『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 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 play 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.
Keywords: Chemical weathering; Proglacial area; Sr isotopes; Soil
mineralogy; Water chemistry』
1. Introduction
2. Materials and methods
2.1. Study area
2.2. Soil sampling
2.2.1. Soil chemistry and physics
2.3. Mineralogy
2.4. Water sampling
2.5. Water chemistry
3. Results
3.1. Physical and chemical soil properties
3.2. Mineralogy
3.3. Water chemistry
4. Discussion
4.1. Physical and chemical soil properties
4.2. Weathering processes indicated by mineral and water analysis
5. Conclusion
Acknowledgements
References