A STABLE ISOTOPIC ANALYSIS OF AN END PLEISTOCENE-AGE STALAGMITE FROM ALMONDA CAVE, WEST-CENTRAL PORTUGAL
We report preliminary results from analysis of a Portuguese stalagmite that has been dated by two U/Th alpha spectrometry techniques to ~12,000 10,000 yr BP (note that the chronology is should not be considered robust as it is based only on two alpha spectrometry dates from a low U sample). This stalagmite, identified as ALM-04-01, is 21 cm long and is composed of clear, coarsely crystalline calcite. ALM-04-04, was collected from Almonda Cave, which is part of an expansive karst system formed in Mesozoic dolomites and limestones in west-central Portugal.
ALM-04-01 was sampled for stable isotopic analysis at 5 mm intervals. Oxygen isotopic values average 3.2 per mil PDB, but vary by ~1 per mil during two discrete intervals. The ?18O values of speleothems are determined by the ?18O of the infiltrating water (meteoric precipitation) and the temperature of the cave, but may be modified by a variety of variables including kinetic effects. It remains unclear at this point whether the observed oxygen isotopic fluctuations represent rapid and large-scale changes in climate, possibly linked to those observed in marine cores.
The carbon isotopic composition of the bedrock hosting the cave averages2 per mil, and because stalagmite ?13C values reflect carbon from both the bedrock and the soil, the -9 per mil stalagmite carbon isotopic value likely corresponds to C3 vegetation over the cave.