A comparative evaluation of the CF:CS and CRS models in ²¹⁰Pb chronological studies applied to hydrographic basins in Brazil

The Constant Flux: Constant Sedimentation (CF:CS) and Constant Rate of Supply (CRS) of unsupported⧸excess ²¹⁰Pb models have been applied to a ²¹⁰Pb data set providing of eighteen sediments profiles sampled at four riverine systems occurring in Brazil, South America: Corumbataí River basin (S1=Site 1, São Paulo State), Atibaia River basin (S2=Site 2, São Paulo State), Ribeirão dos Bagres basin (S3=Site 3, São Paulo State) and Amazon River mouth (S4=Site 4, Amapá State). These sites were chosen for a comparative evaluation of the performance of the CF:CS and CRS models due to their pronounced differences on the geographical location, geological context, soil composition, biodiversity, climate, rainfall, and water flow regime, among other variable aspects. However, all sediments cores exhibited a common denominator consisting on a database built from the use of the same techniques for acquiring the sediments major chemical composition (SiO₂, Al₂O₃, Na₂O, K₂O, CaO, MgO, Fe₂O₃, MnO, P₂O₅, TiO₂ and LOI-Loss on Ignition) and unsupported⧸excess ²¹⁰Pb activity data. In terms of sedimentation rates, the performance of the CRS model was better than that of the CF:CS model as it yielded values more compatible with those expected from field evidences. Under the chronological point of view, the CRS model always provided ages within the permitted range of the ²¹⁰Pb-method in the studied sites, whereas the CF:CS model predicted some values above 150 years. The SiO₂ content decreased in accordance with the LOI increase in all cores analyzed and such inverse relationship was also tracked in the SiO₂–LOI curves of historical trends. The SiO₂–LOI concentration fluctuations in sites S1 and S3 also coincided with some Cu and Cr inputs in the drainage systems. - Highlights: • Comparative evaluation of the excess ²¹⁰Pb CF:CS and CRS models. • Superior performance of the CRS model for evaluating sedimentation rates. • Superior performance of the CRS model for chronological applications. • Use of SiO₂–LOI concentration fluctuations for indicating heavy metal releases