Evidence for deposition of 10 million tonnes of impact spherules across four continents 12,800 y ago

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 impactrelated 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.Clovis-Folsom | lechatelierite | tektite | wildfires

Giant comets and mass extinctions of life

“…It has been proposed that bombardment episodes are caused by the disintegration of giant comets in short-period, Earth-crossing orbits (Clube & Napier 1984). A more recent worldwide extinction of species at 12,800 BP, accompanied by a climate cooling which lasted for over 1000 years, has likewise been attributed to an encounter with the debris from such a body (Napier 2010;Wittke et al 2013;Moore et al 2014), although this remains a controversial proposition (van Hoesel et al 2014). Models that neglect the injection of large comets into the near-Earth environment (e.g Chapman 2004, Boslough et al 2013 fail to yield large-scale catastrophic disruption of this sort in the recent past, but it remains to be seen how often such comets arrive and what effect they may have on the biosphere over geological as well as human timescales; this is the subject of the present paper.…”

Evaluating the link between the sulfur-rich Laacher See volcanic eruption and the Younger Dryas climate anomaly

“…The consequences of the impact (possibly a cometary airburst) were hypothesised to have destabilised the Laurentide Ice Sheet, cooled NH climate, and contributed to the megafaunal extinction characteristic of the period (Firestone et al, 2007;Kennett et al, 2009). The discovery of significant amounts of impactderived spherules scattered across North America, Europe, Africa, and South America at ∼ 12.80 ± 0.15 ka BP further supports the Younger Dryas Impact Hypothesis (YDIH) (Wittke et al, 2013), as does apparently extraterrestrially derived platinum, found initially in Greenland ice (Petaev et al, 2013) and subsequently globally (Moore et al, 2017), coincident with the YD onset. However, other research questions the evidence of an impact, focussing on perceived errors in the dating of the YD boundary layer (Holliday, 2015;Meltzer et al, 2014), the misidentification of terrestrially derived carbon spherules, shocked quartz, and nanodiamonds as extraterrestrial (Pinter et al, 2011;van Hoesel et al, 2015;Tian et al, 2011), the non-uniqueness of YD nanodiamond evidence , and inconsistencies regarding the physics of bolide trajectories and impacts (Boslough et al, 2013).…”

“…Conversely, the LSE could also be directly responsible for some of the evidence of an impact, a possibility that has not been investigated thoroughly due to the widespread, but incorrect, concept that the eruption predates the evidence of an impact by ∼ 200 years. An impact event (independently or in conjunction with YD cooling) is suggested to explain North American megafaunal extinctions (Firestone et al, 2007;Wolbach et al, 2018a, b), though recent research has built a compelling case that anthropogenic factors such as overhunting and disease were largely responsible for the demise of many species of large mammalian fauna (Sandom et al, 2014;Bartlett et al, 2016;van der Kaars et al, 2017;Cooper et al, 2015;Metcalf et al, 2016). Notably, megafaunal extinction dates appear correlated with the timing of human colonisation of the Western Hemisphere (Surovell et al, 2016), a result inconsistent with an impactdriven extinction.…”

“…The Younger Dryas Impact Hypothesis (YDIH) is supported by the discovery of iridium, shocked quartz, platinum, and millions of tons of impact spherules at the YD boundary layer (Kennett et al, 2009;Wittke et al, 2013;Wu et al, 2013). Wolbach et al (2018a) use charcoal and soot evidence from 152 sedimentary sequences and elevated concentrations of platinum at 26 sites (including Greenland) to argue that a cometary impact at the beginning of the YD triggered the largest wildfires of the Quaternary, consuming 9 % of global terrestrial biomass. The YDIH has proven remarkably controversial, and different researchers suggest either terrestrial origins for the same evidence, that the evidence is not unique to the YD boundary, or that the YD boundary layer was misidentified (e.g.…”

“…Most recently, Wolbach et al (2018a, b) catalogue extensive evidence of global wildfires they argue were ignited by the impact of fragments of a > 100 km diameter comet. According to Wolbach et al (2018a, b), 9 % of global biomass burned, considerably more than that following the end of the Cretaceous impact event. Despite this growing catalogue of evidence of an impact event, the YDIH remains extremely controversial.…”