Fields, and 9 co-authors, reported in 2021 that the Late Devonian was a protracted period of low speciation resulting in biodiversity decline, culminating in extinction events near the Devonian–Carboniferous boundary. Recent evidence indicates that the final extinction event may have coincided with a dramatic drop in stratospheric ozone, possibly due to a global temperature rise. The authors investigated an alternative possible cause for the postulated ozone drop: a nearby supernova explosion that could inflict damage by accelerating cosmic rays that can deliver ionizing radiation for up to ∼100∼100 ky.
The authors concluded that the end of Devonian extinctions were triggered by supernova explosions at ∼20 pc, somewhat beyond the “kill distance” that would have precipitated a full mass extinction. Such nearby supernovae are likely due to core collapses of massive stars; these are concentrated in the thin Galactic disk where the Sun resides. Detecting either of the long-lived radioisotopes 146Sm or 244Pu in one or more end of Devonian extinction strata would confirm a supernova origin, and point to the core-collapse explosion of a massive star, and probe supernova nucleosynthesis. Other possible tests of the supernova hypothesis are discussed.
Some hundreds or thousands of years after the optical and ionizing outburst, the cosmic ray and dust bombardment of Earth would begin, with several possible effects. Cosmic ray ionization of the atmosphere and accompanying electron cascades may lead to more frequent lightning, increased nitrate deposition, and wildfires. The increased nitrate flux might have led to CO2 drawdown via its fertilization effect, thereby cooling the climate. There is evidence for cooling during the first stage of the DCB, although this occurred an estimated 300 ky before the radiation damage attested by the data on pollen and spores. Any increases in soot and carbon deposits during the end of Devonian could have been generated by increases in wildfires.
Cosmic rays striking the atmosphere produce energetic muons that can penetrate matter to a much larger depth than UV-B radiation. The radiation dose due to muons at Earth’s surface and in the oceans at depths of ≤1 km could exceed, for many years, the current total radiation dose at Earth’s surface from all sources. Therefore, in addition to comparing the effects of muons and UV-B radiation at or near the surface, they could be considered in end of Devonian extinctions of megafauna living at depth.
Finally, if there was one CCSN at the DCB, there may have been more, which may have been responsible for the Kellwasser and additional events. These could show evidence for ozone depletion and the other signatures above.
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