U–Th dating of speleothems with high initial 230Th using stratigraphical constraint.
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Uranium–thorium dating of speleothem calcite has proved to be exceptionally useful in anchoring late Quaternary paleoenvironmental records, and is arguably the most robust geochronometer available over much of this period. Nonetheless, correction for the effect of 230Th incorporated at the time of deposition is extremely important, and has been approached in a number of ways. The use of stratigraphical constraint, that is the a priori requirement that any axial sequence of true ages must exhibit a monotonic increase away from the outer surface, is surprisingly uncommon given its advantages over other techniques. Here a simple numerical implementation of this technique is described, and contrasted with conventional isochron methods. Current practice in the correction of speleothem age determinations for initial Th is reviewed in the light of continued improvements in analytical precision, and is found to be inconsistent.