『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 if 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 (＜2μm), 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 silts 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 1M 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.』

1. Introduction
2. Methods
　2.1. Field site and sampling
　2.2. Characterization of bulk soils and size fractions
　2.3. Sequential extraction
　2.4. Sample digestion and isotope analysis
3. Results
　3.1. Bulk soils
　3.2. Size fractions
　3.3. Sequential extraction
4. Discussion
　4.1. The evolution of the Fe isotope signature of Fe(III)-(hydr)oxides with time
　4.2. iron isotope fractionation processes in the weathering environment
　4.3. Implications for Fe isotope fractionation in the soil-plant Fe cycle
5. Conclusions
Acknowledgements
Appendix A. Supplementary data
References