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

Abstract

Most 210 Pb dating models assume that atmospheric flux of excess 210 Pb ( 210 Pb exc ) 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 210 Pb exc flux, initial 210 Pb exc activity, and sediment accumulation rate (SAR). 210 Pb exc 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 210 Pb exc flux and SAR was found for nine of the ten cores. Thus, in most of the studied aquatic systems, 210 Pb exc flux to the sediment was governed primarily by flux of matter, rather than by direct atmospheric 210 Pb exc deposition. Errors in chronology and SAR, attributable to varying 210 Pb exc 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.

Sediments in the Tema Harbour (Ghana): chemical pollution and sedimentation rates

“…The in situ radioactive decay of 226 Ra in the water column also produces 210 Pb referred to as supported 210 Pb or 210 Pb supp (Oldfield and Appleby, 1984;Appleby, 2008;Nehyba et al, 2011;Abril and Brunskill, 2014 Ra and other members of the decay chain preceding it, while the 210 Pb exc represents that part of the measured total 210 Pb specific activity in sediment that exceeds the specific activity in secular equilibrium with 226 Ra (Pfitzner et al, 2004;Hollins et al, 2011). The fluxes of 210 Pb in sediments are linked to wet and dry atmospheric deposition Appleby, 2008;Pfitzner et al, 2004;Mabit et al, 2014) as well as surface run-off and erosion from the catchment (Mabit et al, 2008;.…”

“…18: Pb dating models is unrealistic, particularly in sediment cores with very high porosities . Thus, using statistical analysis of a database of laminated sediment cores, Abril and Brunskill (2014) reconstructed historical records of initial 210 Pb exc specific activity, 210 Pb exc flux onto the SWI and SARs, which revealed wide temporal fluctuations. Their results also showed a linear relationship between the 210 Pb exc fluxes and SAR.…”

“…The TERESA model is based on the findings that (1) the flux of 210 Pb exc to the SWI varies with time and (2) the flux of 210 Pb exc is governed by the flux of matter instead of direct atmospheric 210 Pb exc deposition, resulting in a statistically significant correlation between the flux of 210 Pb exc and the flux of matter/sediment accumulation rates (Abril and Brunskill, 2014). Other important assumptions in the application of the TERESA model are that (1) 210 Pb exc is a particle-bound tracer, which is continually deposited onto the SWI, (2) no postdepositional redistribution occurs and (3) the sedimentary sequence is continuous with none of its layers missing.…”

“…Thus, a sediment core cut into N slices (labelled i = 1, 2, … N) is assumed to have resulted from the accumulated mass flow onto the SWI over the time interval ∆T i with an average mass flow (SAR) value of w i (Abril, 2016). If w is the magnitude of the arithmetic mean of all the w i for the N slices, then the values of w i are almost normally distributed around w with a standard deviation σ w (Abril and Brunskill, 2014). This results in a linear correlation between the 210 Pb exc fluxes and SAR.…”

“…For example, atmospheric 210 Pb and 137 Cs may first deposit onto soils and subsequently transported into aquatic systems (Abril and Garcia-Le6n, 1994;Mabit et al, 2014), in which case the flux of matter into the aquatic system will exert a greater control on the 210 Pb exc flux onto the bottom sediments than direct atmospheric 210 Pb exc deposition (Abril and Brunskill, 2014). Since the fluxes of matter is affected by storm runoff, floods and tidal events (Palinkas et al, 2005;Díaz-Asencio et al, 2009), which may be siteand time-specific, the application of a particular model at different sites would require a case by case assessment of sediment cores Abril and Brunskill, 2014). Moreover, the assumption of no post-depositional mobility of sediment-bound tracers under the application of 210 Pb dating models may also not hold in reality due to potential diffusion and advection of the tracers within the sediment pore water (Appleby and GarciaLe6n, 1997) under the influence of the sediment porosity ).…”