Beryllium-7 and lead-210 chronometry of modern soil processes: The Linked Radionuclide aCcumulation model, LRC. (1st May 2016)

• Record Type: Journal Article
• Authors: Landis, Joshua D.; Renshaw, Carl E.; Kaste, James M.
• Is Part Of: Geochimica et cosmochimica acta. Volume 180(2016:May 01)
• Page Start: 109
• Page End: 125
• Publication Date: 2016-05-01
• Subjects: Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2016.02.013
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: Soil systems are known to be repositories for atmospheric carbon and metal contaminants, but the complex processes that regulate the introduction, migration and fate of atmospheric elements in soils are poorly understood. This gap in knowledge is attributable, in part, to the lack of an established chronometer that is required for quantifying rates of relevant processes. Here we develop and test a framework for adapting atmospheric lead-210 chronometry ( 210 Pb; half-life 22 years) to soil systems. We propose a new empirical model, the Linked Radionuclide aCcumulation model (LRC, aka "lark"), that incorporates measurements of beryllium-7 ( 7 Be; half-life 54 days) to account for 210 Pb penetration of the soil surface during initial deposition, a process which is endemic to soils but omitted from conventional 210 Pb models (e.g., the Constant Rate of Supply, CRS model) and their application to sedimentary systems. We validate the LRC model using the 1963–1964 peak in bomb-fallout americium-241 ( 241 Am; half-life of 432 years) as an independent, corroborating time marker. In three different soils we locate a sharp 241 Am weapons horizon at disparate depths ranging from 2.5 to 6 cm, but with concordant ages averaging 1967 ± 4 via the LRC model. Similarly, at one site contaminated with mercury (HgT ) we find that the LRC model is consistent with the recorded history of Hg emission. The close agreement of Pb, Am and Hg behavior demonstrated here suggests that organo-metallic colloid formation and migration incorporates many trace metals in universal soil processes and that these processes may be described quantitatively using atmospheric 210 Pb chronometry. The 210 Pb models evaluated here show that migration rates of soil colloids on the order of 1 mm yr −1 are typical, but also that these rates vary systematically with depth and are attributable to horizon-specific processes of leaf-litter decay, eluviation and illuviation. We thus interpret 210 Pb models to quantify (i) exposure of the soil system to atmospheric aerosol deposition in the context of (ii) organic carbon assimilation, colloid production, and advection through the soil profile. The behavior of some other elements, such as Cs, diverges from the conservative colloid behavior exemplified by Pb and Am, and in these cases the value of empirical 210 Pb chronometry models like LRC and CRS is as a comparator rather than as an absolute chronometer. We conclude that 210 Pb chronometry is valuable for tracing colloidally-mediated transport of Pb and similarly-refractory metals, as well as the mobile pool of carbon in soils.