Impact of low denudation rates on soil chemical weathering intensity: A multiproxy approach
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 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 have important implications for the global carbon cycle and the climate through the consumption of atmospheric CO2 by silicate weathering (review in Goudie and Viles, 2012).
The relationship between denudation and weathering is traditionally studied through mass-balance 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 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 von Blanckenburg, 2012 and in Ferrier et al., 2016) but the conclusions remain diverse and highlight a controversy. While some 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 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 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 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 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 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 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 clay content (fraction < 2 μm), the cation exchange capacity) and combined with the silicon (Si) 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, Opfergelt et al., 2010, Opfergelt et al., 2011, Opfergelt et al., 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.
Section snippets
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 (Fig. 1). Three catchments with comparable lithology and catchment size have been selected along a gradient of denudation rates (Bellin et al., 2014): from north-west to south-east, in the Sierra de las Estancias (EST), Sierra de
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 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 horizon,
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 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 soils display higher total C content (1.1–1.6%) than Sierra de los Filabres (FIL-1) and Sierra de las Estancias (EST) soils (0.2–1.3%). The presence of inorganic carbon (Ci) was identified in
Multiproxy analysis of the soil weathering intensity
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 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 of the present study is represented as a web graph (Fig. 4). Five weathering indexes (TRB, Fed/Fet, quartz
Conclusions
This study investigated the impact of denudation rates on soil chemical weathering intensity in a semi-arid 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 (δ
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 her help with mineral quantification based on X-ray diffraction patterns. The manuscript benefited from helpful
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