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Less well known is the discovery that fragments of a disintegrating ~100km-diameter comet collided with the Earth some 12,800 years ago in what is known as the Younger Dryas period (named after a signature Arctic flower). The collision triggered a rapid return to glacial conditions which lasted about 1,400 years, interrupting the gradual warming of the planet after the Last Glacial Maximum around 20,000 years ago. In a recent two-part publication, Wolbach (and 31 co-authors) presented a detailed analysis of evidence of this most unusual climatic episode gathered over the last decade (Wolbach et al. 2018, Jour. Geology, v 126: 165-184; 185-205). Data was gathered from ice-cores in Greenland, Russia and Antarctica as well as from lake, marine and terrestrial sediments. Contemporaneous layers of charcoal and dust in these geographically dispersed cores confirm this cosmic impact event. These specific layers are enriched in platinum and other impact-related elements. They also contain glassy spherules and nano-diamonds, and are anomalously high in ammonia, nitrate, and other compounds that represent a major period of extensive biomass burning. Sea levels rose a few meters due to major melting of the North American Ice Cap and this surge of fresh water disturbed the oceanic circulation that began a period of cooling.

Evidence points to numerous fragments of a disintegrating comet detonating above and/or colliding with ice-sheets, oceans, and land on at least four continents centered on North America. The radiant and thermal energy from multiple explosions triggered extensive multiple wildfires that are estimated to have burned about 10% of the planet’s biomass, considerably more than that accompanying the meteorite impact that caused the demise of the dinosaurs. The burning created long-lived atmospheric soot, blocking most sunlight and creating an impact winter and acid rain. The reduced vegetation caused a major crisis in the ecosystem that may have contributed to many megafaunal extinctions including mammoths, mastodons, ground sloths and American horses, along with many birds and smaller mammals. Human population declined for about a thousand years and the demise of the Clovis hunters ensued. This synchronicity of multiple events makes the Younger Dryas interval one of the most unusual climatic/ecological episodes during the last two million years. It also raises the importance of supporting the Near Earth Asteroid Survey in defense of future serious impacts on our planet.”

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