CONCORDANT CHRONOMETERS OF MODERN SOIL PROCESSES: ALIGNING 7BE:210PB, 228TH:228RA, 241AM BOMB-PULSE METHODS

Anthropogenic forcing is changing soil-vegetation systems with a range of implications, for example altering rates of carbon mineralization and accumulation of contaminants such as mercury (Hg). Understanding modern change in soil biogeochemical processes requires age models that are accurate and precise over annual-decadal timescales. Here I describe efforts to establish concordant chronometers of modern soil processes using a suite of short-lived radionuclides that includes ⁷Be, ²¹⁰Pb, ²⁴¹Am, ²²⁸Th and ²²⁸Ra. The Linked Radionuclide aCcumulation (LRC) chronometer, based on atmospheric deposition of ⁷Be and ²¹⁰Pb to vegetation and soil (the ⁷Be:²¹⁰Pb ratio), accurately predicts the depth of the 1963-4 bomb-pulse peak of ²⁴¹Am (plutonium daughter) in diverse soils, peat and saltmarsh, with corresponding calendar year of 1964.1 (age = 58 ±3 years, mean ±1SD, n=14 soil profiles). At shorter timescales, the ²²⁸Th:²²⁸Ra chronometer, which is based on assimilation of ²²⁸Ra by vegetation and in vivo ingrowth of daughter ²²⁸Th in foliage, litter and humus, might provide a key corroborator of LRC in the 0-5 year window, that includes soil Oi and Oe horizons. Preliminary results show good agreement between LRC and ²²⁸Th:²²⁸Ra methods, differing by 0.10 ±1.2 years (mean ±1 SD; n=26 soil horizons). Concordance of these three independent radionuclide chronometers thus suggests that the LRC model can provide insights into metal-carbon pedogenic processes at the patch scale over subannual to centennial timescales.