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

Live Fe-60 has recently been reported in a deep-ocean ferromanganese crust. Analysis of the isotopic ratios in the sample suggests that the measured F e-60 abundance exceeds the levels generated by terrestrial and cosmogenic s ources, and it has been proposed that the excess of Fe-60 is a signature of a supernova that exploded near the earth several Myr ago. In this paper, w e consider the possible background sources, and confirm that the measured F e-60 is significantly higher than all known backgrounds, in contrast with t he reported abundance of live Mn-53 in the same sample. We discuss scenario s in which the data are consistent with a supernova event at a distance D s imilar to 30 pc and an epoch t(SN) similar to 5 Myr ago. We propose tests t hat could confirm or refute the interpretation of the Fe-60 discovery, incl uding searches for Be-10, I-129 and Sm-146. Such a nearby supernova event m ight have had some impact on the earth's biosphere, principally by enhancin g the cosmic-ray flux. This might have damaged the earth's ozone layer, enh ancing the penetration of solar ultraviolet radiation. In this connection, we comment on the Middle Miocene and Pliocene mini-extinction events. We al so speculate on the possibility of a supernova-induced "cosmic-ray winter'' , if cosmic rays play a significant role in seeding cloud formation. (C) 19 99 Elsevier Science B. V. All rights reserved.