Age-Dating of Foliage and Soil Organic Matter: Aligning ²²⁸Th:²²⁸Ra and ⁷Be:²¹⁰Pb Radionuclide Chronometers over Annual to Decadal Time Scales

The ²²⁸Th:²²⁸Ra ratios of foliage and organic soil horizons evolve with time following a predictable radioactive decay law and thus provide a new chronometer for absolute age-dating of plant and soil organic matter. Preferential uptake of ²²⁸Th (t_0.5 = 1.9 years) and ²²⁸Ra (t_0.5 = 5.9 years) by canopy tree species, ferns, and mosses, drives disequilibrium in the ²³²Th–²²⁸Ra-²²⁸Th radioactive decay series within forest vegetation and organic soils. With examples from northeastern USA, we verify a new ²²⁸Th:²²⁸Ra age model by demonstrating its concordance with the fallout radionuclide chronometer ⁷Be:²¹⁰Pb in the 0 to 5-year time frame [R² = 0.87, RMSE = 0.5 years]. At our locality, canopy tree species assimilate ²²⁸Th with a typical initial ratio (²²⁸Th:²²⁸Ra)₀ ∼ 0.3, but in several instances, both deciduous and coniferous tree species show a preference for Th over Ra with (²²⁸Th:²²⁸Ra)₀ exceeding 5. While the ²²⁸Th:²²⁸Ra system is restricted to organic soil horizons, concordance of ²²⁸Th:²²⁸Ra with established ⁷Be:²¹⁰Pb and ²⁴¹Am bomb-pulse chronometers establishes a coherent age-dating system of soil organic matter based on three independent chronometers and five particle reactive metals, and spanning 0–200 years in time scale that encompasses both organic and mineral soils to depths of up to 30 cm. Concordance indicates that these metals all follow common processes of organometallic colloid formation and migration and, in conjunction with ¹⁴C, may open new opportunities to understand soil pedogenic processes that regulate the storage of carbon and atmospheric metals such as Pb and Hg.