Impact-related microspherules in Late Pleistocene Alaskan and Yukon “muck” deposits signify recurrent episodes of catastrophic emplacement

The high-temperature, melt-quenched and accretionary microspherules, and Pt enrichments associated with sediments from the megafaunal fossils of this study (Figs 47, S1–S3), are well-established indicators of extraterrestrial impact events30,50,53. In light of this evidence, much of the death, dismemberment, burial, and preservation of megafaunal bones and partial carcasses within the frozen Beringian mucks is perhaps better explained by the catastrophic effects of cosmic impacts rather than by more commonplace causes of mortality such as illness, accident, advanced age and/or predation. In the following discussion, we reinterpret parts of the mucks and their contents as blast deposits and attempt to resolve the apparent paradoxes and inconsistencies outlined above. Examples to be applied come from studies of blast effects associated with the ~50 ka B.P. Meteor Crater impact33, 1908 Tunguska airburst54, ≥1945 nuclear tests55, and 1980 Mount St. Helens eruption56.

The blast or shock waves associated with ground impacts or airbursts cause an almost instantaneous rise in air pressure to peak overpressures as the wave fronts expand at supersonic velocities. The subsequent mass movement of air generates blast winds, which travel more slowly than the blast waves but can be just as damaging, if not more so33,57. In general terms, the area (A) damaged by an explosion is proportional to the cube root of its yield (E) squared33, or A E2/3. The amount of damage is also a function of the explosion’s height, and for a given yield there is an optimum blast height at which the damage is maximized. For the 1908 Tunguska airburst, the blast height was ~6–9 km58, near the optimum height for a ~10–20 megaton (MT) blast, and trees of the Siberian taiga were knocked down33 over an area of ~2150 km2. Intense thermal radiation was also associated with the Tunguska event. Within a radius of ~10–15 km from the epicenter the sides of trees facing the atmospheric impact were scorched, and ignited treetops were reported to have burned in many places for up to 24 hr54.

Kring33 quantified the air blast for the Meteor Crater impact using scaling relationships derived from nuclear bomb tests. Given an estimated explosive yield of ~20–40 MT for the impact of an ~50-m-diameter iron asteroid59, the air blast would have flattened trees within a ~16–22 km radius of ground zero, and damaged them over an area of ~4100–8500 km2. Within a radius of ~3–5 km, the ephemeral blast wind (>1000 km/hr) would likely have “scoured the surface of loose debris, plants, animals, and soil”, and, although decreasing in intensity with distance, would have remained “fairly large” up to distances of ~20–40 km33. Both types of impact events would have killed megafauna from various levels of blast injuries.

Primary blast injuries are caused by the nearly instantaneous change in environmental pressure from passage of the blast wave. In large mammals, the organs most commonly affected are the lungs, ears, bowel, and central nervous and cardiovascular systems57. The blast wave can also fracture bones and cause dismemberment including traumatic amputations and decapitations. From the exposure of a wide range of animals to nuclear tests (e.g., steers, burros, sheep, goats, monkeys, pigs, dogs, cats, rabbits, chickens, rats, hamsters, and mice), it was determined that larger animals generally endure higher overpressures better than smaller ones, and that animals affected by primary blast injuries usually die within a few minutes55.

Secondary blast injuries are associated with the subsequent blast wind and result from projectiles that have been picked up and energized by it. Sufficiently large fragments can also cause direct limb amputations and decapitations. Tertiary blast injuries result when victims are hurled against the ground or fixed objects. Often, victims tumble along the ground sustaining multiple injuries. Miscellaneous blast injuries include inhalation of dust, and burns from the explosion itself or from fires ignited by it57.

A modern example of a catastrophic explosion causing death and dismemberment of large mammals was the 1980 eruption of Mount St. Helens. An equivalent explosive yield for the eruption is estimated to have been ~35 MT, and the “blast zone” of downed trees and destroyed habitat56 covered an area >350 km2. Animals within the blast zone died from suffocation, severe blast injuries, and falling timber and/or pumice blocks60. Of the 24 sites where remains of elk (Cervus elaphys) and black-tailed deer (Odocoileus hemionus) were found, 5 were examined ~1 year later by Lyman56. All 5 sites were located ~15 km away from the Mount St. Helens vent, and at this distance mature trees had been snapped off and splintered near their base and completely stripped of their branches. The volcanic crater could not be seen from two of the sites, where intervening ridges probably acted as deflectors, but the crater was visible at the other three sites where they would have been subjected to the full force of the blast56.

Portions of cervid skeletons were found buried under ~20–60 cm of volcanic ash and no soft tissue was preserved. Long bones at the sheltered sites were not broken, but those at the exposed sites were fractured, in some cases shattered, and were generally more disarticulated and widely scattered. Bone fracturing was nonselective as unbroken examples of all skeletal elements could be found. Apparently the elk and deer at the exposed sites were engulfed in the eruption’s ash cloud, hurled along with trees, rock fragments and pumice blocks, and deposited some distance away at their recovery locations. Carnivore damage to the bones was minimal because many of the bones were buried, and scavengers within the blast zone would have been killed as well56.

The blast injuries sustained by cervids at Mount St. Helens apparently match much of the damage exhibited by the vertebrate remains found within Beringian muck and Yedoma deposits. Broken and shattered bones, especially within the frozen mummies (e.g., Berezovka mammoth, Lyuba, Blue Babe), have been attributed to retransportation and diagenetic processes5,46. Pfizenmayer45, however, noted that bleeding had occurred at the break of the Berezovka mammoth’s right foreleg, and inferred it had still been alive at the time. The mammoth, therefore, was initially thought to have fallen into a “crevasse”45, but instead could have been thrown and/or tumbled across the ground by blast winds, come to rest in a surface depression or pit, and been covered over by blast-mobilized sediment. The segment of a bison’s tail found under the Berezovka mammoth’s foot could simply have been clipped off a nearby grazer by the blast and deposited along with the mammoth’s carcass. That a healthy bull mammoth was instantly killed while grazing, subjected to multiple broken bones and internal injuries, and rapidly buried along with a body part from another animal, is consistent with the effects of a large explosive event.

The smallest frozen mummies preserved in underground nests and burrows (e.g., ground squirrels, pikas, mice) could have been killed because of their higher susceptibility to blast overpressures. At the surface, baby mammoths (e.g., Dima, Lyuba), with smaller cross sections, would have sustained less blast damage than adults. The baby mammoth Effie, however, could have been blown apart in closer proximity to an explosive center. The disarticulated and scattered bones along with partial carcasses and the relatively abundant mummified limbs are likely indicative of the damaging effects of airbursts and/or ground impacts. The overall abundance of Beringian fossil bones, often including those of different species within a single bed, do not conform with normal modes and rates of attrition, but are in accord with the expected aftermaths of cosmic impact events.

The mixture of vertebrate remains, plant debris, and loess within the Beringian mucks is consistent with ground-surface scouring, transport, and the chaotic redeposition of these materials in creek valleys by blast winds (Fig. 3b). Trees exposed to such winds would have been stripped of their branches, leaves, and bark, and either knocked down or broken off and splintered, like those found in the muck deposits. The finely comminuted plant material distributed throughout the mucks might have resulted from pulverization of less durable plant debris (leaves, grasses, bark, etc.) within turbulent blast winds, and its carbonization could have been initially caused by scorching and/or burning from intense thermal radiation associated with atmospheric passage of hypervelocity impactors. Stacked layers of killed forests indicate that creek-valley silt accumulation was not gradual or uniform, but that it was deposited cyclically in large amounts over short intervals of time. In addition, less severely injured animals could have suffocated in the dense clouds of airborne silt (e.g., Dima, Lyuba).

The buried forest layers, range of radiocarbon dates, and microspherules associated with the vertebrate fossils indicate that deposition of the Alaskan and Yukon mucks would have required more than one impact event between ~48 and 18 ka B.P. (Fig. 9; Table S1). A possible mechanism for repetitive impacts on Earth is the cyclical intersection of its orbit with meteoroid streams containing numerous objects large enough (~10–100 m) to generate Tunguska-class airbursts and ground impacts. Since the late 1970s, a group of British astronomers34,61,62 has calculated that such a scenario likely affected Earth throughout Late Pleistocene time during formation of the Taurid Complex.

Figure 9
Figure 9

Megafaunal regional or global extinction-event age ranges, as identified by Cooper et al.25 in Late Pleistocene Eurasian and North American ancient DNA and paleontological data sets, plotted along with climate intervals determined from the GICC05 δ18O ice-core record (black curve) from Greenland71,72. Red bars, with taxonomic names, indicate the youngest AMS 14C dates for the events (±2 SD), and green (Eurasia) and black (North America) bars indicate GRIWM-based estimates of last-occurrence temporal ranges (95% confidence intervals); the Gaussian-resampled, inverse-weighted McInerney (GRIWM) method incorporates sampling density and dating errors in order to estimate the most plausible temporal ranges25. Light gray boxes (1–12) indicate Dansgaard-Oeschger (D-O) interstadial warming events73, and darker gray boxes indicate Heinrich cold events (H1-H5)74. Blue dots and values represent the AMS 14C dates for the skull fragments of this study (Table S1), the light blue area delimits the Last Glacial Maximum (LGM), HOL abbreviates the Holocene Epoch, YD denotes the Younger Dryas stadial, and ranges for Marine Isotope Stages (MIS) 1–3 are shown atop the plot. Reproduced with permission of the American Association for the Advancement of Science.

The massive Taurid Complex is known to consist of several dynamically related meteoroid streams, several Apollo-type asteroids, and Comet 2 P/Encke, which has a ~5-km diameter and orbital period of 3.3 yr61,63. These objects are most likely the remnant debris of a large comet (~50–100 km) from the trans-Neptunian region that was injected into a short-period, sub-Jovian, low-inclination Earth-crossing orbit63,64 by gravitational fields of the outer planets. Such injections of large comets into the inner solar system possibly occur at a mean rate of roughly once per 30 to 100 kyr64.

Spontaneous splitting is a major factor in comet disintegration and may occur anywhere along its orbit, with some preference for near-perihelion breakup, and would lead to hierarchical fragmentation of the progenitor comet into numerous ~1 km-sized bodies in short-period orbits, many smaller objects in the ~10–100 m range, and large quantities of dust34,64. Substreams of meteoroids having similar but separate orbits within the complex are evidence of this cascading mode of fragmentation, which would likely have occurred over a period of ~10–100 kyr63,64. Using the observed dispersion of material in the Taurid Complex, Steel and Asher62 have calculated an overall age of at least 20–30 kyr, similar to the older age limit of megafaunal fossils containing impact-related microspherules. The continued dispersal of Taurid material by collisions, and by gravitational and radiative effects, has created a broad sporadic stream surrounding the complex65.

Jupiter’s gravity would predominantly cause the Taurid-object orbits to precess, and dynamical calculations have shown that Earth would encounter these orbits at ~3000 yr intervals34,64. Over the ~30-kyr period spanned by the skull-fragment ages (Table S1), it is expected that one or more encounters might have individually generated the atmospheric impact of hundreds to thousands of Tunguska-sized objects on a hemispheric scale. Such bombardment episodes could have lasted for several hours and occurred at certain times of the year, every few years, over a period of one to two centuries34,64,65.

The Late Pleistocene megafaunal extinction events appear to have occurred over a time interval between ~46 and 24 ka B.P. prior to the LGM, and again over a shorter interval between ~15 and 11 ka B.P. near the end of the Pleistocene Epoch47,66. Recently, Cooper et al.25 investigated all megafaunal species, or major clades, with comprehensive radiocarbon-dated series, to determine the timings of regional or global extinctions, and invasions or replacements by conspecific or congeneric populations (Fig. 9). They determined 31 such events widely distributed across Eurasia and North America that involved animals having diverse ecological roles and life histories25. The lack of megafaunal extinction events during the LGM, and to a lesser extent the YD stadial, suggests that cold environmental conditions were not an important driver of the extinctions, and Cooper et al.25 found that the population transitions were instead significantly correlated with rapid climate shifts associated with the Dansgaard-Oeschger (D-O) interstadial warmings (Fig. 9). Because of the apparent lack of ecological turnovers related to similar interstadial events that frequently occurred before ~46 ka, when modern humans were mostly absent from Eurasia and North America, Cooper et al.25 concluded that humans likely played a significant role in enhancing the effects of rapid climate change on Late Pleistocene megafaunal extinctions and population transitions25.

The presence of two geochemically distinct microspherule populations found by us in Late Pleistocene skull fragments points to at least two episodes of cosmic impact, and/or different target source rocks, and the skull ages (~18–48 ka B.P.; Table S1; Fig. 9) indicate that perhaps a greater number of such episodes (~2–4) occurred. It is also unclear how many separate extinction events there were both before and after the LGM. Cooper et al.25 noted a distinct cluster of events between ~37 and 32 ka, corresponding to D-O interstadials 5–7, and the question arises whether these extinctions with overlapping ages could have resulted from a single short-lived event. By comparing our 14C dates with the extinction age ranges in Fig. 9, it appears plausible that there could have been fewer discrete impact-related extinction events prior to the LGM, possibly occurring around 40–42 ka, 34–37 ka, and 27–30 ka, with another at the onset of the YD stadial (~13 ka)26,27,28.

Modern humans have also been implicated in the Late Pleistocene megafaunal extinctions23,24,25, but the temporal correspondence between these events both in Eurasia and North America seems to argue against their involvement (Fig. 9). The megafaunal extinctions and population transitions during MIS3 occurred in North America before modern humans had arrived, and in Eurasia over an extended period well after their arrival, whereas the terminal Pleistocene extinctions (MIS1-2) appear roughly coincident with modern humans’ arrival in North America, but occurred in Europe long after they had become established there. It would appear that some mechanism other than intensive human hunting drove the extinctions in both locations over the same time intervals. Moreover, the lack of evidence for ecological disturbances associated with interstadial warmings prior to ~46 ka25 suggests that rapid climate change might also have played a more limited, if any, role in the Late Pleistocene megafaunal extinctions.

The range in degree of fossil preservation, from weathered bones to frozen mummies, likely indicates the depth and/or timing of burial. Mummified carcasses and body parts were probably rapidly buried and frozen at deeper levels, whereas well-preserved bones were buried close enough to the surface to allow decomposition of the attached soft tissues before freezing, while weathered bones were exposed at the surface before burial by subsequent depositional events. Fraser and Burns9, however, noted “the position of bones of various ages near the lower contact of the Silt unit suggests…redeposition by events of considerable magnitude”. With the lack of evidence in the mucks for fluvial transport, such redeposition might have been accomplished by one or more impact-generated blast winds. During a bombardment episode, blast winds could have excavated and mixed bones of various ages, including those from animals that had already died of more common causes. Thus, the extensive breakage and disarticulation of skeletal elements in the Alaskan and Yukon mucks could have resulted from a relatively few episodes of cosmic bombardment.

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