Soils are the product of a complex suite of chemical, biological, and physical processes. In spite of the importance of soils for society and for sustaining life on earth, our knowledge of soil formation rates and of the influence of biological activity on mineral weathering and geochemical cycles is still limited. In this paper we provide a description of the Damma Glacier Critical Zone Observatory and present a first synthesis of our multidisciplinary studies of the 150-yr soil chronosequence. The aim of our research was to improve our understanding of ecosystem development on a barren substrate and the early evolution of soils and to evaluate the influence of biological activity on weathering rates. Soil pH, cation exchange capacity, biomass, bacterial and fungal populations, and soil organic matter show clear gradients related to soil age, in spite of the extreme heterogeneity of the ecosystem. The bulk mineralogy and inorganic geochemistry of the soils, in contrast, are independent of soil age and only in older soils (>100 yr) is incipient weathering observed, mainly as a decreasing content in albite and biotite by coincidental formation of secondary chlorites in the clay fraction. Further, we document the rapid evolution of microbial and plant munities along the chronosequence.

Knowledge of the lithium (Li) isotope fractionation factor during clay mineral formation is a key parameter for Earth system models. This study refines our understanding of isotope fractionation during clay formation with essential implications for the interpretation of field data and the global geochemical cycle of Li. We synthesised Mg-rich layer silicates (stevensite and saponite) at temperatures relevant for Earth surface processes. The resultant solids were characterised by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR) to confirm the mineralogy and crystallinity of the product. Bulk solid samples were treated with ammonium chloride to remove exchangeable Li in order to distinguish the Li isotopic fractionation between these sites and structural (octahedral) sites. Bulk solids, residual solids and exchangeable solutions were all enriched in 6 Li compared to the initial solution. On average, the exchangeable solutions had δ 7 Li values 7 lower than the initial solution. The average difference between the residue and initial solution δ 7 Li values (∆ 7 Li residue−solution) for the synthesised layer silicates was-16.6±1.7 at 20 • C, in agreement with modelling studies, extrapolations from high temperature experimental data and field observations. Three bonding environments were identified from 7 Li-NMR spectra which were present in both bulk and residual solid 7 Li-NMR spectra, implying that some exchangeable Li remains after treatment with ammonium chloride. The 7 Li-NMR peaks were assigned to octahedral, outer-sphere (interlayer and adsorbed) and pseudo-hexagonal (ditrigonal cavity) Li. By combining the 7 Li-NMR data with mass balance constraints we could calculate a fractionation factor, based on a Monte Carlo minimum misfit method, for each bonding environment. The calculated values are-21.5±1.1 ,-0.2±1.9 and 15.0±12.3 for the octahedral, outer-sphere and pseudo-hexagonal sites respectively (errors 1σ). The bulk fractionation factor (∆ 7 Li bulk−solution) is dependent on the chemistry of the initial solution. The higher the Na concentration in the initial solution the lower the bulk δ 7 Li value. We suggest this is due to Na outcompeting Li for interlayer sites and as interlayer Li has a high δ 7 Li value relative to octahedral Li, increased Na serves to lower the bulk δ 7 Li value. Three experiments conducted at higher pH exhibited lower δ 7 Li values in the residual solid. This could either be a kinetic effect, resulting from the higher reaction rate at high pH, or an equilibrium effect resulting from reduced Li incorporation in the residual solid and/or a change in Li speciation in solution. This study highlights the power of 7 Li-NMR in experimental studies of clay synthesis to target site specific Li isotope fractionation factors which can then be used to provide much needed constraints on field processes.

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The direction and magnitude of magnesium (Mg) isotope fractionation attendant to the formation of clay minerals is fundamental to the use of Mg isotopes to decipher the biogeochemical cycling of Mg in the critical zone and for the oceanic Mg budget. This study provides experimental data on the Mg fractionation factor for two smectitegroup minerals (stevensite and saponite) at temperatures relevant for Earth surface processes. The resultant solids were characterised by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR) to confirm the mineralogy and crystallinity of the product. A series of experiments were performed to asses the impact of temperature and pH on isotope fractionation. Bulk solid samples were treated with ammonium chloride to remove exchangeable Mg in order to distinguish the Mg isotopic fractionation between these sites and octahedral sites.All bulk and residual solids were enriched in 24 Mg compared to the initial solution and δ 26 Mg values of the exchangeable pool were lower than, or within error of, the initial solution. Final solutions were either within error of, or enriched in, 26 Mg compared to the initial solution, depending on the fraction of Mg removed from solution (f Mg ). For experiments with small or negligible f Mg , increasing the pH resulted in a higher reaction rate and reduced fractionation from the initial solution. This could point to a kinetic effect, but the composition of the residual solid (Mg/(Li+Mg) ratio) was also dependent on pH. The change in the composition was reflected in the wavenumber of the Mg 3 -OH stretch in FT-IR data, which is a proxy for bond strength, and suggests an equilibrium control. An equilibrium control is further supported by the observation of reduced fractionation compared to the initial solution with increasing temperature. Rayleigh and batch fractionation models were fitted to the data giving fractionation factors of 0.9991 and 0.9990 respectively.We compare our results with existing field and experimental data and suggest that the apparent contradictions surrounding the direction of Mg isotope fractionation into phyllosilicate minerals could be due to the similarity of Mg bond lengths between clay octahedral sites and dissolved Mg. Thus small changes in mineral structure or initial solution conditions may result in a change in bond length sufficient to alter the direction of fractionation, implying that the magnitude and direction of Mg isotope fractionation into clay minerals could be dependent on local field conditions. Alternatively, if the precipitation of secondary clay minerals in the field preferentially incorporates light Mg, as observed in this experimental study, this implies the contribution of carbonate weathering to dissolved Mg fluxes has been underestimated, with major implications for the global biogeochemical cycle of Mg.