  It is known that a gamma ray burst (GRB) originating near the Earth could be
devastating to life. The mechanism of ozone depletion and subsequent increased
UVB exposure is the primary risk, but models also show increased nitrification
culminating in nitric acid rainout. These effects are also expected after
nearby supernovae and extreme solar proton events. In this work we considered
specifically whether the increased nitric acid rainout from such events is a
threat to modern terrestrial ecosystems. We also considered its potential
benefit to early terrestrial Paleozoic ecosystems. We used established critical
loads for nitrogen deposition in ecoregions of Europe and the US and compared
them with previously predicted values of nitric acid rainout from a typical GRB
within our galaxy. The predicted rainout was found to be too low to harm modern
ecosystems, however, it is large compared with probable nitrate flux onto land
prior to the invasion of plants. We suggest that this flux may have contributed
nutrients to this invasion if, as hypothesized, the end-Ordovician extinction
event were initiated by a GRB or other ionizing radiation event.
