U and Pb variability in older speleothems and strategies for their chronology

Speleothem proxy records, coupled with U–Th chronologies, are widely used in studies of palaeoclimate, landscape and faunal evolution. The prospect of extending such studies beyond ∼500 ka has fuelled considerable interest in the U–Pb dating method. While theoretical mechanisms for the transport and incorporation of both U and Pb in speleothem calcite remains a matter of debate, it is clear that natural samples span a broad spectrum of compositional variation, and likely contain Pb components with multiple origins and/or transport pathways. Based upon these observations, optimal methods for sample selection are proposed and a variety of techniques for age estimation are explored. Laser ablation ICPMS and autoradiography are ideal rapid screening methods for reconnaissance studies. Common Pb isotopic variability is a key parameter in the choice of analytical strategy for age determination. In large U–Pb studies where invariant common Pb can be established, common Pb corrected ages can be produced for individual sample aliquots promoting rapid throughput and permitting the establishment of detailed age-depth models. Even with these technologies, however, age resolution becomes an issue beyond a few million years, at which time alternative strategies for developing internal speleothem chronologies, such as lamina counting, must be considered in unison with U–Pb dating.

► Natural variations in U and Pb content in speleothems.
► Optimal strategies for sample screening.
► Alternative methods for isochron construction.
► Limitations on age models through time.