Stable isotope record in mollusca and pedogenic carbonate from Late Pliocene soils of Central Italy

The Late Pliocene (younger than 2.5 Ma) alluvial fan succession of Toppetti Quarry, near Todi, Umbria, Central Italy, includes many palaeosols. Pedogenic carbonate concretions and mollusc remains were separated from 73 palaeosols collected along 150 m of exposure and analysed for their oxygen and carbon stable isotope content. δ13C and δ18O of the pedogenic carbonate and land snail shells suggest that no dramatic change in local climate took place. Values of the pedogenic carbonates are similar to other data reported for the Mediterranean region. δ18O data suggest a mean annual temperature and/or δ18O similar to modern climatic conditions. Carbon isotope data from both land snails and pedogenic carbonate are consistent with a prevailing type C3 vegetation. A mixture of forest and C3 grassland, sustained by a mean annual precipitation between 300 and 1000 mm, mainly in the cool periods of the year, seems to be the most likely scenario. The limited variation shown by the isotope composition of the biogenic and inorganically precipitated carbonate indicates that the environmental variations are referable mainly to change of humidity rather than temperature.