On Deep-Ocean Fe-60 as a Fossil of a Near-Earth Supernova

Abstract

Live ${}^{60}$Fe has recently been reported in a deep-ocean ferromanganese crust. Analysis of the isotopic ratios in the sample suggests that the measured ${}^{60}$Fe abundance exceeds the levels generated by terrestrial and cosmogenic sources, and it has been proposed that the excess of ${}^{60}$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 ${}^{60}$Fe is significantly higher than all known backgrounds, in contrast with the reported abundance of live ${}^{53}$Mn. We discuss scenarios in which the data are consistent with a supernova event at a distance $D\sim 30$ pc and an epoch $t_{\mathrm{SN}}\sim 5$ Myr ago. We propose tests that could confirm or refute the interpretation of the ${}^{60}$Fe discovery, including searches for ${}^{10}$Be, ${}^{129}$I and ${}^{146}$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.

18 pages, AASTeX, 1 ps figure

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