A STABLE ISOTOPIC ANALYSIS OF AN END PLEISTOCENE-AGE STALAGMITE FROM ALMONDA CAVE, WEST-CENTRAL PORTUGAL

Little is known about the climate and vegetation of western Iberia during the last deglaciation. However, cores of marine sediments taken off the coast of southwestern Portugal suggest that southward shifts in the North Atlantic polar front caused two major sea surface cooling events between 17,000 and 9,000 cal-14C yr BP. During these cool periods, deciduous trees decreased in favor of steppe taxa, likely because of increased aridity. The relative importance of cooler air temperatures over Portugal in effecting this vegetation change remains unclear.

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 averages–2 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.