Impact of low denudation rates on soil chemical weathering intensity: A multiproxy approach

Accepted Manuscript Impact of low denudation rates on soil chemical weathering intensity: A multiproxy approach

Yolanda Ameijeiras-Mariño, Sophie Opfergelt, Jérôme Schoonejans, Veerle Vanacker, Philippe Sonnet, Jeroen de Jong, Pierre Delmelle PII: DOI: Reference:

S0009-2541(17)30121-3 doi: 10.1016/j.chemgeo.2017.03.007 CHEMGE 18275

To appear in:

Chemical Geology

Received date: Revised date: Accepted date:

18 August 2016 24 February 2017 4 March 2017

Please cite this article as: Yolanda Ameijeiras-Mariño, Sophie Opfergelt, Jérôme Schoonejans, Veerle Vanacker, Philippe Sonnet, Jeroen de Jong, Pierre Delmelle , Impact of low denudation rates on soil chemical weathering intensity: A multiproxy approach. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Chemge(2016), doi: 10.1016/j.chemgeo.2017.03.007

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ACCEPTED MANUSCRIPT Impact of low denudation rates on soil chemical weathering intensity: a multiproxy approach Yolanda Ameijeiras-Mariñoa, Sophie Opfergelta,*, Jérôme Schoonejansb, Veerle Vanackerb, Philippe Sonneta, Jeroen de Jongc, Pierre Delmellea a

Université catholique de Louvain, Earth and Life Institute, ELIe, L7.05.10 1348 Louvain-la-Neuve, Belgium.

[email protected], [email protected], [email protected], b

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[email protected] Université catholique de Louvain, Earth and Life Institute, George Lemaître Center for Earth and Climate

Research, L4.03.08 1348 Louvain-la-Neuve, Belgium. [email protected], [email protected] c

Université Libre de Bruxelles. Department of Earth and Environmental Sciences, CP160/02. Avenue F.D.

Roosevelt 50, 1050 Brussels, Belgium. [email protected] *Corresponding author: [email protected]

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Abstract

Quantifying the influence of denudation, i.e., physical erosion and chemical weathering, on soil

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weathering intensity is an important component for a comprehensive understanding of element biogeochemical cycles. The relation between the weathering intensity and the denudation rate is not clear and requires further investigation in a variety of climatic and erosional settings. Here, in the

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Betic Cordillera (southern Spain), we assess the soil chemical weathering intensity with a multiproxy approach combining different indicators of chemical weathering of the soil: the Total Reserve in Bases

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(TRB), the content in Fe-oxides, the quartz and clay content, the soil cation exchange capacity (CEC), and the silicon (Si) isotope composition of the clay-sized fraction. Our multiproxy approach

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demonstrates that in this semi-arid environment at low denudation rates, an increasing denudation rate decreases the soil weathering intensity, whereas Si mobility remains limited. Our results converge with previous conclusions based on chemical mass balance methods in the same geological setting.

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Mass balance methods, and particularly Chemical Depletion Fractions (CDF), are based on the immobility of a refractory element (commonly zirconium, Zr) relative to major cations in soils. Interestingly, our study suggests that a weathering index such as the TRB may provide a useful complement to assess soil chemical weathering intensity in eroding landscapes where the application of chemical mass balances may be hampered by potential Zr mobility in the soil or by heterogeneity of Zr concentrations in the bedrock. Keywords: Chemical weathering; soil weathering intensity; low denudation rates; semi-arid environment; Betic Cordillera; silicon isotopes

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ACCEPTED MANUSCRIPT 1.

Introduction

The Critical Zone is the uppermost part of Earth surface where chemical, biological, physical and geological processes interact to support life (National Research Council, 2001; Brantley et al., 2007). Soil cover influences the interactions occurring within this upper layer and it can be thought of as a feed-through reactor, with a thickness controlled by the balance between the removal of material by

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denudation processes, i.e., the total loss of material from soils by both physical and chemical processes, and the advance of the weathering front down to the bedrock (e.g., Heimsath et al., 1997; Anderson et al., 2007). We can therefore expect denudation and weathering processes to be closely linked. Understanding the relationship between denudation and weathering processes is of great importance, as they control soil physical and chemical properties. Moreover, weathering processes

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have important implications for the global carbon cycle and the climate through the consumption of

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atmospheric CO2 by silicate weathering (review in Goudie and Viles, 2012). The relationship between denudation and weathering is traditionally studied through mass-balance

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calculations using the chemical depletion fraction (CDF, dimensionless) that represents the enrichment or depletion of an immobile element within the soil column relative to the parent material (e.g., Riebe

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et al., 2004). The relation between the weathering intensity (CDF) and the denudation rate is not clear. Previous work has tackled this relation (e.g., global compilations in Dixon and Blanckenburg, 2012

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and in Ferrier et al., 2016) but the conclusions remain diverse and highlight a controversy. While some

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studies have shown a negative relation between CDF and denudation rate (e.g., Dixon et al., 2012), others show no specific pattern (e.g., Riebe et al., 2004; Dixon et al., 2009). Thus, more work across all climatic and erosional settings is needed to resolve the controversy. The CDF estimations are commonly based on zirconium, which is considered to be conservative in the soil. There are number of studies that have successfully used the CDF based on Zr concentrations to constrain the chemical weathering intensity (e.g., Riebe et al., 2004; Dixon et al., 2009; Dixon et al., 2012; Ferrier et al., 2012; Schoonejans et al., 2016a). In certain environments, such as shown by Kurtz et al. (2000) for volcanic soils in Hawaii, Zr mobility can increase with rainfall, leading to potential

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ACCEPTED MANUSCRIPT underestimation of weathering losses (Hill et al., 2000; Hodson, 2002). As such, other conservative elements (e.g. Hf, Ti, Nb) have been used as an alternative (Kurtz et al., 2000; Little and Lee, 2010). In some areas, the heterogeneity in immobile element concentrations in the parent material might add uncertainty to the mass balance estimates (Ferrier et al., 2012). Soil weathering indexes may provide complementary methods to constrain the relationship between

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denudation and weathering. Chemical weathering indexes such as the Chemical Index of Alteration (CIA) or the Weathering Index of Parker (WIP) have been applied in the past to study the relationship between weathering and soil production (Burke et al., 2007; Larsen et al., 2014).

Here, we test a multiproxy approach to derive soil weathering indexes in a semi-arid region where the

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mass balance approach based on CDF was successfully applied, the Spanish Betic Cordillera (Schoonejans et al., 2016a). The existence of a good correlation between our approach and CDF

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estimates would generate complementary information to quantify chemical weathering intensity in specific environments where the use of CDF might be prevented (i.e., mobility of the reference

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elements and/or heterogeneity in the parent material). Our approach combines physico-chemical soil properties, mineralogy and isotope geochemistry to derive soil weathering indexes and estimate the

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soil weathering intensity. Five weathering indexes classically used in soil science are considered (the Total Reserve in Bases, TRB; Herbillon, 1986), the amount of Fe-oxides, the amount of quartz, the

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clay content (fraction < 2 µm), the cation exchange capacity) and combined with the silicon (Si)

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isotope composition of the clay-sized fraction. The Si isotope composition of the clay-sized fraction can be used as a weathering index as Si isotopes respond to soil chemical weathering and clay formation (Ziegler et al., 2005a; Ziegler et al., 2005b; Georg et al., 2007; Opfergelt et al., 2009, 2010, 2011, 2012; Bern et al., 2010; Pogge von Strandmann et al., 2012; Cornélis et al., 2014). The application of Si isotopes to assessing the response of chemical weathering to physical denudation has been suggested previously (Georg et al., 2007; Opfergelt and Delmelle, 2012) although no publications have explored this application of Si isotopes so far.

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ACCEPTED MANUSCRIPT 2.

Environmental setting

The study site is located in the Betic Cordillera in Southeast Spain, Almería province, the southernmost extreme of the European Alpine belt. The cordillera is subdivided in the External and Internal Zones. This study focuses on the eastern part of the Internal Zone (Figure 1). Three catchments with comparable lithology and catchment size have been selected along a gradient of

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denudation rates (Bellin et al., 2014): from north-west to south-east, in the Sierra de las Estancias (EST), Sierra de los Filabres (FIL-1) and Sierra Cabrera (CAB). The FIL-1 and CAB catchments belong to the Nevado-Filábride geological complex, whereas the EST catchment is part of the Alpujárride complex. Both complexes underwent similar metamorphic evolutions at different geological times (López Sánchez-Vizcaíno et al., 2001). According to Junta de Andalucía (2004) the

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main lithology of these catchments is mica schist with a local occurrence of quartzite and phyllite. In-

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situ produced 10Be denudation rates for the eastern Betic Cordillera (Vanacker et al., 2014) range from 34 ± 24 mm/kyr for Sierra de las Estancias (n = 5), 54 ± 25 mm/kyr for Sierra de los Filabres (n = 8) to 164 ± 74 mm/kyr for Sierra Cabrera (n = 3). The spatial pattern and magnitude of 10Be based

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denudation rates are consistent with tectonic uplift constrained by Braga et al. (2003) and Masana et

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al. (2005) based on marine deposits and trenching observations (Table 1, Bellin et al., 2014). Average annual precipitation ranges from 275 mm/yr in CAB to 425 mm/yr in EST, while mean

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annual temperature ranges from 17 °C in CAB to 12 °C in EST (García Lorca, 2009). Evapotranspiration varies between 900 mm/yr for CAB and 794 mm/yr for EST (Junta de Andalucía,

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2008). Following the UNEP’s aridity index (ratio evapotranspiration/rainfall; Barrow, 1992) the study sites are characterized by a semi-arid climate (Table 1). In the three catchments, the dominant vegetation type is Mediterranean shrub characterised by a sclerophyllous and thorny vegetation (locally known as matorral), with some remnants of Quercus trees at higher altitudes (Bellin et al., 2011; Table 2). In each catchment (or its close vicinity), two profiles were sampled on exposed ridgetops to avoid the complexities of soil-forming processes associated with lateral transport of chemical fluids and soil

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ACCEPTED MANUSCRIPT particles along slope. The sampling sites have similar slope gradients, between 14 and 28°. The sampling sites were selected avoiding locations with clear anthropogenic disturbances such as quarrying or terracing activities. These soil profiles present no evidences of strong anthropogenic perturbations (Schoonejans et al., 2016a). Materials and methods

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3.

3.1. Sampling and pre-treatments

Soil description and sampling was conducted in September 2013, as part of a larger sampling campaign described in Schoonejans et al. (2016a) and Schoonejans et al. (2016b). The soil profiles were sampled in the Sierra de las Estancias, Sierra de los Filabres and Cabrera (Table 1). The soil

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thickness was evaluated: EST-A is the deepest soil (47 cm) and EST-B, CAB and FIL-1 (both A and B) have similar soil depth (20-30 cm). The CAB and FIL-1 soil profiles are characterized by only one

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horizon, while EST soils have two horizons.

Soil samples were air-dried and sieved through a 2 mm size mesh. The < 2 mm fraction was analysed

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for the total element content, cation exchange capacity (CEC), and Fe content following selective

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extractions. The soil mineralogy, particle size distribution, pH and Si isotopic composition of the clay fraction (< 2 µm) were determined on a set of samples selected as representative from the top and

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bottom horizons (eight samples in total: EST-A-U1, EST-A-U3, EST-B-U1, EST-B-U3, FIL-1-A-U1,

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FIL-1-B-U1, CAB-A-U2, CAB-B-U2). Bedrock samples were collected below the soil-bedrock boundary, right under the soil profile. The weathering rinds of rock samples were removed via sawing in order to measure the most unweathered parent material. The bedrock samples were analysed for their total element content, mineralogy and Si isotopic composition. 3.2. Physico-chemical characterization of soils The selected samples were characterized for their particle size distribution; we applied the USDA Textural Soil Classification (1987). The sand fraction (50 µm – 2 mm) was recovered after dispersion

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ACCEPTED MANUSCRIPT of the fine earth fraction with an ultrasonic disperser (Branson Sonifier 250) and wet sieving. The separation of silt (2 µm - 50 µm) and clay fractions (< 2 µm) was achieved by dispersion with Na+resin (Rouiller et al., 1972) and followed by 24 h cycles of decantation according to the Stokes law. The separated sand-sized and clay-sized fractions were then characterized for their mineralogy and Si isotopic composition.

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The cation exchange capacity (CEC) of the soil was measured following standard procedures (Page et al., 1982) through soil saturation with ammonium acetate and posterior desorption and quantification of the ammonia retained on the negative charges of soil, originating from both organic matter and clay minerals.

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The soil pH in DI-water was measured on the set of selected samples (section 3.1) using a ratio of 25 mL of water for 5 g of soil.

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The total carbon content (CT) in soils was measured using a Vario Max dry combustion CN analyser instrument (Elementar Analysensysteme GmbH, Germany; uncertainty 0.1 %). The inorganic carbon

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content (Ci) was measured based on the modified-pressure calcimeter method (Sherrod et al., 2002). This method consists in measuring the pressure change produced by the release of CO2 in gaseous

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phase when a known volume of HCl reacts with the fine earth fraction of the soil sample. This method

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to measure Ci can only be applied to samples presenting an observable chemical reaction at room temperature after the addition of a few drops of HCl 10%, i.e., in this case only in CAB soils.

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The total element content was measured in our soil samples and bedrocks. The method consists in measuring the element content by ICP-AES (< 3 % uncertainty relative to standard material) after lithium metaborate and tetraborate fusion of the sample at 1000 °C and dissolution of the fusion beads in HNO3 1 N (Chao and Sanzolone, 1992). The loss on ignition (LOI) is assessed at 1000 °C and the total element content is expressed in reference to the soil dry weight at 105 °C. The accuracy of the measurement is checked by testing the method with the reference material BHVO-2 (Basalt, Hawaiian Volcanic Observatory, USGS).

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ACCEPTED MANUSCRIPT A selective extraction of Fe with Dithonite-Citrate-Bicarbonate (DCB) was performed to quantify soil Fe-oxides (Mehra and Jackson, 1960) and the Fe extracted (Fed) was quantified by ICP-AES. 3.3. Mineralogical characterization by X-ray Diffraction The primary and secondary crystalline minerals in bedrock, bulk soil (fine earth), and sand and clay-

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sized fractions were identified after X-ray diffraction analysis (XRD, Cu Kα, Bruker, D8 Advance). Bulk soils, sand-sized fractions and bedrock were analysed as finely milled powder without pretreatment. Clay-sized fractions were processed with hydrogen peroxide (H2O2) to remove organic matter, DCB to remove Fe oxides and analysed after KCl and MgCl2 saturation. Samples saturated with K+ were heated to 105, 300 and 550 °C and those saturated with Mg2+ were solvated with

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ethylene glycol (Robert and Tessier, 1974). The changes on the minerals structure (reflected on the XRD patterns) following heating and saturation allow the identification of the clay type (non-

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quantitative). The obtained bedrock and bulk soil diffraction patterns were studied with the software Diffrac Plus (by Bruker; composed of EVA 2.5 and TOPAS 2.0) for qualitative identification and

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3.4. Silicon isotopes analysis

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semi-quantitative analysis (uncertainty on mineral content < 1 %).

The clay-sized and sand-sized fractions and the bedrock samples were analysed for their Si isotopic

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composition. The soil fractions and bedrock samples were dissolved by alkaline fusion at 750 °C with NaOH in a silver crucible and recovered in 1 % HNO3 suprapur. Silicon was then purified using a

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cation exchange resin (Georg et al., 2006). The Si isotope composition in solutions was measured with a Nu Plasma II Multicollector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS) at the Université Libre de Bruxelles (ULB). Measurements were performed in dry plasma mode using a Cetac Aridus-2 desolvating sample introduction device and in medium resolution. The instrumental mass bias was corrected for using Mg doping and the sample-standard bracketing technique. Data are expressed in relative deviations of Si/28Si ratios from NBS-28 standard using the common δ-notation (‰, Eq.1):

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δ30 Si =

30 Si ( 28 ) Si sample 30

Si ( 28 ) ( Si NBS−28

− 1 ∙ 1000

[Eq.1]

)

Each single δ-value (n) represents one sample run and two bracketing standard runs. The total sample

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preparation, including dissolution by alkaline fusion, was replicated at least two times for two sandsized fractions (20 % of the sand-sized fractions) and five clay-sized fractions (65 % of the clay-sized fractions). Accuracy and reproducibility (δ30Si) were checked over a period of 10 months on reference materials for Si isotopes: Diatomite (+1.32 ± 0.20 ‰, 2SD, n = 25), Quartz Merck (-0.02 ± 0.12 ‰, 2SD, n = 18) and the USGS Hawaiian basalt BHVO-2 (-0.30 ± 0.11 ‰, 2SD, n = 9), which yielded

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isotope compositions indistinguishable from previously published values (Abraham et al., 2008; Reynolds et al., 2007).

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4. Results

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4.1. Soil general properties

The pH values of the set of selected soil samples (section 3.1) are alkaline (7.6 to 9.1; Table 3). The

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Sierra Cabrera (CAB) soils display the highest values (8.9 and 9.1; Table 3). The total carbon content in the soils here studied is low, ranging from 0.2 to 1.6 % (Table 3) and decreases with depth. CAB

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soils display higher total C content (1.1 - 1.6 %) than Sierra de los Filabres (FIL-1) and Sierra de las

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Estancias (EST) soils (0.2 - 1.3 %). The presence of inorganic carbon (Ci) was identified in CAB soils: its content ranges from 0.1 to 0.9 % and increases with depth. The Ci content is slightly higher for CAB-A than for CAB-B. The higher pH and Ci values in Sierra Cabrera (CAB) soils, together with the higher Ca content in CAB soils (2.2 ± 0.8 %; Electronic Annex, Table A.1) than in Sierra de los Filabres (FIL-1) and Sierra de las Estancias (EST) soils (0.4 ± 0.1 %; Table 3) support the presence of secondary pedogenic carbonates in CAB soils. The amount of Ca associated with secondary pedogenic carbonates is not related to the parent material. Therefore, a correction is applied to the total Ca content in Sierra

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ACCEPTED MANUSCRIPT Cabrera (CAB) soils before assessing the soil weathering degree with the TRB (Electronic Annex A.1). A correction is also applied to the amount of Fe selectively extracted by DCB (Fed) in the CABA soil given that secondary carbonates including Fe are dissolved by the extraction and affect the calculation of the Fed/Fet ratio for this soil (Electronic Annex A.2).

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4.2. Mineralogy of bedrock and soils The four different bedrock samples (EST-A-UR, EST-C-R, FIL-1-A-R, CAB-R) are characterized by a similar primary mineralogy composed mainly of quartz, muscovite, biotite, clinochlore and plagioclase (Table 4). This similarity in mineralogy is consistent with the chemical composition of the four bedrock samples (Si content: 25.2 - 27.8 %; Fe content: 4.72 - 5.98 %; Table 3), and supports the

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observation that the lithology is similar across the three catchments. In addition to the primary minerals, the presence of secondary minerals, i.e., kaolinite, vermiculite (total of 5 to 8 %, semi-

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quantitative; Table 4), has been observed in the bedrock samples. The primary minerals in bulk soils are dominated by quartz and muscovite, with a lower proportion of

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biotite, clinochlore and minerals from the plagioclase series (Table 4). This is consistent with the primary mineral assemblage of the bedrock, supporting our hypothesis that the bedrock samples can

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be considered as the parent material of the soils studied. Quartz content increases from Sierra Cabrera

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(CAB, higher denudation rates) to Sierra de las Estancias (EST, lower denudation rates) soils (25 to 77 %; Table 4). The abundance of muscovite is lower in the EST catchment (from 3 to 14 %; Table 4)

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compared to the FIL-1 (Sierra de los Filabres, intermediate denudation rates) and CAB catchments (from 10 to 22 %; Table 4), and the presence of biotite decreases from CAB (14 - 15 %; Table 4) to EST (from 2 to 6 %; Table 4). Clinochlore is more abundant in CAB soils (from 7 to 10 %; Table 4), which is associated with a higher content of this mineral in the CAB bedrock (15 %; Table 4). The minerals of the plagioclase series are present in low abundance (2.9 ± 1.2 %, n = 7), except for FIL-1B (14 %; Table 4). The presence of calcite in Sierra Cabrera (CAB) soils confirms the occurrence of pedogenic carbonates in this catchment (section 4.1); that presence of calcite is not considered to be of external origin. Dust deposition cannot be completely ruled out in these soils, since the Betic

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ACCEPTED MANUSCRIPT Cordillera is exposed to aeolian dust input from the Sahara and Sahel (Castillo et al., 2008; Scheuvens et al., 2013). However, the influence of dust deposition is considered as limited (Schoonejans et al., 2016b) since the soils have been sampled in ridge tops typically exposed to wind and less favourable for dust deposition, and since there is no evidence for Si- or Al- enrichment at the top of the profiles as would be expected when levels of mineral dust input are important (dust from northern Africa and the

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Western Sahara: ~50-60% SiO2; Avila et al., 1998; Castillo et al., 2008). The sand-sized fractions mineral assemblage has been qualitatively described. The X-ray patterns are dominated by quartz, with a relative increase of the quartz presence from soils characterized by higher denudation rates (CAB) to those characterized by lower denudation rates (EST). Other primary minerals are also detected on the X-ray diffraction pattern (muscovite, clinochlore, biotite and

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plagioclase).

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The qualitative identification of the mineral assemblage (section 3.3; Table 4) from the clay-sized fractions indicates that they are dominated by secondary minerals (chlorite, kaolinite, illite and

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vermiculite). Some quartz is also observed. The semi-quantification in the bulk soil indicates that the amount of kaolinite decreases from the soils with high denudation rates (CAB) to soils with low

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denudation rates (EST) (16 to 4%; except for EST-B-U3, 9 %; Table 4). A qualitatively higher content of chlorite in the clay fraction is noticed for CAB compared to the other sites. This is directly

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associated with the occurrence of clinochlore (a mineral of the chlorite group) in CAB rock (Table 4).

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4.3. Indexes of chemical weathering Six weathering indexes are used to characterize the weathering degree of these soils: the Total Reserve in Bases (TRB), the ratio Fe-oxides to total Fe (Fed/Fet), the quartz content, the clay content, the cation exchange capacity (CEC) and the silicon isotopic signature of the clay-sized fraction (δ30Siclay-sized fraction).

These indexes are expected to evolve with weathering as follows:

- The TRB is the sum of the alkaline and alkaline-earth cations (Ca2+, Mg2+, K+ and Na+, cmolc/kg) following Eq.2 (Herbillon, 1986) and is expected to decrease as cations are leached with increasing weathering.

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ACCEPTED MANUSCRIPT 𝑇𝑅𝐵𝑠𝑜𝑖𝑙 = [𝐶𝑎2+ ] + [𝑁𝑎+ ] + [𝐾 + ] + [𝑀𝑔2+ ] (𝑐𝑚𝑜𝑙𝑐 /𝑘𝑔)

[Eq.2]

- The amount of iron oxides (Fed,) in reference to the total iron content (Fet) is expected to increase with weathering, the ratio Fed/Fet is, then, expected to increase as weathering increases.

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- Quartz is a mineral highly resistant to weathering. As a consequence, it can be expected that the quartz content in soil increase with increasing weathering intensity.

- The clay content is expected to increase with increasing weathering intensity.

- In this study case, given the low organic carbon content in these soils (see Sections 4.1 and 4.3.5),

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the CEC is expected to increase with weathering degree with increasing clay content.

4.3.1. Total Reserve in Bases (TRB)

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- The δ30Siclay-sized fraction is expected to be more negative as weathering and desilication increase.

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Figure 2a shows the evolution of TRB with depth for the study sites. EST-A shows a clear evolution

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with depth, with increasing values of TRB at higher depths. There is no clear trend with depth for the other studied soil profiles. CAB (higher denudation rates) is the site presenting the higher TRB values

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although FIL-1 (intermediate denudation rates) values are very close to them. EST (lower denudation rates) site has the lower TRB values of all soil profiles studied, and present different trends with depth

sides.

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between the A and B side. FIL-1 and CAB soils show no significant differences between A and B

4.3.2. Iron oxides content The evolution of the Fed/Fet ratio in the soils is presented in Figure 2b. EST (lower denudation rates) site presents the highest Fed/Fet values and the two EST profiles follow parallel evolutions of the Fed/Fet ratio, decreasing with depth, although EST-A presents higher Fed/Fet ratio than EST-B. In CAB and FIL-1 soils (higher and intermediate denudation rates respectively), there is no observable pattern

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ACCEPTED MANUSCRIPT in the Fed/Fet ratio with depth. FIL-1-A presents a higher Fed/Fet ratio than CAB-A, CAB-B, and FIL1-A. 4.3.3. Quartz content The quartz content in EST (lower denudation rates) is up to three times higher than in CAB (high

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denudation rates, 77 % for EST-A-U1 versus 25 % for CAB-A-U2; Table 4). The quartz content in EST (from 65% to 77%) is also higher than in FIL-1 soils (intermediate denudation rates, FIL-1-A, 45%; FIL-1-B, 60 %; Table 4). In FIL-1 and CAB catchments, the side B presents higher quartz content than the side A. CAB is the catchment presenting the lower content on quartz (CAB-A, 25%; CAB-B, 36%; Table 4).

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4.3.4. Clay content

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The clay content is comparatively higher (Table 4) in EST-B (13.8 ± 0.4 % and 14.9 ± 0.6 %) and FIL-1-B (13.9 ± 0.3 %) than in CAB (10.6 ± 0.7 % and 9.2 ± 0.3 %) and FIL-1-A (10.2 ± 0.3 %). The clay content in EST-A is more variable, ranging from 8.4 ± 0.3 % in the surface horizon to 20.3 ± 0.7

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% in the deepest horizon. This variation in EST-A is accompanied by a decreasing content of the sand-

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sized fraction from the surface to the deeper horizon (Table 4).

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4.3.5. Cation exchange capacity (CEC)

The CEC is not directly a weathering index. This parameter reflects the presence of negative charges

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in soil, due to the presence of secondary clay minerals and organic matter. Since these soils are characterized by low organic matter content (averages for soils of each catchment: EST, 0.5 % ± 0.4; FIL-1, 0.9 % ± 0.2; CAB, 0.9 % ± 0.2; for CAB, this is the difference between the CT and the Ci; Table 3) the CEC is considered to be mainly controlled by the clay presence, and is therefore compared with other weathering indexes. The CEC values range from 1.6 cmolc/kg in CAB-B soil up to 15 cmolc/kg in EST-A (Figure 2c, Table 3). The CEC values in CAB (higher denudation rates) soils are lower (1.6 - 5 cmolc/kg) than in the other two catchments and they are homogenous with depth, although the values for CAB-A (4.6 - 5 cmolc/kg) are significantly higher than for CAB-B (1.6 - 1.9

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ACCEPTED MANUSCRIPT cmolc/kg). FIL-1-A presents CEC values lower (6.6- 8.4 cmolc/kg) than FIL-1-B (8.3- 9.8 cmolc/kg) and EST (lower denudation rates) soils (except EST-A shallower horizon: samples EST-A-U1 and U2). FIL-1-B and EST soils present similar CEC values, a common pattern which was also observed for the clay content (Table 4). The horizons in EST-A soils are characterized by distinct CEC values, with lower values in the shallowest horizon (EST-A-U1, 6.9 cmolc/kg, EST-B-U2, 6.2 cmolc/kg) than

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in the deepest horizon (EST-A-U3, 10.5 cmolc/kg; EST-A-U4, 14.0 cmolc/kg; EST-A-U5, 15.0 cmolc/kg).

4.3.6. Silicon isotope composition of bedrock, and sand-sized and clay-sized fractions of soils The Si isotope compositions of the bedrock, the sand-sized and the clay-sized fractions were

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determined (Table 3 and Figure 3). The Si isotope composition of the bedrock (from - 0.39 to - 0.28 ‰; Table 3; Figure 3) is consistent with values reported for schist and shale (e.g., between - 1.1 and -

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0.1 ‰, Ding et al., 1996; between -0.5 and 0.0‰, André et al., 2006; between -0.82 and 0.0 ‰, Savage et al., 2013). The four bedrock samples present no significant differences in their δ30Si values:

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this is consistent with the geochemical similarity of parent material across sites (section 4.2). The Si isotope compositions of the sand-sized fractions range from - 0.25 to 0.04 ‰ (Table 3; Figure

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3) and are not significantly different between catchments. The sand fraction is dominated by quartz but also contains other minerals such as micas, plagioclases and chlorite (Table 4), which may explain the

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variability in δ30Si. The presence of quartz is known to have a strong influence on δ30Si in shales

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(Savage et al., 2013) due to the high variability of the quartz isotopic signature across different geological settings (e.g., δ30Si in ‰ of -0.09 in sandstone, Georg et al., 2009; -0.33 in sandstone and 0.36 in paragneiss, Steinhoefel et al., 2011; - 0.2 in granites, Savage et al., 2012). The Si isotope composition of the clay-sized fractions (from - 0.67 to - 0.37 ‰; Table 3; Figure 3) is generally lighter than the δ30Si of the sand-sized fractions of the corresponding soil profile, except the clay-sized fraction of EST-A-U1 which cannot be distinguished from the corresponding sand fraction (given the large variability among the replicates; Table 3). The average Si isotope composition of the clay-sized fraction is not significantly different between the catchments (CAB: - 0.49 ± 0.01 ‰, n = 2;

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ACCEPTED MANUSCRIPT FIL-1: - 0.61 ± 0.09 ‰, n = 2; EST: -0.43 ± 0.07 ‰, n = 4; where n is the number of samples considered for the catchment). The only exception is FIL-1-B sample (- 0.67 ± 0.11 ‰, 2SD; Table 3; Figure 3), which presents a significantly lighter δ30Si value than samples from EST-B (- 0.37 ± 0.07 ‰, - 0.42 ± 0.07 ‰, 2SD; Table 3; Figure 3) and the deepest sample from EST-A (-0.41 ± 0.09 ‰,

5. Discussion 5.1. Multiproxy analysis of the soil weathering intensity

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2SD; Table 3; Figure 3).

The six weathering indexes determined in the soils (TRB, Fed/Fet, quartz content, clay content, CEC, δ30Siclay-sized fraction) are combined and compared to assess the variation of the weathering intensity

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between catchments (Table 5). For the eight fully characterized samples (EST-A-U1, EST-A-U3, EST-B-U1, EST-B-U3, FIL-1-A-U1, FIL-1-B-U1, CAB-A-U2, CAB-B-U2), the multiproxy approach

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of the present study is represented as a web graph (Figure 4). Five weathering indexes (TRB, Fed/Fet, quartz content, clay content and CEC) agree that the soils from the catchment with the lowest

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denudation rates (EST) are more weathered than the soils from the catchment with the highest denudation rates (CAB). The weathering degree in FIL-1 catchment (intermediate denudation rates) is,

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according to one weathering index (CEC), similar to the weathering degree of EST, and according to

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one weathering index (TRB) similar to or close to the weathering degree of CAB. According to the three other weathering indexes (Fed/Fet, quartz content, clay content), A and B sides of FIL-1

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catchment present distinct weathering degrees (Table 5). The weathering degree of the A side (FIL-1A) is close to EST according to Fed/Fet and close to CAB according to the quartz and the clay content (Table 5). The weathering degree of the B side (FIL-1-B) is close to CAB according to Fed/Fet and close to EST soils according to the quartz and the clay content (Table 5). This suggests that the FIL-1 catchment is an intermediate situation, a transition between CAB and EST catchments. Overall, five weathering indexes (TRB, Fed/Fet, quartz content, clay content and CEC) support that the catchment characterized by the lowest denudation rate (EST; 34 ± 24 mm/yr; Bellin et al., 2014; Vanacker et al.,

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ACCEPTED MANUSCRIPT 2014) is more intensely weathered than the catchment with the highest denudation rate (CAB; 164 ± 74 mm/yr; Bellin et al., 2014; Vanacker et al., 2014). The EST-A profile is the most weathered profile (Table 5). This profile presents two horizons well differentiated, with contrasted mineralogical (Fed/Fet, quartz content), and physico-chemical (clay content, TRB, CEC) characteristics (Table 5). Based on the differences observed with depth in clay

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content (section 4.3.4) and CEC (section 4.3.5) it is concluded that this profile is characterized by clay illuviation from the upper A horizon and clay accumulation in the B horizon due to soil development. According to the web graph (Figure 4), the δ30Siclay-sized fraction is not a weathering index that allow differentiating weathering intensity between catchments. The lighter Si isotope composition of the

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clay-sized fraction relative to the sand-sized fractions in all sites agrees with the commonly accepted view that light Si isotopes are preferentially incorporated in secondary weathering phases (Ziegler et

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al., 2005a; Ziegler et al., 2005b; Georg et al., 2007; Opfergelt et al., 2009, 2010, 2011, 2012; Bern et al., 2010; Cornélis et al., 2014). The δ30Si of the clay-sized fraction indicates no significant variation

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between catchments (Table 5). As a consequence, there are no significant correlations between the δ30Siclay-sized fraction values and the other weathering indexes (Figure 5). Considering that Si isotopes are

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preferentially incorporated in secondary phases with increasing weathering (Ziegler et al., 2005a; Ziegler et al., 2005b; Georg et al., 2007; Opfergelt et al., 2009, 2010, 2011, 2012; Bern et al., 2010;

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Cornélis et al., 2014), a difference of the δ30Siclay-sized fraction between catchments of distinct weathering intensity could have been expected. The clay-sized fraction (< 2 µm) was physically separated and is

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not exclusively composed of secondary minerals. Micro-grains of quartz (characterized by a δ30Si heavier than clay minerals; e.g., Savage et al., 2013) may partially contribute to reduce the differences in Si isotope composition between catchments. An additional factor reducing Si isotope fractionation with increasing weathering may be related to Si mobility in soils (Steinhoefel et al., 2011; Opfergelt et al., 2012). Chemical weathering is characterized by element mobility and element loss with mineral weathering, with the cations Ca, Mg, Na, and K being the more mobile elements. The decrease in the soil TRB values from the catchment presenting higher denudation rates (CAB) to the lowest denudated catchment (EST) supports cation loss (Ca, Mg, Na, K) with increasing weathering (Figure

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ACCEPTED MANUSCRIPT 2a). Silicon mobility in soils can be assessed using the Tau Si index (τSi; Figure 6, data published in Schoonejans et al., 2016a). The Tau index expresses the relative gain (positive values) or loss (negative values) of an element with weathering relative to an element considered chemically immobile, in this case Zr (Brimhall and Dietrich, 1987). It appears that the studied soils are characterized by a limited Si loss (generally < 10 %, except for sample FIL-1-B-U1 up to 30 %; Figure

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6), which suggests a low Si mobility in soils likely due to the semi-arid climatic conditions in the area. Silicon mobility is an important factor controlling Si isotope fractionation, with lighter Si isotopes accumulating in soils with increasing Si mobility and increasing Si loss (τSi increasingly negative) with increasing weathering (Steinhoefel et al., 2011; Opfergelt et al., 2012). The only clay-sized fraction presenting a significantly lighter δ30Si value than the other samples (FIL-1-B-U1) is also characterized

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by a higher Si loss (more negative τSi value; Figure 6). This is consistent with observations on shales investigated as part of the variability of Si isotopes in the upper continental crust (Savage et al., 2013),

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and in which no correlation was observed between δ30Si and weathering indexes such as CIA despite the presence of secondary minerals in the shales supporting the effect of weathering. The lack of Si

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isotope fractionation in shales was explained by the authors by the low desilication and the accumulation of detrital quartz that obscured the influence of isotopically lighter secondary minerals.

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The low Si mobility and limited Si loss in the soils from the Betic Cordillera are suggested to be key factors to explain the limited difference in the δ30Si clay-sized fraction between catchments despite the

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catchments.

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range in weathering intensity (as indicated by the weathering indexes) observed among the

5.2. Controls on soil weathering intensity Soil weathering intensity in eroding landscapes is often assessed using mass balance approaches based on chemical depletion fractions (CDF). In the Betic Cordillera, Schoonejans et al. (2016a) investigated the relationship between denudation rates and chemical weathering intensity using the CDF (assuming Zr immobility). A significant correlation exists between the CDF (calculated by Schoonejans et al., 2016a) and three weathering indexes from this study: the TRB (Figure 7a; R = - 0.81), the Fed/Fet (Figure 7b; R = 0.81) and the quartz content (Figure 7c; R = 0.98). The relationship is less clear with

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ACCEPTED MANUSCRIPT the CEC (R = 0.42; Figure 7d), especially in EST due to clay illuviation (section 5.1). The relation between the CDF and the clay content is not presented given the bias induced by the identified pedogenic processes of clay illuviation affecting the clay content (section 5.1). The present study based on the weathering indexes and the study based on the CDF (Schoonejans et al., 2016a) converge to conclude that the soil weathering intensity in the Betic Cordillera increases (from CAB to EST)

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with decreasing catchment denudation rate (CAB = 164 ± 74 mm/kyr; EST = 34 ± 24 mm/kyr; Bellin et al., 2014; Vanacker et al., 2014).

The agreement between the conclusions from three weathering indexes based on different soil constituents (TRB, Fed/Fet and quartz content) and the conclusions from the CDF in the Betic Cordillera has important implications. First, it supports the validity of the mass balance technique

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(CDF) in this environment to investigate weathering processes. Moreover, our study critically

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highlights that a multiproxy approach can be a powerful complement to CDF to assess weathering intensity; this might be useful in settings with mobilization of Ti and Zr within weathered profiles (Du et al., 2012) or with heterogeneity in the bedrock of the concentration of the element used as reference.

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Finally, as part of the controversy on the link between denudation rate and chemical weathering intensities, the present study agrees with the observed negative relation between CDF and denudation

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rate (e.g., Dixon et al., 2012), and not with the hypothesis that there is no specific pattern to relate

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CDF and denudation rate (e.g., Riebe et al., 2004; Dixon et al., 2009). Further investigations considering more climatic and erosional settings would be needed to resolve the controversy.

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5.3. Implications for other eroding landscapes Weathering indexes such as the TRB, independent from the Zr content, may provide a potential complement to quantify the relationship between chemical weathering and denudation processes in landscapes where the use of the CDF is questioned because of Zr mobility or heterogeneity of immobile elements in the parent material. In order to evaluate the potential of the TRB to constrain weathering intensity in eroding landscapes, we plotted the TRB (calculated from Eq.2) as a function of the CDF for a compilation of soil data

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ACCEPTED MANUSCRIPT from different lithological settings: granite from San Gabriel Mountains (Dixon et al., 2012) and Swiss Alps (Norton and von Blanckenburg, 2010), gneiss from Swiss Alps (Norton and von Blanckenburg, 2010), and shale from Susquehanna/Shale Hills Critical Zone Observatory (Jin et al., 2010). The last study site has a lithology that is comparable to the mica schist in our study sites in the Betic Cordillera.

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It appears that the slope of the correlation between the TRB and the CDF measured in soils depends on the lithology of the parent material (Figure 8). In soils derived from mica schist (Betic Cordillera) and shale (Jin et al., 2010), the TRB decreases when the CDF increases, whereas in soils derived from granite (Dixon et al., 2012; Norton and von Blanckenburg, 2010) or gneiss (Norton and von Blanckenburg, 2010), the TRB increases when the CDF increases. The Betic Cordillera and the San

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Gabriel Mountains are both characterized by a semi-arid climate, but display opposite trends in the

relationship between CDF and TRB.

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relationship between CDF and TRB, which suggests that climate likely has a limited control on the

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We hypothesize that the mineralogy of the parent material, and in particular the amount of feldspar, exerts a strong control on the evolution of the TRB as a function of the CDF. In the data presented in

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Figure 8, the feldspar content ranges from ~ 0 % in the shale (Jin et al., 2010), 3 – 6 % in the mica schist from the Betic Cordillera (Table 4), 59 % in the granite from San Gabriel Mountains (Dixon et

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al., 2012), 62 % in the granite from the Swiss Alps (Norton and Blanckenburg, 2010), and up to 70 % in the gneiss (Norton and Blanckenburg, 2010). Feldspars are a group of primary weatherable minerals

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rich in cations. When the amount of feldspar in the bedrock is low, chemical weathering removes mobile elements (decreasing TRB) as they are released from the parent material, leading to an enrichment in Zr (CDF increase). This behaviour can be observed in soils that are derived from parent material with a low content in feldspar minerals (having less than 6 % of feldspar minerals such as shale and mica schist; negative slopes of the CDF-TRB relation; Figure 8). When the feldspar content in the parent material is higher, release of cations such as Ca or Na from feldspar weathering may be faster than Ca or Na removal by leaching, leading to a relative enrichment in these cations in the soil. Based on Eq. 2 (not corrected for mass losses), an increase in cations such as Ca, Na, K, or Mg would

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ACCEPTED MANUSCRIPT lead to an increase of the TRB in the soil, while the removal of other mobile elements would lead to a Zr enrichment, and hence a CDF increase. This behaviour is observed in soils that are derived from bedrock with high feldspar content (59 – 70 % feldspar; granite and gneiss; positive slopes of the CDF-TRB relation; Figure 8). Our data compilation suggests that the importance of feldspar weathering for soil development, may explain differences in the relationship between TRB and CDF.

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Exploring the potential of the TRB as a complementary weathering index to the CDF in eroding landscapes requires further analyses.

An important caveat to consider is that the TRB should not be used as a single weathering index but always as a complement to CDF. The TRB is a sum of of the moles of charge per gram of soil of Ca, Na, K and Mg (Eq. 2) and does not account for mass loss or gain occurring in the soil profile relative

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to the parent material. This is mathematically different from CDF which is an index normalized to an

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immobile element expressing the mass loss or gain of an element relative to the parent material. The TRB presents the advantage to complement the information from the CDF in certain environments where the immobility of the element used for the normalization in the CDF is questioned or when the

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concentration in this element is considered as heterogeneous in the parent material. This study highlights that in eroding landscapes, a weathering index such as the TRB can provide a useful

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complementary approach to the classically used CDF to investigate the relationship between

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6. Conclusions

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denudation rate and chemical weathering intensity.

This study investigated the impact of denudation rates on soil chemical weathering intensity in a semiarid environment characterized by low denudation rates using physico-chemical soil properties, mineralogy and Si isotopes as weathering indexes. More specifically, this multiproxy approach uses the Total Reserve in Bases (TRB), the amount of Fe-oxides (Fed/Fet), the quartz content, the clay content, the cation exchange capacity (CEC), and the Si isotope composition of the clay-sized fraction (δ30Siclay-sized fraction) as weathering indexes.

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ACCEPTED MANUSCRIPT Five of the six weathering indexes (TRB, Fed/Fet, quartz content, clay content, CEC) converge to demonstrate that differences in denudation rates produce significant differences in the soil chemical weathering intensity in this semi-arid environment with low denudation rates: the higher the denudation rates, the lower the soil weathering intensity. Using the sixth weathering index (δ30Siclay-sized fraction),

there is no contrast in the soil weathering intensity between catchments. A limited Si loss in

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these soils (generally < 10 %) suggest a low Si mobility with weathering in this semi-arid environment, and probably explains that no difference in δ30Siclay-sized fraction between catchments is observed.

Given the agreement between our multiproxy approach and the mass balance methods (CDF) in the same study site, our data demonstrate that a multiproxy approach based on physico-chemical soil

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properties and mineralogy provides a consistent way to assess soil weathering intensity as a response

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to soil denudation. Weathering indexes such as the TRB, which can be easily obtained from the total element composition, may provide a potentially interesting complement to assess chemical weathering intensity in certain regions where the use of the CDF might be questioned by mobility of the reference

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element (e.g., Zr) or when the concentration in the reference element in the bedrock is considered heterogeneous. This study calls for more investigations on the complementarity between CDF and

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TRB in settings with contrasted lithology, climate, and denudation rates.

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ACCEPTED MANUSCRIPT Acknowledgements We thank A. Iserentant and C. Givron from UCL for their help with soil characterization. We also want to thank R. Ortega for his help during field work, M. Bravin (UCL) for his help with the quantification of the total and inorganic carbon, N. Mattielli for managing the MC-ICP-MS facilities at ULB in Brussels, and A. Guevara (DEMEX, Escuela Politécnica Nacional in Quito, Ecuador) for

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her help with mineral quantification based on X-ray diffraction patterns. The manuscript benefited from helpful discussions with J.T. Cornélis and G. Govers. We also want to thank the editor, J. Gaillardet, and P. Savage and one anonymous reviewer for their constructive comments that greatly improved this manuscript. Y.A.M. and J.S. are funded by the Belgian Science Policy Office (BELSPO) in the framework of the Inter University Attraction Pole project (P7/24): SOGLO – The

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soil system under global change, and S.O. is funded by the FNRS, Belgium.

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Masana E., Pallàs R., Perea H., Ortuño M., Martínez-Díaz J. J., García-Meléndez E. and Santanach P. (2005) Large Holocene morphogenic earthquakes along the Albox fault, Betic Cordillera, Spain. J. Geodyn. 40, 119-133. Mehra O. P. and Jackson M. L. (1960) Iron oxide removal from soils and clays by a dithionite-citrate system buffered with sodium bicarbonate. In Proc. 7th Natl. Conf. Clays Clay Minerals. Washington. pp. 317-327. National Research Council (2001) Basic Research Opportunities in Earth Science, edited. The National Academies Press, Washington, DC, pp.168. Norton K. P. and von Blanckenburg F. (2010) Silicate weathering of soil-mantled slopes in an active Alpine landscape. Geochim. Cosmochim. Acta 74, 5243–5258. Opfergelt S., de Bournonville G., Cardinal D., André L., Delstanche S. and Delvaux B. (2009) Impact of soil weathering degree on silicon isotopic fractionation during adsorption onto iron oxides in basaltic ash soils, Cameroon. Geochim. Cosmochim. Acta 73, 7226-7240. Opfergelt S., Cardinal D., André L., Delvigne C., Bremond L. and Delvaux B. (2010) Variations of δ30Si and Ge/Si with weathering and biogenic input in tropical basaltic ash soils under monoculture. Geochim. Cosmochim. Acta 74, 225-240. Opfergelt S., Georg R. B., Burton K. W., Guicharnaud R., Siebert C., Gislason S. R. and Halliday A. N. (2011) Silicon isotopes in allophane as a proxy for mineral formation in volcanic soils. Appl. Geochem. 26, S115–S118. Opfergelt S., Georg R. B., Delvaux B., Cabidoche Y.-M., Burton K. W. and Halliday A. N. (2012) Silicon isotopes and the tracing of desilication in volcanic soil weathering sequences, Guadeloupe. Chem. Geol. 326-327, 113-122. Opfergelt S. and Delmelle P. (2012) Silicon isotopes and continental weathering processes: assessing controls on Si transfer to the ocean. CR Geosci. 344, 723–738. Page A. L., Miller R. H. and Keeney D. R. (1982) Methods of soil analysis, Part 2. Chemical and microbiological properties. American Society of Agronomy and Soil Science Society of America, Madison, Wisconsin, USA. Pogge von Strandmann P. A. E., Opfergelt S., Lai Y.-J., Sigfusson B., Gislason S. R. and Burton K. W. (2012) Lithium, magnesium and silicon isotope behavior accompanying weathering in a basaltic soil and pore water profile in Iceland. Earth Planet. Sci. Lett. 339–340, 11–23. Reynolds B. C., Aggarwal J., André L., Baxter D., Beucher C., Brzezinski M. A., Engström E., Georg R. B., Land M., Leng M. J., Opfergelt S., Rodushkin I., Sloane H. J., van den Boorn S. H. J. M., Vroon P. Z. and Cardinal D. (2007) An inter-laboratory comparison of Si isotope reference materials. J. Anal. Atom. Spectrom. 22, 561–568. Riebe C. S., Kirchner J. W. and Finkel R. C. (2004) Erosional and climatic effects on long-term chemical weathering rates in granitic landscapes spanning diverse climate regimes. Earth Planet. Sci. Lett. 224, 547-562. Robert M. and Tessier D. (1974) Méthode de préparation des argiles des sols pour des études minéralogiques. Ann. Agron. 25, 859–882. Rouiller J., Burtin G. and Souchier B. (1972) La dispersion des sols dans l’analyse granulométrique. Méthode utilisant les résines échangeuses d’ions. Bull. EN-SAIA, Nancy 14, 193-205. Savage P. S., Georg R. B., Williams H. M., Turner S., Halliday A.N. and Chappell B. W. (2012) The silicon isotope composition of granites. Geochim. Cosmochim. Acta 92, 184–202. Savage P.S., Georg R.B., Williams H.M., Halliday A.N. (2013) The silicon isotope composition of the upper continental crust. Geochim. Cosmochim. Acta 109, 384-399. Scheuvens D., Schütz L., Kandler K., Ebert M., Weinbruch S. (2013) Bulk composition of northern African dust and its source sediments — A compilation. Earth-Science Reviews 116, 170-194.

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ACCEPTED MANUSCRIPT

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CE

PT

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Schoonejans J., Vanacker V., Opfergelt S., Ameijeiras-Mariño Y. and Christl, M. (2016a) Kinetically limited weathering at low denudation rates in semi-arid climatic conditions. J. Geophys. Res. Earth Surf. 121, 336-350. Schoonejans J., Vanacker V., Opfergelt S., Granet M. and Chabaux F. (2016b). Coupling uranium series and 10Be cosmogenic radionuclides to evaluate steady-state soil thickness in the Betic Cordillera. Chem. Geol. 446, 99-109. Sherrod L. A., Dunn G., Peterson G. A. and Kolberg R. L. (2002) Inorganic carbon analysis by modified pressure-calcimeter method. Soil Sci. Soc. Am. J. 66, 299-305. Steinhoefel G., Breuer J., von Blanckenburg F., Horn I., Kaczorek D. and Sommer M. (2011) Micrometer silicon isotope diagnostics of soils by UV femtosecond laser ablation. Chem. Geol. 286, 280-289. USDA Textural Soil Classification Study Soil Mechanics Level 1 (1987) US Department of Agriculture-Soil conservation service. Vanacker V., Bellin N., Molina A., Kubik P.W. (2014) Erosion regulation as a function of human disturbances to vegetation cover: a conceptual model. Landsc. Ecol. 29, 293–309. Ziegler K., Chadwick O. A., Brzezinski M. A. and Kelly E. F. (2005a) Natural variations of δ30Si ratios during progressive basalt weathering, Hawaiian Islands. Geochim. Cosmochim. Acta 69, 4597-4610. Ziegler K., Chadwick O. A., White A.F. and Brzezinski M. A. (2005b) δ30Si systematics in a granitic saprolite, Puerto Rico. Geology 33, 817-820.

25

ACCEPTED MANUSCRIPT Figure captions Figure 1. Geological map of the internal zone of the Betic Cordillera with the three selected catchments and pictures of each catchment. The same colour code is used in all graphs: a) Sierra de las Estancias (EST), in green, lies on the intermediate units of the Alpujárride complex. b) Sierra de los Filabres (FIL-1), in blue, lies on the Mulhacén unit of the Nevado-Filábride complex. c) Sierra

SC RI PT

Cabrera (CAB), in red, lies on the Veleta unit of the Nevado-Filábride complex.

Figure 2. Evolution of three weathering indexes with depth in the soil profiles of CAB, FIL-1 and EST catchments: a) the Total Reserve in Bases (TRB) or the sum of the total content in Ca, Mg, Na, K; b) the Fe-oxides content expressed as the ratio between the iron extracted by dithionite-citrate-

NU

bicarbonate (Fed) in reference to total Fe content (Fet); c) the cation exchange capacity (CEC). Colours correspond to the intensity of the denudation rates: red sites for higher (CAB), blue sites for medium

MA

(FIL-1) and green sites for lower (EST) denudation rates. Open symbols stand for the profile on the A side of each catchment and closed symbols for the profile on the B side. The error bars represent the

ED

uncertainty on the measurement. Catchment code (CAB, FIL-1, EST) as in Figure 1. Figure 3. Silicon isotopic compositions (δ30Si) of the: a) bedrock; b) sand-sized fraction; c) clay-sized

PT

fraction. For bedrock (a), red symbols represent CAB sample, blue symbols represent FIL-1 sample

CE

and green symbols (both open and closed) represent EST samples. For sand-sized (b) and clay-sized fractions (c), colour code as in Figure 2. In EST profile (green), the diamonds represent the shallowest

AC

horizon (EST-A-U1, EST-B-U1) and the triangles represent the deepest horizon (EST-A-U3, EST-BU3). The error bars represent the 2SD on the mean values of the different replicates. Catchment code (CAB, FIL-1, EST) as in Figure 1. Figure 4. Web graph representing the multiproxy approach of this study, using six weathering indexes (TRB, Fed/Fet, quartz content, clay content, CEC, δ30Siclay-sized fraction) to assess the weathering intensity of eight fully characterized soil samples (EST-A-U1, EST-A-U3, EST-B-U1, EST-B-U3, FIL-1-A-U1, FIL-1-B-U1, CAB-A-U2, CAB-B-U2). Each arm of the web graph represents a weathering index, with increasing weathering intensity from the center to the external part of the graph. For each

26

ACCEPTED MANUSCRIPT weathering index, the sample with the value corresponding to the highest weathering intensity is considered as 100% of the weathering intensity, and the other samples are expressed as a percentage of variation from 100%. Colour code: green for EST, blue for FIL-1, red for CAB. Catchment code as in Figure 1. Figure 5. Silicon isotope compositions (δ30Siclay-sized fractions) of the clay-sized fraction as a function of

SC RI PT

four of the studied weathering indexes: a) Total Reserve in Bases (TRB); b) proportion of iron oxides relative to total iron (Fed/Fet); c) quartz content; d) cation exchange capacity (CEC) for the selected samples. Vertical error bars represent the 2SD on the mean δ30Si values. Weathering indexes are single values for the corresponding depth with the horizontal error bars representing the uncertainty on the

NU

measurement. Colour code as in Figure 2. Catchment code (CAB, FIL-1, EST) as in Figure 1. Figure 6. The Si isotopic signature of the clay-sized fraction (δ30Siclay-sized fraction) as a function of the Si

MA

gain and loss expressed as a Tau index (τSi; data published in Schoonejans et al. (2016a). Vertical error bars represent the 2SD on the mean δ30Si values and horizontal bars represent the error associated to

ED

Tau calculation. Colour code as in Figure 2. Catchment code (CAB, FIL-1, EST) as in Figure 1. Figure 7. Comparison between the Chemical Depletion Fraction (CDF) as calculated in Schoonejans

PT

et al. (2016a) and four of the studied weathering indexes: a) Total Reserve in Bases (TRB); b)

CE

proportion of iron oxides relative to total iron (Fed/Fet); c) quartz content; d) cation exchange capacity (CEC). The relation between the CDF and the clay content is not presented given the bias induced by

AC

the identified pedogenic processes of clay illuviation (section 5.1). Error bars represent the uncertainty on the measurement. Colour code as in Figure 2. Catchment code (CAB, FIL-1, EST) as in Figure 1. Figure 8. The Total Reserve in Bases (TRB, the sum of the total content in Ca, Mg, Na, K) as a function of the Chemical Depletion Fraction (CDF) in different locations: Red diamonds for granitic soils from Dixon et al. (2012); blue triangles for gneissic soils from Norton and von Blanckenburg (2010); green crosses for granitic soils from Norton and von Blanckenburg (2010); orange stars for shale derived soils from Jin et al. (2010) (CDF values recalculated using the Zr content in the shale parent material); purple dashes for data from the present study for mica schist derived soils from the

27

ACCEPTED MANUSCRIPT Betic Cordillera (CDF data from Schoonejans et al., 2016a). The TRB values have been calculated according to Equation 2 with the total element content. Samples from Norton and von Blanckenburg (2010) have been separated in two groups according to the parent material. Different correlations are observed: Dixon et al. (2012; R = 0.75), Norton and von Blanckenburg (2010; gneiss, R = 0.77), Norton and von Blanckenburg (2010; granite, R = 0.92), Jin et al. (2010; R = -0.90), this study (R = -

AC

CE

PT

ED

MA

NU

SC RI PT

0.81).

28

ACCEPTED MANUSCRIPT Table 1. Main characteristics of the studied catchments: code, uplift rates ([1] Masana et al., 2005; [2] Booth-Rea et al., 2004; [3] Braga et al., 2003), catchment denudation rates ([4] Bellin et al., 2014), surface area, mean altitude of the catchment, annual mean temperature (T, [5] García Lorca, 2009), annual mean rainfall (P, [5] García Lorca, 2009), annual mean potential evapotranspiration (PET, [6] Junta de Andalucía, 2008), aridity index calculated as the ratio between rainfall and potential evapotranspiration (Barrow, 1992). More information on the derivation of the climatic variables can be found in Schoonejans et al. (2016a)

Sierra

Catchment code

Uplift rate (mm/kyr)

Catchment denudation rate[4] (mm/kyr)

Area (km2)

Catchment mean altitude (m)

Annual mean[5] T(°C)

Sierra de las Estancias

EST

10-40[1]

34 ± 24

0.21

1179

12.5

Sierra de los Filabres

FIL-1

70-110[2],[3]

54 ± 25

0.26

811

Sierra Cabrera

CAB

170[3]

164 ± 74

1.98

D E

M

T P E

C C

A

29

Annual mean PET[6] (mm)

Aridity Index (P/PET)

425 ± 25

794

0.54 (Semi-arid)

15.5

375 ± 25

750

0.50 (Semi-arid)

17

275 ± 25

900

0.31 (Semi-arid)

T P

I R

C S U

N A 541

Annual mean P[5] (mm)

ACCEPTED MANUSCRIPT Table 2. Main characteristics of the studied soil profiles (side A and B of the catchment): altitude, soil depth, vegetation type and distribution of the soil horizons with depth. Sierra

Sampling site

Altitude (m)

Soil depth (cm)

Horizons depth

EST-A

1187

47

A (0 – 15 cm), B (15 – 47 cm)

EST-B

1187

25

A (0 – 15 cm), B (15 – 25 cm)

FIL-1-A

776

22

A (0 – 22 cm)

FIL-1-B

776

17

A (0 – 17 cm)

CAB-A

525

19

CAB-B

525

18

Sierra de las Estancias

Sierra de los Filabres

C S U

Sierra Cabrera

N A

D E

M

T P E

C C

A

30

I R

T P

A (0 – 19 cm) A (0 – 18 cm)

ACCEPTED MANUSCRIPT Table 3. Main characterization of the soil (<2mm fraction; U samples) and bedrock samples (R samples): sampling depth; pHH20; major elements content (Al, Ca, Fe, K, Mg, Mn, Na, P, Si, Ti, expressed in element mass %; Zr expressed in mg/kg); loss on ignition (LOI, %); cation exchange capacity (CEC, cmolc/kg); total carbon content (CT, %); inorganic carbon content (Ci, %); Fe extracted with DCB (Fed, g/kg). For bedrock and the separated sand-sized (50 µm – 2 mm) and clay-sized (< 2 µm) fractions: average Si isotopic signature (δ30Si) with standard deviation (2SD) and number of replicates (n). For CAB soils, major element content and Fed values are corrected for the presence of secondary pedogenic carbonates (See Electronic Annex A.1 and A.2). Sierra

Sample

Sampling depth

pHH2O

cm

Sierra de las Estancias

Sierra de los Filabres

Sierra Cabrera

EST-A-U1

6-8

EST-A-U2

12 - 14

EST-A-U3

20 - 22

EST-A-U4

28 - 30

EST-A-U5

32 - 34

EST-B-U1

3-5

EST-B-U2

9 - 11

EST-B-U3

18 - 21

EST-B-U4

25 - 27

EST-A-UR EST-C-R

FIL-1-A-U1

2-4

FIL-1-A-U2

6-8

FIL-1-A-U3

Al

Ca

Fe

K

Mg

Mn

Na

P

Si

Ti

% 7.59

Zr

LOI

CEC

CT

Ci

Fed

Sand-sized fraction

mg/kg

%

cmolc/kg

%

%

g/kg

δ30Si (‰)

2SD

n

δ30Si (‰)

2SD

n

-0.18

0.23

5

- 0.53

0.18

5

0.04

0.11

2

- 0.41

0.09

4

- 0.01

0.11

2

- 0.37

0.07

3

- 0.14

0.12

5

- 0.42

0.07

2

- 0.32*

0.16

5

- 0.28*

0.04

2

- 0.14

0.15

10

- 0.54

0.16

6

- 0.08

0.08

2

- 0.67

0.11

2

- 0.37*

0.08

6

- 0.25

0.14

3

- 0.50

0.09

5

- 0.22

0.14

3

- 0.48

0.14

5

- 0.39*

0.07

4

4.73

0.25

2.19

0.99

0.39

0.03

0.33

0.01

37.4

0.37

305

4.35

6.91

1.29

8.13

5.53

0.28

2.64

1.04

0.48

0.02

0.26

0.01

36.6

0.44

284

3.71

6.25

0.48

9.15

7.87

0.49

4.01

1.57

0.74

0.02

0.29

0.01

32.5

0.46

264

4.65

10.5

0.41

8.71

0.50

4.7

1.83

0.89

0.03

0.28

0.01

30.6

0.43

222

5.48

14.0

0.30

9.92

0.63

5.26

2.37

1.01

0.03

0.67

0.01

28.1

0.49

238

6.17

15.0

0.29

6.19

0.30

2.92

1.37

0.79

0.03

0.40

0.01

34.8

0.39

309

4.75

8.35

0.79

5.92

0.41

2.65

1.00

0.72

0.02

0.26

0.08

35.3

0.37

287

5.17

11.6

0.81

5.63

0.45

2.78

0.75

0.77

0.02

0.16

0.01

35.9

0.35

205

4.72

11.0

0.31

6.46

5.62

0.56

2.90

0.75

0.84

0.03

0.19

0.01

35.8

0.36

227

4.27

10.3

0.22

6.58

92.5

12.7

0.64

4.72

3.86

0.20

0.01

0.80

0.06

26.8

0.58

210

3.15

82.5

13.2

0.67

5.19

3.90

0.22

0.01

1.05

0.07

25.8

0.65

232

3.50

8.85

0.32

4.51

2.44

0.58

0.04

1.28

0.03

29.8

9.51

0.31

4.83

2.56

0.62

0.04

1.28

0.03

29.1

9 - 11

9.31

0.32

4.78

2.65

0.64

0.05

1.26

0.03

29.3

FIL-1-A-U4

11 - 13

9.55

0.30

4.76

2.62

0.61

0.04

1.28

0.03

29.1

FIL-1-B-U1

3-5

8.25

0.33

4.15

1.70

0.56

0.03

1.70

0.02

FIL-1-B-U2

6-8

8.30

0.31

3.96

1.73

0.56

0.03

1.67

0.02

FIL-1-B-U3

10 - 12

8.70

0.32

4.08

2.02

0.67

FIL-1-B-U4

15 - 16

9.20

0.28

FIL-1-A-R

43

12.3

0.60

CAB-A-U1

1-2

CAB-A-U2

6

CAB-A-U3

11

CAB-A-U4

17 - 19

CAB-B-U1

4-6

CAB-B-U2

7-9

CAB-B-U3 CAB-B-U4

7.83

7.73

7.98

7.73

7.66

14.6 8.68 7.22

0.55

220

5.65

8.41

1.14

10.2

0.56

208

5.30

6.57

0.96

10.8

0.57

235

5.11

7.03

0.93

10.7

0.58

218

5.17

6.95

0.93

11.6

31.0

0.58

319

5.03

9.84

0.97

7.79

31.3

0.55

332

4.76

9.19

0.88

7.62

D E

PT

M

14.7

C S U

N A

0.02

1.54

0.03

30.6

0.53

289

4.98

9.68

0.73

6.31

4.14

2.25

0.84

0.02

1.13

0.03

30.4

0.52

277

4.50

8.30

0.46

7.13

5.97

3.31

0.88

0.03

1.79

0.12

25.2

0.57

204

3.89

6.54

12.6

0.867

2.61

1.06

0.0696

0.555

0.0614

23.9

0.839

213

8.43

4.98

1.18

0.455

12.9

13

0.956

E C 6.46

2.82

1

0.0732

0.558

0.0565

23.3

0.83

220

8.83

4.93

1.30

0.547

12.0

12.9

0.661

6.82

2.83

0.951

0.0861

0.542

0.0594

23.2

0.792

220

10.42

4.62

1.57

0.868

7.11

13

0.626

6.44

2.78

0.738

0.099

0.549

0.042

23.8

0.73

220

9.13

5.02

1.12

0.563

12.3

0.328

6.05

2.75

1.03

0.0486

0.671

0.0796

24.9

0.779

266

7.09

1.94

1.23

0.132

10.6

11.6

0.293

5.61

2.6

1.04

0.0435

0.605

0.0758

26.2

0.718

213

7.29

1.79

1.42

0.336

8.10

10 - 11

11.6

0.368

5.85

2.56

1.08

0.0395

0.592

0.0807

25.9

0.677

208

8.19

1.70

1.62

0.530

10.2

15 - 19

11.4

0.535

5.27

2.48

1.05

0.0426

0.639

0.084

26.1

0.597

233

8.87

1.57

1.63

0.530

7.19

11.6

0.265

5.98

2.71

1.15

0.0439

1.09

0.1051

26.0

0.632

222

5.24

CAB-R 40 *Bedrock Si isotopic signature

8.86

9.12

AC

31

T P

I R 12.5

Clay-sized fraction

ACCEPTED MANUSCRIPT Table 4. Additional characterization of eight selected soil samples (one horizon per profile for CAB and FIL-1, two horizons per profile for EST) and bedrock samples: sampling depth; particle size distribution of the fine earth fraction (< 2mm); mineralogy of primary minerals ([1] Q = Quartz, M = Muscovite, B = biotite, Cl = clinochlore, P = plagioclase group), and mineralogy of secondary minerals ([2] K = kaolinite, Z = zeolite, V = vermiculite, Ca = calcite, I = illite). Particle size distribution Type of sample

Sampling depth

Sand (50 µm – 2 mm)

cm

%

EST-A-U1

6-8

52.1

39.5

8.4

EST-A-U3

20-22

32.3

47.5

20.3

65

EST-B-U1

3-5

40.1

46.1

13.8

EST-B-U3

18-21

25.4

59.7

FIL-1-A-U1

2-4

45.3

44.5

FIL-1-B-U1

3-5

53.6

CAB-A-U2

6

21.5

CAB-B-U2

7-9

26.7

EST-A-UR

92.5

-

A M

EST-C-R

82.5

-

Sierra de los Filabres

FIL-1-A-R

43

Sierra Cabrera

CAB-R

Sierra

Bulk soil

Sample

Silt (50 µm – 2 µm)

Primary minerals [1]

T P

%

4

2

4

5

-

-

-

14

6

4

2

4

2

3

-

66

10

4

6

3

6

3

2

-

14.9

69

3

2

8

2

9

3

4

-

10.2

45

22

10

5

5

5

4

4

-

13.9

60

10

4

3

14

4

2

3

-

10.6

25

17

14

10

2

16

3

4

4

9.2

36

20

15

7

2

8

3

1

2

-

37

25

12

2

5

4

3

2

-

5

-

32

22

10

6

6

5

5

3

-

5

-

27

30

8

8

6

2

2

3

-

8

-

32

23

10

15

3

5

3

2

-

3

Clay (< 2µm)

Sierra Cabrera

D E

Sierra de las Estancias

E C 40

32.5 67.9 64.1

PT -

M

-

AC

32

U N

B

I R

77

SC

Sierra de las Estancias

Bedrock

Q %

Sierra de los Filabres

Secondary minerals [2]

8

Cl

P

K

Z

V

Ca

I

ACCEPTED MANUSCRIPT Table 5. Qualitative comparison of the soil weathering intensity assessed using the different weathering indexes: Total Reserve in Bases (TRB); iron oxide content (ratio Fed/Fet); quartz content; clay content; cation exchange capacity (CEC), and Si isotope composition of the clay fraction (δ30Siclay-sized fraction). The weathering intensity is evaluated for the eight selected soil horizons (section 3.1; two for EST-A and B profiles). For each weathering index, the relative intensity of weathering in one horizon is qualitatively assessed relative to the other horizons following a scale of increasing weathering intensity: = very low; - low; + high; ++ very high; +++ the highest. Sierra

Code

TRB

Fed/Fet

Quartz content

Clay content

Horizon A

+++

+++

+++

=

Horizon B

+

++

++

+++

Horizon A

++

++

++

++

Horizon B

++

+

++

-

+

-

FIL-1-B

-

-

CAB-A

=

CAB-B

-

EST-A Sierra de las Estancias EST-B

FIL-1-A

Sierra de los Filabres

Sierra Cabrera

D E

-

T P E

C C

A

33

+

T P

δ30Siclay-sized fraction

+

=

+++

=

++

=

++

++

=

-

+

=

I R

SC

U N

A M -

CEC

++

++

-

=

-

-

=

-

=

=

=

ED

MA

NU

SC RI PT

ACCEPTED MANUSCRIPT

AC

CE

PT

Figure 1

34

AC

CE

PT

ED

MA

NU

SC RI PT

ACCEPTED MANUSCRIPT

Figure 2

35

AC

Figure 3

CE

PT

ED

MA

NU

SC RI PT

ACCEPTED MANUSCRIPT

36

MA

NU

SC RI PT

ACCEPTED MANUSCRIPT

AC

CE

PT

ED

Figure 4

37

MA

NU

SC RI PT

ACCEPTED MANUSCRIPT

AC

CE

PT

ED

Figure 5

38

SC RI PT

ACCEPTED MANUSCRIPT

AC

CE

PT

ED

MA

NU

Figure 6

39

AC

CE

PT

ED

MA

NU

SC RI PT

ACCEPTED MANUSCRIPT

Figure 7

40

MA

NU

SC RI PT

ACCEPTED MANUSCRIPT

AC

CE

PT

ED

Figure 8

41

ACCEPTED MANUSCRIPT Highlights •

At low denudation rates, the soil chemical weathering intensity increases within a range of decreasing denudation rates

•

A multiproxy weathering approach provides a complement to mass-balance methods to assess the soil weathering intensity The response of Si isotopes to weathering depends on Si mobility in soils, despite

CE

PT

ED

MA

NU

SC RI PT

contrasts in soil weathering intensity

AC

•

42