Paleozoic Extinctions in Cosmoclimatological Context: ‘Non-Bolide’ Extraterrestrial Causes for Global Chilling

Abstract. The Paleozoic Era experienced 4 major mass extinctions; i.e., end-Ordovician, Late Devonian, end-Guadalupian, and end-Permian episodes. As a cause of significant biodiversity decline, non-biological environmental change on global scale was inevitable; nonetheless, popular claims of bolide impact and/or large igneous province (LIP) with too many ad-hoc assumptions have not yet been accepted as common/universal explanations for the Paleozoic extinctions. Recent research on extinction causes evolved through two stages; i.e., the heyday of the bolide impact scenario in the 1980s, and the overtaking by a LIP-mantle plume scenario in the 1990–2000s. Lately, we may sense a return trend to extraterrestrial causes since the late 2000s, which is not a simple revival of the old bolide-impact model but a new proposal for a cosmoclimatological scenario relevant to extra-solar processes; i.e., supernovae explosions and relevant migration of dark clouds over the Solar System. This short article reviews the current status of extinction-related research, which emphasizes two key issues; i.e., the categorization of extinction causes and new perspectives on non-bolide extraterrestrial causes. The categorizing of extinction causes at four distinct levels is effective in separating “global triggers” on the Earth's surface from more essential “ultimate cases” within the Earth and/or on outside of the planet. Causes of extinction can be grouped into four distinct categories in a hierarchy, from small to large scale: i.e., Category 1 – direct kill mechanism for each local biota, Category 2 – background change in global environment, Category 3 – major geological phenomenon on the planet's surface, and Category 4 – ultimate cause from the interior and exterior of the planet. Recent advances in He isotope analysis for extinction-related sedimentary records suggest extraterrestrial causes, not of bolide impact but of the encounter with a dark cloud (nebula). Emerging new perspectives of cosmoclimatology leads to an alternative extinction scenario; e.g. 1) increased flux of galactic cosmic radiation (GCR) with extensive cloud cover and 2) passage of a dark cloud (nebula) enriched with micro-dusts (IDPs) enveloping the Solar System. Both meteoric cloud coverage and IDP-screen can induce lowering/shutdown of solar irradiance, which may drive global cooling and sea-level drop associated with biodiversity decline. The past star-burst events detected in the Milky Way Galaxy apparently coincide in timing with the cooling episodes associated with major extinctions of the Paleozoic, i.e., at the end-Ordovician, Late Devonian, and Late Permian. Given such astronomical processes associated with global cooling in the past, much older global freezing episodes, i.e., Proterozoic snowball Earth events developed under high atmospheric CO2 levels, can be likewise explained. The study of mass extinctions on the Earth is entering a new stage under new astrobiological perspectives.