Radiometric dating of recent sediments: On the performance of 210Pb-based CRS chronologies under varying rates of supply

Radiometric dating was a revolutionary contribution to the study of sedimentary processes. Empirical data from varved sediments show that unsupported (210)pb (Pb-210(exc)) fluxes vary over time while they statistically correlate with sediment accumulation rates (SAR). This contradicts the basic assumption of the Constant Rate of Supply (CRS) model, which is the most widely used technique for recent sediments. This work is aimed at assessing the CRS model-errors by quantifying the effects on chronologies of various patterns of temporal variability of fluxes. Results are discussed through applications with synthetic and real cores for which and independent varve chronology or a set of reference dates are available. Periodic harmonic and random fluctuations in fluxes tend to cancel out positive and negative deviations in CRS ages, but they produce spurious variability in SARs. Persistent changes in fluxes lead to cumulated and unacceptable deviations of the CRS ages, which cannot be satisfactorily compensated by the use of a piecewise CRS model. The raw and piecewise CRS models can still be powerful tools of widespread use when the conditions for their applicability are well understood. The paper shows how the analysis of clusters in the Pb-210(exc) vs mass depth profile, along with the estimation of equivalent constant fluxes pre and post-dating a known reference date, are powerful methods to prevent misapplications of the CRS model.