Chronologie Th/U des concrétions calcaires des varves du lac glaciaire de Deschaillons (Wisconsinien inférieur)

Abstract

Late Pleistocene Lake Deschaillons varves from the central St. Lawrence Lowlands represent a lithostratigraphic unit whose upper limit is defined by the overlying Gentilly Till, which was laid down during the last Laurentide Ice advance over the area. Calcareous concretions are imbedded in the varves. These concretions reflect two distinct phases of CaCO3 precipitation. The first phase was during the early diagenesis of the interstitial organic matter. This phase is characterized by high uranium contents (~ 8 μg∙g−1), low carbon- and oxygen-isotope composition (−22 > δ13C > −24‰, −16 > δ18O > −9‰, vs. PDB), and 14C activities lower than 3% (vs. 13.56 dpm∙g−1). The second phase of CaCO3 precipitation occurred during postglacial times, as shown by the higher 14C activity (~30%) and stable-isotope composition (δ13C > −16‰; δ18O > −9‰). No significant U uptake was observed in the corresponding concretions. Th and U activity ratios were determined. The results allow evaluation of the analytical artefacts resulting from partial leaching of the detrital particles incorporated in the concretions by the weak acid treatment used to solubilize calcite. Since the early diagenetic concretions contain a significant but variable detrital fraction, 230Th/232Th and 234U/232Th isochrons from activity ratios measured in several samples are not representative of the age of the carbonate fraction. Isochrons from activity ratios of leachate and residue from individual samples yield, however, a reproducible slope (mean value: 0.52 ± 0.01) representative of the 230Th/234U activity ratio of the pure carbonate fraction. An age of 79 800 ± 1400 years is calculated for the relevant phase of CaCO3 precipitation. It is considered, within a few hundred years, the age of the Lake Deschaillons episode, which would therefore correspond to the first Wisconsinian ice advance in southern Quebec, at the transition between isotopic stages 5a and 4 of the oceanic 18O stratigraphy.