U-Th Chronologies of Stalagmites from Two Caves in Western Portugal

April 29th, 2006

Stalagmites from two caves, Almonda Cave and Glory Hole Cave, located near Nazare in westcentral 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 U-Th mass spectrometry. Most stalagmites 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, 18-16 ka, although the dates on all but the youngest stalagmite are complicated by high abundances of 232Th (and a high uncertainty in initial 230Th/232Th ratios). We corrected for unsupported 230Th using an initial 230Th/232Th ratio of 4.5 ppm (± 50%), the average crustal silicate value, although this correction factor may be more than an order of magnitude too low; future 232Th/230Th vs 234U/230Th isochron studies will be used to better constrain initial Th ratios. The only Pleistocene-age stalagmite that was precisely dated grew from 18,020 ± (620) – 15,700 ± 450, 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 occurred coincident with Heinrich Events 5 and 6 (45 and 60 ka, respectively).

Charles Trodick, ’07 LeClaire, IA
Major: Geology
Rhawn Denniston
Cornell College

Sponsor: Rhawn Denniston

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