The Search for Supernova Signatures in an Ice Core

It has been suggested that ice cores may preserve detectable enhancements of some terrestrially rare radioisotopes, Be, Al, Cl, resulting from a near-Earth core-collapse supernova(SN) [1]. Both Be and Cl are also produced by atmospheric cosmic ray spallation and hence are influenced by processes that modulate the Earth’s cosmic ray flux. Previous studies [2], [3] have suggested that enhancements occurred in the Be and Cl fluxes at ∼35 ky and at ∼60 ky for Be. Thus we have searched for potential SN condensates with Al amongst grains filtered from the 308.6m Guliya ice core recovered from the Qinghai-Tibetan plateau in China [3]. We searched for potential core-collapse SN condensate grains corundum (Al2O3), hibonite (CaAl12O19) and spinel (MgAl2O4) (see [4]) in Guliya grain samples from the following time periods: ∼2-10 ky, ∼25-27 ky, ∼34-36 ky, ∼53-57 ky, ∼59-62 ky and ∼6872 ky. These minerals are rare among terrestrial rocks and fine-grained atmospheric dust of terrestrial origin. Furthermore, they are insoluble in the acids employed in the sample preparation process and therefore separable from other minerals, such as silicates, that have high terrestrial abundances. Candidate SN condensate grains were identified among their terrestrial diluents employing a procedure developed at the University of Chicago for detecting presolar grains in meteoritic samples [5]. A set of 37 grains from the ∼34-36 ky, ∼53-57 ky and ∼59-62 ky samples were analyzed with the NanoSIMS at Washington University to measure their oxygen isotopic ratios. The preliminary results indicate that