Iron speciation and isotope fractionation during silicate weathering and soil formation in an alpine glacier forefield chronosequence

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Abstract

The chemical weathering of primary Fe-bearing minerals, such as biotite and chlorite, is a key step of soil formation and an important nutrient source for the establishment of plant and microbial life. The understanding of the relevant processes and the associated Fe isotope fractionation is therefore of major importance for the further development of stable Fe isotopes as a tracer of the biogeochemical Fe cycle in terrestrial environments. We investigated the Fe mineral transformations and associated Fe isotope fractionation in a soil chronosequence of the Swiss Alps covering 150 years of soil formation on granite. For this purpose, we combined for the first time stable Fe isotope analyses with synchrotron-based Fe-EXAFS spectroscopy, which allowed us to interpret changes in Fe isotopic composition of bulk soils, size fractions, and chemically separated Fe pools over time in terms of weathering processes. Bulk soils and rocks exhibited constant isotopic compositions along the chronosequence, whereas soil Fe pools in grain size fractions spanned a range of 0.4‰ in δ56Fe. The clay fractions (<2 μm), in which newly formed Fe(III)-(hydr)oxides contributed up to 50% of the total Fe, were significantly enriched in light Fe isotopes, whereas the isotopic composition of silt and sand fractions, containing most of the soil Fe, remained in the range described by biotite/chlorite samples and bulk soils. Iron pools separated by a sequential extraction procedure covered a range of 0.8‰ in δ56Fe. For all soils the lightest isotopic composition was observed in a 1 M NH2OH–HCl–25% acetic acid extract, targeting poorly-crystalline Fe(III)-(hydr)oxides, compared with easily leachable Fe in primary phyllosilicates (0.5 M HCl extract) and Fe in residual silicates. The combination of the Fe isotope measurements with the speciation data obtained by Fe-EXAFS spectroscopy permitted to quantitatively relate the different isotope pools forming in the soils to the mineral weathering reactions which have taken place at the field site. A kinetic isotope effect during the Fe detachment from the phyllosilicates was identified as the dominant fractionation mechanism in young weathering environments, controlling not only the light isotope signature of secondary Fe(III)-(hydr)oxides but also significantly contributing to the isotope signature of plants. The present study further revealed that this kinetic fractionation effect can persist over considerable reaction advance during chemical weathering in field systems and is not only an initial transient phenomenon.

Introduction

The study of stable Fe isotopes has become a growing field of Earth and environmental sciences over the past decade, aiming at the development of Fe isotopes as a tracer for the biogeochemical Fe cycle (Dauphas and Rouxel, 2006, Johnson et al., 2008). Iron is an important nutrient for almost all living organisms (Marschner, 1995) and its high redox activity triggers many processes in terrestrial ecosystems such as mineral weathering, nutrient cycling, and contaminant mobility (Borch et al., 2010).

Iron isotope fractionation patterns investigated in soils indicated a preferential mobilization of light Fe isotopes in various soil forming processes. Iron dissolution and translocation under reducing conditions in soils of Hawaii (Thompson et al., 2007), Switzerland, and Germany (Wiederhold et al., 2007a) resulted in Fe-depleted zones, which were depleted in light Fe isotopes. In Podzols, Fe mobilization and translocation by organic ligands were shown to favor light Fe isotopes (Wiederhold et al., 2007b). Several studies found an enrichment of light Fe isotopes in organic matter rich topsoil horizons (Fantle and DePaolo, 2004, Emmanuel et al., 2005, Wiederhold et al., 2007b), which can be explained by the preferential incorporation of light Fe isotopes in the aboveground biomass of plants (Guelke and von Blanckenburg, 2007). In a recent study we demonstrated that the overall light isotope signature of plant biomass is the result of fractionation in two steps, one before the actual Fe uptake, presumably in the rhizosphere soils or soil solutions, and one at the selective uptake system at the root plasma membrane. However, the fractionation mechanisms in soils could not be identified conclusively (Kiczka et al., 2010b).

One approach for a better understanding of Fe isotope fractionation in soils is the determination of Fe isotope signatures of operationally defined soil Fe pools (e.g., poorly-crystalline and crystalline Fe(III)-(hydr)oxides) by sequential extraction procedures (Brantley et al., 2004, Fantle and DePaolo, 2004, Thompson et al., 2007, Wiederhold et al., 2007a, Wiederhold et al., 2007b, Buss et al., 2010, Guelke et al., 2010). Iron pools separated by sequential extractions span a much larger range in their isotopic composition than bulk soils, with Fe(III)-(hydr)oxide pools mostly exhibiting light Fe isotope signatures. These signatures were attributed to in-situ Fe mineral transformations, such as mineral weathering and crystallization, or to micro- and medium scale ligand- or redox-induced dissolution and precipitation cycles. However, in these studies, the long time periods of soil development resulted in soil Fe pools being influenced by more than one soil forming process, which significantly hampered the interpretation of the Fe isotope fractionation associated with individual processes. In particular, fractionation effects during silicate weathering processes were likely overprinted by secondary reactions. In old soil systems, export of Fe with an unknown isotopic composition further complicated the interpretation with respect to fractionation mechanisms. A more detailed investigation of the Fe isotope fractionation involved in silicate weathering and the initial soil formation processes therefore requires the investigation of young well-constrained ecosystems. A mechanistic understanding of the dominant fractionation processes further necessitates time-resolved data on the development of Fe isotope signatures of soil Fe pools, information which was mostly not available in previous studies. Proglacial environments provide an ideal setting to study isotope fractionation in such weathering reactions, because the general retreating trend of glaciers on the northern hemisphere gradually exposed fresh unweathered rock to the atmosphere leading to the formation of soil chronosequences. In addition, the lack of significant bulk Fe losses during initial pedogenesis in such young oxic soil ecosystems (Egli et al., 2001a) simplifies the interpretation of the obtained Fe isotope ratios.

The dissolution of Fe(II)-bearing silicates in igneous rocks such as biotites, chlorites, amphiboles, and pyroxenes (Dietrich and Skinner, 1984) is one of the most important weathering reactions. The concomitant isotope fractionation effects may therefore have a major influence on the Fe isotope signature of secondary minerals or biomass. Laboratory studies demonstrated that hornblende dissolution in the presence of bacteria and organic ligands resulted in solutions, measured at one time point, which were enriched in light Fe isotopes (Brantley et al., 2001, Brantley et al., 2004). Leaching of a biotite-granite and a tholeiitic basalt with HCl and oxalic acid produced early solutions enriched in light Fe isotopes. A gradual transition towards less fractionated δ56Fe values during leaching of the granite by HCl was modeled by isotope mixing between different Fe pools (Chapman et al., 2009). Recently, we showed in laboratory experiments that early released fractions during proton- and ligand-promoted dissolution of biotite and chlorite were also enriched in light Fe isotopes by up to 1.4‰ in δ56Fe compared with the bulk isotopic composition with a transition towards less fractionated values with progressing Fe release (Kiczka et al., 2010a). The evolution of Fe isotope ratios in solution was modeled by a kinetic isotope effect and different enrichment factors were estimated for Fe release from different surface sites and for different dissolution mechanisms. In these experiments only the dissolution step was investigated and the question remained if these fractionation effects are preserved in field systems or if isotope fractionation in natural systems is dominated by other processes, such as precipitation, sorption, or equilibration between species in solution. In soils, the weathering of biotite can be described as a direct structural modification process during which the stepwise replacement of interlayer K+ by hydrated cations leads to the formation of hydrobiotite, regularly interstratified biotite–vermiculite mixed-layers, and vermiculite (Banfield and Eggleton, 1988). At the same time, Fe(II) is oxidized and partly released from the mineral structure, resulting in the precipitation of Fe(III)-(hydr)oxides.

The aim of this study was to close the gap between controlled laboratory studies on isolated processes (Kiczka et al., 2010a) and a static view of the end product of soil development in the field as well as to provide an integrated picture of fractionation processes in the soil–plant environment (Kiczka et al., 2010b). We think that chronosequences in young weathering environments, where process rates are extremely high, are ideally suited to study the development of Fe isotope signatures as a dynamic process. Specifically, the objectives of the present study were to test (i) if Fe isotope fractionation during the initial weathering of biotite and chlorite in a natural field system results in a distinct isotope signature of newly formed Fe(III)-(hydr)oxides, (ii) if this can be related to kinetic fractionation during mineral dissolution observed in laboratory experiments or if other processes, e.g., precipitation or adsorption, dominate, and (iii) if fractionation effects and processes change with progressive weathering. We therefore used the Damma glacier soil chronosequence in the Central Alps Switzerland as a model system to investigate the changes in speciation and isotopic composition of Fe in bulk soils and in different soil Fe pools during initial soil formation on granite. We combined for the first time stable Fe isotope analyses with synchrotron-based Fe-EXAFS spectroscopy and evaluated physical and chemical separation procedures for their selectivity and suitability for Fe isotope analysis of different soil Fe pools.

Section snippets

Field site and sampling

The field site of this study is the forefield of the retreating Damma glacier in the Central Alps of Switzerland (N46°38.177′, E008°27.677′; 1950–2050 m) (Fig. 1). It also serves as field site for the interdisciplinary research project “BigLink” focused on linking climate change, biogeochemical weathering, soil formation, and initial ecosystem evolution (Bernasconi et al., 2011), to which the present study is associated. The parent rock of the area is the Central Aar granite, which contains

Bulk soils

Soil pH and soil C content changed significantly along the Damma glacier chronosequence with increasing time since deglaciation as presented in Table 1 and Fig. 2. Soil pHH2O dropped from 5.7 at site 1 close to the glacier front to 4.1 at the reference site 11 and soil C content increased from less than 1 g kg−1 at site 1 to 93 and 46 g kg−1 at site 11 in 0–5 and 5–10 cm depth, respectively. However, these trends were not a linear function of deglaciation time but showed discontinuities after the

The evolution of the Fe isotope signature of Fe(III)-(hydr)oxides with time

The interpretation of Fe isotope signatures of physically or chemically separated soil Fe pools requires a careful calibration of these pools to mineral phases present in the field and any method-induced fractionation has to be prevented (Wiederhold et al., 2007b, Guelke et al., 2010). In the present study, a reductive dissolution step under mildly acidic conditions (1 M hydroxylamine–HCl in 25% v/v acetic acid, pH 1.5, 48 h) was used to selectively dissolve poorly-crystalline

Conclusions

The investigation of Fe speciation and isotopic composition of bulk soils and soil Fe pools along the Damma glacier chronosequence demonstrated that significant Fe isotope fractionation takes place during the weathering of Fe(II)-bearing phyllosilicates. Oxic weathering in soils produces Fe phases with distinct isotopic signatures, with newly formed Fe(III)-(hydr)oxides being enriched in light Fe isotopes leading to a gradual enrichment of heavy Fe isotopes in residual Fe(III)-bearing

Acknowledgements

We thank the staff of the ETH MC-ICPMS Lab for machine maintenance and support during isotope measurements, Kurt Barmettler for support in the laboratory, Michael Plötze for the separation of clay from BigLink samples, Emmanuel Lemarchand for the dissolution of BigLink soil and clay samples, the entire BigLink team for soil sampling and discussions, and Robin S. Smith for help with the optical microscopy. Peter Brack and Olivier Jacquat are acknowledged for providing reference phases for XAS

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    Current address: Laboratoire de Géologie de Lyon, ENS Lyon, CNRS and UCBL, 46 Allée d’Italie, F-69364 Lyon, France.

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