Geological Isotope Anomalies as Signatures of Nearby Supernovae

Nearby supernova explosions may cause geological isotope anomalies via thedirect deposition of debris or by cosmic-ray spallation in the earth'satmosphere. We estimate the mass of material deposited terrestrially by thesetwo mechanisms, showing the dependence on the supernova distance. A number ofradioactive isotopes are identified as possible diagnostic tools, such asBe-10, Al-26, Cl-36, Mn-53, Fe-60, and Ni-59, as well as the longer-livedI-129, Sm-146, and Pu-244. We discuss whether the 35 and 60 kyr-old Be-10anomalies observed in the Vostok antarctic ice cores could be due to supernovaexplosions. Combining our estimates for matter deposition with results ofrecent nucleosynthesis yields, we calculate the expected signal from nearbysupernovae using ice cores back to $\sim 300$ kyr ago, and we discuss usingdeep ocean sediments back to several hundred Myr. In particular, we examine theprospects for identifying isotope anomalies due to the Geminga supernovaexplosion, and signatures of the possibility that supernovae might have causedone or more biological mass extinctions.Comment: 26 pages, 2 postscript figures. Uses AASTeX (version 4). To appear in Astrophys.