Physical, geochemical and mineralogical indicators of aging in Quaternary soils of Central Italy

In this paper, a set of physical, geochemical and mineralogical indicators are proposed for the correlation of soil age. The parameters studied allowed the highlighting of trends in soil aging as well as the relating of these trends to specific soil forming processes. Soil aging was marked by a progressive increase in clay content and a decrease in silt/clay ratio, a slight reduction in the CEC of clay and a striking increase in free iron content. Clay mineralogy transformation over time was marked by a progressive increase in kaolinite and vermiculite, as well as a decrease in illite and chlorite. The most outstanding pedological processes, i.e. crystalline free iron content increase, clay neo-genesis and illuviation, glossic horizon and fragipan formation, were all found to be related to kaolinite content. Element content, namely that of Fe, Cr, Pb, Zn, Ti and K increased from the Holocene to the Upper and Middle Pleistocene soil horizons and, as far as Fe, Cr, Pb and Zn are concerned, up to those attributed to the Lower Pleistocene. Al and Ca content decreased along with time, especially in soils older than Holocene. The trends of silt/clay ratio, CEC of clay and free iron were linear, that is not influenced by changes of weathering conditions during Pleistocene, while vermiculite and kaolinite content, total iron, chromium and lead distinguished the stronger weathering environment occurring during Lower Pleistocene in southern Europe. The tendencies of the remaining indicators revealed intermediate phases of mineral transformation or were influenced by rather fast processes. Besides soil age, the indicators resulted as being affected by clay impoverishment, which occurred within eluvial horizons and bleached streaks of fragipan, reducing and oxidizing conditions, and lithological and chronological discontinuities within profiles.