Stable isotope analyses on the last 30 ka molluscan fauna from Pampa grassland, Bonaerense region, Argentina

Fossil and living shells of terrestrial and freshwater molluscs collected f rom various sites in the Bonaerense and Patagonia regions of Argentina were analysed for their stable isotope content as a possible tool for palaeocli matic and palaeoenvironmental reconstruction. The oxygen isotope compositio n of calcareous shell from freshwater samples was interpreted in terms of a change in moisture condition and the related evaporative effect on surfici al bodies of water, linked to a climatic shift. The carbon isotope composit ion mainly records changes in biological productivity. Interpretation of th e isotopic composition of the land snail carbonate is more complex and worl dwide data from living shells have been used to decipher to some extent the palaeoenvironmental record contained in their isotopic composition. Overal l, the interpretations deduced from isotopic data of land snails as well as from nearby freshwater molluscs are quite comparable. The isotopic data su ggest that arid conditions occurred between 35 and 15 ka B.P, and warmer co nditions around 35-25 ka B.P. A shift toward a new phase comparable with th e present meteorological and climatic situation probably began around 9 ka B.P. and it was linked to some short but severe climatic fluctuations, thus suggesting that high climatic stress characterised this period. Moreover, a trend toward slightly arid conditions is suggested by isotopic data, poss ibly sometimes after 5 ka. The differences arising from the geographical lo cation and the short duration of each single series studied still hamper an extensive interpretation of the isotopic data. (C) 1999 Elsevier Science B .V. All rights reserved.