THE ROLE OF PALEOSOLS IN PALEOENVIRONMENTAL STUDIES: GENESIS AND DEVELOPMENT OF APENNINE MOUNTAIN SOILS DURING THE HOLOCENE.
Creators: name: MARIANI, GUIDO STEFANO
Publisher: name: Universit� degli Studi di Milano
Year: 2016

description: The aim of this research is to conduct an investigation on the soils, paleosols and landforms of the area of Mt Cusna ridge (Northern Apennines - Italy) in order to define the relationship between geomorphic evolution and soil development, to characterise the main pedogenetic processes and factors acting on soils in past and present times, and to provide new information to understanding the Holocene climatic variations in the area. For this purpose, several sets of field, laboratory and microscopic analyses were carried out. A comprehensive survey of the study area allowed the production of a geomorphological map (scale 1:10000, attached to this thesis) expanding and revising the existing cartography (Panizza et al., 1982). At the same time, also the soils were surveyed and described, in order to provide a detailed characterisation of the soil types of the area. Selected soil profiles were sampled and underwent a wide set of laboratory analyses, including measurements of pH, exchangeable bases and cation exchange capacity (CEC), organic carbon content, total nitrogen and exchangeable phosphorus; along these iron oxides were investigated with the measurement of total iron and its extractable forms; grain size analyses and x-ray diffractions on the clay fraction were also carried out; finally, micromorphology of soil thin sections and SEM observations and analyses were also carried out on a subset of significant horizons. Geomorphological survey results allowed to compare the active and inactive processes. Glacial and periglacial processes, though mainly inactive since the beginning of the Holocene, still give an important forcing on the present landscape through their deposits, produced during the Last Glacial period. During the Holocene, different phases of stability and instability could be detected from the activation an reactivation of slope dynamics as the result of both climate fluctuations and structural constraints. The result is a very diversified landscape in which erosion and deposition alternate in space and time. The present day conditions are characterised by a prevalence of washout and erosion activity, mainly on lithologies more susceptible to surface processes. Soils were attributed to different landscape units, highlighting differences in processes and development between them. Degree of pedogenesis, in fact, varies from weakly developed soils on the highest and steeper areas to deep and more weathered profiles at lower elevations and/or on flatter surfaces. Among the latter, the presence of a paleosurface, characterised by the presence of paleosol units truncated and buried by colluvial deposits, has been detected in the stable areas above 1650 m a.s.l.. Laboratory geopedological analyses showed how soil in the area are characterised by being mainly silty-clayey, with low values of pH and organic C which tends to concentrate at the surface and in many cases also in the uppermost horizon of buried soil units. Iron oxides are usually present in its crystalline form, with some exceptions in peculiar soils. The crystalline/total iron ratio (weathering index) is generally low, higher inside paleosurface buried units. Clay fraction mineralogy shows the presence of quartz and chlorite inherited from parent material; neoformed clay minerals consist in illite and mixed layer clays. Micromorphological analysis mainly involved paleosurface horizons. Colluvial units show the presence of variable quantities of pedorelicts (Brewer, 1967) in their groundmass, as well as features pointing to multiple depositional events. The buried unit shows different phases of clay illuviation relatable to environmental changes. In flat areas a 2Ab horizon is found between the two units, showing accumulation of excrements and organic material; features of frost action are also present. Particular conditions are related to higher elevations as well as areas with steeper slopes. Data obtained from field and analytical approaches allowed to outline the main pedogenetic processes acting in the area. Pedogenesis started since the glacial retreat: clay mineralogy and iron oxide content are compatible with a soil formation taking place during the Holocene. The main active process in the area is Brunification (Duchaufour, 1983), which drive the development of Regosols and Cambisols (FAO, 2014); these soils are better developed at lower elevations on flat areas and stable deposits, whereas soils on steeper slopes and higher elevations show evidences of a weaker pedogenesis. Luvisols (FAO, 2014) were also formed in the past and are mainly preserved as paleosols of the buried units related to the paleosurface; moreover the clayey pedofeatures of these paleosols allowed the identification of three different clay illuviation phases, preceding the Subboreal climatic recrudescence 14C dated (Compostella et al., 2012; Giraudi, 2014). During the Late Holocene in the area appears Podzolisation (Duchaufour, 1983) as a secondary process, as testified by the presence of cryptopodzolisation features in soil profiles from different areas. Traces of this process could be also found inside the 2Ab horizon, marking the top of the buried unit in some soil profiles surveyed on the paleosurface; this horizon can be characterised as an accumulation of insect excrements and organic material developed in cold conditions. A change in vegetation cover detected by anthracological assemblages (Compostella et al., 2012) seems to confirm this hypothesis. 14C dating (Compostella et al., 2012) and frost features inside the 2Ab horizon date its burial to the Little Ice Age (LIA), which marks a phase of general erosion causing colluvial deposition also in flatter areas, which probably happened in multiple events through time. The colluvial layers show apparent pedogenesis caused by the presence of pre-weathered soil material and signs of homogenisation probably related to cryoturbation processes. Finally, the presence of frost and solifluction features inside these recent soils point to the characterisation of the LIA as a drier period in which winter precipitation were less abundant and snow cover thinner. This study outlined the existence of complex interactions between pedogenic, geomorphic and environmental processes throughout the Holocene. The influence of these aspects on soil features could be detected and used to describe and interpret the present landscape in the light of its modifications through time.