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

Aragonite-Calcite Relationships in Speleothems (Grotte De Clamouse, France): Environment, Fabrics, and Carbonate Geochemistry

In Grotte de Clamouse (France), aragonite forms in a variety of crystal habits whose properties reflect the conditions of formation. Prolonged degassing and evaporation yield needle aragonite, which is more enriched in 18O and 13C than aragonite ray crystals, which form near isotopic equilibrium. At present, aragonite ray crystals form at the tops of stalagmites at very low discharge (0.00035 ml/ min), and when fluid Mg/Ca ratio is \u3e 1.1. Temperature and evaporation do not seem to have a significant role in their formation. The presence of aragonite in stalagmites should be indicative of a decrease in drip rate related to either dry climate conditions or local hydrology. Fossil aragonite was in part replaced by calcite in a time frame \u3c 1.0 ka, possibly through the combined effects of dissolution of aragonite, and precipitation of calcite, which preferentially nucleated on calcite cements that had previously formed between aragonite rays. Commonly, the replacement phase inherited the textural and chemical characteristics of the precursor aragonite prisms and needles (and in particular the δ13C signal and U content), and preserved aragonite relicts (up to 16 weight %). The isotope signal of different aragonite habits may reflect conditions of formation rather than climate parameters. The real extent of aragonite-to-calcite transformation may be underestimated when replacement calcite inherits both textural and chemical properties of the precursor