Many soils older than the Holocene have experienced several changes, and possess properties that may be due to the complex effect of several stages of soil development; these soils are called polygenetic. It is still rather unclear, however, if, as time elapses, pedogenic processes tend to diverge generating different soils, or converge towards increasingly similar soils. We studied five pedons (37°60′N, 13°90′E) exposed to present weathering simultaneously since the Holocene but located on Upper Miocene or Holocene parent material. Their XRD and FTIR clay mineralogy reveal an overall homogeneity; smectites, calcite and gypsum reach the thermodynamic equilibrium, a slight undersaturation of the illitic phase occurs in all soil profiles. Also soil micromorphology confirms a general similarity: vertic features detected as anisotropic lines and aureoles occur in all soils. Net mass chemical fluxes, based on mean translocation values per profile, excluded vertical (depth to the soil profile) and horizontal (topographical positions of profiles) pedogenic domains. Here, we show that the elapsed time of weathering has obliterated the effects of past sea transgression, revealing the vertic character independently on the age of its parent material in the case of soils that develop from Holocene, Messinian and Tortonian deposits, with a rate of pedogenesis plausibly in the order of 1/10 mm per year. The soils of today reveal a strong homogeneity, in particular, in the mineralogical composition of the very fine earth; twenty-five per cent kaolinite and 75% interstratified illite/smectite are independently inherited from the parent material and are thermodynamically very stable. Vertic properties here could be considered to be in a steady-state condition nevertheless they experienced significant environmental disturbance.

Free access to this article is valid until March 31, 2016 at http://authors.elsevier.com/a/1SWh-8MoFTn5O5