Fe AS A FOSSIL OF A NEAR-EARTH SUPERNOVA

Live Fe has recently been reported in a deep-ocean ferromanganese crust. Analysis of the isotopic ratios in the sample suggests that the measured Fe abundance exceeds the levels generated by terrestrial and cosmogenic sources, and it has been proposed that the excess of Fe is a signature of a supernova that exploded near the earth several Myr ago. In this paper, we consider the possible background sources, and confirm that the measured Fe is significantly higher than all known backgrounds, in contrast with the reported abundance of live Mn in the same sample. We discuss scenarios in which the data are consistent with a supernova event at a distance D ∼ 30 pc and an epoch tSN ∼ 5Myr ago. We propose tests that could confirm or refute the interpretation of the Fe discovery, including searches for Be, I and Sm. Such a nearby supernova event might have had some impact on the earth’s biosphere, principally by enhancing the cosmic-ray flux. This might have damaged the earth’s ozone layer, enhancing the penetration of solar ultraviolet radiation. In this connection, we comment on the Middle Miocene and Pliocene mini-extinction events. We also speculate on the possibility of a supernova-induced “cosmic-ray winter”, if cosmic rays play a significant role in seeding cloud formation. Subject headings: supernovae — nuclear reactions, nucleosynthesis: abundances