Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling

Discussion

Age of the YDB.

The YDB at the 10 Clovis- and equivalent-age sites has been well dated to ≈12.9 ka, but the reported ages of the Carolina Bays vary. However, the sediment from 15 Carolina Bays studied contain peaks in the same markers (magnetic grains, microspherules, Ir, charcoal, carbon spherules, and glass-like carbon) as in the YDB at the nearby Topper Clovis site, where this assemblage was observed only in the YDB in sediments dating back >55 ka. Therefore, it appears that the Bay markers are identical to those found elsewhere in the YDB layers that date to 12.9 ka. Although the Bays have long been proposed as impact features, they have remained controversial, in part because of a perceived absence of ET-related materials. Although we now report that Bay sediments contain impact-related markers, we cannot yet determine whether any Bays were or were not formed by the YD event.

Peaks in Markers.

We investigated whether peaks in YDB markers might be attributed to terrestrial processes. The 25 sites examined represent a wide range of depositional environments (fluvial, lacustrine, eolian, alluvial, colluvial, and glacial), soil conditions (aerobic/well drained to anaerobic/saturated), sediment composition (dense clay to gravelly sand), climatic regimes (semiarid to periglacial), and biomes (grasslands to forests). The presence of identical markers found under such a wide range of conditions argues against formation by terrestrial processes and is consistent with an impact origin. We also examined whether the YDB might represent an interval of reduced deposition, allowing the accretion of interplanetary dust particles enriched in ET markers, such as Ir, Ni, and ET helium. At Blackwater Draw, based on 24 calibrated ¹⁴C dates from 13.30 to 10.99 ka, Haynes (31, 32) suggested that any hiatus at the level representing the YDB most likely lasted less than a decade, which is insufficient to have produced a local Ir bulk sediment level that is >100× crustal abundance. Furthermore, abundances of microspherules and magnetic grains decrease with increasing distance from the Great Lakes region (see SI Fig. 12). This nonrandom distribution is unlikely to be due to terrestrial factors or interplanetary dust storms, but it is consistent with airburst/impacts over northern North America.

Magnetic Microspherules and Grains.

High concentrations of microspherules (glass, clinopyroxene, spinel, or metallic) are accepted as evidence for at least 11 older ET impact events (33). Alternately, microspherules are sometimes associated with volcanism, but when YDB microspherules were analyzed by SEM/x-ray fluorescence and compared with known cosmic and volcanic microspherules (34, 35), they appear to be nonvolcanic in origin. Analysis suggests an ET origin, but because of high titanium (Ti) concentrations, the microspherules differ from typical meteoritic ones.

The magnetic grains and microspherules are anomalously enriched in Ir and Ti (see Table 1 and SI Table 5) and are enriched in water (up to 28 at.%), especially at northern sites. TiO₂/FeO ratios of microspherules (0.48 ratio) and magnetic grains (0.76) are 4- to 250-fold higher than Alaskan terrestrial magnetite (<0.12 ratio in 347 samples) (36), crustal abundance (0.13) (25), CI chondrites (0.003) (23), and K/T impact layers (0.07) (12). These ratios and the similarity in composition of YDB magnetic microspherules and magnetic grains (e.g., high Ti) from many sites across North America cannot be explained at this time, but the YDB abundance of microspherules and magnetic grains most likely resulted from the influx of ejecta from an unidentified, unusually Ti-rich, terrestrial source region and/or from a new and unknown type of impactor.

Carbon-Rich Markers.

At Murray Springs, Haynes (37) first reported the presence of glass-like or “vitreous” carbon in the black mat. In addition, he chemically analyzed the black mat layer, concluding that it most likely resulted from the decomposition of charred wood and/or a prolonged algal bloom, both of which could result from event-related processes (e.g., climate change and biomass burning). Some black mats have no algal component, only charcoal. The widespread peaks of charcoal in or near the YDB, and their association with soot and polycyclic aromatic hydrocarbons at specific sites, provide strong evidence for extensive wildfires. We propose that glass-like carbon, carbon spherules, and nanodiamonds were produced in the YDB by high temperatures resulting from the impact and associated biomass burning.

Ir Anomaly.

Ir concentrations in sediments and ocean cores are high for many accepted impact events, such as for the K/T and Chesapeake Bay (≈36 Ma) (38). However, Ir values in the YDB bulk sediment are lower than at many K/T sites (e.g., 9.1 ppb at Gubbio, Italy) (12), suggesting much less Ir in the YD impactor. The evidence indicates an Ir anomaly in both the YDB bulk sediment and the magnetic fraction; however, for Ir in the bulk sediment, the level of uncertainty remains high (±50–90%), in contrast to the magnetic fraction, where values have higher certainty (up to ±10%), and are, therefore, more compelling. In 169 measurements at 14 sites up to ≈9,200 km apart, Ir was detected only in the YDB sediments, YDB magnetic fraction, and the black mat. Ir never was detected above or below these layers, lessening concerns about the high uncertainties, while providing strong evidence that Ir concentrations are above background in the YDB or black mat. The relatively low Ir and Ni peaks associated with the YDB are more consistent with the generally proposed composition of comets and inconsistent with the high-Ir content typical of most stony, nickel–iron, or chondritic meteorites.

Alternately, Ir peaks are found at major geologic boundary layers with no confirmed impacts, and at least some of those Ir concentrations may have resulted from volcanism. However, no major North American volcanic episode is known at 12.9 ka, and, according to Koeberl (39), such events produce Ir abundances of <0.5 ppb, much less than we find in the YDB. Therefore, the high concentrations of Ir do not appear to be of volcanic origin.

We also considered microbial concentration from Ir-rich adjacent sediment, such as occurred in experiments by Dyer et al. (40), who cultured microbes in Ir-rich igneous rocks and meteoritic material. However, at all sites analyzed, non-YDB sediment levels of Ir are very low (<0.1 ppb and possibly <0.02 ppb) and are insufficient to account for Ir levels up to 5,000× crustal abundance. Given the association of high Ir with a suite of other event-related markers, an ET connection is more plausible.

Ice Core Evidence.

Large increases in Ir and Pt occurred during the Younger Dryas as recorded in the GRIP (Greenland) ice core by Gabrielli et al. (41), who attributed these increases to increased cosmic input. Although sample resolution in the ice core was too low to permit us to specifically link the onset of these increased fluxes with the timing of the YD event, the evidence is consistent with the YD event.

As evidence for biomass burning, Mayewski et al. (42, 43) reported large ammonium and nitrate spikes in the Greenland GISP2 ice core at the onset of the YD. These GISP2 data are consistent with strong geochemical evidence in the GRIP ice core for massive biomass burning at the YD onset, especially a major ammonium spike, in association with peaks in nitrate, nitrite, formate, oxalate, and acetate (44). Altogether, the YD onset was one of the most robust intervals of biomass burning inferred from the Greenland ice cores, although the source of this burning signal must have been far more remote than sources today, because much of the modern forested Arctic region was then covered by ice. The cause of this biomass burning is consistent with the YD event.

Radioactive Elements.

Some megafaunal bones in the YDB are highly radioactive relative to other stratigraphic intervals, as occurred for some bones at the K/T boundary (see SI Figs. 13 and 14). In addition, high concentrations of U and Th were found in the YDB sediment at six of six Clovis-age sites analyzed and in four of four Bays with a paleosol, just as were found in the impact layers at Chesapeake Bay (38) and the K/T (see SI Fig. 15) (45). Because the heavy minerals, zircon, monazite, and garnet, along with Ti-rich minerals, such as titanite, ilmenite, and rutile, sometimes contain high concentrations of U and Th, we investigated whether lag deposits of those minerals might be the source of high radioactivity. We conclude that lag deposits may explain the high YDB radioactivity at some sites but not at others. Ilmenite, rutile, and titanite are possible carriers given that they comprise up to ≈2% of all sediments, but zircon, monazite, and garnet are unlikely, because they represent <0.1% each (see SI Figs. 16 and 17). The elevated levels of U and Th may result from multiple processes related to the impacts/airbursts, including formation of lag deposits, as well as the dispersal of ejecta from the impactor and/or the target area.

Nature of the Event.

The evidence points to an ET event with continent-wide effects, especially biomass burning, but the size, density, and composition of the impactor are poorly understood. Even so, current data suggest that this impactor was very different from well studied iron, stony, or chondritic impactors (e.g., at the K/T boundary). The evidence is more consistent with an impactor that was carbon-rich, nickel–iron-poor, and therefore, most likely a comet. Although the current geologic and geochemical evidence is insufficient to fully understand impact dynamics, we can offer speculation for future work.

Toon et al. (46) suggest that an impact capable of continent-wide damage requires energy of 10⁷ megatons, equivalent to an impact by a >4-km-wide comet (figure 1 in ref. 46). Although an impactor that size typically leaves an obvious large crater, no such late Pleistocene crater has been identified. The lack of a crater may be due to prior fragmentation of a large impactor, thereby producing multiple airbursts or craters. Hypervelocity oblique impact experiments (P.H.S., unpublished data) indicate that a low-impedance surface layer, such as an ice sheet, can markedly reduce modification of the underlying substrate if the layer is equal to the projectile's diameter. These results suggest that if multiple 2-km objects struck the 2-km-thick Laurentide Ice Sheet at <30°, they may have left negligible traces after deglaciation. Thus, lasting evidence may have been limited to enigmatic depressions or disturbances in the Canadian Shield (e.g., under the Great Lakes or Hudson Bay), while producing marginal or no shock effects and dispersing fine debris composed of the impactor, ice-sheet detritus, and the underlying crust.

Toon et al. (46) also noted that if airbursts explode with energy of 10⁷ megatons at optimum height, they will cause blast damage over an area the size of North America that is equivalent to a ground impact of 10⁹ megatons (figure 5 in ref. 46). Such airbursts effectively couple the impactor's kinetic energy with the atmosphere or surface (47, 48), producing devastating blast waves well above hurricane force (70 m·s⁻¹) (46). In 1908, at Tunguska, Siberia, a object <150 m in diameter, either a carbonaceous asteroid or a small, burned-out comet, produced a <15-megaton airburst with an intense fireball (10⁷ °C) that scorched ≈200 km² of trees and leveled ≈2,000 km² of forest yet produced no crater or shock metamorphism (49). A debris shower from a heavily fragmented comet (11) would have produced an airburst barrage that was similar to, although exponentially larger than Tunguska, while causing continent-wide biomass burning and ice-sheet disruption, but again possibly, without typical cratering.

Environmental Effects.

The YD event would have created a devastating, high-temperature shock wave with extreme overpressure, followed by underpressure, resulting in intense winds traveling across North America at hundreds of kilometers per hour, accompanied by powerful, impact-generated vortices (50–52). In addition, whether single or multiple objects collided with Earth, a hot fireball would have immersed the region near the impacts and would have been accentuated if the impact angles were oblique (46, 53). For comparison, Svetsov (48) calculated that a Tunguska-sized airburst would immerse the ground with a radiation flux severe enough to ignite 200 km² of forest within seconds. Thus, multiple, larger airbursts would have ignited many thousands of square kilometers. At greater distances, the reentry of high-speed, superheated ejecta would have induced extreme wildfires (53), which would have decimated forests and grasslands, destroying the food supplies of herbivores and producing charcoal, soot, toxic fumes, and ash. The number of ET airbursts or impacts necessary to induce the continent-wide environmental collapse at 12.9 ka is unknown.

Climate.

A number of impact-related effects most likely contributed to the abrupt, major cooling at the onset of the YD and its maintenance for >1,000 years. Cooling mechanisms operating on shorter time scales may have included (i) ozone depletion, causing shifts in atmospheric systems in response to cooling, with the side-effect of allowing increased deadly UV radiation to reach survivors on the surface (46); (ii) atmospheric injection of nitrogen compounds (NO_x), sulfates, dust, soot, and other toxic chemicals from the impact and widespread wildfires (46), all of which may have led to cooling by blockage of sunlight, with the side-effect of diminished photosynthesis for plants and increased chemical toxicity for animals and plants (46); and (iii) injection of large amounts of water vapor and ice into the upper atmosphere to form persistent cloudiness and noctilucent clouds, leading to reduced sunlight and surface cooling (46). Although these cooling mechanisms tend to be short-lived, they can trigger longer-term consequences through feedback mechanisms. For example, noctilucent clouds can reduce solar insolation at high latitudes, increasing snow accumulation and causing further cooling in a feedback loop. The largest potential effect would have been impact-related partial destabilization and/or melting of the ice sheet. In the short term, this would have suddenly released meltwater and rafts of icebergs into the North Atlantic and Arctic Oceans, lowering surface-ocean salinity with consequent surface cooling. The longer-term cooling effects largely would have resulted from the consequent weakening of thermohaline circulation in the northern Atlantic (54), sustaining YD cooling for >1,000 years until the feedback mechanisms restored ocean circulation.

Clovis and Megafauna.

The impact-related effects would have been devastating for animals and plants. For humans, major adaptive shifts are evident at 12.9 ka, along with an inferred population decline, as subsistence strategies changed because of dramatic ecological change and the extinction, reduction, and displacement of key prey species (55, 56). Many sites indicate that both Clovis people and extinct megafauna were present immediately before the YD event, but, except in rare cases, neither appears in the geologic record afterward. At Murray Springs, butchered, still-articulated mammoth bones, Clovis tools, and a hearth were found buried directly beneath the black mat, indicating that it buried them rapidly (37). YDB markers, including Ir at 51 ppb, occur inside an extinct horse skull at the Wally's Beach Clovis kill-site (57), again suggesting rapid burial following the YD event. It is likely that some now-extinct animals survived in protected niches, only later to become extinct because of insufficient food resources, overhunting, climate change, disease, flooding, and other effects, all triggered or amplified by the YD event.