EnvironmentNanodiamonds in the Younger Dryas Boundary Sediment Layer:
We report abundant nanodiamonds in sediments dating to 12.9 ± 0.1 thousand calendar years before the present at multiple locations across North America. Selected area electron diffraction patterns reveal two diamond allotropes in this boundary layer but not above or below that interval. Cubic diamonds form under high temperature-pressure regimes, and n-diamonds also require extraordinary conditions, well outside the range of Earth's typical surficial processes but common to cosmic impacts. N-diamond concentrations range from 10 to 3700 parts per billion by weight, comparable to amounts found in known impact layers. These diamonds provide strong evidence for Earth's collision with a rare swarm of carbonaceous chondrites or comets at the onset of the Younger Dryas cool interval, producing multiple airbursts and possible surface impacts, with severe repercussions for plants, animals, and humans in North America.
A few days ago I expressed the opinion that climate change alone explanations are a bit suspicious to me. This particular hypothesis has a bit of a skyhook feel to me. The scenario is not logically impossible, and it could be sufficient as a causal factor, but the reasons for megafaunal extinctions have to consider the broader context of separate events over long periods of time correlated with the arrival of humans. Though climate change is sufficient, I would offer that the arrival of modern humans was necessary.
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Of course all the megafauna disappeared in South America as well: but not on islands like Cuba, Hispaniola, Wrangel and St. Paul (one of the Pribilofs). Steller's Sea Cow disappeared everywhere except the Komandorski islands...
Posted by: gcochran | January 3, 2009 3:11 AM
That's fascinating about the islands. It's an especially potent point against the climate hypothesis, if there are terrestrial forms that lasted on rather small islands, because obviously they are far less able to migrate during climate change than are terrestrial animals on the continent.
Granted, I guess(?) islands might be buffered by the ocean against certain kinds of climate change.
The odd thing for the human hypothesis, which I assume we are all leaning toward, is that wiki claims there were people on Cuba
The earliest inhabitants of Cuba were the Guanajatabey people,[1] who migrated to the island from the forests of the South American mainland as long ago as 5300 BC.[2]
another site discusses natives on Hispaniola
One long technical article on the [native] population comes in the with the low estimate of 100,000. Several other modern scholars seem to lean more forcefully in the area of 300,000 to 400,000.
I didn't try any other islands.
Posted by: Eric J. Johnson | January 3, 2009 6:48 PM
far less able to migrate during climate change
Ie, in response to climate change, so as to not go extinct.
Posted by: Eric J. Johnson | January 3, 2009 6:54 PM
The megafauna disappered later in Cuba: "Whatever the cause, it is extraordinary that dozens of genera of large mammals became extinct during the late Quaternary throughout the Western Hemisphere, including 90% of the genera of the xenarthran suborder Phyllophaga (sloths). Radiocarbon dates directly on dung, bones, or other tissue of extinct sloths place their "last appearance" datum at ≈11,000 radiocarbon years before present (yr BP) or slightly less in North America, ≈10,500 yr BP in South America, and ≈4,400 yr BP on West Indian islands. " From
"Asynchronous extinction of late Quaternary sloths on continents and islands" PNAS.
Anyhow, there have been other interglacial periods, but there were no associated mass extinctions. The idea that a meteorite drove the Pleistocene extinctions in Noram is silly, and I say this as a big proponent of asteroid strikes - both on dinosaurs and people who piss me off.
Posted by: gcochran | January 3, 2009 7:48 PM
Thanks - cool paper. This line of argument is pretty convincing. PDF here:
http://bill.srnr.arizona.edu/classes/596b/Steadman.pdf
up to at least 3,000 kg for the largest [American giant sloth], Megatherium americanum
Wow, I didn't know they were that huge.
Deferred extinction of insular sloths resembles the situation with mammoths (Mammuthus), where isolated populations survived on Wrangel Island (Chukchi Sea, northeastern Asia) and St. Paul Island (Bering Sea) into the early to mid-Holocene, which is several millennia later than on the Eurasian or American continents (49, 50).
I scoped the abstracts of refs 49 and 50, not having the full texts. Unfortunately they don't address when sapiens reached Wrangel or St Paul. Wiki[wrangel island] states: "radiocarbon dating shows the human inhabitation roughly coeval with the last mammoths on the island circa 1700 BCE, though no direct evidence of mammoth hunting has been found." I'm having a harder time finding a human arrival date for St Paul.
[Sloth extinction on islands millennia after extinction on continents] is not compatible with glacial-interglacial climate change forcing these extinctions, especially given the great elevational, latitudinal, and longitudinal variation of the slothbearing continental sites.
[...]
Particularly informative is the large deposit of N. shastense dung in Rampart Cave (36°06'N, 113°56'W, elev. 535 m)
Damn... that is some informative dung.
Posted by: Eric J. Johnson | January 4, 2009 3:19 AM
Razib: Humans are part of the effected mega-fauna (the Clovis culture termination) covered by the comet impact hypothesis. The climate change from draining the vast glacial lakes into the Atlantic would be a consequence of the impact melting or blasting a new drainage. It is a matter of accounting for the abundances of impact indicators in the 'black mat' layers for now but in previous glacial terminations no Younger Dryas type climate reversals are observed.
Posted by: C W Lucas | January 5, 2009 12:52 PM
I'm just a high school educated layman but this may be the impact structure you folks are talking about:
http://theholocenecomet.spaces.live.com/
Take a look.
Posted by: Dennis Cox, Fresno, Ca. | January 25, 2009 2:39 PM