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Initial Stages of Soil and Clay Minerals Formation in the Morteratsch Proglacial Area (upper Engadine, Switzerland)

Abstract:

Investigations in Alpine soils indicate that mineral weathering is much faster in ‘young’ soils (< 1000 years) than in ‘old’ soils (10,000 years). However, 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 rock-forming minerals weathering and clay minerals formation during this time span. Due to the continuous retreat of the Morteratsch glacier (Upper Engadine, Swiss Alps), the proglacial area offers a full time sequence from 0 to 160 years old surfaces. Reduced elevation stacks, low slope and absence of glacier advancement within this time span and soil formation process interruption, contributed to the choice of Morteratsch proglacial valley as case study, as well. The area is also well documented regarding vegetation and soils. The morainic material was produced through glacial transport within a small area of relatively homogeneous parent material. The lithostratigraphic unit is the Bernina-crystalline, mainly constituted of diorite and granodiorite; consequently, the glacial till shows an acidic character. Accordingly, special emphasis has been given to the weathering of biotite, the ‘chlorite’ group, amphiboles and apatite in the soils. Mineralogical measurements were carried out for the clay as well as for the bulk soil fraction using XRD, DRIFT and total chemical analyses. In addition to the soil mineralogy, analyses of the main stream water, tributary rivers and the rainwater were performed with a special focus on Sr isotopes (87Sr/86Sr). The water measurements supported the findings of mineral analyses. The high Ca/Sr as well as 87Sr/86Sr ratios confirmed that biotite and other Ca bearing minerals are weathering and transforming at very high rates in the proglacial area. Also in cryic, ice-free environments, chemical weathering rates are high leading to the formation and transformation of clay minerals (Simas et al., 2006). Biotite (clay fraction) transforms relatively quickly into hydrobiotite and in a following step into expandable minerals or into HIV. Also the process of ‘chlorite group’ minerals (clay fraction) transformation into HIV seems to be a fast process. Disseminated calcite in granitoid rocks, not only confined to carbonate geology, also plays a role in subglacial environments (White et al., 1999). It is, however, not known for how long such an influence is significant and measurable. The high Ca/Na and Ca/Sr ratio in the stream and tributary waters showed that calcite significantly contributes to the Ca solubility although the ion activity product calculations clearly demonstrate that the waters are undersaturated with respect to calcite (Blum et al., 2002). White, A.F., Bullen T.D., Vivit, D.V., Schulz, M.S., Clow, D.W. 1999. The role of disseminated calcite in the chemical weathering of granitoid rocks. Geochimica et Cosmochimica Acta, 63, Issues 13-14, July 1999, p.1939-1953 Simas, F.N.B., Schaefer, C.E.G.R., Melo, V.F., Guerra, M.B.B., Saunders, M., Gilkes, R.J., 2006. Clay-sized minerals in permafrost-affected soils (Cryosols) from King George Island, Antarctica. Clays and Clay Minerals, 54, 721-736. Blum, J.D., Klaue A., Nezat, C.A., Driscoll, C.D., Johnson, C.E., Siccama, T.G., Eagar, C., Fahey, T.J., Likens, G.E. 2002. Mycorrhizal weathering of apatite as important calcium source in base-poor ecosystems. Nature, 417, 13 June 2002, p. 729-731

Authors:

Mavris, Christian and Egli, Markus and Plötze, Michael and Blum, Joel and Haeberli, Wilfried

Index Terms:

glacier; weathering; ClayGroup; Blum, Joel; Egli, Markus; Haeberli, Wilfried; Mavris, Christian; Plotze, Michael

Further Information:

Date published: 2009