Optimizing subsampling strategies for U/Th dating and geochemical proxies in carbonate speleothems

Since they provide the chronological frame on which paleoclimate and paleoenvironmental reconstructions are built, radiometric ages of carbonate speleothems, especially U/Th dating, are of fundamental importance to speleothem and paleoclimate sciences. Based on 17 aragonite-calcite speleothems from the Pont-de-Ratz Cave (S. France), we propose an integrated petrographical and geochemical approach for optimizing subsampling in speleothems. Portable energy-dispersive X-ray fluorescence was used as a fast and non-destructive in-situ trace-element screening of the speleothems. These geochemical results are combined with petrographical and mineralogical information to produce distribution maps showing the growth discontinuities and the identified primary and secondary (diagenetic) products on the speleothem slabs. Because they provide a clear and precise spatial distribution of primary and secondary carbonate phases at the sample scale, these maps constitute a valuable tool for selecting the position of subsamples for radiometric dating or environmental proxy analysis. The efficiency and reliability of our integrative approach is convincingly demonstrated by the remarkable coherence of U/Th ages both within and between speleothem samples.