Nuclear bombs, tested in the atmosphere, spewed out plutonium which settled on the ground in easily detectable amounts.

Source: https://www.theguardian.com/science/2023/jan/01/theres-been-a-fundamental-change-in-our-planet-hunt-on-for-spot-to-mark-the-start-of-the-anthropocene-epoch

Russia’s aid to China’s plutonium reactors proves that when Russian President Vladimir Putin and Chinese President Xi Jinping declared a “no limits” partnership in February 2022, they really meant it.

Source: https://www.dailysignal.com/2023/03/16/russia-helps-china-accelerate-nuclear-buildup-us-unprepared-counter/?utm_source=rss&utm_medium=rss&utm_campaign=russia-helps-china-accelerate-nuclear-buildup-us-unprepared-counter

The radioactive core, which used British and Canadian plutonium, was flown out later, and installed in the bomb on Plym very close to the scheduled detonation.

Source: https://www.thehindu.com/children/the-first-british-atomic-bomb/article67333631.ece

The scientists plan to measure plutonium levels at the bottom of Crawford Lake to determine a specific start date of the proposed new epoch.

Source: https://www.newser.com/story/337576/start-of-a-new-geological-epoch-could-be-represented-by-this-lake.html

They found ways to purify dispersed uranium in the ground and to even form a highly explosive new element from uranium named plutonium, the explosive core stuff of nuclear bombs.

Source: https://www.counterpunch.org/2023/08/11/splitting-the-atom-threatening-the-world-the-american-tragedy-of-robert-oppenheimer/