Published in Nature: ANU-led international research has found evidence of a series of massive supernova explosions near our solar system, which showered the Earth with radioactive debris.
The scientists found radioactive iron-60 in sediment and crust samples taken from the Pacific, Atlantic and Indian Oceans.
The iron-60 was concentrated in a period between 3.2 and 1.7 million years ago, which is relatively recent in astronomical terms.
It corresponds with a time when the Earth cooled moving from the Pliocene into the Pleistocene period.
The debris spread over 1.5 million years suggests a series of supernovae explosions that created heavy elements and radioactive isotopes strewn into the cosmic neighbourhood.
The researchers believe that the supernovae were less than 300 light years away, and therefore close enough to be visible during the day.
The team also found evidence of iron-60 from an older supernova around eight million years ago, coinciding with global fauna changes in the late Miocene.
Some theories suggest cosmic rays from the supernovae could have increased cloud cover.
The iron-60 atoms, which have a half-life of 2.6 million years, reached Earth in minuscule quantities. Their analysis therefore required extremely sensitive techniques, including a separation of the interstellar iron-60 from other terrestrial isotopes - work done using the using the Heavy-Ion Accelerator at ANU.
The dating of the cores, determined from the decay of other present radioactive isotopes, showed the fallout had only occurred in two time periods: 3.2 to 1.7 million years ago and eight million years ago. A possible source of the supernovae is an ageing star cluster, which has since moved away from Earth, independent work led by TU Berlin has proposed in a parallel publication. The cluster has no large stars left, suggesting they have already exploded as supernovae, throwing out waves of debris.
In addition to ANU researchers, the study involved scientists from Austria, Japan, Israel, and Germany.