  We explore the idea that detectable excursions in 26Al may arise from direct
deposition by any bolide, and excursions in 14C and 10Be abundances in the
atmosphere may result from long-period comet impacts. This is very different
from the usual processes of production by cosmic rays within Earths atmosphere.
Long-period comets experience greatly increased cosmic ray flux beyond the
protection of the suns magnetic field. We report the computed amount of 14C,
10Be, and 26Al present on long-period comets as a function of comet mass. We
find that the amount of nuclide mass on large long-period comets entering the
Earths atmosphere may be sufficient for creating anomalies in the records of
14C and 10Be from past impacts. In particular, the estimated mass of the
proposed Younger Dryas comet is consistent with its having deposited sufficient
isotopes to account for recorded 14C and 10Be increases at that time. The
26Al/10Be ratio is much larger in extraterrestrial objects than in the
atmosphere, and so, we note that measuring this ratio in ice cores is a
suitable definitive test for the Younger Dryas impact hypothesis, even if the
hypothetical bolide is not a long-period comet and/or did not contribute to the
14C and 10Be increases.
