Stable isotopes of pedogenic carbonates from the Somma-Vesuvius area, southern Italy, over the past 18 kyr: palaeoclimatic implications

Stable isotopes were measured in the carbonate and organic matter of palaeo sols in the Somma-Vesuvius area, southern Italy in order to test whether th ey are suitable proxy records for climatic and ecological changes in this a rea during the past 18000 yr. The ages of the soils span from ca. 18 to ca. 3 kyr BP. Surprisingly, the Last Glacial to Holocene climate transition wa s not accompanied by significant change in delta O-18 of pedogenic carbonat e. This could be explained by changes in evaporation rate and in isotope fr actionation between water and precipitated carbonate with temperature, whic h counterbalanced the expected change in isotope composition of meteoric wa ter. Because of the rise in temperature and humidity and the progressive in crease in tree cover during the Holocene, the Holocene soil carbonates clos ely reflect the isotopic composition of meteoric water. A cooling of about 2 degreesC after the Avellino eruption (3.8 ka) accounts for a sudden decre ase of about 1 parts per thousand in delta O-18 of pedogenic carbonate reco rded after this eruption. The delta C-13 values of organic matter and pedog enic carbonate covary, indicating an effective isotope equilibrium between the organic matter, as the source of CO2, and the pedogenic carbonate. Carb on isotopes suggest prevailing C-3 vegetation and negligible mixing with vo lcanogenic or atmospheric CO2. Copyright (C) 2000 John Wiley & Sons, Ltd.