U-TH CHRONOLOGIES OF STALAGMITES FROM ALMONDA CAVE, WESTERN PORTUGAL

Stalagmites from Almonda Cave, located near Nazare in semi-arid west-central Portugal, are being investigated in order to reconstruct the region's paleoenvironmental history. Here we report our initial attempts to construct growth histories for these samples using uranium-series dating. The stalagmites collected from Almonda Cave, one of the deepest and most extensive cave systems in Portugal, are characterized by intervals of growth separated by multiple hiatuses. Where dated, these growth pulses appear centered at 74, 64-60, 45-44, and 18-16 ka, although the dates on all but the youngest stalagmite are complicated by high abundances of ²³²Th and an uncertainty in initial ²³⁰Th/²³²Th ratios. We corrected for detrital ²³⁰Th using an initial ²³⁰Th/²³²Th atomic ratio of 4.5 ppm (±50%), the average crustal silicate value. The Pleistocene-age stalagmite most precisely dated grew from 18.0±0.6 to 15.7 ±0.4 ka, coincident with the onset and termination of ice-rafted debris associated with Heinrich Event 1 in core S075-6KL from the Iberian margin (Boessenkool et al., 2001). The remaining dates also suggest, albeit with large uncertainties, that stalagmite growth may have also been coincident with Heinrich Events 5 and 6 (45 and 60 ka, respectively).

Based on a coupled analysis of both pollen and the oxygen isotopic composition of dinoflagellate cysts from S075-6KL, Boessenkool et al. (2001) argued that reduced sea surface temperatures associated with Heinrich events increased aridity in western Portugal. However, in arid and semi-arid climates, meteoric precipitation exerts a primary control on speleothem growth (Polyak and Asmerom, 2001), and thus this apparent correlation of speleothem growth and Heinrich events is surprising and may reflect a more complex control of sea surface temperatures on moisture balances along western Iberia than has been proposed.