uraniumthoriumdatering

Uranium-thorium dating, or U-Th dating, is a radiometric technique used to determine the age of calcium carbonate materials such as speleothems (stalagmites and stalactites), coral, travertine, and related carbonate deposits. It is most effective for ages from roughly 1,000 up to several hundred thousand years, providing a time scale for late Quaternary environments that extends beyond the practical range of radiocarbon dating.

Principle and basis: The method relies on the radioactive decay chain from uranium-238 to thorium-230, passing through uranium-234. When carbonate minerals form, they typically incorporate uranium but contain little thorium. Over time, uranium-234 decays to thorium-230; the amount of thorium-230 increases as uranium-234 decays. By measuring the present-day abundances or activities of uranium-238, uranium-234, and thorium-230, scientists calculate the time elapsed since formation. The half-life of thorium-230 is about 75,000 years, which defines the practical dating window. In many samples the initial thorium-230 is negligible, but if detrital thorium or inherited thorium is present, corrections or isochron approaches are used to account for it.

Methods and measurements: Samples are prepared by drilling or dissolving carbonate materials, and the isotopic ratios are measured with mass spectrometry techniques such as TIMS or MC-ICP-MS, or in situ with laser ablation ICP-MS. Precision depends on sample quality, closed-system behavior, and the ability to correct for initial or contaminant thorium.

Applications and limitations: U-Th dating is widely used to date stalagmites and other speleothems to reconstruct paleoclimate, as well as corals and carbonate terraces. Limitations include potential open-system alteration, detrital thorium contamination, and low uranium content, which can reduce precision. Proper sample selection and correction strategies are essential for robust ages.