Forest soils function as a critical global sink for CO2 and other anthropogenic pollutants such as heavy metals Hg and Pb, which are captured from the atmosphere by foliar uptake and subsequently delivered to soil through litterfall. (1−4) Our understanding of soil dynamics that promote long-term storage versus rerelease of carbon and metals from soil requires accurate chronometers that can reconstruct in situ rates of carbon and metal accumulation, especially over annual–decadal time scales that govern anthropogenic forcing and societal response to ecosystem impacts. (5) Carbon and metal cycles are in fact inextricably linked through the formation of organometallic complexes, a critical process that ultimately controls the fate and persistence of both carbon and heavy metals in soils. (2,6,7) The metal–carbon link begins at the atmosphere–biosphere interface with irreversible sorption of atmospheric metals by foliage and forest floor (leaf litter) and continues as heavy metals including Hg and Pb remain firmly fixed during litter decomposition, development of soil organic matter (SOM), and formation of organometallic complexes with admixtures of pedogenic clays and Mn-, Al-, and Fe-oxyhydroxides. The metal–carbon link proceeds during export of colloids to storage in deep mineral soil. (8-16) Age-dating carbon in the organometallic complex is possible using the percent modern (pM) or delta 14C chronometer, which exploits the pulse in atmospheric 14C concentration created by nuclear bomb testing of the 1950–1970s. (17,18) However, delta 14C dating of soils is complicated by uncertain lag times of 5-20 years between carbon assimilation and SOM age due to plants’ use of stored carbon for leaf or root tissue production, assimilation of respired soil CO2, and growth of old fine roots in young SOM. (19-22) As a result, soil Δ14C ages do not provide accurate accumulation rates of atmospheric metals such as Hg or Pb, or rates of organometallic complex transformation and advection through the soil column, where lag times are unknown or long relative to the time scale of interest.

Short-lived radionuclide chronometers present a new alternative for the chronometry of vegetation–soil systems. The natural fallout radionuclides (FRNs) ⁷Be (cosmogenic, half-life 54 days) and ²¹⁰Pb (radiogenic, half-life 22.3 years) are two divalent metals with strong affinity for organic matter, and together are used to construct the Linked Radionuclide aCcumulation (LRC) model based on the ⁷Be:²¹⁰Pb ratio. (23) The FRNs are produced in the atmosphere, and upon deposition by wet and dry aerosol processes, irreversibly sorb to both organic and mineral matter at the earth surface. (8,24) ²¹⁰Pb by itself is widely used to reconstruct lake sediment and peat environmental archives and is arguably the most important chronometer of historical sedimentary systems and anthropogenic change. (25-27) The LRC model is an update to conventional ²¹⁰Pb age models that uses ⁷Be to correct for penetration of ²¹⁰Pb into the soil or sediment, as for example rainfall percolating into soils, or sedimentary systems where “nonideal” deposition of ²¹⁰Pb to the sedimentary bed occurs. (28,23,29) The LRC model is corroborated by a second metal chronometer system based on the plutonium daughter ²⁴¹Am (half-life 421 years), a tetravalent particle-reactive metal, another product of the bomb-pulse, and a robust time marker for 1963–64 onset of the Anthropocene. (30,31,23,32) However, at an age of 60 years and soil depths of 3–10 cm, the ²⁴¹Am peak is far removed from the soil/sediment surface, where assimilation of carbon and atmospheric metals occurs. This leaves a gap in LRC verification at the annual–decadal time scales that are critical for monitoring anthropogenic forcing and societal response to pollutants including CO2, Pb, and Hg.

The ²²⁸Th:²²⁸Ra chronometer is a promising new option for chronometry over the annual–decadal time scale. The daughter–parent pair ^{228Th (half-life 1.91 years) and 228Ra (half-life 5.85 years) is produced in the radiogenic 232}Th decay series, which is a universal and ubiquitous component of soil forming minerals and one of the principal sources of natural environmental radiation. The ²³²Th decay chain proceeds as follows:

²³²Th>²²⁸Ra>²²⁸Ac>²²⁸Th>²²⁴Ra>²²⁰Rn>²¹⁶Po>²¹²Bi>²¹²Pb>²⁰⁸Tl>²⁰⁸Pb

A natural chronometer is created in biological systems ranging from algae to vascular plants which accumulate minute amounts of Ra²⁺ as a putative Ca²⁺ analog, whereas Th⁴⁺ is geochemically immobile and taken up to a far lesser degree. (33−35) When 228Ra is assimilated in excess of both the 232Th parent and the 228Th daughter, disequilibrium in the decay chain is created within the biological medium. Subsequent in vivo production of 228Th by decay of 228Ra establishes a chronometer as the ²²⁸Th:²²⁸Ra ratio evolves over time based on the isotopes’ different half-lives, from an initial value approximately 0 toward a transient equilibrium value of approximately 1.5 over a period of about 10 years, provided these simplifying assumptions that (1) 228Ra is assimilated into plants only during an initial growth period and (2) that neither 228Th nor 232Th is assimilated to any degree. An analytical solution for age (t) then follows from classical Bateman equations...

...We hypothesized that a ²²⁸Th:²²⁸Ra chronometer should be viable in forest soils due to accumulation of leaf litter and SOM during subsequent leaf litter decomposition. Following foliar uptake of ²²⁸Ra during plant growth, active assimilation of ²²⁸Ra would cease following senescence, and organic matter should subsequently remain a closed system during decomposition.