A soil chronosequence study of the Reno valley, Italy: Insights into the relative role of climate versus anthropogenic forcing on hillslope processes during the mid-Holocene

Abstract

Relatively few soil chronosequences with good age control exist for Late Pleistocene and Holocene deposits in the Mediterranean region of Europe, yet surface soils and their young buried counterparts can provide meaningful information about the timing and processes of landscape response to external forcing such as climate or anthropogenic change. Here we present the results of a study that examines a well-dated chronosequence of nineteen soil exposures in surficial deposits ranging in age from hundreds of years to hundreds of thousands of years located in the Reno River Valley near Bologna, Italy. The soil-forming climate of the Reno Valley sits on a threshold between that which facilitates the accumulation of CaCO 3 and that which facilitates the translocation and formation of clay minerals and iron oxides. Once formed, secondary iron oxides are relatively immobile in oxidizing conditions in the soil profile and therefore consistently record age trends, with Feo/Fed ratios decreasing predictably with soil age for all soils in the chronosequence. The morphology of secondary carbonate accumulation and its depth of leaching also vary predictably with soil age, but only for soils younger than about ~ 12,000 years. In these young soils, horizons that contain secondary carbonate progress from relatively shallow, Stage I Bk horizons in late Holocene-aged soils to deeper Stage III Bk horizons in latest-Pleistocene aged soils. In soils older than approximately 12,000 cal ybp, however, carbonate dissolution during wetter climates and the addition of colluvium and/or dust to terrace surfaces become important factors in the preservation of carbonate in the profile, and carbonate content is not predictable by age alone. Without significant depositional additions, these older soils become decalcified. Pedogenic carbonate precipitated during drier climates is susceptible to dissolution during wetter periods, as evidenced by discontinuous carbonate coatings on grains in some soils. Therefore overall measurable pedogenic carbonate content is spatially and temporally variable as a function of input of fresh carbonate and locally-controlled soil moisture conditions. Particle size does not decrease predictably with soil age for Reno Valley soils and appears to be strongly influenced by parent material variability in younger soils and by dust additions in older soils. Our detailed examination of soil stratigraphy in the Reno Valley revealed evidence of significant aggradation of colluvium and tributary fans in the middle Holocene between ~ 6000 and ~ 4000 ybp. This aggradation pre-dates Bronze Age agricultural expansion, and appears to be instead attributable to the climate conditions of the middle Holocene that have been documented elsewhere in Italy for this time period. This study demonstrates that late Pleistocene and Holocene surface soils in the Mediterranean region of Europe can effectively help elucidate rates and processes of landscape evolution.