Brown Bag: Tyrone Daulton

Abstract: In North America 12,900 years ago as the last glacial period was nearing an end, the period of deglacial warming known as the The Bølling-Allerød interstadial ceased abruptly with temperatures plummeting approximately ten degrees within a decade.  A period of near glacial conditions resumed for another ~1,300 years, an interval known as the Younger Dryas (YD) stadial. Conclusion of the YD was abrupt with temperature increasing to present-day Holocene-interglacial levels within a few decades.  In North America at least 17 genera of megafauna (e.g., mammoths, mastodons, giant short-faced bears, saber-tooth tigers, and other species) became extinct near the YD onset. North America's earliest known human populations arrived and dispersed prior to the YD, and their Clovis lithic technology disappeared from the sedimentary record also near the YD onset.  While the YD climate reversal as well as the geologically abrupt Pleistocene megafauna extinctions and disappearance of the Clovis culture are not disputed, their causes are matters of intense debate  A recent controversial hypothesis suggests that an extraterrestrial body, either a fragmented chondritic meteorite or comet, detonated as airburst(s) and/or impact(s) over North America, igniting continent-wide wildfires and injecting a large mass of dust into the atmosphere.  The energy deposited by the bolides is speculated to have induced partial melting of the Laurentide ice sheet disabling the North Atlantic thermohaline circulation and initiating the YD stadial. The combined environment impact of such an event would have adversely affected animal populations. While the YD impact hypothesis appears to explain the North American late Pleistocene megafaunal extinctions, disappearance of Clovis culture, and abrupt Younger Dryas (YD) climate reversal, did an impact actually occur 12,900 years ago?  The physical evidence purported to support the YD impact hypothesis is reviewed.  This evidence includes elevated concentrations of "impact markers" in sediments of YD onset age such as: iridium, magnetic microspherules, fullerenes enriched in trapped ³He, charcoal/soot, carbon spherules, glass-like carbon, and nanometer-sized diamonds.