Search for supernova 60Fe in the Earth's microfossil record

Approximately 2.8 Myr before the present, our planet was subjected to the debris of a supernova explosion. The terrestrial proxy for this event was the discovery of live atoms of 60Fe in a deep-sea ferromanganese crust. The signature for this supernova event should also reside in magnetite (Fe3 O4) microfossils produced by magnetotactic bacteria extant at the time of the Earth-supernova interaction, provided the bacteria preferentially uptake iron from fine-grained iron oxides and ferric hydroxides. Using estimates for the terrestrial supernova 60Fe flux, combined with our empirically derived microfossil concentrations of a deep-sea drill core, we deduce a conservative estimate of the 60Fe fraction as 60Fe/Fe= 3.6 x 10-15. This value sits comfortably within the sensitivity limit of present accelerator mass spectroscopy capabilities. The implication is that a biogenic signature of this cosmic event resides, and is detectable, in the Earth's fossil record. In this talk we describe this idea in further detail and present our magnetic scanning data from a Pacific Ocean drill core (provided by the Ocean Drilling Program, ODP) that allows us to quantify the mass concentration of Fe within the microfossils.