In:
Proceedings of the National Academy of Sciences, Proceedings of the National Academy of Sciences, Vol. 109, No. 28 ( 2012-07-10)
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.
Type of Medium:
Online Resource
ISSN:
0027-8424
,
1091-6490
DOI:
10.1073/pnas.1204453109
Language:
English
Publisher:
Proceedings of the National Academy of Sciences
Publication Date:
2012
detail.hit.zdb_id:
209104-5
detail.hit.zdb_id:
1461794-8
SSG:
11
SSG:
12
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