As evidence shows, before 12,800 years, there was an impact between Earth and an extra-terrestrial body. In this post we summarize information extracted from official publications.
Firestone et al. (2007) and Kennett et al. (2009a, 2009b) presented a new hypothesis based on the identification of shock impact proxies (including nanodiamonds) consistent with a cosmic impact (crater forming or aerial detonation) at the onset of the Younger Dryas cooling episode ~12.9 ka. Impact markers are found in a thin layer, called the Younger Dryas Boundary (YDB), at 51 locations across North America, Greenland, Europe, and Syria, although not all markers are present at every site. This hypothesis has been met with contentious arguments from some members of the impact community, mostly based on probability statistics and absence of a crater.
The most significant lines of evidence for impact in the YDB are: presence of (1) nanodiamonds (ND) and other exotic carbon phases, (2) compositions of magnetic and silicate spherules (Figs. above and below), (3) spherule morphology (aerodynamic shapes, accretionary and collisional features), and (4) high-temperature melt products.
Five allotropes of diamonds (cubic, hexagonal, p-diamond, n-diamond, and i-carbon) are produced by experimental high explosive detonations (Yamada and Sawoaka, 1994) and by carbon vapor deposition. All five have been found at the YDB, and at the Cretaceous-Tertiary Boundary (KTB), now known as the Cretaceous-Paleogene (KPg) boundary, produced by the Chicxulub impact. Most diamond allotropes have also been observed in other impact craters as well as in shocked meteorites. (See poster by West et al. 2011, this meeting, for nanodiamond details). Other carbon phases (aciniform soot, fullerenes, carbon spherules, glass-like carbon and graphite) have been observed in detonation experiments. All of these are found together with nanodiamonds in the YDB and KPg boundary, but nowhere else together (Gilmour, 1998).
For the many spherules found in the YDB, bulk compositions and REE abundances are unique and inconsistent with the compositions of micrometeorites and volcanic or anthropogenic spherules. YDB magnetic spherules range from ~1 to 500 μm in diameter, averaging about 60 μm, and typically appear as highly reflective, black spheroids. Although we refer to them as “spherules,” shapes such as ovals, doublets, dumbbells, and tear-drops occur frequently. SEM observation of outer surfaces of all magnetic spherules analyzed reveals a surficial crystalline pattern that is typically dendritic or polygonal like a soccer-ball. These are indicative of melting with rapid quenching (Petaev, 2004), which precludes diagenetic, biogenic, or detrital origin.
Within an impact plume, melt droplets, rock particles, dust, and partially melted debris collide with a wide range of velocities. High-speed collisions can be destructive, resulting in annihilation or surface scarring that leaves small craters (Prasad and Kledehar, 2003) or constructive, whereby partially molten and plastic spherules grow by accretion of smaller melt droplets (Kyte, 2010).
Microtektites commonly show the effects of high velocity interparticle collisions (Prasad and Kledehar, 2003; Prasad et al., 2010), which resulted in microcraters that show brittle fracturing and lower velocity craters that have less discernible cratering features, but have a higher population of oblique impacts that formed elongated craters and very low impact “furrows” (Fig. above). Other moderately high velocity impacts occur when spherules collide with partial penetration and partial melting (Fig. below).
With very low velocities, collisions became accretionary and range in characteristics from disrupted projectiles with outward splatter to partial burial in and flattening of projectiles on the accreting host. The least energetic accretions are exemplified by gentle welding together of tacky projectiles and/or hosts. Accretion impacts are the most common collisions observed in 36 Meteor Crater glassy impactites and 246 YDB spherules, microtektites, and impactites, examples are given in Figures below.
In addition to spherule accretions, another type is in the form of irregular melt drapings or splatter (Mirsa et al., 2009) (Fig. below).
Melt drapings are also common on Meteor Crater and YDB spherules and impactites that are also illustrated in the slideshow Figures. (Low velocity (<500 m sec-1) micro-impact craters are found on the surface of YDB glassy spherules, as well as evidence for penetrating collisions among these objects. Such features are known only among microtektites and impact-melt spherules (Prasad and Kledehar, 2002).
Some glassy objects from the YDB are aerodynamically shaped into teardrops and dumbbells (among other shapes), morphologies not found in micrometeorite collections. These shapes are common to microtektites and macrotektites from the Southeast Asian tektite strewnfields and to melt materials from the Trinity nuclear detonation.
Melting SiO2 produces flow-textured lechatelierite grains, in addition to SiO2 spherules, which are found in the YDB. These grains are unique only to impact and fulgurite production due to the formational need of very high temperature flash heating (2200 to 5000º K).
Perhaps the most convincing evidence for producing high temperature melt glasses from an aerial burst is from the detonation of the world’s first atomic bomb at the Alamogordo Bombing Range, NM in 1945. The device was detonated atop a 100-foot tower and melted about 1 to 3 cm of the desert soil for a radius of 1000 feet. The blast site, since called Trinity, was littered with green glass fragments, small glass beads, teardrop and dumbbell shaped “microtektite”-like glasses that were ejected miles from the site. These glasses, called trinitite (Fig. above), are similar to the Australasian tektites and YDB glassy objects in shapes and rapid melting/cooling rates. The very high temperatures of many thousands of degrees and very short melting time of 3 seconds for the Trinity explosion are very similar to the conditions postulated for aerial burst flash melting of tektites and YDB glasses (Figure below).
The above characteristics are individually and collectively unique to impact pressures and/or very high temperatures and extremely rapid quenching rates. They cannot be products of forest fires or micrometeorite origins. The existence of a crater appears unnecessary to form the YDB impact proxies based on the comparison to similar proxies found associated with the widely accepted aerial burst at Tunguska (1017 J energy), the proposed, larger aerial burst(s) responsible for the Australian tektite strewn field (1021 J energy) (Wasson, 2003), formation of Libyan Desert glass, and the known Trinity nuclear airburst. We propose a potential solution to the YD impact mechanism. Napier (2010) suggested that the Earth encountered a dense trail of material from a large disintegrating comet at 12.9 ka, giving rise to catastrophic cluster impacts. Napier provided compelling evidence that such a comet entered the inner planetary system between 20,000 and 30,000 years ago.
(Source: “New Physical Evidence for a Cosmic Impact with the Earth at 12.9 ka”, by T. E. Bunch et al., 2010)
We report the discovery in Lake Cuitzeo in central Mexico of a black, carbon-rich, lacustrine layer, containing nanodiamonds, microspherules, and other unusual materials that date to the early Younger Dryas and are interpreted to result from an extraterrestrial impact. These proxies were found in a 27-m-long core as part of an interdisciplinary effort to extract a paleoclimate record back through the previous interglacial. Our attention focused early on an anomalous, 10-cm-thick, carbon-rich layer at a depth of 2.8 m that dates to 12.9 ka and coincides with a suite of anomalous coeval environmental and biotic changes independently recognized in other regional lake sequences. Collectively, these changes have produced the most distinctive boundary layer in the late Quaternary record. This layer contains a diverse, abundant assemblage of impact-related markers, including nanodiamonds, carbon spherules, and magnetic spherules with rapid melting/quenching textures, all reaching synchronous peaks immediately beneath a layer containing the largest peak of charcoal in the core. Analyses by multiple methods demonstrate the presence of three allotropes of nanodiamond: n-diamond, i-carbon, and hexagonal nanodiamond (lonsdaleite), in order of estimated relative abundance. This nanodiamond-rich layer is consistent with the Younger Dryas boundary layer found at numerous sites across North America, Greenland, and Western Europe. We have examined multiple hypotheses to account for these observations and find the evidence cannot be explained by any known terrestrial mechanism. It is, however, consistent with the Younger Dryas boundary impact hypothesis postulating a major extraterrestrial impact involving multiple airburst(s) and and/or ground impact(s) at 12.9 ka.
SEM images of magnetic impact spherules
Synchronous peaks in multiple YDB markers dating to 12.9 ka were previously found at numerous sites across North and South America and in Western Europe. At Lake Cuitzeo, magnetic impact spherules, CSps, and NDs form abundance peaks within a 10 cm layer of sediment that dates to the early part of the YD, beginning at 12.9 ka. These peaks coincide with anomalous environmental, geochemical, and biotic changes evident at Lake Cuitzeo and in other regional records, consistent with the occurrence of an unusual event. Analyses of YDB acid-resistant extracts using STEM, EDS, HRTEM, SAD, FFT, EELS, and EFTEM indicate that Lake Cuitzeo nanoparticles are dominantly crystalline carbon and display d-spacings that match various ND allotropes, including lonsdaleite. These results are consistent with reports of abundant NDs in the YDB in North America and Western Europe.
Although the origin of these YDB markers remains speculative, any viable hypothesis must account for coeval abundance peaks in NDs, magnetic impact spherules, CSps, and charcoal in Lake Cuitzeo, along with apparently synchronous peaks at other sites, spanning a wide area of Earth’s surface. Multiple hypotheses have been proposed to explain these YDB peaks in markers, and all but one can be rejected. For example, the magnetic impact spherules and NDs cannot result from the influx of cosmic material or from any known regular terrestrial mechanism, including wildfires, volcanism, anthropogenesis, or alternatively, misidentification of proxies. Currently, only one known event, a cosmic impact, can explain the diverse, widely distributed assemblage of proxies. In the entire geologic record, there are only two known continent-wide layers with abundance peaks in NDs, impact spherules, CSps, and aciniform soot, and those are the KPg impact boundary at 65 Ma and the YDB boundary at 12.9 ka.
(Source: “Evidence from central Mexico supporting the Younger Dryas extraterrestrial impact hypothesis”, by Isabel Israde-Alcántara et al., 2011)
It has been proposed that fragments of an asteroid or comet impacted Earth, deposited silica-and iron-rich microspherules and other proxies across several continents, and triggered the Younger Dryas cooling episode 12,900 years ago. Although many independent groups have confirmed the impact evidence, the hypothesis remains controversial because some groups have failed to do so. We examined sediment sequences from 18 dated Younger Dryas boundary (YDB) sites across three continents (North America, Europe, and Asia), spanning 12,000 km around nearly one-third of the planet. All sites display abundant microspherules in the YDB with none or few above and below. In addition, three sites (Abu Hureyra, Syria; Melrose, Pennsylvania; and Blackville, South Carolina) display vesicular, high-temperature, siliceous scoria-like objects, or SLOs, that match the spherules geochemically. We compared YDB objects with melt products from a known cosmic impact (Meteor Crater, Arizona) and from the 1945 Trinity nuclear airburst in Socorro, New Mexico, and found that all of these high-energy events produced material that is geochemically and morphologically comparable, including: (i) high-temperature, rapidly quenched microspherules and SLOs; (ii) corundum, mullite, and suessite (Fe3Si), a rare meteoritic mineral that forms under high temperatures; (iii) melted SiO2 glass, or lechatelierite, with flow textures (or schlieren) that form at > 2,200 °C; and (iv) particles with features indicative of high-energy interparticle collisions. These results are inconsistent with anthropogenic, volcanic, authigenic, and cosmic materials, yet consistent with cosmic ejecta, supporting the hypothesis of extraterrestrial airbursts/impacts 12,900 years ago. The wide geographic distribution of SLOs is consistent with multiple impactors.
Abundance peaks in SLOs were observed in the YDB layer at three dated sites at the onset of the YD cooling episode (12.9 ka). Two are in North America and one is in the Middle East, extending the existence of YDB proxies into Asia. SLO peaks are coincident with peaks in glassy and Fe-rich spherules and are coeval with YDB spherule peaks at 15 other sites across three continents. In addition, independent researchers working at one well-dated site in North America and one in South America have reported YDB melt glass that is similar to these SLOs. YDB objects have now been observed in a total of eight countries on four continents separated by up to 12,000 km with no known limit in extent. The following lines of evidence support a cosmic impact origin for these materials.
Our research demonstrates that YDB spherules and SLOs have compositions similar to known high-temperature, impact-produced material, including tektites and ejecta. In addition, YDB objects are indistinguishable from high-temperature melt products formed in the Trinity atomic explosion. Furthermore, bulk compositions of YDB objects are inconsistent with known cosmic, anthropogenic, authigenic, and volcanic materials, whereas they are consistent with intense heating, mixing, and quenching of local terrestrial materials (mud, silt, clay, shale).
Dendritic texturing of Fe-rich spherules and some SLOs resulted from rapid quenching of molten material. Requisite temperatures eliminate terrestrial explanations for the 12.9-kyr-old material (e.g., framboids and detrital magnetite), which show no evidence of melting. The age, geochemistry, and morphology of SLOs are similar across two continents, consistent with the hypothesis that the SLOs formed during a cosmic impact event involving multiple impactors across a wide area of the Earth.
Lechatelierite and Schlieren
Melting of SLOs, some of which are > 80% SiO2 with pure SiO2 inclusions, requires temperatures from 1,700–2,200 °C to produce the distinctive flow-melt bands. These features are only consistent with a cosmic impact event and preclude all known terrestrial processes, including volcanism, bacterial activity, authigenesis, contact metamorphism, wildfires, and coal seam fires. Depths of burial to 14 m eliminate modern anthropogenic activities as potential sources, and the extremely high melting temperatures of up to 2,200 °C preclude anthropogenic activities (e.g., pottery-making, glass-making, and metal-smelting) by the contemporary cultures.
The YDB objects display evidence of microcratering and destructive collisions, which, because of the high initial and differential velocities required, form only during cosmic impact events and nuclear explosions. Such features do not result from anthropogenesis or volcanism.
Light photomicrographs of YDB objects. (Upper) SLOs and (Lower) magnetic spherules. A = Abu Hureyra, B = Blackville, M = Melrose.
Our observations indicate that YDB objects are similar to material produced in nuclear airbursts, impact crater plumes, and cosmic airbursts, and strongly support the hypothesis of multiple cosmic airburst/impacts at 12.9 ka. Data presented here require that thermal radiation from air shocks was sufficient to melt surface sediments at temperatures up to or greater than the boiling point of quartz (2,200 °C). For impacting cosmic fragments, larger melt masses tend to be produced by impactors with greater mass, velocity, and/or closeness to the surface. Of the 18 investigated sites, only Abu Hureyra, Blackville, and Melrose display large melt masses of SLOs, and this observation suggests that each of these sites was near the center of a high-energy airburst/impact. Because these three sites in North America and the Middle East are separated by 1,000–10,000 km, we propose that there were three or more major impact/airburst epicenters for the YDB impact event. If so, the much higher concentration of SLOs at Abu Hureyra suggests that the effects on that settlement and its inhabitants would have been severe.
(Source: “Very high-temperature impact melt products as evidence for cosmic airbursts and impacts 12,900 years ago”, by Ted E. Bunch et al., 2102)
Airbursts/impacts by a fragmented comet or asteroid have been proposed at the Younger Dryas onset (12.80 ± 0.15 ka) based on identification of an assemblage of impact-related proxies, including microspherules, nanodiamonds, and iridium. Distributed across four continents at the Younger Dryas boundary (YDB), spherule peaks have been independently confirmed in eight studies, but unconfirmed in two others, resulting in continued dispute about their occurrence, distribution, and origin. To further address this dispute and better identify YDB spherules, we present results from one of the largest spherule investigations ever undertaken regarding spherule geochemistry, morphologies, origins, and processes of formation. We investigated 18 sites across North America, Europe, and the Middle East, performing nearly 700 analyses on spherules using energy dispersive X-ray spectroscopy for geochemical analyses and scanning electron microscopy for surface microstructural characterization. Twelve locations rank among the world’s premier end-Pleistocene archaeological sites, where the YDB marks a hiatus in human occupation or major changes in site use. Our results are consistent with melting of sediments to temperatures >2,200 °C by the thermal radiation and air shocks produced by passage of an extraterrestrial object through the atmosphere; they are inconsistent with volcanic, cosmic, anthropogenic, lightning, or authigenic sources. We also produced spherules from wood in the laboratory at >1,730 °C, indicating that impact-related incineration of biomass may have contributed to spherule production. At 12.8 ka, an estimated 10 million tonnes of spherules were distributed across ∼50 million square kilometers, similar to well-known impact strewnfields and consistent with a major cosmic impact event.
YDB spherules from 18 sites. SEM images illustrate the wide variety of sizes, shapes, and microstructures of YDB spherules. Diameters are in yellow.
An increasing body of evidence suggests that major cosmic airbursts/impacts with Earth occurred at the onset of the Younger Dryas (YD) episode, triggering abrupt cooling and causing major environmental perturbations that contributed to megafaunal extinctions and human cultural changes. (Note that “airburst/impact” is used to refer to a collision by a cosmic body with Earth’s atmosphere, producing an extremely high-energy aerial disintegration that may be accompanied by numerous small crater-forming impacts by the fragments.) The impact hypothesis originated from observations of peaks in Fe-rich and Al-Si–rich impact spherules, nanodiamonds, and other unusual impact tracers discovered in the Younger Dryas boundary layer (YDB), a sedimentary stratum typically only a few centimeters thick. The hypothesis was first proposed by Firestone et al. and expanded upon by Kennett et al., Kurbatov et al., Anderson et al., Israde et al., Bunch et al., and Jones and Kennett. Formerly, the date of the impact event was reported as 10.9 ± 0.145 ka (radiocarbon), calibrated as 12.9 ± 0.10 ka B.P., using the then-standard calibration curve IntCal04. (Unless otherwise noted, all dates are presented as calibrated or calendar kiloannum.) Using the most recent curve, IntCal09, the same radiocarbon date calibrates as 12.8 ± 0.15 ka.
YDB impact field, based on data from 27 locations. In the YDB strewnfield (red), there are 18 YDB sites in this study (red dots; see table on Right). Eight independent studies have found spherules and/or scoria-like objects at nine additional sites (blue dots) located in Arizona, Montana, New Mexico, Maryland, South Carolina, Pennsylvania, Mexico, and Venezuela. The largest accepted impact strewnfield, the Australasian (purple), is shown for comparison with each strewnfield covering ∼50 million square kilometers or ∼10% of the planet. Table shows location of sites and lists site details (A, archeological material; B, black mat; C, charcoal; M, megafaunal remains, present either at the sampling location or in the vicinity). Also given are stratigraphic settings (Strat: A, alluvial; C, colluvial; E, eolian; G, glacial; and L, lacustrine) and relative physical stability of depositional paleoenvironments (Env: A, active, e.g., riverine, lacustrine, or eolian; I, inactive).
The analyses of 771 YDB objects presented in this paper strongly support a major cosmic impact at 12.8 ka. This conclusion is substantiated by the following:
Spherules and SLOs are (i) widespread at 18 sites on four continents; (ii) display large abundance peaks only at the YD onset at ∼12.8 ka; (iii) are rarely found above or below the YDB, indicating a single rare event; and (iv) amount to an estimated 10 million tonnes of materials distributed across ∼50 million square kilometers of several continents, thus precluding a small localized impact event.
Spherule formation by volcanism, anthropogenesis, authigenesis, and meteoritic ablation can be rejected on geochemical, morphological, and/or thermochemical grounds, including the presence of lechatelierite (>2,200 °C).
Spherule formation by lightning can be eliminated due to magnetic properties of spherules and the paucity of lightning melt-products (e.g., fulgurites) above or below the YDB.
Morphologies and compositions of YDB spherules are consistent with an impact event because they (i) are compositionally and morphologically similar to previously studied impact materials; (ii) closely resemble terrestrial rock compositions (e.g., clay, mud, and metamorphic rocks); (iii) often display high-temperature surface texturing; (iv) exhibit schlieren and SiO2 inclusions (lechatelierite at >2,200 °C); (v) are often fused to other spherules by collisions at high-temperatures; and (vi) occasionally display high-velocity impact cratering.
High-temperature incineration of biomass (>1,730 °C) produced laboratory spherules that are similar to YDB spherules, providing a complementary explanation that some unknown percentage of YDB spherules may have formed that way.
Abundances of spherules covary with other YDB impact proxies, including nanodiamonds, high-temperature melt-glass, carbon spherules, aciniform carbon, fullerenes, charcoal, glass-like carbon, and iridium.
The geographical extent of the YD impact is limited by the range of sites available for study to date and is presumably much larger, because we have found consistent, supporting evidence over an increasingly wide area. The nature of the impactor remains unclear, although we suggest that the most likely hypothesis is that of multiple airbursts/impacts by a large comet or asteroid that fragmented in solar orbit, as is common for nearly all comets. The YD impact at 12.8 ka is coincidental with major environmental events, including abrupt cooling at the YD onset, major extinction of some end-Pleistocene megafauna, disappearance of Clovis cultural traditions, widespread biomass burning, and often, the deposition of dark, carbon-rich sediments (black mat). It is reasonable to hypothesize a relationship between these events and the YDB impact, although much work remains to understand the causal mechanisms.
(Source: “Evidence for deposition of 10 million tonnes of impact spherules across four continents 12,800 y ago”, by James H. Wittke et al., 2013)
A major cosmic-impact event has been proposed at the onset of the Younger Dryas (YD) cooling episode at ≈12,800±150 years before present, forming the YD Boundary (YDB) layer, distributed over 150 million km2 on four continents. In 24 dated stratigraphic sections in 10 countries of the Northern Hemisphere, the YDB layer contains a clearly defined abundance peak in nanodiamonds (NDs), a major cosmic-impact proxy. Observed ND polytypes include cubic diamonds, lonsdaleite-like crystals, and diamond-like carbon nanoparticles, called n-diamond and i-carbon. The ND abundances in bulk YDB sediments ranged up to ≈500 ppb (mean: 200 ppb) and that in carbon spherules up to ≈3700 ppb (mean: ≈750 ppb); 138 of 205 sediment samples (67%) contained no detectable NDs. Isotopic evidence indicates that YDB NDs were produced from terrestrial carbon, as with other impact diamonds, and were not derived from the impactor itself. The YDB layer is also marked by abundance peaks in other impact-related proxies, including cosmic-impact spherules, carbon spherules (some containing NDs), iridium, osmium, platinum, charcoal, aciniform carbon (soot), and high-temperature melt-glass. This contribution reviews the debate about the presence, abundance, and origin of the concentration peak in YDB NDs.We describe an updated protocol for the extraction and concentration of NDs from sediment, carbon spherules, and ice, and we describe the basis for identification and classification of YDB ND polytypes, using nine analytical approaches. The large body of evidence now obtained about YDB NDs is strongly consistent with an origin by cosmic impact at ≈12,800 cal BP and is inconsistent with formation of YDB NDs by natural terrestrial processes, including wildfires, anthropogenesis, and/or influx of cosmic dust.
The Map Map showing 24 sites containing Younger Dryas Boundary (YDB) nanodiamonds. The solid line defines the current known limits of the YDB field of cosmic-impact proxies, spanning 50 million km2 (Wittke et al. 2013), including the study of Mahaney et al. (2010) in Venezuela (open circle). Numbered sites are from this study: (1) Lake Cuitzeo, Mexico (Israde-Alcántara et al. 2012b); (2) Daisy Cave, California; (3) Arlington Canyon, California (Kennett et al. 2009b); (4) Murray Springs, Arizona (Kennett et al. 2009a); (5) Lindenmeier, Colorado; (6) Bull Creek, Oklahoma (Kennett et al. 2009a); (7) Blackville, South Carolina; (8) Topper, South Carolina (Kennett et al. 2009a); (9) Kimbel Bay, North Carolina; (10) Newtonville, New Jersey; (11) Melrose, Pennsylvania; (12) Sheriden Cave, Ohio; (13) Gainey, Michigan (Kennett et al. 2009a); (14) Chobot site, Alberta, Canada (Kennett et al. 2009a); (15) Lake Hind, Manitoba, Canada (Kennett et al. 2009a); (16) Kangerlussuaq, Greenland (Kurbatov et al. 2010); (17) Watcombe Bottom, Isle of Wight, United Kingdom; (18) Lommel, Belgium; (19) Ommen, Belgium; (20) Lingen, Germany; (21) Santa Maira, Spain; (22) Abu Hureyra, Syria. In addition, independent researchers have reported NDs at six sites, indicated by letters, four of which are in common: (a) Indian Creek, Montana (Baker et al. 2008); (b) Bull Creek, Oklahoma (Madden et al. 2012; Bement et al. 2014); (c) Sheriden Cave, Ohio (Redmond and Tankersley 2011); (d) Newtonville, New Jersey (Demitroff et al. 2009); (e) Lommel, Belgium (Tian et al. 2011); (f) Aalsterhut, Netherlands (van Hoesel et al. 2012).
The Younger Dryas (YD) impact hypothesis proposes that a major cosmic-impact event occurred at the Younger Dryas Boundary (YDB) 10,900±145 radiocarbon years before present (RCYBP), a time corresponding to the onset of the YD cooling recorded in Greenland Ice Sheet cores and other sequences (Firestone et al. 2007). The published IntCal radiocarbon curve has recently been revised (Reimer et al. 2013) and provides a calibrated age for this radiocarbon date of ≈12,830±130 cal BP at 1 standard deviation (σ). This differs from earlier calibrated ages for the YDB of 12,900±100 cal BP, used by Firestone et al. (2007), and 12,800±150 cal BP, more recently used by Wittke et al. (2013). Because this latest adjustment represents a difference of only ≈30 yr, we continue to use an age of 12,800±150 cal BP for the YDB. We emphasize that, although the calendar calibration has changed, the radiocarbon age has remained the same.
The proposed impact deposited the YDB layer, which contains many cosmic-impact proxies, including magnetic and glassy impact spherules, iridium, fullerenes, carbon spherules, glass-like carbon, charcoal, and aciniform carbon, a form of soot (Firestone et al. 2007; Wittke et al. 2013). In North America and the Middle East, Bunch et al. (2012) identified YDB melt-glass that formed at high temperatures (1730# to 12200#C), as also reported by three independent groups, Mahaney et al. (2010) in South America and Fayek et al. (2012) and Wu et al. (2013) in North America. This study focuses solely on nanodiamonds (NDs), and so, for independent discussions of other proxies, see Haynes et al. (2010) and Paquay et al. (2009), who found no evidence for the platinum-group elements iridium or osmium. Alternately, Wu et al. (2013) found large YDB anomalies in osmium.
Also, in a Greenland ice core, Petaev et al. (2013) found a large YDB abundance peak in the platinum-group element platinum. Surovell et al. (2009) found no YDB peaks in magnetic spherules, whereas LeCompte et al. (2012) found large, well defined YDB spherule peaks at sites common to the study by Surovell et al. Also, critical overviews of the YDB hypothesis are presented in Pinter et al. (2011) and Boslough et al. (2012).
Recently, the YDB cosmic impact was independently confirmed by Petaev et al. (2013), who reported compelling evidence from a well-dated Greenland Ice Core Project (GISP2) ice core exhibiting a sharp abundance peak in platinum precisely at the YD onset (12,877±3.4 cal BP). Those authors’ mass-balance calculations indicate that the platinum peak resulted from a major cosmic impact event by an impactor estimated to be at
least 1 km in diameter. Similarly, Wittke et al. (2013) estimated that the tonnage of YDB ejecta (spherules and melt-glass) is comparable to that ejected from the 10.5-km-wide Bosumtwi Crater, likely produced by a 1-km-wide impactor. The GISP2 platinum peak is coeval with the abrupt onset (≈1.5 yr) of the atmospheric changes that mark the YD climatic episode in the North Greenland Ice Core Project (NGRIP) ice core at 12,896 cal BP (Steffensen et al. 2008). The discovery of such an unequivocal impact proxy at the YD onset in the Greenland record was predicted by the YDB impact hypothesis when it was initially introduced (Firestone et al. 2007).
The comprehensive impact proxy assemblage in the YDB layer also includes NDs and diamond-like carbon, which were discovered within carbon spherules, glass-like carbon, and bulk sediment. The polymorphs of carbon extracted from bulk sediment and carbon spherules include cubic NDs and hexagonal lonsdaleite-like crystals as well as unique carbon allotropes, called n-diamonds and icarbon.
These latter two types of nanocrystals, almost as hard as cubic NDs, are frequently used in thin, polycrystalline films for industrial applications requiring hardness and abrasion resistance (Wen et al. 2007). Ongoing investigations have been examining whether these polymorphs are simply cubic diamonds with atomic substitution of carbon by hydrogen or other elements (Wen et al. 2011) or are new forms of diamond-like carbon (Hu et al. 2012). Regardless, the nanoparticles in question form under exotic temperatures and pressures not present naturally at the Earth’s surface or lower atmosphere but similar to conditions related to cosmic impact (Wen et al. 2007) and are unlike other forms of carbon typically found naturally on Earth. For simplicity, we refer to all forms as NDs, even though n-diamonds and i-carbon may actually be only diamond-like. YDB NDs were most likely formed from terrestrial carbon, based on their carbon isotopic composition (Tian et al. 2011; Israde-Alcántara et al. 2012b), and are similar to NDs formed during the cosmic impact at the Cretaceous-Paleogene boundary (K-Pg, formerly referred to as the K-T; Gilmour et al. 1992).
The YDB carbon spherules that contain NDs are morphologically and compositionally similar to younger carbon spherules first reported in near-surface forest soils of Europe by Rösler et al. (2005), who first suggested an impact-related origin of the particles. Later, some of the same authors (Yang et al. 2008) stated, “Whether this would have occurred during or before any impact is still unclear for now”. Carbon spherules have been proven to form in cosmic-impact events, as shown by the discovery of a <1100-yr-old meteorite crater in Alberta, Canada (Newman and Herd 2013). Some carbon spherules are fused to fragments of the meteorite, indicating that they formed upon impact. Their morphology includes an exterior shell around a highly vesicular interior, identical to YDB spherules and carbon spherules found in Europe. Carbon spherules have also been reported from experiments using hypervelocity impacts into carbonrich substrates, duplicating cosmic-impact conditions (Heymann et al. 2006). Furthermore, carbon spherules containing NDs have been demonstrated to form from tree sap under laboratory conditions that duplicate the temperature, pressure, and redox values within an impact fireball (Israde-Alcántara et al. 2012b).
Following the identification of NDs by Kennett et al. (2009a, 2009b), Daulton et al. (2010) attempted to replicate that discovery at two well-known archaeological sites, Murray Springs, Arizona, and Arlington Canyon, California. Daulton et al. (2010) found no YDB NDs and concluded that their findings cast doubt on the presence of YDB NDs, although they pointed out that YDB NDs might “occur inhomogeneously and only in some of the YD-boundary carbons and hence are not observed in our study”. Daulton et al. (2010) also noted that other minerals, including nanocrystalline copper and copper oxide, could be misidentified as several of the proposed diamond polytypes, because of crystallographic similarities between copper and diamond.
Later, an independent YDB study by Tian et al. (2011) confirmed the discovery of cubic YDB NDs at Lommel, Belgium, in the charcoal-rich YDB layer in the upper part of a layer that is known regionally as the Usselo Horizon. The intersection between the Usselo layer and regional overlying cover sands has been long recognized as representing the onset of the YD climate change (Van Geel et al. 1989). At Lommel, cubic NDs were embedded in carbon particles but with no otherNDpolytypes, and no NDs were observed above or below the YDB layer. As with previous studies, the authors did not examine bulk sediment for NDs. Tian et al. (2011) concluded that the NDs alone did not represent indisputable evidence for a cosmic impact, but they did not exclude one.
Israde-Alcántara et al. (2012b) used multiple analytical techniques to demonstrate that the YDB NDs from Lake Cuitzeo, Mexico, are cubic NDs, n-diamonds, i-carbon, and lonsdaleite-like crystals. Israde-Alcántara et al. (2012b) also identified several problems and limitations of the study by Daulton et al. (2010), who reported an absence of YDB NDs in carbon spherules at Murray Springs and Arlington Canyon. First, Daulton et al. (2010) searched for and failed to find NDs within carbon spherules at Murray Springs, but neither Firestone et al. (2007) nor Kennett et al. (2009a) reported finding carbon spherules at that site, making the related absence of NDs unsurprising. Our investigations showed that carbon spherules are most common in regions having conifer trees at 12,800 cal BP, not in scrubby grasslands, as existed at Murray Springs at that time (Haynes and Huckell 2007). Second, at both sites Daulton et al. (2010) searched for NDs in charcoal, which has never been reported by any workers to contain NDs. Third, Daulton et al. (2010) did not examine bulk sediment, the only source of NDs at Murray Springs reported by Kennett et al. (2009a).
Kennett et al. (2009b) reported NDs in carbon spherules at Arlington Canyon, California; Daulton et al. (2010) found no NDs there either, but there was a major flaw in their sample acquisition. The same coauthors of Daulton et al. (2010) claimed, in Pinter et al. (2011), to have acquired their samples from a location “identical or closely proximal to the location” examined by Kennett et al. (2009a). Contradicting that statement, Wittke et al. (2013) noted that the Universal Transverse Mercator coordinates of their sampling sites show conclusively that their purported continuous sequence was actually collected as four separate discontinuous sections, separated by up to 7000 m horizontally from the sampling location of Kennett et al. (2009a, 2009b). Therefore, Scott et al. (2010) did not sample the YDB at the location studied by Kennett et al. (2009a) and did not acquire a dated, continuous profile across the YDB at any Arlington Canyon location. These mislocated sediment samples collected by Scott et al. (2010) were subsequently used in several different studies by the same group of authors (Daulton et al. 2010; Scott et al. 2010; Pinter et al. 2011). Their incorrect stratigraphic locations apply to all those investigations, explaining their inability to detect YDB NDs, cosmic-impact spherules, and ND-rich carbon spherules at Arlington Canyon.
Daulton (2012) also questioned the identification of lonsdaleite (hexagonal diamond), suggesting that some particles exhibited in Kennett et al. (2009b) appear to be graphene-graphane aggregates. Van Hoesel et al. (2012), Madden et al. (2012), and Bement et al. (2014) also reported finding graphene-graphane clusters with diffraction patterns similar
to those of lonsdaleite. Boslough et al. (2012) suggested that some of the reported lonsdaleite from Lake Cuitzeo might instead be other minerals. We discuss these points below in “Identification of Lonsdaleite-Like Crystals.” Daulton (2012) and Boslough et al. (2012) questioned whether YDB NDs are robust cosmic-impact markers. However, cubic NDs are widely accepted to have formed during the K-Pg impact event and were not found in sediment before or after the event (Carlisle and Braman 1991; Gilmour et al. 1992; Hough et al. 1997, 1999). Those NDs are found at six coeval sites across North America: two in Colorado and one each in Mexico, Montana, and Alberta, Canada. The K-Pg NDs were reported to range in size from 1 nm to 30 mm, whereas YDB NDs are smaller, spanning a narrower range of ≈1 to ≈2.9 mm, perhaps because that older impact was larger and more energetic than the YDB event. Van Hoesel et al. (2012) observed cubic NDs within particles of glass-like carbon at the Geldrop-Aalsterhut site in the Netherlands. The NDs were found in a few-centimeter-thick, charcoal-rich interval at the upper boundary of the Usselo layer, the top of which is widely accepted as representing the onset of the YD cooling episode (Van Geel et al. 1989). They reported NDs only in glass-like carbon in the bottom 1 cm of that interval and did not examine bulk sediment for the presence of NDs.
Recently, Bement et al. (2014) discovered an abundance peak in YDB n-diamonds (190 ppm) at Bull Creek, Oklahoma, independently confirming the discovery there of YDB NDs (100 ppb) by Kennett et al. (2009a). They did not observe cubic NDs, as Kennett et al. (2009a) did, and neither group observed lonsdaleite at Bull Creek. In addition, Bement et al. (2014) observed an ND abundance peak of similar amplitude to their YDB peak in two contiguous samples of late Holocene surface sediments (0–10 and 10–20 cm below surface). They suggested that this younger NDpeak may have been produced by a nearby cosmic-impact event within the past several thousand years. This discovery may correlate with that of Courty et al. (2008), who discovered melt-glass and spherules at widely distributed sites in Syria, Spain, and Peru, localities separated by up to 13,000 km, as evidence for a ≈4000-yr-old Northern Hemispheric impact event. Bement et al. (2014) concluded from sedimentological evidence that the peak ND accumulations in the YDB and younger strata did not result from changes in climate, deposition rates, lag deposits, or human site usage. Their results refute the hypothesis that the NDs simply resulted from cosmic influx that deposited them as a lag deposit at the YDB over an extended interval of time (Haynes et al. 2010; Pinter et al. 2011; Boslough et al. 2012). Instead, Bement et al. (2014) concluded the evidence is consistent only with cosmic-impact events.
In summary, abundant NDs within or near the YDB layer have been reported by four independent groups (Redmond and Tankersley 2011; Tian et al. 2011; van Hoesel et al. 2012; Bement et al. 2014). In addition, NDs have been reported independently in three conference presentations (at Indian Creek, MT, by Baker et al. 2008; at Newtonville, NJ, by Demitroff et al. 2009; and at Bull Creek, OK, by Madden et al. 2012). These investigations independently confirm the presence of an ND abundance peak in the YDB layer, which has also been shown to be associated with a diversity of other cosmic impact proxies. Research continues into the specific origin of the various YDB ND polytypes and the presence of lonsdaleite.
(Source: “Nanodiamond-Rich Layer across Three Continents Consistent with Major Cosmic Impact at 12,800 Cal BP”, by Charles R. Kinzie et al., 2014)
Research-Selection for NovoScriptorium: Isidoros Aggelos