Scientists from the Technical University of Munich found higher-than-expected levels of iron-60, a rare radioactive isotope, in a sample of deep-sea sediment from the Pacific Ocean. That appears to be the mark of a supernova, says Brian Fields, a visiting assistant professor of astronomy at the University of Illinois in Champaign. He notes that the cosmic rays of a supernova would weaken the ozone layer that guards species against ultraviolet radiation.

In a paper that will be published in the journal New Astronomy, Fields and physicist John Ellis of the European lab CERN note that the iron-60 appears to have been deposited around the same time the earth was undergoing "mini-extinctions," in which species die out at a higher-than-normal rate. The two such mini-extinctions under examination occurred, respectively, 3 million years ago and 13 million years ago, wiping out various types of zooplankton and other tiny marine life.

Based on the amount of iron-60 produced in a typical supernova, Fields and Ellis estimate that the star would have exploded roughly 100 light years away. Although only a tiny amount of iron-60 was found in the sample, Fields and Ellis say even this amount is more than can be accounted for by the normal influx of cosmic rays to Earth.

However, they note, confirming that a supernova occurred will require finding elevated levels of other radioactive isotopes, such as plutonium-244, in deep-sea sediment. Studies aimed at detecting such isotopes already are under way.