Evidence for a Massive Extraterrestrial Airburst over North America 12.9 ka Ago
A carbon-rich black layer commonly referred to as a black mat, with a basal age of approximately 12.9 ka, has
been identified at over 50 sites across North America1. The age of the base of the black mat coincides with
the abrupt onset of Younger Dryas cooling and megafaunal extinctions in North America. In situ bones of extinct
mammals, including mammoths, mastodons, ground sloths, horses, camels, many smaller mammals and
birds, and Clovis tool assemblages occur below the black mat but not within or above it. In this paper, we provide
evidence for an ejecta layer at the base of the black mat from an extraterrestrial impact event 12.9 ka ago. We
have investigated nine terminal Clovis-age sites in North America and a comparable site in Lommel, Belgium
that are all marked by a thin, discrete layer containing varying peak abundances of (1) magnetic
grains/microspherules containing iridium concentrations up to 117 ppb, (2) charcoal, (3) soot, (4) vesicular
carbon spherules, (5) glass-like carbon, and (6) fullerenes enriched in 3He. This layer also extends
throughout the rims of at least fifteen Carolina Bays, unique, elliptical, oriented lakes and wetlands scattered
across the Atlantic Coastal Plain whose major axes point towards the Great Lakes and Canada. Microspherules,
highly enriched in titanium, were found only in or near the YD boundary (YDB) layer with greatest deposition rates
(35 per cm2) occurring near the Great Lakes. Magnetic grains also peak in the YDB with maximum deposition
near the Great Lakes (30 mg/cm2). Magnetic grains near the Great Lakes are enriched in magnetite (4
mg/cm2) and silicates (23 mg/cm2) but contain less ilmenite/rutile (1 mg/cm2) than distant sites
where ilmentite/rutile deposition ranges up to 18 mg/cm2. Analysis of the ilmenite/rutile-rich magnetic grains
and microspherules indicates that they contain considerable water, up to 28 at.% hydrogen, and have
TIO2/FeO, TIO2/Zr, Al2O3/FeO+MgO, CaO/Al2O3, REE/chondrite, K/Th, FeO/MnO ratios
and SIO2, Na2O, K2O, Cr2O3, Ni, Co, Ir, Th, U, and other trace element abundances that are
inconsistent with all terrestrial and extraterrestrial sources except for Lunar Procellarum KREEP terrain (PKT).
We propose that the YDB layer is the ejecta layer from an airburst over the Laurentide Ice Sheet that deposited
local, low-speed terrestrial material near the airburst site and KREEP-like, high-speed projectile material farther
away, leaving little or no permanent crater. The associated blast wave and thermal pulse would have contributed
to the megafaunal extinctions and destabilized the Laurentide Ice Sheet, loading the atmosphere with dust, soot,
NOx, and water vapor and triggered the YD cooling.
1 Haynes, C. V., Jr. in Murray Springs: a Clovis site with multiple activity areas in the San Pedro Valley, Arizona. C. V. Haynes, Jr. and Bruce B. Huckell, eds. Tucson: Univ. of Arizona Press, in press, 2007.
Extraterrestrial Markers Found at Clovis Sites Across North America
We present evidence for an extraterrestrial (ET) impact event with Earth at about 12.9 ka, which, we hypothesize, caused abrupt environmental changes that contributed to Younger Dryas (YD) cooling, major ecological reorganization, broad-scale extinctions, and the rapid human behavioral shifts evident at the end of the Clovis Period. Terminal Clovis-age sites in North American are marked by a thin, discrete layer, the YD boundary layer (YDB) with varying peak abundances of an array or ET markers, including magnetic grains, microspherules, carbon spherules, soot, fullerenes with ET helium, and iridium. We will present data from a number of well- known Clovis sites and other locations in support of the YD impact event. Gainey, north of Detroit, Michigan, is a PaleoAmerican campsite that was located a few tens of kilometers from the southern margin of the Laurentide Ice Sheet at 12.9 ka. Gainey gave its name to the distinctive fluted point style found there, and Gainey sediments contained some of the highest abundances of YDB markers found, suggesting that the YD impact was centered nearby. Murray Springs, near Sierra Vista, Arizona is one of the best known Clovis mammoth kill-sites. A distinctive carbonaceous layer, called a "black mat" and thought to be of algal origin, drapes conformably over the mammoth bones (Haynes and Huckell, 2007). A thin layer (about 2 mm), the YDB, containing Younger Dryas impact event markers, lies at the base of the black mat and immediately overlies the mammoth bones and Clovis artifacts. Blackwater Draw, New Mexico is southwest of the town of Clovis, which gave its name to the type of projectile points first found there. It was a PaleoAmerican hunting site on the bank of a spring-fed waterhole, and the black mat drapes over bones of butchered mammoths and Clovis artifacts. Topper, located on a high bank of the Savannah River near Allendale, South Carolina, was a Clovis-age flint quarry containing thousands of artifacts. YDB markers occur within an approximate 5-cm interval immediately above the distinct layer of Clovis artifacts. Wally`s Beach at St. Mary Reservoir, southwestern Alberta, Canada, at 12.9 ka, supported many species of now-extinct megafauna, including mammoths, camels, and horses (Kooyman, et al., 2001). YDB markers, including iridium at 51 ppb, occur inside an extinct horse skull at the Wally's Beach Clovis kill-site, suggesting rapid burial following the YD event. One other Clovis site examined was the Chobot site near Edmonton, AB, Canada. We also examined other sites that date to 12.9 ka: glacial Lake Hind in MB, Canada; an ice-aged drumlin at Morley, AB, Canada; Daisy Cave on the Channel Islands of California; and Lommel in Belgium. At all sites, the YDB layer was evident, dated to around 12.9 ka, and contained abundant ET markers.
The End Pleistocene Extinction Event - What Caused It?
It is well established that the last catastrophic faunal extinction in the geologic past coincided with the end of the Pleistocene during the deglaciation between the last glacial episode and the present Holocene interglacial. The cause of this extinction has been debated for many years but remains highly controversial in part because of limitations of available data, but also because the two major hypotheses that have been long invoked, climate change and human overhunting, have continued to present significant problems. Recently, Firestone et al.  have reported strong evidence for an extraterrestrial (ET) impact including peak abundances of metallic microspherules and magnetic grains with elevated iridium, glass-like carbon, carbon microspherules, soot and charcoal in a carbon-rich black layer dating close to ~12.9 ka (referred to as Younger Dryas Black Layer or YDB) in numerous terminal Clovis-age sites across North America. To assess the YDB impact hypothesis and its potential effects on the megafauna and humans in North America [2-4], we have analyzed several well-dated suites of samples in search of fullerenes with ET noble gases and other impact debris (e.g. microspherules). These sites include Daisy Cave (DC) on California's San Miguel Island, Murray Springs (MS) in Arizona and Blackwater Draw (BWD) in New Mexico, all well known previously described archaeological and paleontological sites, with established chronologies spanning the YDB (~12.9 k) [5-7]. To further assess the environmental effects of the impact on the Pleistocene fauna and human activity, we examined the YDB layer at each of these locations for evidence of wildfires triggered by the ET event. If there were wildfires, a group of high molecular weight aromatic compounds or polycyclic aromatic hydrocarbons (PAHs) should be present in the associated soot and charcoal material within the YDB. By examining the distributions of the PAHs, the source of these compounds, wildfire or human activity, can be determined. The recognition of global wildfires at the time of the K/T impact event was first proposed based on the presence of elemental carbon (mainly soot) associated with ET tracers in the boundary layer [8-10]. Soot produced by severe wildfires at the YDB would have severely affected the ecological niches, reduced primary productivity, eliminated food sources and added to the effects of the ejecta cloud . Here, we present our evidence in support of an ET impact and its effects on the environment at the YDB and compare these results to other well-known impact events in the geologic record.  Firestone et al. (2006) Proceeding of the National Academies of Science (submitted).  Martin, P.S. Twilight of the Mammoths Ice Age Extinctions and the Rewilding of America. University of California Press, Berkeley 250 pp. (2005).  Haynes, C.V. (2002) World Archeology 33,391-416.  Grayson, D. K. and Meltzer, D.J. (2002) Journal of World Prehistory 16, 313-359.  Erlandson, J.M. et al. (1996) Radiocarbon 38,355-373.  Haynes, C.V.et al., (1999) Geoarcheology: An International Journal, 14, 455-470. Tayler, R.E., Haynes, C.V. and Stuvier, M. (1996) Antiquity 70, 515-525.  Wolbach, W.S., Lewis, R.S. and Anders, L.E. (1985) Science 230,167-170.  Wolbach et al. (1988) Nature 334,57-669.  Venkatesan, M.I. and Dahl, J. (1989) Nature 338,57-60.
Mineralogical and Noble Gas Evidence for an ET Impact at the Younger Dryas
We report mineralogical and noble gas evidence for an ET impact associated with the Younger Dryas (YD) event and the mass extinction of various megafauna throughout the Americas approximately 12,900 years ago. We examined numerous well established, 14C dated Clovis sites across the US including detailed sediment profile analyses at Daisy Cave, CA (DC), Murray Springs, AZ (MS), Topper, SC, and Blackwater Draw, NM (BWD). He, Ne, and Ar analysis of magnetic separates, bulk sediments, HF-acid treated residue, and fullerenes coupled with a detailed mineralogical and compositional analysis of magnetic separates by SEM suggest a marked increase in the accretion of extraterrestrial debris in sediments at the YD boundary layer. A preliminary characterization of the magnetic separates consistently observed across numerous sites indicates an extremely high abundance of relatively unoxidized, trace metal rich metallic grains with extremely unusual compositions (e.g. Pt (5-98%), Sn (25-28%) , Ni (1-11%), and Cu (12-90%). These compositions are not easily explained by ordinary terrestrial processes. A few examples include the YD layer at DC which contains numerous metallic grains including Fe-Ni, Cu-Ni, Fe-Sn-Ni, and Pt grains. Metallic iron grains are observed at BWD and Topper, while the Topper site also contains iron spherules, Fe-Ni metallic grains, and Fe-Ni oxides. At this point there is no evidence for a significant concentration of iron spherules, Fe-Ni oxides nor unusual metallic grains away from the proposed impact layer. The noble gas composition and isotopic ratios of fullerenes located at the YD layers contain trapped He with ET isotopic signatures (0.3-3.5 ncc/g 3He and 3He/4He 30- 220 Ra) while Argon ratios (40Ar/36Ar= 230-255) reflect a mixture of planetary and air components. Bulk sediments were treated with HF acid to remove silicates, leaving a carbonaceous residue. Acid treated residues from two boundary layers contained up to 5 ncc/g 3He and 3He/4He from 15- 374 Ra. The DC acid residue exhibits a significant galactic cosmic ray input with 3He/4He=374-800 Ra. Bulk and magnetic sediments away from the boundary layer contain only 1-10 pcc/g 3He in the bulk sediment with 3He/4He from 0.1 to 4.6 Ra. These are consistent with the background IDP flux. An increase in 3He concentrations (5-8 ncc/g) and 3He/4He (10-25Ra) of magnetic grains is observed near the YDB which likely reflects an increase in the flux of IDPs associated with the impactor. The results indicate that the noble gas carrier is concentrated in the carbonaceous phase. Specifically, helium remains trapped in fullerenes as has been seen in other impact events (e.g. PT). The metallic Fe phase is either relatively devoid of ET noble gases or has been degassed during atmospheric entry.
Triggering of the Younger Dryas Cooling by Extraterrestrial Impact
The enigmatic Younger Dryas (YD) cooling episode (12.9-11.5 ka) is unique to late Quaternary deglacials; its timing is unexplainable by orbital forcing because Northern Hemisphere insolation was increasing at that time. The abrupt onset of YD cooling coincided with, and was likely partially in consequence of reduction in north Atlantic thermohaline circulation (THC). Triggering of YD cooling has often been attributed to major diversion of freshwater outflow from the continental interior via newly opened outlets resulting from ice sheet melt-back. However, an alternative primary trigger for YD cooling is actively being sought because of problems related to the timing of outlet openings and the relative importance of the magnitude and timing of freshwater input into the ocean. We present evidence in support of the hypothesis that the primary trigger for YD cooling may have instead been an extraterrestrial impact (the YD impact event) over northern North America at 12.9 ka. This impact event is recorded in a diverse range of impact-related proxies within the widely distributed YDB layer that indicates continent-wide impact effects. YD cooling likely resulted both from a combination of short-term atmospheric processes due to the impact itself that reduced solar radiation and from reorganization of ocean circulation. The hypothesis also posits that the impact caused partial destabilization of northern ice sheets and the opening of freshwater conduits; ice sheet melting due in part to the deposition of impact related dust (i.e. albedo) on the ice sheets; freshening of Arctic and northern Atlantic surface waters, and resulting strong reduction in north Atlantic THC. The impact hypothesis for YD triggering explains the timing enigma of the YD episode and is consistent with much existing data: 1) Abrupt and dramatic switch in Lake Agassiz outflow at 12.9 ka away from the Mississippi River's southern outlet to newly-opened outlets to the east and/or north; 2) A major and abrupt drop in ice-margin lake levels at precisely the time of the impact, based on the chronostratigraphy at Lake Hind and supported by Lake Agassiz sediment records; 3) Evidence at the onset of YD cooling in northern Atlantic and Arctic sediment cores of widespread IRD (during Heinrich event H0) reflecting ice-rafting armadas, freshwater discharges and reduced sea-surface salinity that resulted from the partial destabilization and melting of the ice sheet; 4) Evidence for an anomalous peak in Greenland ice sheet margin melting at the onset of the YD during a time of abrupt ocean and atmosphere cooling. The YD impact event supports the concept that extraterrestrial impacts, even of such limited geographic extent, can significantly and abruptly affect global climate change. Comparison of the YD event with other even larger meltwater outflow events of the last deglacial that had limited affects on the THC, suggests that the YD cooling was not triggered solely by salinity-driven ocean circulation changes. Instead, short-term climate pertubations due to the impact likely played a key role in triggering initial cooling that was reinforced or enhanced by the ocean circulation changes. Thus, the YD climate episode would not have occurred in the absence of the YD extraterrestrial impact event.
Dynamic Proxies of Ocean Circulation in the North Atlantic During the Younger Dryas
The North Atlantic Deep Water (NADW) is a major component of the Atlantic's meridional overturning circulation, which is strongly linked to climate through the sea-to-air heat transfer by water transported from low to high latitudes. Changes in this circulation system have been implicated in the abrupt climate reversal of the Younger Dryas. Previous studies using nutrient proxies such as δ13C and Cd/Ca show a nutrient enrichment in the North Atlantic during the Younger Dryas, reflecting a reduction in the volume of nutrient-depleted NADW. Although valuable, water mass tracers cannot constrain the rate of overturning; a crucial factor in the overall heat flux of deep water formation. Dynamic proxies such as 231Pa/230Th disequilibria and the grain size of deep sea sediments provide tools to measure changes in the vigor of ocean circulation. 231Pa/230Th ratios act as a proxy for the export rate of subsurface waters from the North Atlantic. Changes in the non-cohesive sortable silt (SS) size fraction (10-63μm) of terrigenous sediments reflect variations in the current strength as a result of the relative entrainment capacity of flow velocity. Here we compare the grain size record from site 984, along the Rekjanes Ridge, with 231Pa/230Th data from core GGC5 on the Bermuda Rise. Site 984 is well situated to monitor both the modern deep water overflows and the intermediate depth waters of the glacial period, whereas core GGC5 offers a more basin-wide measure of circulation export. These records indicate similarly robust overturning circulation during the last glacial maximum and Holocene. In contrast, the deglacial period reveals significant reductions in the circulation. The Younger Dryas exhibits the most dramatic decrease in grain size throughout the 20,000 year record, and the 231Pa/230Th data indicate a reduction in export rate that is rivaled only by the first Heinrich iceberg discharge event. The reduction in current strength during the Younger Dryas is concurrent with an increase in δ18O and polar fauna at both sites, indicating colder surface temperatures. The dynamic circulation proxies demonstrate that decreases in the rate of circulation occurred in association with deglacial melting, supporting the hypothesis that freshwater influx can have a dampening effect on the rate of overturning. This confirms the close connection between ocean circulation and such rapid climate oscillations as the Younger Dryas.