Fallout Radionuclides (FRNs) and the 7Be:210Pb chronometer
Fallout Radionuclides are a suite of natural and man-made radioisotopes that are produced in the atmosphere. The moniker seems borrowed from Cold War nuclear bomb testing which dispersed the anthropogenic FRNs – notably the long-lived ones including 137Cs, 241Am (a 241Pu daughter), and 239,240Pu – globally through the atmosphere. They are a legacy contaminant now found in every environment on Earth, and candidate for demarcating ‘Anthropocene’ as the geologic age of humankind.
The naturally-produced FRNs include 7Be and 210Pb, and many others such as 10Be, all produced by various processes in the atmosphere. 7Be (half-life 54 days) is cosmogenic, produced when cosmic radiation splits atoms of N and O in the upper atmosphere. 210Pb (half-life 22.3 years) is radiogenic, produced by the 238U decay chain in soils, sediment and rock through the emanation of 222Rn (half-life 3.8 days) to the atmosphere where Rn subsequently decays to 210Pb. To be precise, this atmospherically-produced 210Pb is typically called ‘excess’ or 210Pbex since it is present in excess of the activities of its long-lived precursors 238U or 226Ra (half-life 1602 years) which are restricted to the terrestrial environment.
The FRNs are powerful tracers of biogeochemical cycles. All are ubiquitous in the environment. The natural FRNs are produced at approximately constant rates through time, and the anthropogenic as a ‘pulse’ spanning the nuclear testing area of approximately 1950-1970, but with a sharp peak in 1964. The FRNs are particle-reactive metals that sorb to ambient aerosols following their production. They trace atmospheric circulation until deposited, primarily by rainfall and snow. There they sorb to vegetation, soils and sediments and subsequently trace soil processes, erosion and stream processes.
We focus on the 7Be:210Pb chronometer, which we are developing for understanding ecosystem cycling of aerosols, carbon and legacy contaminant metals like Pb and Hg in atmosphere-biosphere exchange.
Short-Lived Radiogenic Isotopes (SLRs), 228Th:228Ra chronometer
There are four primary sources of natural radiation in the environment: 238U (half-life 4.5×109 years), 235U (0.7×109 years) and 232Th (14×109 years) decay chains, and primordial 40K (1.3×109 years). The decay chains include many both short- and long-lived isotopes as intermediates as primordial 238U, 235U and 232Th decay to stable isotopes 206Pb, 207Pb, and 208Pb, respectively. Many of the radioactive intermediates are environmentally important, scientifically useful, and readily measured by gamma spectroscopy.
We are developing a novel chronometer based on 228Th:228Ra, a parent-daughter pair in the 232Th decay chain with 1.9 and 5.8 year half-lives, respectively, and a ~10-year useful timeframe in vegetation, organic soils and sediments.