Radiometric dating of recent sediments: beyond the boundary conditions

Abstract

Most mathematical models for radiometric dating of recent sediments are particular solutions of a unique physical problem: the advective-diffusive transport of a particle-bound radiotracer within a sediment profile that undergoes accretion. Regardless of the particular assumptions about fluxes, sedimentation rates and the diffusion term, all models assume ideal deposition as a boundary condition at the sediment water interface, i.e. new radioactive input will be deposited above the previously existing material. In sediments with very high porosities, this assumption may be unrealistic, because a fraction of the incoming flux may penetrate rapidly through the connected pore spaces. This process will be referred to as non-ideal deposition. This paper reviews evidence from literature data, discusses the basic processes involved, and establishes the mathematical basis to incorporate non-ideal deposition into one-phase radiometric dating models, as depth-distributed local sources. Through analytical and numerical solutions, this work demonstrates that such penetration patterns can explain excess 210 Pb subsurface maxima, often observed in sediment cores, as well as penetration of 137 Cs to depths greater than expected from sedimentation rates and diffusion. These ideas are illustrated using examples from the literature in which sediment porosities were [90 %. Implications for radiometric dating include: (1) spurious accelerations in sedimentation rate inferred when applying the constant rate of supply model, and (2) erroneous chronologies, developed when using the maximum depth at which 137 Cs can be measured as a chronostratigraphic marker.

Reviews and syntheses: <sup>210</sup>Pb-derived sediment and carbon accumulation rates in vegetated coastal ecosystems – setting the record straight

“…(2), (3) A linear combination of solutions for CF-CS and CMZ-CS with coefficients g and (1 − g), being g [0, 1] Abril et al (1992) SIT: sediment isotope tomography…”

Deeper snow increases the net soil organic carbon accrual rate in moist acidic tussock tundra:210Pb evidence from Arctic Alaska

“…Applications of fallout radionuclides in studies of sediment, soil, and peat accumulation ( 7 Be, 137 Cs, 210 Pb, 241 Am) have been explored by a number of authors (Robbins and Edgington 1975;Münnich 1989, 1991;Urban et al 1990;Kaste, Heimsath, and Bostick 2007;Kaste et al 2011;Abril and Gharbi 2012;Kaste 2014, 2016;Shotyk et al 2015;Olid et al 2016;Davies et al 2018;Gonzalez-Meler et al 2018). The prior studies most relevant to the current study are those dealing with accumulation of soil and peat.…”

Why would we use the Sediment Isotope Tomography (SIT) model to establish a 210 Pb-based chronology in recent-sediment cores?

“…In some cases the behavior of the dissolved fraction of radionuclides requires handling separate equations for the dissolved and the particle-bound phases (Robbins and Jasinski, 1995;Abril and Gharbi, 2012). Almost all models assume ideal deposition as a boundary condition at the sediment water interface (SWI), i.e.…”

“…new radioactive inputs will be deposited above the previously existing material. Abril and Gharbi (2012) have shown how this assumption may be unrealistic in sediment cores with very high porosities.…”