On deep-ocean ⁶⁰Fe as a fossil of a near-earth supernova

Live ⁶⁰Fe has recently been reported in a deep-ocean ferromanganese crust. Analysis of the isotopic ratios in the sample suggests that the measured ⁶⁰Fe abundance exceeds the levels generated by terrestrial and cosmogenic sources, and it has been proposed that the excess of ⁶⁰Fe 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 ⁶⁰Fe is significantly higher than all known backgrounds, in contrast with the reported abundance of live ⁵³Mn 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 t_SN~5 Myr ago. We propose tests that could confirm or refute the interpretation of the ⁶⁰Fe discovery, including searches for ¹⁰Be, ¹²⁹I and ¹⁴⁶Sm. 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.