 Some scientists have proposed that this event triggered extensive biomass burning, a brief impact winter, the Younger Dryas abrupt climate change, contributed to extinctions of late Pleistocene megafauna, and resulted in the end of the Clovis culture.The evidence given by proponents of a bolide or meteorite impact event includes "black mats", or strata of organic-rich soil that have been identified at dozens of Clovis culture archaeological sites in North America.Proponents have reported materials including nanodiamonds, metallic microspherules, carbon spherules, magnetic spherules, iridium, platinum, platinum/palladium ratios, charcoal, soot, and fullerenes enriched with helium-3 that they interpret as evidence for an impact event that marks the beginning of the Younger Dryas.