Younger Dryas Boundary Research Articles

Very high-temperature impact melt products as evidence for cosmic airbursts and impacts 12,900 years ago

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 (Fe(3)Si), a rare meteoritic mineral that forms under high temperatures; (iii) melted SiO(2) 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.

Evidence from central Mexico supporting the Younger Dryas extraterrestrial impact hypothesis.

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 2.8m that dates to 12.9ka 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 12.9ka.

Nanodiamonds do not provide unique evidence for a Younger Dryas impact

Microstructural, δ(13)C isotope and C/N ratio investigations were conducted on excavated material from the black Younger Dryas boundary in Lommel, Belgium, aiming for a characterisation of the carbon content and structures. Cubic diamond nanoparticles are found in large numbers. The larger ones with diameters around or above 10 nm often exhibit single or multiple twins. The smaller ones around 5 nm in diameter are mostly defect-free. Also larger flake-like particles, around 100 nm in lateral dimension, with a cubic diamond structure are observed as well as large carbon onion structures. The combination of these characteristics does not yield unique evidence for an exogenic impact related to the investigated layer.

Confirmation of the Younger Dryas boundary (YDB) data at Murray Springs, AZ

We are pleased that Haynes et al. (1) have verified our discovery of a 12.9-ka peak in magnetic grains from the Clovis-age surface (YDB) at the Murray Springs site (2) that is even higher (8.2 g/kg) than we reported (2.6 g/kg), consistent with variable deposition. Haynes et al. (1) also reported much larger microspherule concentrations in the YDB, up to 37,000/kg, compared with 109/kg that we found in bulk sediment. Their elemental analysis of magnetic grains is similar to …

Reply to Firestone et al.: No confirmation of impact at the lower Younger Dryas boundary at Murray Springs, AZ

Firestone et al. (1) imply that our finding of 0.82% magnetic particles in one of our Murray Springs samples, 9-5-3MS07 (2), compared with their 0.26% substantiates their claim of an anomalous spike at the Clovis-age surface. However, they ignore our sample 26MS07 (2, figure 4B) that has only 0.072% magnetic particles also from the Clovis surface about 3 m away. Such differences are to be expected on the bed of a fluvial channel. As we pointed out to Allen West when he made his first collection, magnetite and maghemite sand grains are ubiquitous in the late Quaternary alluvium at Curry Draw, including the bed load of the modern channel where these minerals form numerous streaks of black sand. Therefore, because one can find orders of magnitude differences in the concentrations of magnetic sand on the Clovis surface, we see no anomalous concentration.

Geochemical data reported by Paquay et al. do not refute Younger Dryas impact event

In the 12,900-year-old Younger Dryas boundary layer (YDB), Firestone et al. (1) reported inferred extraterrestrial (ET) iridium peaks in sediments and magnetic separates that coincide with elevated abundances in potential impact markers, including microspherules and nanodiamonds (2). Paquay et al. (3) tested YDB sediments but could not reproduce previous iridium concentrations. However, their results seem problematic because standardization uncertainties ranged up to ±140%, and reproducibility varied up to ≈400% (tables S2 and S1, respectively, in ref. 3). Despite these uncertainties, they documented iridium peaks at Murray Springs, AZ (profile B1) and Lake Hind, AB, Canada (batches 1 + 2) at the same stratigraphic levels as spikes in nanodiamonds, magnetic grains, and microspherules (Fig. 1). Although their iridium values are ≈7% of our median values, theirs rise to ≈33% of lower limits and to >300% above background. Additionally, although they reported sedimentary osmium ratios (187Os/188Os) that seem terrestrial, they failed to analyze YDB magnetic separates, in which previous iridium concentrations were much higher and in which osmium ratios might possibly reveal an ET component, as Sharma et al. (4) reported from YDB-aged Pacific and Atlantic ferromanganese crusts. Paquay et al. (3) also speculated that wildfires created the YDB nanodiamonds, contradicting ≈100 years of research demonstrating that hexagonal and cubic diamonds form only under extreme temperature and/or pressure regimes (5), as occurred during the K/T impact and as proposed for the YDB. Overall, the results of Paquay et al. (3) are useful contributions to the YDB discussion, reinforcing the importance of additional research into the cooccurrence of iridium and osmium anomalies with inferred impact proxies.

The Murray Springs Clovis site, Pleistocene extinction, and the question of extraterrestrial impact

Some of the evidence for the recent hypothesis of an extraterrestrial impact that caused late Pleistocene megafaunal extinctions [Firestone et al. (2007) Proc Natl Acad Sci USA 104:16016-16021] was based upon samples collected at Murray Springs, a Clovis archaeological site in southeastern Arizona. Here we describe sampling and analyses of magnetic separates from within, above, and below the lower Younger Dryas boundary (LYDB) black mat at Murray Springs, as well as radiation measurements from the LYDB at Murray Springs and two other well-stratified Clovis sites. The main magnetic fraction at Murray Springs is maghemite. Magnetic microspherules have terrestrial origins but also occur as cosmic dust particles. We failed to find iridium or radiation anomalies. The evidence for massive biomass burning at Murray Springs is addressed and found to be lacking. We could not substantiate some of the claims by Firestone and others, but our findings do not preclude a terminal Pleistocene cosmic event.

Discovery of a nanodiamond-rich layer in the Greenland ice sheet

AbstractWe report the discovery in the Greenland ice sheet of a discrete layer of free nanodiamonds (NDs) in very high abundances, implying most likely either an unprecedented influx of extraterrestrial (ET) material or a cosmic impact event that occurred after the last glacial episode. From that layer, we extracted n-diamonds and hexagonal diamonds (lonsdaleite), an accepted ET impact indicator, at abundances of up to about 5×106 times background levels in adjacent younger and older ice. The NDs in the concentrated layer are rounded, suggesting they most likely formed during a cosmic impact through some process similar to carbon-vapor deposition or high-explosive detonation. This morphology has not been reported previously in cosmic material, but has been observed in terrestrial impact material. This is the first highly enriched, discrete layer of NDs observed in glacial ice anywhere, and its presence indicates that ice caps are important archives of ET events of varying magnitudes. Using a preliminary ice chronology based on oxygen isotopes and dust stratigraphy, the ND-rich layer appears to be coeval with ND abundance peaks reported at numerous North American sites in a sedimentary layer, the Younger Dryas boundary layer (YDB), dating to 12.9 ± 0.1 ka. However, more investigation is needed to confirm this association.

Shock-synthesized hexagonal diamonds in Younger Dryas boundary sediments

The long-standing controversy regarding the late Pleistocene megafaunal extinctions in North America has been invigorated by a hypothesis implicating a cosmic impact at the Allerød-Younger Dryas boundary or YDB (approximately 12,900 +/- 100 cal BP or 10,900 +/- 100 (14)C years). Abrupt ecosystem disruption caused by this event may have triggered the megafaunal extinctions, along with reductions in other animal populations, including humans. The hypothesis remains controversial due to absence of shocked minerals, tektites, and impact craters. Here, we report the presence of shock-synthesized hexagonal nanodiamonds (lonsdaleite) in YDB sediments dating to approximately 12,950 +/- 50 cal BP at Arlington Canyon, Santa Rosa Island, California. Lonsdaleite is known on Earth only in meteorites and impact craters, and its presence strongly supports a cosmic impact event, further strengthened by its co-occurrence with other nanometer-sized diamond polymorphs (n-diamonds and cubics). These shock-synthesized diamonds are also associated with proxies indicating major biomass burning (charcoal, carbon spherules, and soot). This biomass burning at the Younger Dryas (YD) onset is regional in extent, based on evidence from adjacent Santa Barbara Basin and coeval with broader continent-wide biomass burning. Biomass burning also coincides with abrupt sediment mass wasting and ecological disruption and the last known occurrence of pygmy mammoths (Mammuthus exilis) on the Channel Islands, correlating with broader animal extinctions throughout North America. The only previously known co-occurrence of nanodiamonds, soot, and extinction is the Cretaceous-Tertiary (K/T) impact layer. These data are consistent with abrupt ecosystem change and megafaunal extinction possibly triggered by a cosmic impact over North America at approximately 12,900 +/- 100 cal BP.

Nanodiamonds in the Younger Dryas Boundary Sediment Layer

We report abundant nanodiamonds in sediments dating to 12.9 +/- 0.1 thousand calendar years before the present at multiple locations across North America. Selected area electron diffraction patterns reveal two diamond allotropes in this boundary layer but not above or below that interval. Cubic diamonds form under high temperature-pressure regimes, and n-diamonds also require extraordinary conditions, well outside the range of Earth's typical surficial processes but common to cosmic impacts. N-diamond concentrations range from approximately 10 to 3700 parts per billion by weight, comparable to amounts found in known impact layers. These diamonds provide strong evidence for Earth's collision with a rare swarm of carbonaceous chondrites or comets at the onset of the Younger Dryas cool interval, producing multiple airbursts and possible surface impacts, with severe repercussions for plants, animals, and humans in North America.

Standards of evidence and Paleoindian demographics

Buchanan et al. (1) correctly identify accurate Paleoindian population estimates as an important test of the Younger Dryas boundary (YDB) extraterrestrial impact hypothesis (2). They claim that cumulative probabilities of 628 14C-dated components provide evidence for population continuity across the YDB. In fact, only 80 components fall within the interval of interest (≈11,500 to 10,500 14C years). Data quality is essential with small datasets, and their conclusions are invalid for the following reasons. (i) Only 14C dates with measurement precisions 2,000 years. (ii) Only bone dates processed with modern techniques [e.g., XAD (3) or ultrafiltration (4)] are valid because of the catastrophic consequences of poor chemical preparation (3). (iii) Stratigraphic associations between radiocarbon dates and cultural residues need to be demonstrated; e.g., much of the purported pre-11,000 14C years evidence used is now discredited. (iv) Single-component sites do not have the same credibility as multiple-occupation sites. (v) The potential for site discovery is not equal through time; destruction and preservation vary by region and are determined by burial depth, depositional environment, ground water geochemistry, and site type (e.g., kill, processing, or camp). Cumulative probabilities of outdated and inaccurate radiocarbon dates from poorly defined archaeological contexts do not provide meaningful proxies of past human demographics. Much more archaeological and chronological work is required to test the YDB extraterrestrial impact hypothesis.

Wildfire and abrupt ecosystem disruption on California's Northern Channel Islands at the Ållerød–Younger Dryas boundary (13.0–12.9 ka)

Sedimentary records from California's Northern Channel Islands and the adjacent Santa Barbara Basin (SBB) indicate intense regional biomass burning (wildfire) at the Ållerød–Younger Dryas boundary (∼13.0–12.9 ka) (All age ranges in this paper are expressed in thousands of calendar years before present [ka]. Radiocarbon ages will be identified and clearly marked “ 14C years”.). Multiproxy records in SBB Ocean Drilling Project (ODP) Site 893 indicate that these wildfires coincided with the onset of regional cooling and an abrupt vegetational shift from closed montane forest to more open habitats. Abrupt ecosystem disruption is evident on the Northern Channel Islands at the Ållerød–Younger Dryas boundary with the onset of biomass burning and resulting mass sediment wasting of the landscape. These wildfires coincide with the extinction of Mammuthus exilis [pygmy mammoth]. The earliest evidence for human presence on these islands at 13.1–12.9 ka (∼11,000–10,900 14C years) is followed by an apparent 600–800 year gap in the archaeological record, which is followed by indications of a larger-scale colonization after 12.2 ka. Although a number of processes could have contributed to a post 18 ka decline in M. exilis populations (e.g., reduction of habitat due to sea-level rise and human exploitation of limited insular populations), we argue that the ultimate demise of M. exilis was more likely a result of continental scale ecosystem disruption that registered across North America at the onset of the Younger Dryas cooling episode, contemporaneous with the extinction of other megafaunal taxa. Evidence for ecosystem disruption at 13–12.9 ka on these offshore islands is consistent with the Younger Dryas boundary cosmic impact hypothesis [Firestone, R.B., West, A., Kennett, J.P., Becker, L., Bunch, T.E., Revay, Z.S., Schultz, P.H., Belgya, T., Kennett, D.J., Erlandson, J.M., Dickenson, O.J., Goodyear, A.A., Harris, R.S., Howard, G.A., Kloosterman, J.B., Lechler, P., Mayewski, P.A., Montgomery, J., Poreda, R., Darrah, T., Que Hee, S.S., Smith, A.R., Stich, A., Topping, W., Wittke, J.H. Wolbach, W.S., 2007. Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and Younger Dryas cooling. Proceedings of the National Academy of Sciences 104, 16016–16021.].

The effect of vegetation in a climate model simulation on the Younger Dryas

The effect of vegetation on the Younger Dryas (YD) climate is studied by comparing the results of four experiments performed with the ECHAM-4 atmospheric general circulation model (AGCM): (1) modern control climate, (2) simulation with YD boundary conditions, but with modern vegetation, (3 and 4) identical to (2), but with paleo-vegetation. Prescribing paleo-vegetation instead of modern vegetation resulted in temperature anomalies (both positive and negative) of up to 4°C in the Northern Hemisphere mid-latitudes, mainly as an effect of changes in forest cover (change in albedo). Moreover, changes in precipitation and evaporation were found, most notably during December–January–February (DJF) in the tropics and were caused by the replacement of forests by grasslands. These results are consistent with other model studies on the role of vegetation changes on climate and they suggest that it is important in paleoclimate simulation studies to prescribe realistic vegetation types, belonging to the period of interest. However, in our case the addition of YD vegetation did not improve the agreement with proxy data in Europe, as the temperatures were increasing during winter compared to the YD simulation with modern vegetation. It must be noted that this increase was not statistically significant. The model-data mismatch suggests that other factors probably played an important role, such as permafrost and atmospheric dust. We infer that during the last glacial-interglacial transition, the time lag between the first temperature increase and the northward migration of trees, estimated at 500–1000 years, could have delayed the warming of the Eurasian continent. The relatively open vegetation that existed during the early stages of the last glacial-interglacial transition had a relatively high albedo, thus tempering warming up of the Eurasian land surfaces.

Coleopteran evidence for a mosaic of environments at high altitude in the eastern Pyrénées, France, during the climatic transition between the Allerød and Younger Dryas

Late-glacial environmental and climatic implications are inferred from an insect fauna from organic sediments infilling a palaeochannel on the banks of the River Têt, eastern Pyrénées, France. A pine cone in association with the insect fauna has been radiocarbon dated to 10 920 ± 60 yr BP, namely close to the Allerød – Younger Dryas boundary. Two distinct insect associations appear to be recognisable here. One is an assemblage typical of the high altitude forest and a second is characteristic of an alpine grassland. The close coexistence of these two assemblages is attributed to the climatic cooling towards the start of the Younger Dryas Stadial, when the forest cover broke up into remnant patches interspersed by alpine grassland. It is suggested that in a region of such high relief a mosaic of habitats may have been caused by patchy differences in insolation aspect, especially during a period of climatic deterioration. Copyright © 1999 John Wiley & Sons, Ltd.

The global response to Younger Dryas boundary conditions in an AGCM simulation

Geological evidence points to a global Younger Dryas (YD) climatic oscillation during the last glacial/ present interglacial transition phase. A convincing mechanism to explain this global YD climatic oscillation is not yet available. Nevertheless, a profound understanding of the mechanism behind the YD climate would lead to a better understanding of climate variability.