Deep-ocean crusts as telescopes: Using live radioisotopes to probe supernova nucleosynthesis

Live Fe-60 has recently been detected in a deep-ocean ferromanganese crust, isolated in layers dating from about 3Myr ago. Since Fe-60 has a mean life of 2.2 Myr, a near-Earth supernova is the only likely source for such a signal, and we explore here the consequences of a supernova origin. We combine the Fe-60 data with several supernova nucleosynthesis models to calculate the supernova distance as a function of progenitor mass, finding an allowed range of 15 - 120 pc. We also predict the signals expected for several other radioisotopes, which are independent of the supernova distance. Species likely to be present near or above background levels are Be-10, Al-26, Mn-53, Hf-182, and Pu-244. Of these, Hf-182 and Pu-244 are nearly background-free, presenting the best opportunities to provide strong confirmation of the supernova origin of the Fe-60 signal and to demonstrate that at least some supernovae are sources for the r-process. The accuracies of our predictions are hampered by large uncertainties in the predicted Fe-60 yields for supernovae of different masses, so the new crust data motivate a redoubled theoretical attack on this problem.