Microstrutture della crosta calcarea laminare (orizzonte petrocalcico) di due paleosuoli Pleistocenici nell’agro di Cerignola (Foggia)

Abstract

D.Magaldi & M. Giammatteo, Laminar crust microstructures of 2 Pleistocene palaeosols with petrocalcic horizons from
Cerignola (Foggia Province, Southern Italy). (IT ISSN 0394-3356, 2008).
The petrocalcic horizon (caliche, calcareous crust) is a cemented or indurated calcic horizon largely occurring in arid, semiarid and
Mediterranean zones worldwide. Palaeosols with petrocalcic horizons which formed on the Early Pleistocene marine sediments are
common in the Capitanata plain of the Foggia Province, Southern Italy.
This paper reports results of SEM –EDX observation carried out on the laminar crust or platy horizon collected from two Petrocalcic
Calcixerepts ( QS and VM) which formed on terraced sandy sediments in the countryside near Cerignola ( Capitanata plain ) at 100 and
80 m a.s.l. respectively. The soils differentiate for texture and profile development : being coarse and well developed QS, moderately
fine and less developed, VM. Results revealed in all samples the occurrence of features related to calcite micro features (the α and β
fabric after Wright) which are differently spread in the samples of each crust. QS crust posses features mainly related to a fabric ( inorganic origin) which is characterised by common crystallisation of large scalenohedral and rhombohedral calcite individuals, being scarce calcite features of biologic origin. On the contrary features as needle –shaped crystals, rice-grains crystals, tubules and rhizolites
which are attributed to β fabric (biologic origin) are dominant in VM samples together with some silica substitution for calcium carbonate in organic structures. Formation of these crust appears to be controlled by palaeoenvironmental factors. According to information coming from international references, it was assumed that inorganic process as evapotranspiration and vadose water movement mainly acted on the QS crust formation so producing the randomly arranged large crystals and the Ca and Mg enrichment during some arid or semiarid climatic conditions. Instead the VM crust shows more expressed biologic features which are commonly related to be acquired in semi-arid to sub-humid climatic conditions by the action of both soil vegetation and micro-organisms. Moreover, the occurrence of silica mobilisation and precipitation let us to assume that a strong soil pH variation during past time has happened. The two profiles were considered genetically different on the basis of microscopic features of the laminar crust .This fact also suggests that QS profile is older than VM and nearly confirms the significance of laminar crust in the Quaternary palaeoclimatic reconstruction.