Multi-isotopic age assessment of dirty speleothem calcite: an example from Altamira Cave, Spain

Abstract

The dating of dirty speleothem calcite of Late Pleistocene or Holocene age can be problematic. This is notably the case with the 14C method, due to uncertainties in the initial 14C activity of the cave CO2, and with the 230Th–234U method, due to either low U contents in the authigenic calcite, and/or to the presence of heterogeneous detrital particles containing high concentrations of U and Th series isotopes frequently out of secular equilibrium. In this study of a ∼60 cm-thick flowstone sealing archaeological remains of the Magdalenian period in the Altamira Cave (NW Spain), we used a combination of isotopic approaches and analytical techniques to confront dating methods and finally tried to validate chronological interpretations based on stable isotope paleoclimate constraints. U-series data indicate the presence of a relatively important and heterogeneous detrital fraction resulting in 232Th/230Th mass ratios generally too high (0.1–0.25×106) to allow precise TIMS measurements. MC-ICP-MS measurements provided more reliable results and allowed the calculation of 230Th–234U isochron ages of 11.8±0.8 ka in the upper part of the flowstone, and of ∼22 ka, in the lower part of it. In view of the calibrated 14C ages of these layers (∼13 and ∼15 ka BP, respectively), the isochron age of the lower unit of the flowstone seems erroneous, suggesting the presence of an isotopically heterogeneous contaminating fraction in this layer. 226Ra/230Th ratio measurements indicate near secular equilibrium conditions. Seriate stable isotope measurements indicate calcite precipitation in isotopic equilibrium with ambient waters. They suggest that flowstone deposition ended at the very onset of the Younger Dryas. This transition seems to have occurred here at 11.8±0.8 (±2σ) ka (based on Th/U isochron), and 10.7±0.08 (±1σ) uncalibrated 14C kiloyears. The 14C activity for the cave CO2 seems to have been near that of atmospheric CO2 during the precipitation interval. It is concluded that the 14C data provide here relatively reliable ages, in spite of the fact that they partly fall into a period characterized by 14C-plateau effects, but that other isotopic data revealed essential in validating them.