Although a lot of the hot news in archaeology these days is centered on Pre-Clovis, many scholars are focused on the end of the Clovis big game hunters.
Since the appearance of Clovis big game hunters on the North American continent has been redated to include a mere 300-500 years length, researchers have been trying to trace the reasons for its disappearance. One possible reason is the death of all of the big game Clovis was hunting--called the Pleistocene megafauna extinctions. The megafauna that disappeared between 15,000 and 10,000 years ago include mastodons, horses, camels, sloths, dire wolves, tapir, and short-faced bear. Because these megafauna disappeared at roughly the same time as the Clovis people (or at least their lifestyle), it has long been debated whether the Clovis people were the cause of the disappearance through overkill or merely the stressed-out survivors of a difficult climate change.
The Younger Dryas and the End of Clovis
Clovis Spear Point
Photo Credit: John Weinstein © The Field Museum
Climatically, the end of Clovis coincides with the onset of the Younger Dryas period (abbreviated YD), which was substantially colder, dryer and windier, compared to the late Pleistocene and the Early Holocene on either side of the YD. The end of the Pleistocene was a warming trend as the glaciers retreated; and the YD was an abrupt and nasty surprise, a 1000-year-long return to tundra conditions. The YD was one of our ancestors' occasional struggles with abrupt climate change, which in this case was very bad, especially considering the lack of central heating.
One of the geological markers of the Younger Dryas climatic episode is an organic-rich layer of soil called "sapropelic silt", "peaty muds", "paleo-aquolls" and most commonly, "black mat".
Black Mats and Clovis
The first archaeologist to describe the "black mat" on an archaeological site was C. Vance Haynes, who in the 1960s was working on the Murray Springs Clovis site in
northeastern southeastern (sorry, Brian) Arizona, where he noticed a black mat directly overlaying the Clovis occupation.
A black mat is a thin layer of organic material, sometimes described as 'peaty', that Haynes thought at the time represented evidence of a drought. Black mats vary in color and content, but they are always found to have been created in moist to wet conditions such as ponds, elevated or perched water tables, boggy areas, wet meadows and the margins of spring pools. Scientists debate the genesis of these mats, but one possible theory is that in a shallow pond, for example, dead plants and animals filter through the water and drop to the bottom, creating a thick organic layer.
Mastodon Sculpture, Page Museum La Brea Tar Pits, California
Photo Credit: Rons Log
None of the black mats investigated at these Clovis sites contained any Clovis nor any evidence of any of the Pleistocene fauna, although beneath it, and occasionally immediately beneath it, can be found Clovis mammoth kills. The mat at Murray Springs was dated between 9,800 to 10,800 uncalibrated radiocarbon years before the present (RCYBP).
Haynes did some poking around and discovered that black mats are fairly common, and in fact they are extremely common immediately above Clovis sites, or Clovis-aged natural deposits. Similar black mats have been identified at other Clovis sites, each of those had been similarly dated.
A Summary of Black Mat Research
Further investigation in the late 1990s, reported in the peer reviewed journal Quaternary Research revealed that in the Great Basin at least, black mats actually occur between 11,000 and 6300 RCYBP and again between 2300 and modern day RCYBP; in fact they are found today being created at the margins of springs. However, an extensive cluster of them do date near 10,000 RCYBP. Organic material identified within the YD black mat clearly supports the notion that these are wetland deposits.
In 2007 at the American Geophysical Union meetings, a session was given explaining the black mat as having followed the explosive destruction of a comet which was postulated to have broken into pieces over the Laurentide ice shield.
A formal paper, published in the Proceedings of the National Academy of Sciences in September of that year, described a thin sedimentary deposit immediately beneath the black mat, which contained high concentrations of magnetic grains with iridium, magnetic microspherules, glass-like carbon containing nanodiamonds, and fullerenes with extra-terrestrial helium. Firestone et al. argue that the stuff underneath the black mat represents the detritus of an explosive low-density object--a comet--which destablized the Laurentide Ice Sheet. Widespread fires ensued, followed by an accelerated melting of the ice sheet and then a cooling period (the YD), brought on perhaps perhaps by persistent cloudiness. This combination, they claim, led to the megafaunal extinctions and the end of the Clovis big game adaptation.
Most recently, in fact late last week, C. Vance Haynes reported in PNAS that he had increased the database of Clovis sites with black mats to 70. Using the data from 97 Clovis sites all over North America (70 of which contain black mat deposits), Haynes has assembled the following stratigraphic chronology. It should be recalled that this stratigraphy is targeted at the locations of Clovis sites, which are found at stream side or pond locations.
These stratum titles are mine, not Haynes's, by the way, so take them with a grain of salt, and read his article for detailed information. In order from most recent to oldest:
- Altithermal stratum. 9,000-10,000 RCYBP. Drought conditions prevailed, during which Archaic mosaic hunter-gatherer lifestyles predominate.
- Post-Clovis stratum. (black mat layer) 10,000-10,900 RCYBP (or 12,900 calibrated years BP). Wet conditions are in evidence at the sites of springs and lakes. No megafauna except for bison. Post-Clovis include Folsom, Plainview, Agate Basin hunter-gatherers.
- Clovis stratum. 10,850-11,200 RCYBP. Drought conditions prevalent. Clovis sites found with now-extinct mammoth, mastodon, horses, camels, and other megafauna at springs and lake margins.
- Pre-Clovis stratum. 11,200-13,000 RCYBP. By 13,000 years ago, water tables had fallen to their lowest levels since the Last Glacial Maximum. Pre-Clovis is rare, stable uplands, eroded valley sides.
Frozen Spring in Tundra, Arctic National Wildlife Refuge
Photo Credit: Madhav Pai
The evidence seems to be very strong. In a (geological) instant, the extinction of horses, camels, mammoths, mastodons, dire wolves, American lions, and tapirs occurred. At the same time, Clovis patterns of big game hunting ends, and clear evidence of a surge in spring discharge at former Clovis sites occurs. Could those ponds have been frozen?
Haynes is not ready to completely support the cometary origin of this calamity. He points out that microspherules, nanodiamonds and fullerenes are all part of cosmic dust that falls on planet earth all the time. However, it is clear to him (and many of the rest of us) that something very nasty happened circa 10,900 RCYBP that changed the lifeways of the hunter-gatherers of the Americas forever.
Thanks to George Howard for the suggestion.
Firestone, R. B., et al. 2007 Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling. Proceedings of the National Academy of Sciences 104(41):16016-16021. Free to download
Haynes, Jr C. V. 2008 Younger Dryas "black mats" and the Rancholabrean termination in North America. Proceedings of the National Academy of Sciences 105(18):6520–6525.
Quade, Jay, Richard M. Forester, William L. Pratt, and Claire Carter 1998 Black Mats, Spring-Fed Streams, and Late-Glacial-Age Recharge in the Southern Great Basin. Quaternary Research 49(2):129-148. DOI: 10.1006/qres.1997.1959