Modeling the downward transport of ²¹⁰Pb in Peatlands: Initial Penetration-Constant Rate of Supply (IP-CRS) model

The vertical distribution of ²¹⁰Pb is commonly used to date peat deposits accumulated over the last 100-150 years. However, several studies have questioned this method because of an apparent post-depositional mobility of ²¹⁰Pb within somepeat profiles. In this study, we introduce the Initial Penetration-Constant Rate of Supply (IPCRS) model for calculating ages derived from ²¹⁰Pb profiles that are altered by an initial migration of the radionuclide. This new, two-phased, model describes the distribution of atmospheric-derived ²¹⁰Pb (²¹⁰Pb_xs) in peat taking into account both incorporation of ²¹⁰Pb into the accumulating peat matrix as well as an initial flushing of ²¹⁰Pb through the uppermost peat layers. The validity of the IP-CRS model is tested in four anomalous ²¹⁰Pb peat records that showed some deviations fromthe typical exponential decay profile not explained by variations in peat accumulation rates. Unlike themost commonly used ²¹⁰Pb-dating model (Constant Rate of Supply (CRS)), the IP-CRS model estimates peat accumulation rates consistent with typical growth rates for peatlands from the sameareas. Confidence in the IP-CRS chronology is also provided by the good agreementwith independent chronological markers (i.e. ²⁴¹Am and ¹³⁷Cs). Our results showed that the IP-CRS can provide chronologies from peat records where 210Pb mobility is evident, being a valuable tool for studies reconstructing past environmental changes using peat archives during the Anthropocene.