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I was just reading that. Freshly is some time in the last 10 million years; fresh with respect to aeons, or the age of the earth.
Apparently, there were two events!Traces of rare forms of iron and plutonium have been found at the bottom of the Pacific Ocean, after some kind of cataclysm in outer space created this radioactive stuff and sent it raining down on our planet.
The extraterrestrial debris arrived on Earth within the last 10 million years, according to a report in the journal Science. Once it hit the Pacific Ocean and settled to the bottom, nearly a mile down, the material got incorporated into layers of a rock that was later hauled up by a Japanese oil exploration company and donated to researchers.
This latest study of iron that got incorporated into the slowly growing layers of a deep-sea rock confirms that notion — but also suggests yet another dose of interstellar iron arrived around 6 million years ago. "So there's not just one exploding star — there's two," says Fields.
The statement about 244Pu "does not exist naturally on Earth," is not quite correct and is somewhat misleading. The half-life of 244Pu is 80 million years, so if it existed 4 - 4.5 billion years ago, it mostly decayed over 50 half-lives.What's more, in this study the researchers were able to detect atoms of distinctive plutonium-244, which does not exist naturally on Earth. By looking at the quantities of plutonium and iron in the layers of rock, they could compare what they saw with what models predict about the production of these elements by cosmic events like supernovas.
My grocery store uses a similar definition.Freshly is some time in the last 10 million years
That sounds like a lot, but its only 10-15. For every pound of plutonium the earth started with, about a billion atoms remain. Not many, but not zero. And of course, it apparently gets refreshed from space. Other isotopes were refreshed by Oklo. (Or as-yet-undiscovered other Oklos)over 50 half-lives