『Abstract
　Magnesium (Mg) stable isotopes are increasingly used as a weathering proxy in soils and rivers, but the impact of the mineralogy of secondary phases on isotope fractionation remains obscure. A better understanding of the behaviour of Mg isotopes during weathering processes is a mandatory step toward deployment of this new tracer for understanding chemical fluxes exported from the critical zone. Here we investigate isotopic variation in δ²⁶Mg in bulk soils and clay fractions relative to their parent andesite in three soil weathering sequences from Guadeloupe formed under contrasting climatic conditions. soils formed in drier conditions (low precipitation) contain smectite, whereas soils formed under wet conditions (high rainfall) are characterized by halloysite and Fe-oxides or kaolinite.
　All clay fractions have Mg isotopic compositions (δ²⁶Mg -0.41‰ to -0.10‰) similar to or heavier than their parent andesite (δ²⁶Mg -0.47‰) supporting the preferential incorporation of heavy Mg isotopes in secondary Mg-bearing clay minerals with the first direct measurements on clay fractions. Soils with lighter Mg isotope compositions have greater quantities of exchangeable Mg. The data support a contribution from sea spray to the exchangeable Mg pool correlated to the soil weathering degree. This study highlights for the first time that the soil δ²⁶Mg not only depend on δ²⁶Mg of the parent rock, and on any fractionation that might occur, but also on the Mg retention on the exchange complex, which could in turn be controlled by external inputs such as sea spray.

Keywords: andesite weathering; volcanic soils; magnesium isotopes; clay minerals; exchangeable magnesium; Guadeloupe』

1. Introduction
2. Environmental setting
3. Methods
　3.1. Sampling
　3.2. Soil characterisation
　3.3. Magnesium isotope analysis
　　3.3.1. Sample preparation and chromatography
　　3.3.2. Mass spectrometry
　　3.3.3. Precision and accuracy
4. Results
　4.1. Soil weathering degree
　4.2. Magnesium distribution in soil
　4.3. Magnesium isotopic compositions of soils and clay fractions
5. Discussion
　5.1. Weathering impact on magnesium isotope variations in soils
　5.2. Potential contribution from aeolian dust
　5.3. Potential contribution from banana culture
　5.4. Potential contribution from sea spray to exchangeable Mg
　5.5. Implications
6. Concluding remarks
Acknowledgements
Appendix A. Supporting information
References