Abstract: In summary, our observations indicate that Younger Dryas boundary impact materials (spherules and scoria-like objects) are inconsistent with formation by anthropogenesis, volcanism, biomass burning, and/or ablation of meteorites. Instead, as summarized in Table P1 , Younger Dryas boundary objects are indistinguishable from spherules and melt glass produced in nuclear airbursts, impact crater plumes, and extraterrestrial airbursts. The formation of these materials strongly supports the hypothesis of multiple cosmic airbursts/impacts at 12.9 ka. Table P1. Comparison of high-temperature proxies from various sources YDB, Trinity detonation, Meteor Crater, Australasian tektite field, and Tunguska. The range of proxies reported is nearly identical for all events. “n/a” = not tested * Rich in Ni-Fe from impactor. Rich in Fe from target. We compared the Younger Dryas boundary scoria-like objects with melt products from Meteor Crater, Trinity, and Tunguska, and demonstrate that all of these events produced melt glasses that are geochemically and morphologically comparable. Similarities include: ( i ) melted silica glass (called lechatelierite) with distinctive flow textures (called schlieren) that formed at greater than 2,000 °C, and ( ii ) features indicative of high-energy interparticle collisions, such as microcraters. We investigated known high-temperature events, including crater-forming impacts (Meteor Crater, AZ), cosmic airbursts (the Tunguska Siberia event in 1908), and the Trinity nuclear detonation of 1945 ( 4 ). All of these produced fireballs (hot plumes) that formed numerous convective cells with maximum temperatures greater than 5,000 °C, capable of melting sediment and impactor materials. Surface particles are drawn up into the plumes, and the interaction of these particles typically produces moderate- to low-velocity interparticle collisions, which can be destructive (collisions result in surface scarring and small craters) and/or constructive (partially molten objects grow by accretion of smaller molten objects). These high-energy collisions are inconsistent with any known terrestrial mechanism, such as volcanic or anthropogenic processes, because differential velocities are too low ( 5 ). We also used SEM and EDS to examine the scoria-like objects, which are composed of high-temperature fused vesicular glass that is texturally similar to volcanic vesicular glass, called scoria. Most scoria-like objects are irregularly shaped, although they are frequently composed of one or more subrounded, fused glassy objects. They are commonly larger than spherules, ranging in length from 300 μm to 5.5 mm (mean, 1.8 mm), with abundances ranging from 0.06–15.76 g/kg. At all three sites, spherules and scoria-like objects co-occur in the Younger Dryas boundary stratum dating to 12.9 ka. Analyses of the Younger Dryas boundary melt glass indicate that they are a geochemical match for the Younger Dryas boundary spherules, and bulk compositions of scoria-like objects closely match terrestrial surficial sediments (mud, silt, clay, shale). In the Younger Dryas boundary scoria-like objects, we observed melted silica glass (lechatelierite), requiring temperatures from 1,700 °C to 2,200 °C. These temperatures are consistent only with a cosmic impact event or lightning strikes and preclude all known terrestrial processes, such as volcanism, anthropogenesis, and wildfires ( 3 ). Burial depths of 2–14 m eliminate modern anthropogenic activities as potential sources, and the extremely high melting temperatures of up to 2,000 °C preclude pre-Industrial anthropogenic activities (e.g., pottery-making, glass-making, and metal-smelting) by prehistoric cultures. To determine the characteristics and potential origin of the microspherules, we examined surface structures and geochemistry of the spherules with SEM capable of energy dispersive X-ray spectroscopy (EDS). Our analyses revealed that Fe-rich and/or Si-rich magnetic spherules typically appear as highly reflective, black-to-clear spheroids, but occasionally appear as teardrop and dumbbell shapes. For all sites, spherule concentrations peak in the Younger Dryas boundary layer, ranging from 5–4,900 spherules/kg (mean, 940/kg; median, 180/kg), and vary from 10 μm (microns) to 5.5 mm in diameter (mean, 40 μm). Outer surfaces of most spherules exhibit distinctive skeletal (or dendritic) textures indicative of melting and rapid quenching ( 2 ). The textures and morphology of Younger Dryas spherules correspond to those observed in known impact events, such as the 65-million-year-old Cretaceous–Paleogene impact, the Tunguska airburst in 1908, and the 50,000-year-old Meteor Crater impact. In this study, we tested the hypothesis by Firestone et al. ( 1 ) that fragments of an asteroid or comet collided with Earth 12,900 calendar years ago (12.9 ka), depositing an exotic assemblage of impact-related markers, such as microspherules and nanodiamonds, across parts of several continents. Our goals were to search for, analyze, and determine potential origins of the proposed impact proxies from 18 dated sites, spanning 12,000 km across three continents: North America, Europe, and Asia. We discovered that the 18 sites all display a 12.9-ka Younger Dryas boundary layer containing abundance peaks of microspherules. In addition, we discovered vesicular, high-temperature melt-glass, referred to as scoria-like objects, in the Younger Dryas boundary layer at three of those sites (Abu Hureyra, Syria; Melrose, Pennsylvania; and Blackville, South Carolina). Our research indicates that these high-temperature materials support the hypothesis of a cosmic impact at 12.9 ka and are inconsistent with any other origin.

Abstract: 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.

Abstract: Multiple hypotheses have been proposed to explain the assemblage of markers identified in the YDB, and all but one can be rejected. For example, the magnetic spherules and nanodiamonds cannot result from misidentification or from cosmic influx or any known terrestrial mechanism, including wildfires, volcanism, or anthropogenic processes. Currently, only an extraterrestrial impact is capable of explaining the many types and wide distribution of evidence. In the entire geologic record, there are only two known continent-wide layers containing nanodiamonds, magnetic spherules, carbon spherules, and aciniform soot: the Cretaceous-Paleogene impact boundary (KPg) at 65 Ma and the YD boundary at 12,900 BP. Thus, the YDB evidence is uniquely consistent with a major extraterrestrial impact event 12,900 y ago. We propose the following impact model to explain the observed data. A comet or asteroid, greater than several hundred meters in diameter, entered Earth’s atmosphere at a relatively shallow angle ( 〉  5° and 〈  30°). Thermal radiation from the air shock reached Earth’s surface and thermally decomposed terrestrial carbon, silica, and iron below the flight path of the impactor. Nanodiamonds, carbon spherules, and other carbon species formed through a chemical-vapor-deposition-like process similar to TNT detonation, and simultaneously, magnetic spherules formed from melted iron and silica. Seconds later, the air shock lofted the melted materials into the upper atmosphere and distributed them across the Northern Hemisphere. We also observed carbon onions, nanometer-sized nanoparticles constructed of concentric carbon shells from 2 to 10 nm in diameter. Some carbon onions contained nanodiamonds in their cores, and these nanodiamonds could only have formed at high temperatures under oxygen-deficient conditions that are not known to exist on Earth naturally. Their presence is consistent with known conditions within a cosmic impact. Because of the controversial nature of the YD impact debate, we conducted more comprehensive analyses on YDB nanodiamonds than in previous investigations. Analyses included high-resolution transmission electron microscopy (HRTEM), scanning TEM (STEM), and EDX, all of which clearly revealed a single major peak in nanodiamonds centered across two samples at 2.8 and 2.75 m. This interval dates to the YD onset at 12,900 BP ( Fig. P1 ) and is synchronous with the peak in magnetic spherules and carbon spherules. The nanodiamonds varied in diameter from approximately 1 to 10 nm, averaging to approximately 4 nm, and were often embedded in amorphous carbon, as previously reported from Belgium ( 5 ). They reached a maximum abundance of approximately 100 ± 50 ppb at 2.8 m. We identified three of four previously reported nanodiamond variants, of which n-diamond was most abundant, with lesser amounts of i-carbon and lonsdaleite, a widely accepted impact indicator, as previously reported in North America ( 2 ). NDs were rare below the 2.9 m layer (≤ 1 ppb), whereas they were observed at low concentrations of 4 to 10 ppb above the 2.75 m layer, probably due to the geological process of reworking. Speculation that copper, graphene, and graphane had been misidentified as YDB nanodiamonds is refuted by our identification of characteristics consistent with nanodiamonds and inconsistent with those other materials. Analysis by energy-dispersive X-ray spectroscopy (EDX) demonstrates that these spherules are geochemically distinct from volcanogenic and cosmic material. Instead, Cuitzeo magnetic spherules are consistent with the composition of 〉  1,000 tektites (impact-related glass rocks) and magnetic spherules from eleven craters and large areas of tektites formed by extraterrestrial impact into terrestrial rocks. Magnetic spherules range from 25 to 100 μm in diameter (averaging 60 μm), and typically appear as highly reflective, black spheroids. They display surface textures indicative of melting with rapid quenching that precludes typical terrestrial formation processes. These spherules are conspicuous and abundant at 2.8 m (2,000/kg), where they form a sharp peak in the YDB ( Fig. P1 ). Black carbon spherules reach a significant YDB peak of approximately 680/kg at 2.75 m ( Fig. P1 ). These spherules are 20 to 260 μm in diameter (average of 90 μm), ovoid-to-round with cracked roughened surfaces, typically having a thin rind with a spongy interior containing vesicles within a smooth, homogeneous matrix. Charcoal microparticles ( 〉  125 μm) reached a major peak of 77,000 particles/kg (15× background) beginning just after the onset of YD cooling, indicating a major episode in biomass burning. Fig. P1. Images of ( A ) a nanodiamond, ( B ) magnetic spherule, and ( C ) carbon spherule. The graphs below show peak abundances in nanodiamonds (ppb), magnetic spherules (no./kg), carbon spherules (no./kg), and charcoal (no./kg) at or close to onset of the YD (2.8 m; 12,900 BP). Younger Dryas episode marked by gray band and arrows. There is an anomalous interval at 2.8 m that dates to the YD onset and displays unusually high percentages of total organic carbon (TOC)—up to 15.8%. Carbon-rich material in this interval is not the normal plant-derived organic matter dominating the rest of the 27 m core; instead, it represents major contamination of the TOC by radiocarbon-dead or very old carbon (92 wt%). This material may be analogous to the carbon-rich black mat reported across North America that dates to the YD onset ( 4 ). Its source remains enigmatic and unclear. We utilized a total of 22 accelerator mass spectrometry (AMS) 14 C dates and generated an age-depth curve that permitted identification of the Younger Dryas interval. The pollen, diatom, and algal records from Lake Cuitzeo correlate well with records from several regional lakes in Guatemala, Costa Rica, and Panama, as well as with records from the Cariaco Basin and the Greenland Ice Sheet. These records indicate that dramatic environmental, geochemical, and biotic changes occurred throughout the region at the onset of the YD episode at 12,900 BP. The unusual materials were discovered in a 27-m-long lake core from Lake Cuitzeo, the second largest lake in Mexico. Our attention focused on an anomalous, 10-cm-thick, carbon-rich layer at 2.8 m that dates to the YD onset and contains a major fraction of carbon that is radiocarbon dead (i.e., older than 50,000 y). In this layer, we discovered a diverse, abundant assemblage of markers, including nanodiamonds, carbon spherules, and magnetic spherules with rapid melting/quenching textures. The markers peak at the same time immediately beneath a layer containing the largest charcoal peak in the core. Although some workers have been unable to replicate the YDB evidence, others have confirmed it, although sometimes suggesting alternate hypotheses. In Venezuela, Mahaney et al. ( 3 ) independently identified 12,900-year-old abundance peaks in high-temperature melt-rocks, shocked quartz, carbon spherules, and a carbon-rich black mat analogue, concluding that the cause was “either an asteroid or comet event that reached far into South America.” Haynes et al. ( 4 ) observed high concentrations of magnetic spherules and iridium in the YDB at Murray Springs, Arizona, and stated that their findings are “consistent with their [Firestone et al.’s] data.” Tian et al. ( 5 ) observed abundant YDB nanodiamonds in amorphous carbon from Lommel, Belgium, concluding “our findings confirm … the existence of diamond nanoparticles also in this European YDB layer.” We report the discovery of unusual materials in a 10-cm-thick layer of sediment obtained from Lake Cuitzeo in central Mexico consistent with extraterrestrial impacts and/or atmospheric airbursts at the onset of Younger Dryas (YD), a geologically brief period of cold climatic conditions and drought, caused by the collapse of the North American Ice Sheets 12,900 y before present (BP). Firestone et al. ( 1 ) first reported major abundance peaks in magnetic spherules and carbon spherules within a thin layer (0.5 to 〈  5 cm) called the YD boundary layer (YDB), found across North America and Western Europe. This layer is commonly located beneath a broadly distributed, black, organic-rich layer, called the ‘‘black layer.’’ Subsequently, Kennett et al. ( 2 ) observed nanodiamonds in the YDB in North America; they were also reported in the Greenland Ice Sheet in a discrete layer dating approximately to the YD onset. The proposed impacts may have played a role in initiating the abrupt YD cooling at 12,900 BP, caused widespread biomass burning, and contributed to major declines in human populations and to the extinction of Late Pleistocene megafauna, such as mammoths, mastodons, and giant sloths.

Abstract: The Younger Dryas impact hypothesis posits that a cosmic impact across much of the Northern Hemisphere deposited the Younger Dryas boundary (YDB) layer, containing peak abundances in a variable assemblage of proxies, including magnetic and glassy impact-related spherules, high-temperature minerals and melt glass, nanodiamonds, carbon spherules, aciniform carbon, platinum, and osmium. Bayesian chronological modeling was applied to 354 dates from 23 stratigraphic sections in 12 countries on four continents to establish a modeled YDB age range for this event of 12,835–12,735 Cal B.P. at 95% probability. This range overlaps that of a peak in extraterrestrial platinum in the Greenland Ice Sheet and of the earliest age of the Younger Dryas climate episode in six proxy records, suggesting a causal connection between the YDB impact event and the Younger Dryas. Two statistical tests indicate that both modeled and unmodeled ages in the 30 records are consistent with synchronous deposition of the YDB layer within the limits of dating uncertainty (∼100 y). The widespread distribution of the YDB layer suggests that it may serve as a datum layer.