Sugar maples near Ottawa held onto carbon-14 from Cold War nuclear bomb tests for over six decades after the atmosphere cleared, a delay now traced to the trees recycling old stored carbon to grow new leaves.
Above-ground weapons tests by the U.S., Soviet Union and their allies at the height of the Cold War nearly doubled atmospheric carbon-14, leaving a planet-wide spike known as the bomb pulse. In Gatineau Park, Quebec, maple leaves recorded peak saturation of 842 parts per thousand above the reference level in 1964, one year after the Limited Test Ban Treaty ended above-ground testing.
Atmospheric radiocarbon returned to its pre-bomb baseline between 2021 and 2022, but the maples lagged by roughly 2.2 years, give or take three to four months, only hitting that mark around 2024. By 2025, leaf values had dropped to negative 13.9 parts per thousand, confirming that fossil-fuel emissions, which carry almost no carbon-14, had overwhelmed the remaining bomb-pulse signal.
Small positive offsets averaging 7.4 parts per thousand between 1990 and 2019 exposed the mechanism: biological recycling of older carbon, not an atmospheric quirk. Climate reconstructions built from leaves and tree rings have long assumed samples mirror the atmosphere of their growth year, and those models will now need explicit corrections by plant species, sharpening how scientists read unmeasured places and periods.