Bedrock versus superficial deposits in the Swiss Jura Mountains: What is the legitimate soil parent material?

摘要

Soils are generally considered to be the result of in situ weathered products of geological substrata. Nevertheless, the Earth's surface is mainly composed of superficial deposits made of sediments reworked during periods of climate instability (glaciation, aridification, flooding, etc.). But to what extent do these deposits influence present-day soil development and/or lessen the contribution of the geological substratum? In the Swiss Jura Mountains, characterization and distribution of superficial deposits along a toposequence are used in order to decipher the polygenic sources of soil parent materials and question the importance of lithodependence in soil development. The in situ weathering material and the allochtonous deposits are discriminated through analysis of mineralogical composition, grain size distribution of the soil mineral fraction, and multivariate statistical treatment. As expected, calcite is dominant in the autochonous mineral phase, leading to high pH values in deep soil layers. However, the mineralogical composition of the surficial deposits is made of local mineral components mixed with allochtonous aeolian deposits. This exogenous material is dominated by quartz and phyllosilicates, and is reflected by low pH values in soil surficial horizons. The grain size distribution curves of the insoluble residue from bedrocks are unimodal and dominated by fine loam and clays. In contrast, surficial deposits display polymodal curves, demonstrating the contribution of various aeolian episodes and/or sources. Analysis of the surface texture of quartz sand grains shows glacial marks intersected by aeolian shock marks, indicating the posteriority of wind transportation. Loess material originates from Alpine moraines of the Swiss Plateau and is mixed with deposits originating from more proximal Jura moraines, diluting the Alpine signature. Consequently, the soil development does not follow the expected and conventional decalcification process, but is directly influenced by the origin of the primary sediments (even when present only in small amounts) and their evolution.