  We suggest an explanation of a sharp increase in the abundance of cosmogenic
radiocarbon found in tree rings dated AD 775. The increase could originate from
high-energy irradiation of the atmosphere by a galactic gamma-ray burst. We
argue that, unlike a cosmic ray event, a gamma-ray burst does not necessarily
result in a substantial increase in long-lived 10Be atmospheric production. At
the same time, the 36Cl nuclide would be generated in the amounts detectable in
the corresponding ice core samples from Greenland and Antarctica. These
peculiar features allow experimental discrimination of nuclide effects caused
by gamma-ray bursts and by powerful proton events.
