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Abstract: As a consequence of global warming, additional areas will become ice-free and subject to weathering and soil formation. The most evident soil changes in the Alps will occur in proglacial areas where young soils will continuously develop due to glacier retreat. Little is known about the initial stages of weathering and soil formation, i.e. during the first decades of soil genesis. In this study, we investigated clay minerals formation during a time span 0–150 years in the proglacial area of Morteratsch (Swiss Alps). The soils developed on granitic till and were Lithic Leptosols. Mineralogical measurements of the clay (<2μm) and fine silt fraction (2–32μm) were carried out using XRD (X-ray Diffraction) and DRIFT (Diffuse Reflectance Infrared Fourier Transform). Fast formation and transformation mechanisms were measured in the clay fraction. The decreasing proportion of trioctahedral phases with time confirmed active chemical weathering. Since the start of soil formation, smectite was actively formed. Some smectite (low charge) and vermiculite (high charge) was however already present in the parent material. Main source of smectite formation was biotite, hornblende and probably plagioclase. Furthermore, irregularly and regularly interstratified clay minerals (mica–HIV or mica–vermiculite) were formed immediately after the start of moraine exposure to weathering. In addition, hydroxy-interlayered smectite (HIS) as a transitory weathering product from mica to smectite was detected. Furthermore, since the start of soil evolution, kaolinite was progressively formed. In the silt fraction, only little changes could be detected; i.e. some formation of an interstratified mica–HIV or mica–vermiculite phase. The detected clay mineral formation and transformation mechanisms within this short time span confirmed the high reactivity of freshly exposed sediments, even in a cryic environment. [Copyright &y& Elsevier]

Abstract: Proglacial areas in the Alps usually cover a time span of deglaciation of about 150years (time since the end of the ‘Little Ice Age’ in the 1850s). In these proglacial areas soils have started to develop. Due to the continuous retreat of the Morteratsch glacier (Swiss Alps), the corresponding proglacial area offers a continuous time sequence from 0 to 150year-old surfaces. Furthermore, an optimal digital dataset about the development of vegetation and soils is available for this area. The soils have been developing on glacial till having a granitoidic character. We investigated the clay mineral assemblage at 35 sites within the glacier forefield. Smectite can be used as a proxy for weathering intensity in these environments. In the proglacial area, the smectite concentration in the topsoil steadily increased with time of weathering; however, this development displayed a strongly scattered trend. The complex interplay of biological, physical, and chemical processes in pedogenesis and clay mineral evolution limits our ability to predict and interpret landscape dynamics. We consequently tried to analyse the role of topographic and vegetation modifications on the smectite content. Sites not or only slightly prone to erosion (flattening slope ridge, steepening slope ridge) or flat morphology promoted the formation of smectite. In addition, the texture of the soil material influenced soil moisture and hence the degree of weathering and the development of vegetation. Although vegetation is not a fully independent factor, because it is interrelated to the surrounding environmental conditions, it seemed to exert its influence on weathering and, consequently, the formation of smectite. Green alder stands and grass heath, where moister soils develop that have a finer texture or where more organic acids are produced, were correlated with a higher smectite content. Humus forms serve as a proxy for the soil biota and soil organic matter composition. At sites having a Eumoder and a higher soil organic matter content, the smectite concentration was elevated. At these sites, the production of chelating compounds or organic acids in the soil is believed to promote the development of smectites via an intermediate stage of hydroxy-interlayered minerals and the subsequent removal of the hydroxide polymers. In this work we have demonstrated that the topographic signature and the effect of vegetation on the formation of smectite and consequent weathering are pervasive. Our results will serve as a basis for further spatio-temporal modelling. [Copyright &y& Elsevier]