Rare plutonium isotope preserved in Pacific seabed points to an ancient cosmic explosion

The evidence comes from a nearly 2 kg (4.2 pound) ferromanganese crust recovered from the Pacific Ocean in 1976. Growing only millimeter by millimeter over millions of years, the crust preserved tiny traces of radioactive plutonium that allowed researchers to reconstruct an ancient event that took place long before modern humans, or even many of today’s plant and animal groups, existed.

Scientists from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), the Australian Nuclear Science and Technology Organisation (ANSTO), and the Australian National University (ANU) analyzed three rare radioactive isotopes preserved in the crust. Their results suggest that the plutonium did not originate from the nearby supernova explosions that left iron-60 deposits on Earth several million years ago. Instead, the data point to a much older and much rarer stellar event.

“The absence of the curium radioisotope Cm-247, which was also produced in the explosion, tells us it happened a very long time ago. But not more than about 1 billion years ago, otherwise the Pu-244 would also be undetectable,” said Dr. Michael Hotchkis of ANSTO, who performed the plutonium measurements and is a co-author of the study.

The leading explanation is a kilonova, an explosion triggered when two neutron stars collide. These extraordinarily rare events are believed to create about half of the heavy elements in the Universe, including plutonium, uranium, and many other elements too heavy to form inside ordinary stars.

The research began with what appeared to be an ordinary piece of rock. The crust was recovered from a depth of about 4 830 m (15 850 feet) beneath the Pacific Ocean. Because it grows extremely slowly, it acts as a natural archive, preserving a record of material arriving from Earth’s environment and from interstellar space over millions of years.

Researchers divided the sample into nine sections, each representing roughly one million years of growth. Even in pieces weighing about 90 g (3.2 ounces), they expected to find fewer than 100 atoms of plutonium-244.

Detecting such tiny amounts required one of the world’s most sensitive accelerator mass spectrometry systems.

“We only need 100 plutonium atoms in the final sample to capture one of them in the detector. This level of sensitivity is unique worldwide,” Hotchkis said.

The team compared plutonium-244 with iron-60, an isotope known to mark nearby supernova explosions. Iron-60 produced two clear peaks corresponding to supernovae about 2 million and 7 million years ago. Plutonium-244 followed an entirely different pattern.

Instead of appearing in two bursts, plutonium was spread almost evenly throughout the sample. That showed it had been drifting through interstellar space for a very long time before reaching Earth, rather than arriving directly from those relatively recent stellar explosions.

To test that idea, lead author Dr. Dominik Koll extracted another isotope, curium-247, from the same samples. Curium is produced alongside plutonium during the same violent stellar events but decays much faster, making it an ideal natural clock.

Despite using instruments even more sensitive to curium than plutonium, the researchers found no convincing evidence of interstellar curium.

“The instrument sensitivity was not in question. It is even better at detecting curium atoms than plutonium atoms. The only possible explanation is that the cosmic explosion responsible for the plutonium happened so long ago that the curium has already decayed away to practically nothing,” Hotchkis said.

The missing curium allowed the team to place a firm lower limit on the age of the event. Whatever produced the plutonium must have occurred more than 100 million years ago.

“Our results suggest that the plutonium originated from very rare cosmic explosions, such as those that would occur during the merger of two neutron stars or in extremely energetic supernovae. Since then, it has dispersed throughout the interstellar medium,” said Prof. Anton Wallner, head of the Accelerator Mass Spectrometry and Isotope Research Department at HZDR.

The discovery provides new clues about where many of the Universe’s heaviest elements were created. It also shows that Earth’s geological record can preserve evidence of ancient astrophysical events long after they have disappeared from the sky.

Whether that ancient explosion had any direct effect on Earth remains unknown.

“Did this event affect life on Earth? That’s an open question to be investigated in further research,” Hotchkis said.

Researchers are already studying lunar samples that may preserve an even older record of these rare stellar events. Future work at the new HAMSTER accelerator facility in Dresden is expected to extend the search beyond plutonium and curium, offering new insights into nearby cosmic explosions and the origin of the Universe’s heaviest elements.

References:

¹ Radionuclides trapped in a deep-sea sample point to an ancient cosmic event – HZDR – June 15, 2026

² The timing of the last r-process event near Earth from interstellar ⁶⁰Fe, ²⁴⁴Pu and ²⁴⁷Cm deposition on Earth – Koll, D., Fichter, S., Hotchkis, M.A.C. et al. – Nature – June 15, 2026 – https://doi.org/10.1038/s41550-026-02841-6

Reet Kaur

I’m a science journalist and researcher at The Watchers, contributing to the Epicenter edition, where I cover peer-reviewed scientific research and emerging discoveries across Earth and space sciences. With a background in astronomy and a passion for environmental science, I’ve worked in shark and coral conservation in Fiji, conducting reef and shark-behavior research, contributing to mangrove restoration, and earning PADI Open Water and Coral Reef Certifications. I bring a blend of scientific rigor and storytelling to illuminate the discoveries shaping our planet and beyond.

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