Direct and highly sensitive U-Th dating approach for Holocene stalagmites using LA-ICPMS

U-Th dating has frequently been used to determine the timing of Earth’s geological, environmental, and biotic processes from materials with ages from few years to 800 thousand years (ka). Contemporary U-Th dating methods, however, are restricted to a discrete milli-domain (mm) sampling strategy and time-consuming chemical preparation procedures, including dissolution, spiking, extraction and isotope pre-concentration, which are difficult to generate profiles at high spatial resolution. Here we present a direct, high-sensitivity carbonate U-Th dating by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Utilizing a “jet” interface ICPMS setup improved the detection efficiency to a yield of 1-2%. An online additive of a well characterized 229Th-233U-236U spike tracer, admixed with the laser generated carbonate aerosol enabled to monitor and correct for mass discrimination and elemental fractionation effects of U and Th. The efficacy of inter-element, mass discrimination, and peak tailing baseline corrections were critically evaluated and optimized. Sufficient signal intensities of 30-100 cps for 230Th ion beam can be generated for 5,000s-yr-old stalagmites with U contents of 5-19 µg/g, while 2-sigma precision of U/Th dates better than ±200 years. A flowstone sample in secular equilibrium, collected from Northern Calcareous Alps, was analyzed to verify the approach and activity ratios of 1.011 ± 0.066 (2σm) for 230Th/238U were obtained. This approach allows for the reconstruction of accurate age profiles for Holocene carbonates with ages of only 5,000s years and can be applied for better constraining the timing of past events or climatic conditions