A matter of time: advances in isotope dilution and laser ablation U–Th and U–Pb dating of speleothems and other terrestrial carbonates

Carbonate U–Th and U–Pb dating has undergone significant development in recent years, with advances in both solution-based and in situ laser ablation (LA) techniques enabling increasingly refined temporal reconstructions across a wide range of Earth science disciplines. In this review, we provide a synthesis of the developments and current state of U–Th and U–Pb geochronology applied to terrestrial carbonates, and speleothem in particular, with emphasis on methodological developments, analytical limitations, and sample characterisation. We begin by outlining the basic geochemical requirements for successful U–Th and U–Pb dating and highlight the critical role of rigorous sample characterisation and pre-screening, particularly in heterogeneous or diagenetically-altered materials. We review the foundations of solution-based isotope dilution (ID) methods, which remain the benchmark for precision and accuracy, and compare these with the rapidly evolving field of LA-ICP-MS, which offers unique advantages in terms of spatial resolution, throughput, and the ability to target complex or fragile microstructures. We discuss developments in LA instrumentation, data reduction strategies, and reference materials, as well as some persistent limitations. Finally, we consider the future of the field by highlighting potential avenues for research, and offer some recommendations for improved standardisation of protocols, and better matrix-matched reference materials, particularly for LA U–Th dating. We also encourage more intense efforts of interlaboratory intercalibration amongst the U–Th and U–Pb communities to improve standardisation and comparability of results. While at present, the ID technique is often more precise, laser-based methods continue to mature, and the next decade promises broader applicability across the carbonate geochronology community.