Previously, a large platinum (Pt) anomaly was reported in the Greenland ice sheet at the Younger Dryas boundary (YDB) (12,800 Cal B.P.). In order to evaluate its geographic extent, fire-assay and inductively coupled plasma mass spectrometry (FA and ICP-MS) elemental analyses were performed on 11 widely separated archaeological bulk sedimentary sequences. We document discovery of a distinct Pt anomaly spread widely across North America and dating to the Younger Dryas (YD) onset. The apparent synchroneity of this widespread YDB Pt anomaly is consistent with Greenland Ice Sheet Project 2 (GISP2) data that indicated atmospheric input of platinum-rich dust. We expect the Pt anomaly to serve as a widely-distributed time marker horizon (datum) for identification and correlation of the onset of the YD climatic episode at 12,800 Cal B.P. This Pt datum will facilitate the dating and correlating of archaeological, paleontological, and paleoenvironmental data between sequences, especially those with limited age control.

Sediment sequences spanning the 12,800-year-old lower Younger Dryas boundary (YDB) were investigated at three widely separated sites in eastern North America (Parsons Island, Maryland, a Newtonville sandpit in southern New Jersey, and Flamingo Bay, South Carolina). All sequences examined exhibit peak abundances in platinum (Pt), microspherules, and meltglass representing the YDB cosmic impact layer resulting from the airbursts/impacts of a fragmented comet ∼12,800 years ago. The evidence is consistent with the Younger Dryas impact hypothesis (YDIH) recorded at ∼50 other sites across North and South America, Europe, Asia, and the Greenland ice sheet. These sequences were also examined for shock-fractured quartz, based on a recent study suggesting that low-shock metamorphism may result from low-altitude bolide airbursts similar to that observed during near-surface atomic detonations. Now, for the first time in a suite of well-separated sites in North America, we report in the YDB the presence of quartz grains exhibiting shock fractures containing amorphous silica. We also find in the YDB high-temperature melted chromferide, zircon, quartz, titanomagnetite, ulvöspinel, magnetite, native iron, and PGEs with equilibrium melting points (∼1,250° to 3,053°C) that rule out anthropogenic origins for YDB microspherules. The collective evidence meets the criteria for classification as an “impact spherule datum.”

At Abu Hureyra, a well-studied archeological site in Syria, the onset boundary of the Younger Dryas climatic episode ~12,800 years ago has previously been proposed to contain evidence supporting a near-surface cosmic airburst impact that generated temperatures >2000°C. Here, we present a wide range of potential impact-related proxies representing the catastrophic effects of this cosmic impact that destroyed the village. These proxies include nanodiamonds (cubic diamonds, n-diamonds, i-carbon, and lonsdaleite-like crystals); silica-rich and iron-rich micro-spherules; and melted chromite, quartz, and zircon grains. Another proxy, meltglass, at a concentration of 1.6 wt% of bulk sediment, appears to have formed from terrestrial sediments and was found to partially coat toolmaking debitage, bones, and clay building plaster, suggesting that village life was adversely affected. Abundant meltglass fragments examined display remarkably detailed imprints of plant structures, including those of reeds. The nanodiamonds are proposed to have formed under anoxic conditions from the incineration of plant materials during high-temperature, impact-related fires, while geochemical evidence indicates that the micro-spherules formed from the melting of terrestrial sediments. Broad archeological and geochemical evidence supports the hypothesis that Abu Hureyra is the oldest known archeological site catastrophically destroyed by cosmic impact, thus revealing the potential dangers of such events.

At Abu Hureyra, a well-studied archeological site in Syria, the onset boundary of the Younger Dryas climatic episode ~12,800 years ago has previously been proposed to contain evidence supporting a near-surface cosmic airburst impact that generated temperatures >2000°C. Here, we present a wide range of potential impact-related proxies representing the catastrophic effects of this cosmic impact that destroyed the village. These proxies include nanodiamonds (cubic diamonds, n-diamonds, i-carbon, and lonsdaleite-like crystals); silica-rich and iron-rich micro-spherules; and melted chromite, quartz, and zircon grains. Another proxy, meltglass, at a concentration of 1.6 wt% of bulk sediment, appears to have formed from terrestrial sediments and was found to partially coat toolmaking debitage, bones, and clay building plaster, suggesting that village life was adversely affected. Abundant meltglass fragments examined display remarkably detailed imprints of plant structures, including those of reeds. The nanodiamonds are proposed to have formed under anoxic conditions from the incineration of plant materials during high-temperature, impact-related fires, while geochemical evidence indicates that the micro-spherules formed from the melting of terrestrial sediments. Broad archeological and geochemical evidence supports the hypothesis that Abu Hureyra is the oldest known archeological site catastrophically destroyed by cosmic impact, thus revealing the potential dangers of such events.

At Abu Hureyra, a well-studied archeological site in Syria, the onset boundary of the Younger Dryas climatic episode ~12,800 years ago has previously been proposed to contain evidence supporting a near-surface cosmic airburst impact that generated temperatures >2000°C. Here, we present a wide range of potential impact-related proxies representing the catastrophic effects of this cosmic impact that destroyed the village. These proxies include nanodiamonds (cubic diamonds,n-diamonds,i-carbon, and lonsdaleite-like crystals); silica-rich and iron-rich micro-spherules; and melted chromite, quartz, and zircon grains. Another proxy, meltglass, at a concentration of 1.6 wt% of bulk sediment, appears to have formed from terrestrial sediments and was found to partially coat toolmaking debitage, bones, and clay building plaster, suggesting that village life was adversely affected. Abundant meltglass fragments examined display remarkably detailed imprints of plant structures, including those of reeds. The nanodiamonds are proposed to have formed under anoxic conditions from the incineration of plant materials during high-temperature, impact-related fires, while geochemical evidence indicates that the micro-spherules formed from the melting of terrestrial sediments. Broad archeological and geochemical evidence supports the hypothesis that Abu Hureyra is the oldest known archeological site catastrophically destroyed by cosmic impact, thus revealing the potential dangers of such events.

Many studies of hypervelocity impact craters have described the characteristics of quartz grains shock-metamorphosed at high pressures of >10 GPa. In contrast, few studies have investigated shock metamorphism at lower shock pressures. In this study, we test the hypothesis that low-pressure shock metamorphism occurs in near-surface nuclear airbursts and that this process shares essential characteristics with crater-forming impact events. To investigate low-grade shock microstructures, we compared quartz grains from Meteor Crater, a 1.2-km-wide impact crater, to those from near-surface nuclear airbursts at the Alamogordo Bombing Range, New Mexico in 1945 and Kazakhstan in 1949/1953. This investigation utilized a comprehensive analytical suite of high-resolution techniques, including transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). Meteor Crater and the nuclear test sites all exhibit quartz grains with closely spaced, sub-micron-wide fractures that appear to have formed at low shock pressures. Significantly, these micro-fractures are closely associated with Dauphiné twins and are filled with amorphous silica (glass), widely considered a classic indicator of shock metamorphism. Thus, this study confirms that glass-filled shock fractures in quartz form during near-surface nuclear airbursts, as well as crater-forming impact events, and by extension, it suggests that they may form in any near-surface cosmic airbursts in which the shockwave is coupled to Earth’s surface, as has been proposed. The robust characterization of such events is crucial because of their potential catastrophic effects on the Earth’s environmental and biotic systems.

We present evidence that in ~ 1650 BCE (~ 3600 years ago), a cosmic airburst destroyed Tall el-Hammam, a Middle-Bronze-Age city in the southern Jordan Valley northeast of the Dead Sea. The proposed airburst was larger than the 1908 explosion over Tunguska, Russia, where a ~ 50-m-wide bolide detonated with ~ 1000× more energy than the Hiroshima atomic bomb. A city-wide ~ 1.5-m-thick carbon-and-ash-rich destruction layer contains peak concentrations of shocked quartz (~ 5–10 GPa); melted pottery and mudbricks; diamond-like carbon; soot; Fe- and Si-rich spherules; CaCO3 spherules from melted plaster; and melted platinum, iridium, nickel, gold, silver, zircon, chromite, and quartz. Heating experiments indicate temperatures exceeded 2000 °C. Amid city-side devastation, the airburst demolished 12+ m of the 4-to-5-story palace complex and the massive 4-m-thick mudbrick rampart, while causing extreme disarticulation and skeletal fragmentation in nearby humans. An airburst-related influx of salt (~ 4 wt.%) produced hypersalinity, inhibited agriculture, and caused a ~ 300–600-year-long abandonment of ~ 120 regional settlements within a > 25-km radius. Tall el-Hammam may be the second oldest city/town destroyed by a cosmic airburst/impact, after Abu Hureyra, Syria, and possibly the earliest site with an oral tradition that was written down (Genesis). Tunguska-scale airbursts can devastate entire cities/regions and thus, pose a severe modern-day hazard.

Previous article No AccessExtraordinary Biomass-Burning Episode and Impact Winter Triggered by the Younger Dryas Cosmic Impact ∼12,800 Years Ago: A ReplyWendy S. Wolbach, Joanne P. Ballard, Paul A. Mayewski, Andrei Kurbatov, Ted E. Bunch, Malcolm A. LeCompte, Victor Adedeji, Isabel Israde-Alcántara, Richard B. Firestone, William C. Mahaney, Adrian L. Melott, Christopher R. Moore, William M. Napier, George A. Howard, Kenneth B. Tankersley, Brian C. Thomas, James H. Wittke, John R. Johnson, Siddhartha Mitra, James P. Kennett, Gunther Kletetschka, and Allen WestWendy S. Wolbach1. Department of Chemistry and Biochemistry, DePaul University, Chicago, Illinois 60614, USA*Author for correspondence; email: [email protected]. Search for more articles by this author , Joanne P. Ballard2. Department of Geography, University of Tennessee, Knoxville, Tennessee 37996-0925, USA Search for more articles by this author , Paul A. Mayewski3. Climate Change Institute, University of Maine, Orono, Maine 04469, USA Search for more articles by this author , Andrei Kurbatov3. Climate Change Institute, University of Maine, Orono, Maine 04469, USA Search for more articles by this author , Ted E. Bunch4. Geology Program, School of Earth Science and Environmental Sustainability, Northern Arizona University, Flagstaff, Arizona 86011, USA Search for more articles by this author , Malcolm A. LeCompte5. Center of Excellence in Remote Sensing Education and Research, Elizabeth City State University, Elizabeth City, North Carolina 27909, USA Search for more articles by this author , Victor Adedeji6. Department of Natural Sciences, Elizabeth City State University, Elizabeth City, North Carolina 27909, USA Search for more articles by this author , Isabel Israde-Alcántara7. Instituto de Investigaciones Metalúrgicas, departamento de Geología y Mineralogía, Universidad Michoacana de San Nicolás de Hidalgo, CP 58060, Morelia, Michoacán, Mexico Search for more articles by this author , Richard B. Firestone8. Department of Nuclear Engineering, University of California, Berkeley, California 94720, USA Search for more articles by this author , William C. Mahaney9. Quaternary Surveys, 26 Thornhill Avenue, Thornhill, Ontario L4J 1J4, Canada Search for more articles by this author , Adrian L. Melott10. Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas 66045, USA Search for more articles by this author , Christopher R. Moore11. Savannah River Archaeological Research Program, South Carolina Institute of Archaeology and Anthropology, University of South Carolina, New Ellenton, South Carolina 29809, USA Search for more articles by this author , William M. Napier12. Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, Northern Ireland, United Kingdom Search for more articles by this author , George A. Howard13. Restoration Systems, Raleigh, North Carolina 27604, USA Search for more articles by this author , Kenneth B. Tankersley14. Department of Anthropology and Department of Geology, University of Cincinnati, Cincinnati, Ohio 45221, USA Search for more articles by this author , Brian C. Thomas15. Department of Physics and Astronomy, Washburn University, Topeka, Kansas 66621, USA Search for more articles by this author , James H. Wittke4. Geology Program, School of Earth Science and Environmental Sustainability, Northern Arizona University, Flagstaff, Arizona 86011, USA Search for more articles by this author , John R. Johnson16. Santa Barbara Museum of Natural History, Santa Barbara, California 93105, USA Search for more articles by this author , Siddhartha Mitra17. Department of Geological Sciences, East Carolina University, Greenville, North Carolina 27858, USA Search for more articles by this author , James P. Kennett18. Department of Earth Science and Marine Science Institute, University of California, Santa Barbara, California 93106, USA Search for more articles by this author , Gunther Kletetschka19. Faculty of Science, Charles University, Prague, Czech Republic; Institute of Geology, Czech Academy of Science of the Czech Republic, Prague, Czech Republic; and University of Alaska, 903 Koyukuk Drive, Fairbanks, Alaska 99775, USA Search for more articles by this author , and Allen West20. Comet Research Group, Prescott, Arizona 86301, USA*Author for correspondence; email: [email protected]. Search for more articles by this author PDFPDF PLUSFull Text Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Journal of Geology Volume 128, Number 1January 2020 Article DOIhttps://doi.org/10.1086/706265 Views: 393Total views on this site Citations: 5Citations are reported from Crossref HistoryReceived August 27, 2019Accepted August 28, 2019Published online December 05, 2019 © 2019 by The University of Chicago. All rights reserved.PDF download Crossref reports the following articles citing this article:William C. Mahaney, Peeter Somelar, Christopher C. R. Allen Late Pleistocene Glacial-Paleosol-cosmic record of the Viso Massif—France and Italy: New evidence in support of the Younger Dryas boundary (12.8 ka), International Journal of Earth Sciences 112, no.11 (Sep 2022): 217–242.https://doi.org/10.1007/s00531-022-02243-9James Lawrence Powell Premature rejection in science: The case of the Younger Dryas Impact Hypothesis, Science Progress 105, no.11 (Jan 2022): 003685042110642.https://doi.org/10.1177/00368504211064272Martin B. Sweatman The Younger Dryas impact hypothesis: Review of the impact evidence, Earth-Science Reviews 218 (Jul 2021): 103677.https://doi.org/10.1016/j.earscirev.2021.103677Francisco Tello, José R. Verdú, Michele Rossini, Mario Zunino Onthophagus pilauco sp. nov. (Coleoptera, Scarabaeidae): evidence of beetle extinction in the Pleistocene–Holocene transition in Chilean Northern Patagonia, ZooKeys 1043 (Jun 2021): 133–145.https://doi.org/10.3897/zookeys.1043.61706Maxim Usatov Main Belt asteroid as a Possible Younger Dryas impactor, Astronomische Nachrichten 341, no.88 (Sep 2020): 734–740.https://doi.org/10.1002/asna.202013817

At Abu Hureyra (AH), Syria, the 12,800-year-old Younger Dryas boundary layer (YDB) contains peak abundances in meltglass, nanodiamonds, microspherules, and charcoal. AH meltglass comprises 1.6 wt.% of bulk sediment, and crossed polarizers indicate that the meltglass is isotropic. High YDB concentrations of iridium, platinum, nickel, and cobalt suggest mixing of melted local sediment with small quantities of meteoritic material. Approximately 40% of AH glass display carbon-infused, siliceous plant imprints that laboratory experiments show formed at a minimum of 1200°–1300 °C; however, reflectance-inferred temperatures for the encapsulated carbon were lower by up to 1000 °C. Alternately, melted grains of quartz, chromferide, and magnetite in AH glass suggest exposure to minimum temperatures of 1720 °C ranging to >2200 °C. This argues against formation of AH meltglass in thatched hut fires at 1100°–1200 °C, and low values of remanent magnetism indicate the meltglass was not created by lightning. Low meltglass water content (0.02–0.05% H 2 O) is consistent with a formation process similar to that of tektites and inconsistent with volcanism and anthropogenesis. The wide range of evidence supports the hypothesis that a cosmic event occurred at Abu Hureyra ~12,800 years ago, coeval with impacts that deposited high-temperature meltglass, melted microspherules, and/or platinum at other YDB sites on four continents.

The Younger Dryas (YD) impact hypothesis posits that fragments of a large, disintegrating asteroid/comet struck North America, South America, Europe, and western Asia ~12,800 years ago. Multiple airbursts/impacts produced the YD boundary layer (YDB), depositing peak concentrations of platinum, high-temperature spherules, meltglass, and nanodiamonds, forming an isochronous datum at >50 sites across ~50 million km² of Earth's surface. This proposed event triggered extensive biomass burning, brief impact winter, YD climate change, and contributed to extinctions of late Pleistocene megafauna. In the most extensive investigation south of the equator, we report on a ~12,800-year-old sequence at Pilauco, Chile (~40°S), that exhibits peak YD boundary concentrations of platinum, gold, high-temperature iron- and chromium-rich spherules, and native iron particles rarely found in nature. A major peak in charcoal abundance marks an intense biomass-burning episode, synchronous with dramatic changes in vegetation, including a high-disturbance regime, seasonality in precipitation, and warmer conditions. This is anti-phased with northern-hemispheric cooling at the YD onset, whose rapidity suggests atmospheric linkage. The sudden disappearance of megafaunal remains and dung fungi in the YDB layer at Pilauco correlates with megafaunal extinctions across the Americas. The Pilauco record appears consistent with YDB impact evidence found at sites on four continents.

Part 1 of this study investigated evidence of biomass burning in global ice records, and here we continue to test the hypothesis that an impact event at the Younger Dryas boundary (YDB) caused an anomalously intense episode of biomass burning at approximate to 12.8 ka on a multicontinental scale (North and South America, Europe, and Asia). Quantitative analyses of charcoal and soot records from 152 lakes, marine cores, and terrestrial sequences reveal a major peak in biomass burning at the Younger Dryas (YD) onset that appears to be the highest during the latest Quaternary. For the Cretaceous-Tertiary boundary (K-Pg) impact event, concentrations of soot were previously utilized to estimate the global amount of biomass burned, and similar measurements suggest that wildfires at the YD onset rapidly consumed approximate to 10 million km(2) of Earth's surface, or approximate to 9% of Earth's biomass, considerably more than for the K-Pg impact. Bayesian analyses and age regressions demonstrate that ages for YDB peaks in charcoal and soot across four continents are synchronous with the ages of an abundance peak in platinum in the Greenland Ice Sheet Project 2 (GISP2) ice core and of the YDB impact event (12,835-12,735 cal BP). Thus, existing evidence indicates that the YDB impact event caused an anomalously large episode of biomass burning, resulting in extensive atmospheric soot/dust loading that triggered an impact winter. This, in turn, triggered abrupt YD cooling and other climate changes, reinforced by climatic feedback mechanisms, including Arctic sea ice expansion, rerouting of North American continental runoff, and subsequent ocean circulation changes.

The Younger Dryas boundary (YDB) cosmic-impact hypothesis is based on considerable evidence that Earth collided with fragments of a disintegrating ≥100-km-diameter comet, the remnants of which persist within the inner solar system ∼12,800 y later. Evidence suggests that the YDB cosmic impact triggered an “impact winter” and the subsequent Younger Dryas (YD) climate episode, biomass burning, late Pleistocene megafaunal extinctions, and human cultural shifts and population declines. The cosmic impact deposited anomalously high concentrations of platinum over much of the Northern Hemisphere, as recorded at 26 YDB sites at the YD onset, including the Greenland Ice Sheet Project 2 ice core, in which platinum deposition spans ∼21 y (∼12,836–12,815 cal BP). The YD onset also exhibits increased dust concentrations, synchronous with the onset of a remarkably high peak in ammonium, a biomass-burning aerosol. In four ice-core sequences from Greenland, Antarctica, and Russia, similar anomalous peaks in other combustion aerosols occur, including nitrate, oxalate, acetate, and formate, reflecting one of the largest biomass-burning episodes in more than 120,000 y. In support of widespread wildfires, the perturbations in CO2 records from Taylor Glacier, Antarctica, suggest that biomass burning at the YD onset may have consumed ∼10 million km2, or ∼9% of Earth’s terrestrial biomass. The ice record is consistent with YDB impact theory that extensive impact-related biomass burning triggered the abrupt onset of an impact winter, which led, through climatic feedbacks, to the anomalous YD climate episode.

Chapter 8 reviews the evidence for a suspected cosmic impact over North America at the onset of the Younger Dryas climatic period with the near simultaneous extinction of classic Pleistocene megafauna and the Clovis technoculture. The impact related proxies that are used to detect the impact layer, such as spherules, silica-rich glass, nanodiamonds, carbon, and geochemical enrichment of nickel, cobalt, chromium, iridium, platinum and osmium, are considered. This layer is offered as a useful chronostratigraphic marker for recognizing the YDB boundary in the absence of other dating methods.

Large quantities of impact-related microspherules have been found in fine-grained sediments retained within seven out of nine, radiocarbon-dated, Late Pleistocene mammoth ( Mammuthus primigenius ) and bison ( Bison priscus ) skull fragments. The well-preserved fossils were recovered from frozen “muck” deposits (organic-rich silt) exposed within the Fairbanks and Klondike mining districts of Alaska, USA, and the Yukon Territory, Canada. In addition, elevated platinum abundances were found in sediment analysed from three out of four fossil skulls. In view of this new evidence, the mucks and their well-preserved but highly disrupted and damaged vertebrate and botanical remains are reinterpreted in part as blast deposits that resulted from several episodes of airbursts and ground/ice impacts within the northern hemisphere during Late Pleistocene time (~46–11 ka B.P.). Such a scenario might be explained by encounters with cometary debris in Earth-crossing orbits (Taurid Complex) that was generated by fragmentation of a large short-period comet within the inner Solar System.

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.

Holliday (1) rejects age-depth models for the Younger Dryas boundary layer (YDB) in Kennett et al. (2), claiming that they are incorrect for several reasons, including age reversals, high age uncertainties, and use of optically stimulated luminescence (OSL) dating. These same claims previously were presented in Meltzer et al. (3) and were discussed and refuted in Kennett et al. (2). These criticisms apply to nearly all dated archaeological and geological sequences, including the Odessa meteorite impact crater, where paradoxically, Holliday et al. (4) modeled an impact age using OSL dating (>70% of dates used) with large uncertainties (to >6,000 y) and age reversals (>40% of dates are reversals). Thus, Holliday (1) argues against a practice that he and many other researchers have used and continue to use today. In an ideal world, all dates would be in perfect chronological order with high accuracy and certainty, but such scenarios are rarely …

James P. Kennett, Douglas J. Kennett, Brendan J. Culleton, J. Emili Aura Tortosa, James L. Bischoff, Ted E. Bunch, I. Randolph Daniel Jr., Jon M. Erlandson, David Ferraro, Richard B. Firestone, Albert C. Goodyear, Isabel Israde-Alcántara, John R. Johnson, Jesús F. Jordá Pardo, David R. Kimbel, Malcolm A. LeCompte, Neal H. Lopinot, William C. Mahaney, Andrew M. T. Moore, Christopher R. Moore, Jack H. Ray, Thomas W. Stafford Jr., Kenneth Barnett Tankersley, James H. Wittke, Wendy S. Wolbach, and Allen West

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 >50 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.

Significance We present detailed geochemical and morphological analyses of nearly 700 spherules from 18 sites in support of a major cosmic impact at the onset of the Younger Dryas episode (12.8 ka). The impact distributed ∼10 million tonnes of melted spherules over 50 million square kilometers on four continents. Origins of the spherules by volcanism, anthropogenesis, authigenesis, lightning, and meteoritic ablation are rejected on geochemical and morphological grounds. The spherules closely resemble known impact materials derived from surficial sediments melted at temperatures >2,200 °C. The spherules correlate with abundances of associated melt-glass, nanodiamonds, carbon spherules, aciniform carbon, charcoal, and iridium.

Boslough et al. (1) offer no alternate explanation for ∼10 million tonnes of Younger Dryas spherules recovered from 18 sites across ∼50 million square kilometers of North America, Europe, and the Middle East (2). In addition, the authors claim that our hypothesis “demonstrates a misunderstanding of comets.” However, the misunderstanding is theirs alone, because the model they criticize is their own creation and not the one we adopt, which derives from a substantial body of comet literature (e.g., ref. 3). Most Earth-crossing comets arrive from the Oort cloud and the transneptunian region by way of the centaur population, an unstable reservoir orbiting between the giant planets that feeds the Jupiter …