Monday, June 02, 2008

The Carolina Bays and the Younger Dryas impact event - I

In my last post I mentioned the Younger Dryas, a sudden, dramatic cooling of the earth that happened about 12,900 BP and lasted for about 1300 years. What I didn't mention is a mass extinction - mostly in the Americas - that took place at the same time. Many large North American mammals, including camels, mammoths, the short-faced bear and numerous other species disappear at this horizon.


The widespread paleo-Indian Clovis culture, which produced the finely crafted spear points shown above, also disappeared.

In my last post I mentioned the cessation of the North Atlantic Thermohaline Circulation as a likely cause of the Younger Dryas. The most popular theory for many years has been that the sudden cooling caused the mass extinction. But maybe both the cooling and the mass extinction were caused by something else.

That other possible cause is known as the Younger Dryas impact event. This is a controversial issue, and even though I'm biased, I'll try to present the different sides fairly.

Around the United States and in Canada is a layer of nanodiamonds, magnetic grains, carbon spherules, magnetic spherules, charcoal, soot, fullerenes enriched in Helium 3, and other markers of an ET impact. These sites all date to about 12,900 BP, although dating is another controversial subject. At the very bottom of this layer is a "black mat" of organic material - carbon particles and charcoal - which could have been caused by the firestorm immediately following the impact.

This is the request for papers of New Insights into Younger Dryas Climatic Instability, Mass Extinction, the Clovis People, and Extraterrestrial Impacts, issued by Dr. Richard Firestone, for the May 2007 meeting of the American Geophysical Union (AGU) in Acapulco, Mexico. (emphasis mine)

The deglaciation that followed the last ice age period was abruptly and dramatically interrupted ~12,900 years ago by widespread cooling that marks the onset of the Younger Dryas Cool Episode, an apparent climatic anomaly in Quaternary deglaciation behavior. Much evidence shows that the Younger Dryas onset was marked by abrupt changes in ice sheet configuration, diversion of North American flood-waters to the northern Atlantic, the sudden emptying of proglacial lakes, and the reorganization of thermohaline circulation that may have triggered severe cooling. Nevertheless, significant questions have recently emerged about timing and direction of major freshwater flows to the oceans, in turn raising questions about the triggering mechanism for the Younger Dryas. The onset of the Younger Dryas also appears to have coincided with massive, widespread and punctuated changes in animal biota and Paleolithic cultural development centered in North and South America. This is represented by the well-known extinction of the megafauna of the Americas, including mammoths, horses and groundsloths (the most recent of all mass extinctions) and the termination of Clovis and certain other contemporaneous Paleolithic human cultures. The cause of these changes is also highly controversial and much debated, but is likely tied to the severe environmental changes that occurred at the beginning of the Younger Dryas. Nevertheless, some researchers consider these to be coincidental events, while others link the two as cause and effect. Another hypothesis attributes the extinctions to overhunting by Clovis people and other Paleolithic hunters or to pandemics associated with human migrations. However, all these hypotheses appear to fall short in satisfactorily explaining much available evidence. A new hypothesis posits that Younger Dryas cooling was instead triggered by extraterrestrial impacts that caused ice sheet destabilization, flood-water rediversion and changes in ocean circulation. This work offers newly uncovered evidence for ET impact at 12.9 ka including end-Clovis age sediments throughout North America with high levels of Iridium, magnetic and carbon, spherules, glass-like carbon, fullerenes, and ET noble gas ratios often in association with carbonaceous black layers and succeeded by black mats with unusual biota In this session, we invite abstracts that will explore the strengths and weaknesses of existing and new hypotheses that attempt to explain the cause of the Younger Dryas and of associated changes in the global environmental system, the associated extinctions, and of human cultural changes. We welcome all abstracts exploring new perspectives on the chronology, stratigraphic succession and potential interconnections between a wide-range of processes that appear to have been associated with the Younger Dryas Episode. These include abrupt climatic change, ice-sheet deglaciation, flood-water rerouting, surficial geology, iceberg discharge, ocean reorganization including thermohaline circulation, and sea-level change. Also critical is the timing and nature of major extinction, Paleolithic cultural succession and impact-related phenomena.

YouTube has a some videos from the press conference at the May, 2007 AGU symposium. You can also read Abstracts of ten papers presented the symposium.

Here is the abstract of another paper by Dr. Richard Firestone, et al, from the Proceedings of the National Academy of Sciences dated September 2007, Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling.

A carbon-rich black layer, dating to ~12.9 ka, has been previously identified at ~50 Clovis-age sites across North America and appears contemporaneous with the abrupt onset of Younger Dryas (YD) cooling. The in situ bones of extinct Pleistocene megafauna, along with Clovis tool assemblages, occur below this black layer but not within or above it. Causes for the extinctions, YD cooling, and termination of Clovis culture have long been controversial. In this paper, we provide evidence for an extraterrestrial (ET) impact event at ~12.9 ka, which we hypothesize caused abrupt environmental changes that contributed to YD cooling, major ecological reorganization, broad-scale extinctions, and rapid human behavioral shifts at the end of the Clovis Period. Clovis-age sites in North American are overlain by a thin, discrete layer with varying peak abundances of (i) magnetic grains with iridium, (ii) magnetic microspherules, (iii) charcoal, (iv) soot, (v) carbon spherules, (vi) glass-like carbon containing nanodiamonds, and (vii) fullerenes with ET helium, all of which are evidence for an ET impact and associated biomass burning at ~12.9 ka. This layer also extends throughout at least 15 Carolina Bays, which are unique, elliptical depressions, oriented to the northwest across the Atlantic Coastal Plain. We propose that one or more large, low-density ET objects exploded over northern North America, partially destabilizing the Laurentide Ice Sheet and triggering YD cooling. The shock wave, thermal pulse, and event-related environmental effects (e.g., extensive biomass burning and food limitations) contributed to end-Pleistocene megafaunal extinctions and adaptive shifts among PaleoAmericans in North America.


The photo above, of the 1955 Excavation of the Lehner Mammoth site near Hereford, Arizona, shows the bone layer with Clovis artifacts beneath the "black mat". It is from the web page Paleoindian Studies and Geoarchaeology at the University of Arizona, by Byron Cummings and Emil Haury. This text accompanies the above photo.

Soon after the work at Naco, Haury excavated the Lehner mammoth site near Hereford , Arizona . Nearly two months of fieldwork in 1955 and 1956 yielded the remains of nine mammoths, the isolated remains of horse, bison, and tapir, 13 Clovis points, eight flake tools, one chopping tool, a small amount of flake debris, and two hearths (Haury et al. 1959). A distinctive black clay layer, coined the “Lehner swamp soil,” buried these deposits and the mammoth remains. Antevs (1959) associated the "swamp soil" with a subhumid climate and ponding, but inaccurate radiocarbon dates prohibited an absolute date for the interval. Vance Haynes later renamed it the "black mat" and several more recent radiocarbon dates indicate that it formed between 9,800 and 10,800 BP (Haynes 2007).

To be continued...

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