Simultaneous Disappearance of Pleistocene Megafauna and associated Clovis Culture
Source: Haynes, C. Vance, Jr., 2008, "Younger Dryas black mats and the Rancholabrean termination in North America", Proceedings of the National Academy of Sciences, 105:18, pp. 6520-6525.
Note: the following is essentially a precis of Dr. Vance's paper, intended here for a general public readership. Those desiring details of the argument (which includes a map of relevant sites all over North America) should consult the original paper. We provide below some Internet links for public guidance, marked in blue in the text.
The Clovis culture is the first well defined cultural technocomplex to occupy North America from 11,500 B.P. to 10,900 B.P. At Clovis sites people interacted with the last of the Rancholabrean megafauna at spring heads, along spring-fed streams, or around ponds as the Pleistocene climate became drier and warmer.
Termination of the Rancholabrean megafauna occurred at the end of this Clovis-age dry period, when Clovis hunters appear to have been most widespread and immediately preceding the abrupt cooling to near glacial conditions of the Younger Dryas (YD). At 27 sites mammoth bones are blanketed by YD black mat deposits. Six of these have Clovis artifacts on the same surface on which the bones rest. The black mat deposits themselves are devoid of in situ megafauna other than bison (Bison bison antiquus) but contain the earliest post-Clovis archaeological evidence represented by in situ Paleoindian artifacts, e.g., Folsom-Midland, Plainview-Goshen, and Agate Basin, commonly in direct association with the bones of extinct bison in the Great Plains and Southwest. In the eastern United States post-Clovis fluted-pointmaking cultures dominated during the YD, whereas the Dalton technocomplex appeared in the Southeast.
The manifest increase in cultural diversity and bison kill sites following Clovis suggests a significant population increase during Younger Dryas time. Of 70 geoarchaeological sites examined, 56 (80%) have skeletal elements of the Rancholabrean megafauna directly underlying the YD black mat. Approximately 38.6% have mammoth remains, 37.1% bison, 8.6% horse, 7.1% camel, 2.9% mastodon, and 8.6% other extinct-species remains, all on the Clovis-age surface. But only bison remains appear in the overlying YD black mat.
No skeletal remains of horse, camel, mammoth, mastodon, dire wolf, American lion, short-faced bear, sloth, tapir, etc., or Clovis artifacts have ever been found in situ within the YD age black mat, and no post-Clovis Paleoindian artifacts have ever been found in situ stratigraphically below it.
Whereas Carbon 14 ages of the youngest Clovis sites overlap with those of the oldest Folsom sites at one sigma, the stratigraphic separation is clear. The megafaunal extinction and the Clovis-Folsom transition appear to have occurred in about 100 years, perhaps much less. Extinction of the Rancholabrean megafauna appear to have been geologically instantaneous, essentially catastrophic. While Graham and Stafford report Carbon 14 age data suggesting that horses and camels became extinct 200 years before mammoths and mastodons, excavations at the Murray Springs and Lehner Clovis sites indicate synchronous extinction of all four of these taxa in addition to dire wolves, American lions, and tapirs.
[Note here one remarkable example from the Curry Draw site, reported by Haynes in1987. In this site's mammoth kill area, "numerous large, shallow depressions, presumably mammoth footprints, occurred in a swath along the right bank, across the channel sand, and up the left bank to where the partially articulated skeleton of an adult female mammoth lay surrounded by Clovis artifacts (Fig. 2). [Figure 2 is a photograph of these multiple round depressions of the ground near the skeleton.] The near perfect preservation of these mammoth tracks in the soft channel sand indicates a lack of discharge sufficient to erase the tracks... yet subsequent deposition of the black algal mat and white marl facies indicates seepy ground and shallow ponds. This microstratigraphy suggests that a brief drop in the water table 11,000 years ago led to a dry or nearly dry stream bed that was followed by a gradual rise in the water table soon after the mammoth crossing. Fossil pollen from the same level at the Lehner Clovis site, 10 mi (16 km) to the south... indicates that the water table rise was a regional event.... Growth of the black mat was such that the mammoth tracks were preserved essentially intact." [Source: C. Vance Haynes, Jr., 1987, " Curry Draw, Cochise County, Arizona", Geological Society of America Centennial Field Guide -- Cordilleran Section.]
Causes of the Pleistocene Extinction?
Nothing in the Quaternary stratigraphic record is more impressive than the abruptness of megafaunal extinction near the end of the Pleistocene. While others argue that Pleistocene extinction was gradual with some elements dying out long before others, and this may indeed be true for a number of taxa, for many forms there are still inadequate geochronological data to accurately determine the exact age of their extinction. The fact remains that the existence of mammoths, mastodons, horses, camels, dire wolves, American lions, short-faced bears, sloths, and tapirs terminated abruptly at the Younger Dryas boundary. This is the Rancholabrean Termination (RT). Only bison survived to the Younger Dryas, probably because they vastly outnumbered other species.
The occurrence of so many Clovis sites with stratigraphic evidence of drought in the interval representing the end of the previous warm period, and the termination of most of the Pleistocene megafauna taxa in an instant before the YD, makes possible several explanations for extinction. P.S. Martins overkill hypothesis posits humans as the sole cause, but could they do it everywhere in the same instant? Lundelius and Graham invoke climate change, but this, like overkill, would seem to require more time than the evidence for stratigraphic abruptness allows. MacPhee and Marx believe hyper disease caused extinction of the megafauna, but natural selection would have left survivors. Perhaps the incredible coincidence of drought, rise of the Clovis population, and extinction at the onset of the glacial cold of the YD indicates multiple causes of extinction. In the San Pedro Valley of Arizona animals under stress gathered at dwindling water sources only to be annihilated by Clovis hunters. However, many relatively young, tender mammoths in the San Pedro Valley died without Clovis impact. Did a long-lasting deep freeze deny water to them? Considering the abruptness and magnitude of the termination, a major environmental and biotic disturbance took place at 10,900 B.P. that requires interpretation
Should an extraterrestrial (ET) cause be considered? Brakenridge and Berger suggest there may be an ET explanation for YD in the form of a supernova. G.R. Brakenridge (1981) points out that supernova Vela occurred sometime between 11,300 and 8,400 years ago. R.B. Firestone et al. (2007) propose that a comet impact 12,900 years ago caused the megafauna extinction and triggered the onset of YD cooling. They documented a total of 14 proxies, in a layer found at the base of the black mat at many locations, indicative of an ET impact and associated major biomass burning. This includes above-background peaks in magnetic spherules, magnetic grains, carbon spherules, glass-like carbon, charcoal, iridium, 3He, and nanodiamonds at the contact boundary at many Clovis-age sites.
So far, by preliminary examinations, I (Haynes) have found microspherules in magnetic fractions separated from microstratigraphic samples at the base of the black mat at Murray Springs. However, micrometeorites and microspherules are components of cosmic dust that is constantly falling to earth. Therefore, this is just the beginning of a scientific study to see if an ET event can be verified to explain the Rancholabrean termination. Further analysis is in progress and other Clovis sites need independent study and verification of this evidence. Until then I remain skeptical of the ET impact hypothesis as the cause of the YD onset and the megafaunal extinction. However, I reiterate, something major happened at 10,900 B.P. that we have yet to understand.