Evidence that excess 210Pb flux varies with sediment accumulation rate and implications for dating recent sediments

Abstract

Most 210Pb dating models assume that atmospheric flux of excess 210Pb (210Pbexc) to the sediment–water interface remains constant over time. We revisited this assumption using statistical analysis of a database of laminated sediments and evaluated the implications for radiometric dating of recent deposits. A bibliographic survey enabled us to create a database with 10 annually laminated sediment cores from a variety of aquatic systems. The database has records of 210Pbexc flux, initial 210Pbexc activity, and sediment accumulation rate (SAR). 210Pbexc flux to sediments varied with time, and 1/3 of the data had relative deviations from the mean value >25 %. There was no statistically significant correlation between activities at the core top and SAR, whereas a statistically significant (p < 0.01) linear regression between 210Pbexc flux and SAR was found for nine of the ten cores. Thus, in most of the studied aquatic systems, 210Pbexc flux to the sediment was governed primarily by flux of matter, rather than by direct atmospheric 210Pbexc deposition. Errors in chronology and SAR, attributable to varying 210Pbexc flux and estimated by the constant rate of supply (CRS) model, were evaluated from its analytical solutions, and tested against SAR values from this database that were derived independently from varves. We identified several constraints for general application of the CRS model, which must be taken into account to avoid its misuse.