Supernova Triggers for End-Devonian Extinctions

<p>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. Here we study 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 <math xmlns="http://www.w3.org/1998/Math/MathML" id="i1" overflow="scroll"><mo>∼</mo><mn>100</mn></math> ky. We therefore propose that the end-Devonian extinctions were triggered by supernova explosions at <math xmlns="http://www.w3.org/1998/Math/MathML" id="i2" overflow="scroll"><mo>∼</mo><mn>20</mn><mo> </mo><mtext mathvariant="normal">pc</mtext></math>, 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 <math xmlns="http://www.w3.org/1998/Math/MathML" id="i3" overflow="scroll"><mrow><mmultiscripts><mtext>Sm</mtext><mprescripts></mprescripts><none></none><mn>146</mn></mmultiscripts></mrow></math> or <math xmlns="http://www.w3.org/1998/Math/MathML" id="i4" overflow="scroll"><mrow><mmultiscripts><mtext>Pu</mtext><mprescripts></mprescripts><none></none><mn>244</mn></mmultiscripts></mrow></math> in one or more end-Devonian extinction strata would confirm a supernova origin, point to the core-collapse explosion of a massive star, and probe supernova nucleosynthesis. Other possible tests of the supernova hypothesis are discussed.</p>