Late pleistocene palaeoclimate reconstruction from a large stalagmite (Central east Sardinia - Italy)

During past few decades speleothems become a powerful tool for Quaternary palaeo-climate reconstruction: in fact, isotopic studies using 14C and U/Th dating combined with stable isotope analysis (oxygen and carbon) allow reliable palaeo-climatotogical and palaeo-environmental reconstructions. In October 2000 a 1,60 meter tall stalagmite was sampled in the Codula llune cave system (Urzulei, central-east Sardinia), at a distance of about 1 km and an altitude of 100m a.s.l.. Drip waters have also been collected three times at four stations inside the Su Palu cave to obtain a precise reference on the modern values of cave water. In the geochemical laboratory of the Parma University the speleothem was cut longitudinally in two pieces, one of which has been used for sampling using a dental diamond drill. Samples were treated according to well established procedures and analysed for stable isotopes. Four samples were collected for age determinations with a plasma spectrometer that were carried out at the University of Bern. The age determinations yielded the following results (Ka B.P.): 134.8 ± 6.1, 106.1 ± 4.7, 101.1 ± 3.8 and 72.9 ± 2.3 at the heights of 0 cm (bottom), 71 cm, 112 cm and 160 cm (top) respectively: all these data are within isotopic stage 5. The isotopic values obtained from the stalagmite do not show large changes from the base of the concretion to about 100 cm. From this point on both oxygen and carbon values evolve clearly towards more positive values. D180 and d13C of the stalagmite have a rather positive correlation suggesting that precipitation of the CaC03 may have occurred under non-equilibrium conditions. Further more detailed measurements are now carried out to investigate this possibility. lt seems that the speleothem recorded a relatively long period of minor climatic changes at the end of which a marked climatic evolution took place. The consistent isotopic changes of both variables (oxygen and carbon) suggest a climatic evolution that caused heavier oxygen isotope values and a marked decrease of the inflow in the cave system of dissolved carbon species of continental (biogenic) origin. The Authors would like to express special thanks to the speleologists of the Gruppo Archeo Speleo Ambientale Urzulei, the Gruppo Speleo Ambientale Sassari, the Unione Speleologica Cagliaritana and the Gruppo Speleo-Archeologico Giovanni Spano of Cagliari for the difficult recovery of the large speleothem