The release and persistence of radioactive anthropogenic nuclides

Abstract

Atmospheric testing of nuclear weapons during the period 1945-1980 ushered in the 'atomic age' and released large quantities of anthropogenic radiogenic nuclides into the atmosphere. These radionuclides were subsequently deposited as fallout to the entire surface of the planet. While many have decayed to negligible levels, long-lived radionuclides persist and will do so for thousands of years. Isotopes of plutonium, 239 Pu (half-life 24 100 years) and 240 Pu (half-life 6563 years), provide the best chronological markers for the onset of this anthropogenic event both now and into the future due to their long half-lives, particle-reactivity, and the fact that they were present in negligible quantities prior to anthropogenic production and release. Chronostratigraphic markers established by distinct Pu concentration profiles and Pu isotope changes in sediment sequences and ice and coral cores can provide high-resolution dating over the last 60 years. However, even though fallout has ceased, it is found that the Pu inventory currently held in surface soil layers and the oceans will continue to supply Pu to sediment deposition zones for millennia and centuries, respectively. The delivery of this Pu will depend on soil erosion and bioturbation rates, and the rate of removal of dissolved Pu from the ocean.The discovery and practical implementation of nuclear fission at Los Alamos in the 1940s lead to the onset of the 'atomic age' and the associated staged release of radiogenic nuclides into the Earth's biosphere. The release processes were initially mostly unrestrained, via testing of nuclear weapons in the atmosphere, but later included releases from other sources such as underground nuclear testing, nuclear weapons fabrication, nuclear fuel reprocessing and radioisotope production, and included both controlled and uncontrolled (accidental) releases. These episodes released anthropogenic radioactivity into the environment through a variety of pathways, and with different release histories.Some of the radionuclides released were already present in the environment prior to the onset of anthropogenic production, but many were not or were present in only trace quantities. For those nuclides which have persisted in the environment over the last few decades, i.e. those with radioactive half

When did the Anthropocene begin? A mid-twentieth century boundary level is stratigraphically optimal

“…Therefore, alternative possibilities for an Anthropocene GSSA are either 1950 CE (as being closer to this date) or 1954 CE to mark the first widespread appearance of artificial radioisotopes in the geological record, part of the clear, globally distributed signal from the more extensive above-ground nuclear testing that took place mainly in the 1950s and early 1960s (Gabrieli et al, 2011;Wolff, 2014;Hancock et al, 2014;Waters et al, 2015). Either of these two possibilities might also be considered as more 'neutral' time references than the Trinity test, even though we stress here the functional stratigraphic, rather than societal, implications.…”

“…the global spread of artificial radionuclides from surface A-bomb explosions (Fairchild and Frisia, 2014;Hancock et al, 2014;Wolff, 2014); doubling of the surface reactive nitrogen reservoir (a result of fertilizer manufacture via the HabereBosch process), reflected in nitrogen isotope changes in far-field lacustrine deposits (Holtgrieve et al, 2011;Wolfe et al, 2013); the creation and wide (global) dispersal of new human-made materials (Ford et al, 2014;Zalasiewicz et al, 2014c) and artefacts that may be regarded as technofossils in the environment e almost all the discarded plastic and aluminium waste in surface sediments date from the midtwentieth century, for instance; rapid expansion in the distribution of artificial deposits on land, associated with urbanization (Ford et al, 2014), and of reworked sediment on continental shelves and slopes, associated with deep-sea trawling (see references in Zalasiewicz et al, 2014a); global dispersal of pollutants associated with expansion of industrial activities, including novel organic contaminants that include persistent organic pollutants (POPs) (Muir and Rose, 2007) and increased concentrations of heavy metals that are relatively rare in nature (Leorri et al, 2014;Gałuszka et al, 2014); a significant 'step' in the rate of increase of anthropogenic biotic change (Wolfe et al, 2013;Wilkinson et al, 2014), including accelerated species invasions on land and in the sea that alter species compositions in a wide spectrum of terrestrial and marine communities, in ways that will leave a clear palaeontological signal as we go into the future; a significant signal in polar ice marked by such indicators as lead from gasoline (Wolff, 2014) of different isotopic characteristics than Roman lead from smelting that forms an earlier signal; acceleration in the burning of hydrocarbons that has produced much of the~120 ppm increase in atmospheric carbon dioxide levels since the mid-twentieth century, and hence much of the associated carbon isotope signal (Al-Rousan et al, 2004); the majority of human-created trace fossils derived from sediment and rock drilling. The drilling for petroleum is often particularly deep.…”

“…Hancock et al, 2014;Wolff, 2014;Waters et al, 2015), as nuclear testing became more widespread. Nevertheless, placing the benchmark at the first nuclear test provides a clear, objective moment in time.…”

“…1) and a well-defined maximum in 1963 CE (in this latter case together with other human-made fallout radionuclides such as Plutonium 239 þ 240 or Americium 241). The short-lived radioisotope Cs-137 has been used extensively to date recent sediments (Pennington et al, 1973;Ritchie and McHenry, 1990;Walker, 2005), although in the next few decades it will be replaced by long-lived Pu-239 as the most distinguishable artificial radionuclide on Earth and the best chronological marker of the Anthropocene (Hancock et al, 2014;Waters et al, 2015).…”

Ecology in an anthropogenic biosphere

“…Humans have facilitated the release and utilization of nuclear energy and radionuclides (Harrison et al 2011, Hancock et al 2014, Zalasiewicz et al 2015. Humans engage in annual soil tillage and produce large-scale earthworks, channels, tunnels, and boreholes (Richter et al 2011, Edgeworth 2014, Zalasiewicz et al 2014, including massive vertically built structures (Frolking et al 2013), and artificial structures interconnected across continents (e.g., canals, roads, railways [Forman andAlexander 1998, Verburg et al 2011]).…”