerals
with higher solubility compared to NA (more plagioclase, bi-
otite, chlorite, or hornblende in LC than in NA where quartz
and potassium feldspar dominate; see e.g., Wilson, 2004;
Bandstra et al., 2008). The high chemical depletion (τ[Na,
Ca] up to −0.9 and 1CIA up to 22; Fig. 5) and the occur-
rence of gibbsite in NA are indicative of distinct dissolution
of primary minerals (especially plagioclase; Fig. 7e) and so-
lute removal of alkali and alkaline earth metals, while im-
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520 F. J. Hampl et al.: Feedbacks between the formation of secondary minerals and the infiltration of fluids
Figure 7. Sieving and pipetting results, semiquantitative XRD results, and approximated magnetite contents of the investigated soil pit
samples in La Campana (LC) and Nahuelbuta (NA). (a) Grain size distribution based on wet sieving and pipetting, (b) semiquantitative
XRD, and (c) magnetic susceptibility results converted to approximate magnetite contents of the LC profile. (d) Wet sieving combined with
pipetting results and (e) semiquantitative XRD results of the NA samples. Semiquantitative XRD results of the investigated bedrock samples
(see Fig. 2) are given below the results of the soil pit samples. In panels (a) and (d), 1: ≤2000 to >1000 µm, 2: ≤1000 to >500 µm, 3: ≤500
to >250 µm, 4: ≤250 to >125 µm, 5: ≤125 to >63 µm.
Figure 8. Minerals in the clay-size fraction of the soil pit profiles in La Campana (LC) and Nahuelbuta (NA). (a) The profile in LC features
abundant expandable clay minerals. (b) NA is characterized by the presence of gibbsite and vermiculite but very minor amounts of expandable
clay minerals. C–S denotes interstratified chlorite–smectite, G denotes gibbsite, HIV denotes hydroxy-interlayered vermiculite, K denotes
kaolinite, K+denotes kaolinite with expandable portions, M–S denotes interstratified mica–smectite, and M–V denotes interstratified mica–
vermiculite.
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F. J. Hampl et al.: Feedbacks between the formation of secondary minerals and the infiltration of fluids 521
mobile Al remains as hydroxide (Al(OH)3=gibbsite). This
depletion is assumed to be the result of more water infiltra-
tion into the subsurface of NA (more precipitation due to hu-
mid climate) compared to LC (less precipitation due to the
Mediterranean climate). The measured τ[P] distribution in
NA is a clear indication of biologically controlled nutrient
uplift and recycling within the topsoil (Jobbágy and Jack-
son, 2004). We assume that the high precipitation rate in NA
leads to more biomass production by plants, which in turn
implies more litter production and a stimulation of biogenic
decay that supplies plants with nutrients. Thus, we concur
with the hypothesis that the ecosystem in NA is thriving on
nutrient recycling rather than on an uptake of nutrients that
were released by biogenic weathering at depth (Oeser and
von Blanckenburg, 2020). Apart from Ca, Na, and P (τval-
ues in Fig. 5), the chemical depletion successively decreases
from the surface towards the bottom part of the investigated
profile in NA. To account for this shallow chemical depletion,
we propose that a secondary-mineral-controlled impeding of
the fluid infiltration to depth plays an important role for the
depth of mineral transformations in NA.
Chemical depletion can be detected throughout the inves-
tigated profile in LC, but the chemical weathering degree
is very low between 2–6m depth (Fig. 5), and the mineral
transformations in this section of the profile are only minor
(Fig. 7b). On the other hand, there is proof of distinct min-
eral dissolution and removal of solutes by the higher mag-
netic susceptibility values in the uppermost decimeters of the
LC profile. This can be related to a residual accumulation
of weathering-resistant magnetite while other minerals like
plagioclase dissolve. The strong chemical depletion in this
part of the profile is also reflected by the low τand elevated
CDF as well as 1CIA values. To account for the detected
weak but deep chemical weathering in LC, we propose that
a secondary-mineral-controlled formation of fluid pathways
facilitates the fluid infiltration to depth and is thus an im-
portant control on the chemical weathering reactions in the
subsurface.
The difference between the profiles is also displayed by the
oxalate- and dithionite-extractable Fe, Si, and Al contents.
While high extractable contents especially within the upper-
most 2 m of the NA profile are interpreted to indicate consid-
erable ongoing (recent) transformations of primary to sec-
ondary minerals, LC shows comparatively little indications
in this regard. This difference underlines the higher degree
of mineral transformations in NA compared to LC which is
also reflected in the mineral content of the clay-size fraction
(see Fig. 9). Oxalate-extractable Al contents frequently ex-
ceed dithionite-extractable Al contents which is indicative
of amorphous phases since oxalate is more effective at ex-
tracting amorphous forms of Al (McKeague and Day, 1966).
Moreover, the highest contents of the clay-size fraction in the
profiles are in good correlation with the elevated extractable
Fe, Si, and Al contents and highlight the pronounced min-
eral transformation in the uppermost part of the profiles. This
size fraction hosts most of the products of primary silicate
weathering. Clay-size minerals of NA mainly correspond to
distinct weathering of plagioclase and biotite, whereas in
LC, they can mainly be associated with chlorite and biotite
weathering (Fig. 9). Feldspar weathers to kaolinite and gibb-
site in NA and biotite weathers to hydroxy-interlayered ver-
miculite (HIV). Chlorite completely dissolved in the NA pro-
file, whereas both chlorite and biotite in LC weather via in-
terstratified clay minerals to smectite. Finally, smectite and
feldspar likely weather to kaolinite in LC (Fig. 9). The min-
eral composition of the clay-size fraction (Fig. 8) is depen-
dent on the bedrock composition (e.g., more chlorite in LC)
and the climate-dependent mineral dissolution (see Fig. 9) in
the study sites. However, we argue that the amount of sec-
ondary minerals is largely a function of the climatic condi-
tions that control the weathering intensity via water avail-
ability in the study sites.
As this study does not consider the entire weathering pro-
file in LC and NA, the interplay between erosion rate and
weathering advance rate (Lebedeva and Brantley, 2020) is
not addressed here. However, the different denudation rates
in the study areas (mean soil denudation rate in LC: ∼61,
in NA: ∼33 t km−2yr−1; Oeser et al., 2018) likely affect the
weathering intensity. Due to the higher denudation rates in
LC compared to NA (Oeser et al., 2018; van Dongen et al.,
2019), we hypothesize that the residence time of weathered
material in the regolith of LC is shorter than in NA. Thus,
there is less time for chemical weathering in LC. In combina-
tion with the lower water availability in LC, this factor might
contribute to the lower weathering intensity in the regolith of
LC compared to NA. This would also concur with the find-
ing that water availability in the soil and soil residence time
are the limiting factors for weathering processes in dry envi-
ronments (Schoonejans et al., 2016). NA, on the other hand,
is characterized by a longer residence time of weathered ma-
terial. Together with the higher water availability, this factor
might contribute to the high weathering intensity in the up-
per regolith of NA. The situation in LC may be comparable to
an incompletely developed profile, while the situation in NA
may be comparable to a completely developed profile (Reis
and Brantley, 2019).
5.2 Weathering-intensifying processes
5.2.1 Porosity increase by weathering-induced
fracturing and its impact on the weathering depth
Ferrous primary minerals of the LC granodiorite can fre-
quently be identified as initiating locations of microcracks.
This observation can be related to weathering-induced frac-
turing (WIF) due to the increase in volume caused by the
oxidation of Fe(II) in Fe(II)-bearing silicates (e.g., Buss et
al., 2008; Behrens et al., 2015; Kim et al., 2017) and the for-
mation of secondary Fe(III) oxyhydroxides (Fletcher et al.,
2006; Lebedeva et al., 2007; Anovitz et al., 2021; Fig. 3d).
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522 F. J. Hampl et al.: Feedbacks between the formation of secondary minerals and the infiltration of fluids
Figure 9. Schematic diagram showing the transformation of pri-
mary minerals to secondary minerals (clay minerals and aluminum
hydroxide) depending on the activities of H+, Si, Al, K, Na, Mg,
and Ca. The depletion of the alkali and alkaline earth metals as well
as the increase of the Al activity are coupled to an increase of the
mineral dissolution and the removal of solutes by a higher subsur-
face fluid flow. Elevated aH+values (i.e., lower pH) increase the
mineral solubility. Modified from Chesworth et al. (2008). HIV de-
notes hydroxy-interlayered vermiculite and “a” denotes thermody-
namic activity
This process generates and increases surface areas of pri-
mary minerals and in turn accelerates weathering reactions
(positive feedback between the formation of secondary min-
erals and the infiltration of fluids (especially O2and water)
to depth; e.g., Røyne et al., 2008). These weathering-induced
fractures consequently facilitate the presence of surface-
derived O2in the deep subsurface (Kim et al., 2017) and
the corresponding transport through the saprolite and soil is
dominated by advection (Lebedeva et al., 2007). The bedrock
of LC is richer in Fe-bearing minerals than the investigated
granite of NA (ca. 25wt % in LC and ca. 10 wt % in NA) and
hosts biotite, hornblende, chlorite, and magnetite as Fe(II)
sources. A considerable amount of the total Fe content is
bound in magnetite (roughly 0.7 wt% of the total Fe2O3con-
tent if the magnetite content of the bulk sample is 1 wt %).
However, we found no microscopic evidence (no oxidation)
nor indications in the magnetic susceptibility results that
the Fe(II) in magnetite is available for weathering reactions.
Thus, we conclude that magnetite is stable under the envi-
ronmental conditions of LC. Of the three remaining Fe(II)-
bearing minerals, biotite was found to be the most impor-
tant one for the generation of WIF in LC (see also Buss et
al., 2008; Bazilevskaya et al., 2013, 2015) due to its volu-
metric expansion during weathering (e.g., Goodfellow et al.,
2016). Although WIF also occurs in NA (Fig. 4b), it does not
seem to significantly increase the permeability of the rock
which can be related to the low Fe(II) content of the domi-
nant bedrock (Fe2O3(total Fe): <3 wt%; Table S1; see Kim
et al., 2017).
Other than that, chlorite is suggested to be an important
mineral in the development of the investigated weathering
profile in LC. The original chlorite content of the bedrock
in LC has been completely transformed into interstratified
chlorite–smectite in the soil pit profile. We suggest that this
transformation plays a significant role for the development of
the LC profile since expandable clay minerals are known to
disaggregate rock by swelling (e.g., Dunn and Hudec, 1966;
Jiménez-González et al., 2008). The ensuing fracturing also
forms new fluid pathways and new access to reactive sur-
faces of primary minerals which in turn fosters weathering
reactions (positive feedback mechanism; see e.g., Røyne et
al., 2008). Even though expandable clay minerals can also
cause sealing of the subsurface (Kim et al., 2017), we do not
regard this effect as significant for LC since clay contents are
very low. However, a minor retardation of the fluid flow from
surface to depth due to the expansion of the interstratified
clay minerals in LC (Fig. 8a) cannot be excluded (see Kim
et al., 2017). In conclusion, we propose that small amounts
of expandable clay minerals like in LC can generate poros-
ity, whereas high amounts of expandable clay minerals can
reduce porosity.
The feedback mechanism of weathering-induced fractur-
ing is presented here for granodiorite. However, the signifi-
cance of this mechanism is not restricted to plutonic rocks.
Weathering-induced fracturing requires Fe(II)-bearing min-
erals such as biotite and/or potentially the presence of ex-
pandable clay minerals that cause the formation of cracks by
volume increase during weathering. This feedback concept is
thus transferable to all igneous, metamorphic, and sedimen-
tary rocks that contain these minerals.
5.2.2 Increase of weathering intensity by biogenic
activity
The formation of secondary minerals such as clay minerals
and aluminum hydroxide is among other factors controlled
by biogenic activity since organic acids and an acidity in-
crease by elevated organic-derived CO2contents accelerate
dissolution rates of primary minerals (see e.g., Lucas, 2001;
Lawrence et al., 2014). This effect needs to be considered for
the organic-rich and acidic subsurface of NA (see Bernhard
et al., 2018). The acidity likely contributes to the high de-
gree of mineral dissolution in NA (see aH+in Fig. 9), which
consequently leads to an increased formation of secondary
minerals.
The depth interval of 120–140 cm in LC is characterized
by lower amounts of biotite and a different clay mineral
composition compared to the surrounding depth intervals
(Figs. 7b, 8a). This depth interval coincides with a discon-
tinuity crossing the entire profile width (Fig. 1d). We in-
terpret this plant-root-containing discontinuity in the sapro-
lite as a fracture remnant since there are no indications of
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F. J. Hampl et al.: Feedbacks between the formation of secondary minerals and the infiltration of fluids 523
a lithological heterogeneity in this zone (e.g., a significant
change in the magnetic susceptibility or of the primary min-
eral content; Fig. 7b, c). To explain the lower biotite content
and the different clay composition in this part of the profile,
we propose an intensification of weathering reactions in the
vicinity of the fracture fostered by the observed plant roots
(e.g., Fimmen et al., 2008; Pawlik et al., 2016; Nascimento et
al., 2021). This weathering-promoting mechanism might ac-
count for the increase in interstratified chlorite–smectite and
the appearance of interstratified mica–vermiculite (Fig. 8a),
while the amount of biotite decreases due to its transforma-
tion to secondary minerals (Fig. 7b).
5.3 Weathering-mitigating processes
5.3.1 O2consumption by Fe-bearing silicates and its
impact on the weathering depth and intensity
The granodiorite of LC hosts an abundance of Fe(II)-bearing
minerals (Fig. 7b). The Fe2O3content of the LC bedrock af-
ter subtraction of the inert magnetite-bound Fe2O3fraction
(since 100 % pure magnetite contains 69 % Fe2O3, 0.94 %
magnetite as analyzed in the LC bedrock equals 0.65 %
magnetite-bound Fe2O3which needs to be subtracted) is
5.34 wt % (for comparison: 2.33 wt % Fe2O3in NA). Since
O2is reduced by the oxidation of mineral-bound Fe(II) (e.g.,
White and Yee, 1985; Perez et al., 2005) and the consequent
formation of secondary minerals, the O2content and hence
oxidative weathering reactions are expected to decrease from
surface to depth. A rapid decrease of the O2concentration to
depth is characteristic for weathering systems in which O2
transport is dominated by diffusion (Behrens et al., 2015).
Given the observed deep fracturing due to Fe(II) oxidation
(i.e., WIF) in LC and the consequent deep connectivity be-
tween the surface and the subsurface (Kim et al., 2017),
the O2transport in LC is most likely dominated by advec-
tion. As a consequence, diffusive O2transport is insignif-
icant in the upper regolith of LC and the O2consumption
by Fe(II) oxidation is not limiting the regolith depth in LC
(compare Bazilevskaya et al., 2013). It has been argued that
WIF and thus a thicker regolith is more likely when the ra-
tio pO2/pCO2in soil water is greater than the ratio of the
capacity for O2consumption to the capacity for CO2con-
sumption in bedrock (Stinchcomb et al., 2018). In the study
sites, decomposition of organic matter is restricted to the top-
soil, likely because organic matter at depth becomes stabi-
lized against microbial decomposition (Scheibe et al., 2023).
Thus, we suggest that the pCO2of water in the deeper profile
part of LC is low (i.e., pO2/pCO2is high), and O2is not be-
ing consumed by organic matter decomposition but is avail-
able for Fe(II) oxidation and hence WIF. The WIF-controlled
connectivity between the surface and the subsurface results
in an O2availability for oxidative weathering processes at
great depth. On the other hand, the weak chemical weather-
ing in LC is in good agreement with the low precipitation rate
(∼350 mm yr−1; Übernickel et al., 2020). The low precipi-
tation rate entails a small infiltration of water to depth which
in turn leads to minor primary mineral dissolution and thus
chemical weathering at depth.
The cracks around weathered biotite in the investigated
samples of LC are (mainly) filled with Fe(III) oxyhydrox-
ides as revealed by the high Fe enrichment detected in elec-
tron microprobe maps (Fig. 3d). Newly formed weathering-
induced fractures make the biotite more accessible to sur-
face inputs like water and O2which promotes the dissolution
of biotite. The solutes formed as a result migrate along the
weathering-induced cracks and precipitate in the vicinity of
the biotite crystal as secondary phases (Fig. 3c). Thus, we
propose that the reactive surface of biotite is partly shielded
from weathering reactants (water, O2) due to the precipita-
tion of secondary minerals (see e.g., Navarre-Sitchler et al.,
2015; Vázquez et al., 2016; Gerrits et al., 2020, 2021). Com-
bined with the low subsurface water availability in LC caus-
ing a low mineral dissolution degree, this shielding might
contribute to the relatively stable biotite content throughout
the LC profile (Table S4).
5.3.2 Reduction of weathering intensity and depth by
damping of fluid flow
The formation of secondary minerals such as clay miner-
als (via amorphous and poorly crystalline precursors; see
Fig. 6) can decrease the porosity (e.g., Bazilevskaya et al.,
2015; Navarre-Sitchler et al., 2015) formed by WIF and dis-
solution. Al-rich phases were found as precipitates in partly
dissolved plagioclase of NA (Fig. 4d–f) and within cracks
which can often be identified as weathering-induced cracks.
We suggest that the abundant presence of clay minerals and
gibbsite in NA restricts the fluid flow through such fractures
and pores. The clay-rich zone in the uppermost meter of the
NA soil pit profile (around 50cm depth; Fig. 7d) likely acts
as a (partially) shielding horizon (impeding vertical flow of
surface inputs to the deep subsurface; see e.g., Lohse and Di-
etrich, 2005). Clay-rich horizons can therefore influence the
dynamics of the subsurface fluid flow and thus mitigate min-
eral transformations and chemical weathering at depth. At
the same time, these conditions foster a long fluid residence
time in the upper regolith and thus promote the precipitation
of secondary minerals such as clay minerals that may im-
pel the weathering of primary minerals in the upper part of
the weathering profile (see Maher, 2010). However, the sea-
sonal sealing of fractures and pore spaces due to an increase
of soil moisture and an ensuing clay expansion (Kim et al.,
2017) is not assumed for NA as expandable secondary min-
erals barely occur in the clay-size fraction of NA (Fig. 8b).
The negative feedback mechanism presented here is
demonstrated for granite. However, the concept is essentially
based on newly formed minerals such as clay minerals that
inhibit the subsurface fluid flow by blocking pathways. This
feedback mechanism can thus be significant for weathering
https://doi.org/10.5194/esurf-11-511-2023 Earth Surf. Dynam., 11, 511–528, 2023

524 F. J. Hampl et al.: Feedbacks between the formation of secondary minerals and the infiltration of fluids
Figure 10. Schematic summary of the two weathering systems. According to our findings, the regolith of La Campana (LC) is dominated
by a positive feedback loop between weathering-induced fracturing (WIF) and the infiltration of fluids to depth. WIF creates deep-reaching
pathways for fluids (water, O2) and hence a good connectivity between the surface and the subsurface. Moreover, the low water availability in
the Mediterranean climate inhibits the formation of large amounts of secondary minerals (i.e., low weathering intensity) that could seal these
pathways. The high denudation rate in LC results in a short residence time of weathered material in the profile and could therefore contribute
to the detected lower weathering intensity (i.e., less chemical weathering). The regolith of Nahuelbuta (NA), on the other hand, was found to
be dominated by a negative feedback loop between the formation of secondary minerals and amorphous phases and the infiltration of fluids
to depth. These secondary solids are consequences of the high water availability in NA that results in intense chemical weathering (i.e., high
weathering intensity). The high weathering intensity entails the formation of abundant secondary minerals and amorphous phases that reduce
the connectivity between the surface and the subsurface. The lower denudation rate and thus longer residence time of weathered material in
NA likely contributes to the more intense chemical weathering; dep. denotes depletion.
systems that develop from all igneous, metamorphic, and
sedimentary rocks where secondary minerals can block the
permeable porosity formed by WIF and primary mineral dis-
solution.
6 Conclusions
In two 6 m deep weathering profiles formed on granitic rock
in two climatic zones (Mediterranean and humid climate), we
found different degrees of elemental loss by chemical weath-
ering and different secondary minerals. Under Mediterranean
climate conditions (La Campana), Fe(II) oxidation, precipi-
tation of Fe(III) oxyhydroxide, and clay swelling lead to frac-
turing and the formation of fluid pathways. This weathering-
induced fracturing (WIF) is likely one of the dominant con-
trols in the development of the upper regolith as it leads to
a deep infiltration of surface inputs (especially water and
O2) which in turn causes further WIF. While the intensity
of chemical weathering at the Mediterranean site is low, it
was detected throughout the entire 6 m deep profile. This
suggests that the weathering front is located at much greater
depth in La Campana. The overall low abundance of sec-
ondary minerals can be explained by the low climate-related
subsurface water availability in La Campana. The lack of
large quantities of secondary minerals ensures that fractures
and porosity generated by WIF remain accessible to water
and gases. Thus, we conclude that the development of the
deep but weak chemical weathering in the upper regolith of
La Campana is significantly controlled by two mechanisms:
(1) a positive feedback loop between the formation of sec-
ondary minerals and the infiltration of fluids to depth induced
by (mainly) biotite weathering (WIF) which leads to a deep
surface–subsurface connectivity for weathering reactants (in
particular O2) and (2) low subsurface water availability re-
sulting in a low amount of secondary minerals which would
otherwise seal this connectivity.
Under humid climate conditions (Nahuelbuta), clay min-
erals, gibbsite as well as amorphous and poorly crystalline
secondary minerals largely formed due to intense plagio-
clase dissolution. We link this intense dissolution to the high
climate-related subsurface water availability in Nahuelbuta.
It is suggested that the secondary minerals impede the flow
of surface inputs to depth. Moreover, the generally lower
amount of Fe(II)-bearing silicates in Nahuelbuta compared
Earth Surf. Dynam., 11, 511–528, 2023 https://doi.org/10.5194/esurf-11-511-2023

F. J. Hampl et al.: Feedbacks between the formation of secondary minerals and the infiltration of fluids 525
to La Campana results in less WIF and thus fewer fluid path-
ways. Therefore, we conclude that the development of the
weathering profile in Nahuelbuta is predominantly governed
by two mechanisms: (1) considerable climate-related subsur-
face water availability and high biogenic activity which lead
to intense weathering of primary minerals in the upper part
of the regolith and (2) a negative feedback loop between the
formation of secondary minerals and the infiltration of flu-
ids to depth induced by (mainly) plagioclase weathering and
the ensuing formation of secondary minerals which leads to
a poor surface–subsurface connectivity for weathering reac-
tants. The main findings and factors that are most relevant
to the development of the different weathering systems are
summarized in Fig. 10.
The relationship between precipitation and the degree
of chemical weathering along the climate gradient of the
Chilean Coastal Cordillera was found to be non-linear and
non-systematic (Oeser and von Blanckenburg, 2020; Schaller
and Ehlers, 2022). We argue that a systematic relationship is
likely concealed by variations in the mineral content of the
bedrock and the associated feedback mechanisms. However,
the investigated feedbacks provide a causal explanation for
the depth of chemical weathering. This study illustrates how
the formation of secondary minerals and the infiltration of
surface-derived fluids to depth are interlinked by positive and
negative feedback loops. We demonstrated that these feed-
back loops and the climatic conditions they occur under are
important controls in the development of the upper regolith.
Data availability. Datasets related to this article can
be found in the data publication Hampl et al. (2022b)
(https://doi.org/10.5880/fidgeo.2022.035). The data publica-
tion is hosted at the GFZ Data Services and can be downloaded by
clicking on “Download data and description” in the field “Files”.
Sample availability. The IGSN-registered samples used in this
article are deposited at the Department of Applied Geochemistry
(Technische Universität Berlin) and are listed in the data publica-
tion of this paper (https://doi.org/10.5880/fidgeo.2022.035, Hampl
et al., 2022b).
Author contributions. FJH: conceptualization, methodology, in-
vestigation, writing – original draft preparation. FS: methodology,
supervision, writing – review and editing. CS: investigation, writing
– review and editing. NS: investigation, writing – review and edit-
ing. CB: funding acquisition, writing – review and editing. FvB:
supervision, writing – review and editing. TN: funding acquisition,
supervision, writing – review and editing.
Competing interests. The contact author has declared that none
of the authors has any competing interests.
Disclaimer. Publisher’s note: Copernicus Publications remains
neutral with regard to jurisdictional claims in published maps and
institutional affiliations.
Special issue statement. This article is part of the special issue
“Earth surface shaping by biota (Esurf/BG/SOIL/ESD/ESSD inter-
journal SI)”. It is not associated with a conference.
Acknowledgements. This work was supported by the German
Research Foundation (DFG) priority research program SPP-1803
“EarthShape: Earth surface shaping by biota” (grant no. NE 687/9-
1) and the EarthShape Coordination (grant nos. EH 329/17-2 and
BL562/20-1). We are grateful to Kirstin Übernickel for the man-
agement of the drilling campaigns and to Andreas Kappler for his
support. We would also like to thank Michael Facklam for his help
in determining the clay content and Katja Emmerich for her valu-
able hints on the clay mineralogy. The authors would also like to
thank Peter Finke, Veerle Vanacker, and Susan L. Brantley for their
valuable comments and suggestions that greatly improved the paper.
Finally, we are grateful to Antonia Roesrath for her help in register-
ing the samples.
Financial support. This research has been supported by the
Deutsche Forschungsgemeinschaft (grant no. NE 687/9-1).
This open-access publication was funded
by Technische Universität Berlin.
Review statement. This paper was edited by Veerle Vanacker and
reviewed by Susan Brantley, Peter Finke, and Veerle Vanacker.
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