The extraterrestrial hypothesis

In 2007 a paper was written by Firestone hypothesizing an extraterrestrial airburst or impact in nothern North America that caused the Younger Dryas event, caused megafaunal extinction, and the termination of the Clovis culture. A carbon-rich black layer dating to 12.9ka has been identified at 50 Clovis-age sites and is contemporaneous with the onset of the Younger Dryas. Bones of extinct Pleistocene megafauna and Clovis tool assemblages occur below the black layer but not in the layer or above it. Clovis-age sites are overlain by a thin, discrete layer with varying peak abundances of magnetic grains with iridium, magnetic microspherules, charcoal, soot, glass like carbon containing nanodiamonds, and fullerenes with extraterrestial helium. The layer also extends through 15 Carolina Bays which are unique elliptical depressions that are oriented northwest across the Atlantic Coastal Plain. Evidence suggests that the extinction of mammalian and avian taxa occured catastrophically at the onset of the Younger Dryas. In North America 35 mammalian genera disappeared. At Murray Springs, AZ (Clovis site), mammoth bones and Clovis tools lie below the black layer but not above it. Firestone believes the extinctions are too broad and ecologically deep to be explained by either overkill or climatic cooling hypotheses. He lays out his evidence very well. Magnetic microspherules measuring 10-250 um peaked in or near the Younger Dryas boundary at 8 of 9 Clovis sites tested. Microspherules are most abundant in the north with a peak of 2144 per kilogram at the Paleoindian site of Gainey, MI. Magnetic grains measuring 1-500 um show a peak at the Younger Dryas boundary at all 10 Clovis sites tested. And again the abudances are higher in the north. Iridium was found at levels up to 5000 times higher than typical crustal abundance at 9 of 14 Clovis sites. And iridium was only found at the black mat layer and not above it. Charcoal displays peaked at the Younger Dryas boundary. High temperature polycyclic aromatic hydrocarbons were found in the Younger Dryas boundary layer but no above it suggesting intense fires occured at locations that were tested. Carbon spherules found at the P/T boundary (a suggested cosmic impact that created a mass extinction 250mya) are similar to the spherules found in the black mat of the Younger Dryas boundary. The spherules contain fullerenes and nanodiamonds which are extremely rare on Earth but are found in meteorites and at extraterrestial impact sites. Evidence of biomass burning is found in the ice cores of Greenland which show large spikes of ammonium and nitrate markers. Some megafuanal bones studied at the Younger Dryas boundary were highly roadioactive relative to bones in stratigraphic layers before the Younger Dryas boundary. All this evidence leads Firestone to the conclusion that a comet hit the Laurentide ice sheet causing huge amounts of meltwater to pour into the North Atlantic and caused the MOS cycle to be disrupted. Biomass burning and mass extinctions took place as well. The comet is hypothesized to have released 10e7 megatons of energy when it impacted and it was probably around 4km in size. But where's the crater? If the comet impacted at less than 30 degrees it would not have left a crater. Or if it broke up on the way to impact it wouldn't necessarily leave large craters. While all this is certainly plausible Firestone makes inferences about the effect this had on the Paleoindians that are not supported by any evidence. He claims major adaptive shifting by Paleoindians occured around 12.9kya. But no evidence of this exists. He claims a population decline and bottleneck occured at this time as well, again no evidence supports this. Other archaleologists attempted to recreate his evidence of iridium, spherules, etc., but couldn't detect anything at the level that Firestone did. Something happened at the 12.9kya Younger Dryas boundary, but it most likely wasn't an extraterrestrial impact. Firestone RB. Evidence for an extraterrestial impact 12900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling. PROCEEDINGS- NATIONAL ACADEMY OF SCIENCES USA, 104, no. 41, (October 9, 2007): 16016-16103