Use of soil chronosequences to assess the rates of Quaternary pedogenetic development: case studies from the Ionian and Tyrrhenian coasts of northern Calabria, Italy

Soil degradation represents a decline of soil quality in relation to specific functions of value for men, which is becoming a global-scale problem of growing relevance. The usually much lower rates of soil formation with respect to the extremely higher ones of soil erosion make the soil ecosystem a poorly renewable natural resource. Rates of pedogenetic development during the Quaternary were studied, by comparing three soil chronosequences developed on different terrace staircases along the Tyrrhenian and Ionian coasts of northern Calabria, respectively. Two chronosequences occur on marine terraces in two (northern and southern) sectors of the Praia area, spanning from the lower to the middle Pleistocene; the third one is located on middle to upper Pleistocene fluvial terraces in the Rossano area, both reaching an elevation of about 200 m asl. All these soils consist of Alfisols, characterized by diffuse rubification (i.e. reddening of the matrix for Fe-oxide staining) and abundant clay coatings related to illuvial processes. A few horizons from some soil profiles occasionally exhibit eluvial tongues (both from Rossano and Praia older profiles), whereas other horizons from much younger profiles in the Praia area show andic-like properties related to a volcanic input. The soils at issue were compared in terms of degree of development on the basis of their main macro- and micromorphological features, physical and chemical properties, such as particle size distribution, silt/clay ratio, total CaCO3, pH(H2O), base saturation, weathering intensity of primary minerals. Moreover, different weathering indices were calculated and compared: the redness rating (RR) based on the Munsell colours; the chemical index of alteration (CIA), calculated as the ratio Al2O3/(Al2O3+CaO+Na2O+K2O), where the total element chemical composition was measured by XRF spectroscopy; the pedogenic iron indices (Fed−Feo)/Fet, Feo/Fed and Fed/Fet, obtained by selective extraction procedures. The above soil properties (SP) and weathering indices (WI) were calculated for each soil profile by weighting their values on both single horizon (h) and total profile thickness (H), as follows: h*(SP or WI)/H. They were plotted as a function of terrace ages and therefore different time spans of soil development. This method permitted to highlight some pedogenetic correlations among the different terraced surfaces and estimate soil chronofunctions. An overall inefficacy of the CIA index is revealed by an extremely low correlation (reaching a maximum of R2 = 0.02 with a linear regression function and even lower with other curves). On the contrary, the best-suited chronofunctions are represented by the linear regression of the (Fed−Feo)/Fet index (R2 = 0.45) and more efficiently by the Fed/Fet exponential (R2 = 0.64) and linear (R2 = 0.66) regression curves, respectively. These results indicate that the rates of soil development for the studied chronosequences tend to be rather constant through time, despite they underwent cyclical climatic changes during the Quaternary. The presence of some outlier values that do not fit the calculated regression curves can be explained as they belong to one soil profile of the southern Praia chronosequence, characterized by andic-like properties (and therefore higher amounts of Feo) and much younger age than the corresponding terrace, and an early-buried soil from the Rossano area. Other main discrepancies from the regression curves can be related to soil rejuvenation by erosion or aggradation and/or ageing by soil reworking processes.