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 palaeosols
in the Somma–Vesuvius area, southern Italy in order to test whether they are suitable proxy
records for climatic and ecological changes in this area 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 was not accompanied by significant change in d18O of pedogenic carbonate. This
could be explained by changes in evaporation rate and in isotope fractionation between water
and precipitated carbonate with temperature, which counterbalanced the expected change in
isotope composition of meteoric water. Because of the rise in temperature and humidity and
the progressive increase in tree cover during the Holocene, the Holocene soil carbonates closely
reflect the isotopic composition of meteoric water. A cooling of about 2°C after the Avellino
eruption (3.8 ka) accounts for a sudden decrease of about 1‰ in d18O of pedogenic carbonate
recorded after this eruption. The d13C values of organic matter and pedogenic carbonate covary,
indicating an effective isotope equilibrium between the organic matter, as the source of CO2,
and the pedogenic carbonate. Carbon isotopes suggest prevailing C3 vegetation and negligible
mixing with volcanogenic or atmospheric CO2.

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