Nitrate Deposition following an Astrophysical Ionizing Radiation

TL;DR

This study assesses the impact of nitrate deposition from astrophysical ionizing radiation events like gamma ray bursts on modern ecosystems and early Paleozoic ecosystems, finding it unlikely to harm current life but potentially aiding early plant invasion.

Contribution

It provides the first quantitative analysis of nitric acid rainout from astrophysical events and explores its dual role as a threat and a nutrient source for ecosystems.

Findings

Nitrate rainout from GRBs is too low to threaten modern ecosystems.

The nitrate flux could have supported early plant invasion after the Ordovician extinction.

Predicted nitrate deposition is significant compared to pre-plant invasion levels.

Abstract

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.