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

The 228 Th: 228 Ra 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 228 Th ( t 0.5 = 1.9 years) and 228 Ra ( t 0.5 = 5.9 years) by canopy tree species, ferns, and mosses, drives disequilibrium in the 232 Th- 228 Ra- 228 Th radioactive decay series within forest vegetation and organic soils. With examples from northeastern USA, we verify a new 228 Th: 228 Ra age model by demonstrating its concordance with the fallout radionuclide chronometer 7 Be: 210 Pb in the 0 to 5-year time frame [ R 2 = 0.87, RMSE = 0.5 years]. At our locality, canopy tree species assimilate 228 Th with a typical initial ratio ( 228 Th: 228 Ra) 0 ∼ 0.3, but in several instances, both deciduous and coniferous tree species show a preference for Th over Ra with ( 228 Th: 228 Ra) 0 exceeding 5. While the 228 Th: 228 Ra system is restricted to organic soil horizons, concordance of 228 Th: 228 Ra with established 7 Be: 210 Pb and 241 Am 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 14 C, may open new opportunities to understand soil pedogenic processes that regulate the storage of carbon and atmospheric metals such as Pb and Hg.