A STABLE ISOTOPIC ANALYSIS OF AN END PLEISTOCENE-AGE STALAGMITE FROM ALMONDA CAVE, WEST-CENTRAL PORTUGAL
BOROWSKE, A., Geology, Cornell College, 810 Commons Circle, Cornell College, Mt. Vernon, IA 52314, A-Borowske@cornellcollege.edu, DENNISTON, Rhawn, Geology, Cornell College, 600 1st St West, Mt Vernon, IA 52314, HAWS, Jonathan, Anthropology, Univ of Louisville, Louisville, KY 40292, CARPENTER, Scott J., Department of Geoscience, Univ of Iowa, 121 Trowbridge Hall, Iowa City, IA 52242-1379, and DORALE, Jeff, Geoscience, Univ of Iowa, Iowa City, IA 52242
"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. "
U-TH CHRONOLOGIES OF STALAGMITES FROM ALMONDA CAVE, WESTERN PORTUGAL
TRODICK, Charles, DENNISTON, Rhawn F.2, ASMEROM, Yemane3, POLYAK, Victor J.3, HAWS, Jonathon4, and SOUTO, Pedro5, (1) Geology, Cornell College, 600 1st Street West, Cornell College, Mount Vernon, IA 52314, C-Trodick@cornellcollege.edu, (2) Geology, Cornell College, 600 1st Street West, Mt. Vernon, IA 52314, (3) Earth & Planetary Sciences, Univ of New Mexico, 200 Yale Blvd., Northrop Hall, Albuquerque, NM 87131, (4) Anthropology, University of Louisville, Louisville, KY 40292, (5) Sociedade Torrejana de Espeleologia e Arqueologia, Quinta da Lezíria, 2350, Torres Novas, 2350-743, 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."
No comments:
Post a Comment