End-Paleozoic Mass Extinction: Hierarchy of Causes and a New Cosmoclimatological Perspective for the Largest Crisis

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The largest mass extinction in the Phanerozoic occurred at the boundary between the Paleozoic and Mesozoic eras (about 252 million years ago). The end-Paleozoic extinction that determined the fate of modern animals including human beings occurred in two steps: first around the Middle-Late Permian boundary (G-LB) and then at the Permian-Triassic boundary (P-TB). Biological and non-biological aspects unique to these two distinct events include changes in biodiversity, isotope ratios (C, Sr, etc.) of seawater, sea level, ocean redox state, episodic volcanism, and geomagnetism. This article reviews possible causes proposed for the double-stepped extinction in regard to the current status of mass extinction studies. Causes of extinction can be grouped into four categories in hierarchy, from small to large scale, i.e., Category 1, direct kill mechanism; Category 2, global environmental change; Category 3, trigger on the planet’s surface; and Category 4, ultimate cause. As the G-LB and end-Ordovician extinctions share multiple similar episodes including the appearance of global cooling (Category 2), the same cause and processes were likely responsible for the biodiversity drop. In addition to the most prevalent scenario of mantle plume-generated large igneous provinces (LIPs) (Category 3) for the end-Permian extinction, an emerging perspective of cosmoclimatology is introduced with respect to astrobiology. Galactic cosmic radiation (GCR) and solar/terrestrial responses in magnetism (Category 4) could have had a profound impact on the Earth’s climate, in particular on extensive cloud coverage (irradiance shutdown). The starburst events detected in the Milky Way Galaxy apparently coincide in timing with the cooling-associated major extinctions of the Phanerozoic and also with the Proterozoic snowball Earth episodes. As an ultimate cause (Category 4) for major extinction, the episodic increase in GCR-dust flux from the source (dark clouds derived from starburst) against the geomagnetic shield likely determined the major climate changes, particularly global cooling in the past. The study of mass extinctions on Earth is entering a new stage with a new astrobiological perspective.