From Cosmic Explosions to Terrestrial Fires? A Discussion

Several observational studies have revealed the explosion of a supernova in the early Pleistocene at about 100 pc from the Solar System. The proof of this explosion is based on the signature associated with Fe deposits in Earth’s sediments and on the Moon’s regolith. The supernova remnant was the cause of additional input of galactic cosmic rays (GCRs) in the Solar System. Their propagation from the supernova remnant to Earth and the effects of the associated cascades of secondary particles triggered by their interactionwith Earth’s atmosphere were analyzed by Melott and Thomas (2019). According to their study and previous works cited in their article, high-energy cosmic rays (above 1 TeV) caused a 20-fold increase of irradiation by muons on Earth’s surface and on the ocean and an order of magnitude increase of the atmospheric ionization that could have lasted more than 1,000 years. The increase in irradiation could have contributed to a minor mass extinction in the Pliocene–Pleistocene transition, 2.6 My ago. Melott and Thomas (2019) analyzed the climatic consequences of the increased atmospheric ionization leading tomore frequent lightning and, therefore, to an increase in nitrate deposition and in wildfires. Increased wildfires, evidenced by an increase in soot and carbon deposits over the relevant period, would have contributed to the transition from forest to savanna in northeastern Africa, long argued to have been a factor in the evolution of hominin bipedalism.