Improved U–Th dating of carbonates with high initial 230Th using stratigraphical and coevality constraints

Abstract

With high precision measurements now achievable by MC-ICPMS, the uncertainty on the initial 230Th content can be the major source of uncertainty for U–Th ages of carbonates. The initial 230Th content is usually derived from the 232Th content of the sample and the initial 230Th/232Th ratio calculated using isochron techniques or using stratigraphical constraints along the speleothem growth axis. These two methods are based on different hypotheses and have not been couple previously. Here, we present a new algorithm called STRUTages that combines stratigraphical and coevality constraints in order to obtain the best estimate on the initial 230Th/232Th ratio of each sample. STRUTages and isochron results are compared on a suite of speleothems and a coral core. Comparison of the U–Th and growth-band counted ages of the coral core demonstrates the validity of the STRUTages approach. An Octave (Matlab-compatible) script allowing the use of stratigraphical and coevality constraints is provided.