When in 1981 Louis and Walter Alvarez, the father and son team,
unearthed a tell-tale Iridium-rich sedimentary horizon at the 65 million
years-old Cretaceous-Tertiary boundary at Gubbio, Italy, their find
heralded a paradigm shift in the study of terrestrial evolution. Since
the 1980s the discovery and study of asteroid impact ejecta in the
oldest well-preserved terrains of Western Australia and South Africa, by
Don Lowe, Gary Byerly, Bruce Simonson, Scott Hassler, the author and
others, and the documentation of new exposed and buried impact
structures in several continents, have led to a resurgence of the idea
of the catastrophism theory of Cuvier, previously largely supplanted by
the uniformitarian theory of Hutton and Lyell. Several mass extinction
of species events are known to have occurred in temporal proximity to
large asteroid impacts, global volcanic eruptions and continental
splitting. Likely links are observed between asteroid clusters and the
580 Ma acritarch radiation, end-Devonian extinction, end-Triassic
extinction and end-Jurassic extinction. New discoveries of 3.5 - 3.2
Ga-old impact fallout units in South Africa have led Don Lowe and Gary
Byerly to propose a protracted prolongation of the Late Heavy
Bombardment ( 3.95-3.85 Ga) in the Earth-Moon system. Given the
difficulty in identifying asteroid impact ejecta units and buried impact
structures, it is likely new discoveries of impact signatures are in
store, which would further profoundly alter models of terrestrial
evolution.
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