Scientists call it the “bomb pulse.” At the height of the Cold War, above-ground nuclear weapons tests by the Soviet Union, the U.S., and their respective allies roiled quiet stretches of the Pacific and other test sites around the world. The tests dramatically altered the isotopic composition of Earth’s atmosphere, doubling its carbon-14 levels. Neutrons radiating from these blasts collided with nitrogen gas, transforming some of them into carbon-14 and forming traceable patterns across the planet’s carbon cycle for decades, nearly doubling global levels of this radiocarbon at the bomb pulse’s peak.
In Quebec’s Gatineau Park, across the river from Ottawa, this bomb-pulse carbon-14 recorded in sugar maple leaves reached its highest saturation at 842 parts per thousand above the reference level in 1964, as recorded by the Geological Survey of Canada. But, mysteriously, these maples’ carbon-14 ratios did not drop back down to their pre-Cold War baseline for over six decades—roughly two years after the rest of Earth’s surrounding atmosphere got back to normal. Now researchers at the University of Ottawa have finally figured out why.
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“Plants often use older carbon, stored from previous years, to grow new leaves,” Leonard Wassenaar, a geoscientist at the university, explained in a statement.
