Debris that includes radioactive iron-60 in sediment and crust samples between 3.2 and 1.7 million years ago have been found 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," said lead researcher Anton Wallner, nuclear physicist at the Australian National University (ANU) in Australia.
The team also found evidence of iron-60 from an older supernova around eight million years ago, coinciding with global faunal changes in the late Miocene period, which was 11.6 million to 5.3 million years ago.
"We were very surprised that there was debris clearly spread across 1.5 million years," said Wallner adding, "it suggests there were a series of supernovae, one after another."
The researchers, in the study published in the journal Nature, noted that the supernovae were less than 300 light years away, close enough to be visible during the day and comparable to the brightness of the Moon.
Although Earth would have been exposed to an increased cosmic ray bombardment, the radiation would have been too weak to cause direct biological damage or trigger mass extinctions.
The team was intrigued by first hints of iron-60 in samples from the Pacific Ocean floor, found a decade ago, and then searched for interstellar dust from 120 ocean-floor samples spanning the past 11 million years.
After the extraction of all the iron from the ocean cores, the researchers separated the tiny traces of interstellar iron-60 from the other terrestrial isotopes using the Heavy-Ion Accelerator and found it occurred all over the globe.
The age of the cores was determined from the decay of other radioactive isotopes, beryllium-10 and aluminium-26, using accelerator mass spectrometry (AMS) facilities.
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