PNAS doi: 10.1073/pnas.1301760110
Evidence for deposition of 10 million tonnes of impact spherules across four continents 12,800 y ago
James H. Wittke et al.
Airbursts/impacts by a fragmented comet or asteroid have been proposed at the Younger Dryas onset (12.80 ± 0.15 ka) based on identification of an assemblage of impact-related proxies, including microspherules, nanodiamonds, and iridium. Distributed across four continents at the Younger Dryas boundary (YDB), spherule peaks have been independently confirmed in eight studies, but unconfirmed in two others, resulting in continued dispute about their occurrence, distribution, and origin. To further address this dispute and better identify YDB spherules, we present results from one of the largest spherule investigations ever undertaken regarding spherule geochemistry, morphologies, origins, and processes of formation. We investigated 18 sites across North America, Europe, and the Middle East, performing nearly 700 analyses on spherules using energy dispersive X-ray spectroscopy for geochemical analyses and scanning electron microscopy for surface microstructural characterization. Twelve locations rank among the world’s premier end-Pleistocene archaeological sites, where the YDB marks a hiatus in human occupation or major changes in site use. Our results are consistent with melting of sediments to temperatures >2,200 °C by the thermal radiation and air shocks produced by passage of an extraterrestrial object through the atmosphere; they are inconsistent with volcanic, cosmic, anthropogenic, lightning, or authigenic sources. We also produced spherules from wood in the laboratory at >1,730 °C, indicating that impact-related incineration of biomass may have contributed to spherule production. At 12.8 ka, an estimated 10 million tonnes of spherules were distributed across ∼50 million square kilometers, similar to well-known impact strewnfields and consistent with a major cosmic impact event.
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I suppose it is safe to assume then, that the most recent major bottle-neck in Human DNA must have occurred at that time.
ReplyDeleteSo finally! The impact-deniers will hopefully give up! Yeah, humans with Kalashnikovs killed off all megafauna within several hundreds of years, Clovis dissapeared by chance etc....
ReplyDeleteI suppose it is safe to assume then, that the most recent major bottle-neck in Human DNA must have occurred at that time.
ReplyDeleteNo - this only affected fairly northern latitudes, and unlike LGM, people actually continued to live right at the edge of the ice. Sure, without the Younger Dryas event, expansion would not have halted / slightly reversed, but we are talking about a very small percentage of the total world population at the time.
The back-and-forth may have actually induced some mixing of Western and Eastern European populations (and West Asian ones) that would have otherwise not occurred - in effect making the European gene pool a bit broader and more homogenous than otherwise expected.
"So finally! The impact-deniers will hopefully give up! Yeah, humans with Kalashnikovs killed off all megafauna within several hundreds of years, Clovis dissapeared by chance etc...."
ReplyDeleteDon't get too excited. Mammoths survived on Wrangel Island long after the period covered here so
their extinction on that island cannot possibly be related to any cosmic event 12,000 years ago. Besides which it seems we now have an explanation for how humans drove the megafauna to extinction:
http://www.nzherald.co.nz/world/news/article.cfm?c_id=2&objectid=10884722
Quote:
"Among late-surviving mastodons he has studied, Dan is finding examples of females losing calves (where one pregnancy is immediately followed by another, rather than by two years of lactation) and of males going into musth early (just as young bull elephants do in Africa, when mature males are poached out). Dan had also found examples of mammoths dying in the autumn, a time of year when they should have been in peak condition. Autumn deaths argued for an extrinsic cause of death. For Dan, all this could be pinned on one such cause: overhunting by humans".
And:
"Kill-sites exist that show humans were certainly, at least occasionally, hunting these formidable beasts. But it's hard to argue from those isolated cases that humans were responsible for wiping out entire species. Far from rampaging across the continent, killing every large mammal in sight, it seems ancient hunters may have had a more subtle, but no less terminal impact. Over thousands of years, the level of hunting was just enough to be unsustainable for these huge, slow-breeding behemoths of the ice age".
I think your perspective is motivated by a belief that pre-industrial humans lived in complete harmony with their environment. That belief cannot be supported by the evidence.
How many "Younger Dryas" can you count in Greenland during the last glaciation?
ReplyDelete