Uranium-series and trapped charge dating of tufa and travertines

Speleothems, fresh-water tufa and travertines are generally dated using methods based on uranium-series disequilibria, but also on cosmogenic radionuclides and radiation damage. The basic theory of these methods is introduced and related applications in the study of continental carbonates are shown with examples derived from recent research. U-Th is the most popular method and is based on the measurement of 230Th formed in situ by radioactive decay of uranium (234U and 238U) co-precipitated with CaCO3. In the absence of detrital minerals (see for example pure speleothems) the extent to which the 230Th/234U activity ratio has returned towards unity is a function of time, taking into account also the state of disequilibrium between 234U and 238U. Tufa and travertines, however, are impure mixtures of calcium carbonate and incorporated detrital minerals and deserve the recourse to specific correction schemes to determine the authigenic radionuclide component and thus a fitting age. The method extends back to at least 350,000 years. The disequilibria between other parent-daughter pairs in 238U and 235U decay chains (namely, 234U/238U, 231Pa/235U, 226Ra/230Th) are also used in carbonate dating. 234U/238U method can be applied to dating carbonates as old as 1.25 Ma, assuming that the uranium activity ratio in the parent fluids has remained constant over time. 226Ra/230Th method is used to date continental carbonates younger than 8 ka, presupposing that 226Ra/230Th initial activity ratio can be determined on present-day deposition. Recently, the use of the classic U-Pb method has been extended to speleothems dating, opening new perspectives on the study of older speleothems, not datable using standard U-series methods. The use of radiocarbon dating in carbonate chronology is considered problematic, principally because of potential variability in the contribution of “dead carbon” from the host limestone. Actually, precipitating waters may contain a proportion of carbon inherited from the dissolution of a 14C-free carbonate component in the bedrock. Another problem could arise from the contamination of the material with younger carbon from soil organic matter that might be deposited in the original structure of porous carbonate. In addition to that, bomb-14C may affect modern precipitation, leading to an inconsistently too young age. Because of these problems, relatively few speleothem chronologies are based on 14C ages. Radiation damage dating methods (electron spin resonance, thermoluminescence, optical stimulated luminescence) are based on the time-dependent accumulation of electrons in the crystal lattice of certain common minerals, acting as natural radiation dosimeters. Assuming a constant radiation dose, the extent of trapped electrons is proportional to the accumulated radiation and to the time elapsed since a “zeroing” event. Unfortunately, not all the samples are suited for radiation damage dating: indeed, the presence of cationic impurities such as Mn2+, Fe2+, or Fe3+, humic acids (organic matter), can mask the signal of interest, or interfere with it. These methods can be applied to date materials ranging in age from a few hundreds years to about a million year