A method to anchor floating chronologies in annually laminated speleothems with U–Th dates

Abstract

Stalagmites occasionally present laminae which, when demonstrated to be annual, may be used to construct an annually resolved chronology. Such annually laminated records provide an opportunity to improve the precision of age models based on other dating techniques. Since annually laminated stalagmites sometimes present a complex stratigraphy with lateral variations in lamina thickness associated with changing macroscopic growth shapes, a procedure for lamina counting is developed here, which complements other methods of speleothem lamina counting. Regardless of the complexity of laminae, when the exact date of a laminated section is unknown, lamina counting provides a floating chronology. This paper describes a method to anchor floating chronologies in speleothems using the least-squares fit of the lamina counting to the radiometric dates (typically U–Th). The estimation of uncertainties in the age model is also considered, which accounts for uncertainties in the lamina counting as well as the fit of the lamina count to the radiometric dates. The uncertainty of this fit does not depend on the analytical uncertainty of the radiometric dates or the precision in the lamina counting, and simply considers all the available dates and their distance to the proposed age model. As an example, the method was applied to a stalagmite from Northern Spain and its accuracy was compared with the annually resolved and cross-dated Greenland chronology during the 8.2 ka event. Although this method has been described for stalagmites, it could be applied to other records in which cross-dating techniques would not be suitable.