Initial stages of weathering and soil 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†yr) than in [`]old' soils (~†10,000†yr). 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 at very early stages of soil formation. Due to the continuous retreat of the Morteratsch glacier (Upper Engadine, Swiss Alps), the proglacial area offers a full time sequence from 0 to 150†yr old surfaces. A low slope and the absence of glacier which might have interrupted soil formation processes, contributed to the choice of the Morteratsch proglacial valley for this case study. The area is well documented regarding vegetation and soils. The tectonic unit is the Bernina-crystalline, which is mainly constituted of granitoid rocks. Consequently, the glacial till has an acidic character. Mineralogical measurements were carried out on the soil fraction <†2†mm using XRD and DRIFT for qualitative and quantitative phase analysis. In addition, chemical analyses of the stream water from the main channel, tributaries and of rainwater were performed with a special focus on Ca/Sr and Sr isotope ratios (87Sr/86Sr). Furthermore, the accumulation of organic matter within the time sequence and physical soil properties were measured. Decreasing grain size with time shows active physical weathering processes. Soil organic matter has been accumulated during 150†yr at very high rates. Special emphasis has been given to chemical weathering and to the formation and transformation mechanisms of minerals. Of special interest were biotite, chlorite, epidote, plagioclase and calcite. Biotite has been continuously transformed into illite-like components. Within 150†yr, the concentration of epidote significantly decreased. The high Ca/Sr as well as 87Sr/86Sr ratios in the stream and spring waters confirmed that 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 minerals. Disseminated calcite in granitoid rocks, not confined to sedimentary carbonate rocks, also plays a role in subglacial environments. 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 contributes to the supply of soluble Ca, although the ion activity product calculations clearly demonstrated that the waters were undersaturated with respect to this mineral.