Palaeolithic extinctions and the Taurid Complex
Monthly Notices of the Royal Astronomical Society, Volume 405, Issue 3, 1 July 2010, Pages 1901–1906, https://doi.org/10.1111/j.1365-2966.2010.16579.x
1 INTRODUCTIONThe sudden onset of the Younger Dryas cooling 12 900 yr ago was marked by intense wildfires over North America, major disruption of human culture, and the rapid extinction of 35 genera of North American mammals (Faith & Surovell 2009). A thin carbon-rich black layer of this age has been identified at many sites across North America (Haynes 2008), coincident in age with the Younger Dryas Boundary (YDB).
Recently, several geochemical markers at this layer have been presented which seem to indicate that a major extraterrestrial event was involved in these events (Firestone et al. 2007; Firestone 2009). The markers include following.
Nanodiamonds, found in abundance at 15 YDB sites across North America and north-west Europe, covering a quarter of the Earth's surface. They are mostly found embedded within particles of melted plant resins, confined to the thin black layer, none having yet been found in other strata dating from 55 000 BC to the present. All three diamond allotropes are present in the boundary sediments including lonsdaleite (hexagonal nanodiamonds), which is shock-synthesized and found on Earth only in association with extraterrestrial impacts or inside meteorites (Kennett et al. 2009). The nanodiamonds and lonsdaleite found in meteorites were already present before atmospheric entry (Clarke, Appleman & Ross 1981).
Soot, microspherules and magnetic grains found at high concentrations, the microspherules having trace element abundances comparable to lunar KREEP but not to any other terrestrial or observed meteoritic source (Firestone 2009). The only previously known co-occurrence of soot, nanodiamonds and rapid extinction is at the Cretaceous-Tertiary boundary layer.
To cause destruction on a continental scale by a single impact, the energy required is ∼107 Mt, corresponding to a 4 km wide comet (Toon et al. 1997). Firestone et al. (2007) proposed that such a comet broke up on atmospheric entry and struck the 2-km thick Laurentide ice sheet. They speculated that four deep holes in the Great Lakes might be craters produced by this event. This short-term catastrophe was followed by a sudden return to ice age conditions on Earth which lasted for over a millennium, the cause of which is a matter for speculation (loc. cit.; Franzén & Cropp 2007). An alternative proposal to explain the data is that the Earth encountered a rare swarm of carbonaceous chondrites or comets, yielding multiple airbursts and possible surface impacts (Kennett et al. 2009).
The nature of the event at the YDB remains controversial. For example, it has been argued that the mammoths were in decline for about a millennium before the final extinctions (Gill et al. 2009). Nevertheless, it seems difficult to account for the geophysical markers – nanodiamonds, exotic sediment composition, evidence of a sudden, continent-wide fiery catastrophe, etc. – without the occurrence of some extraordinary extraterrestrial event.
Problems have however been raised from the astronomical perspective. It is difficult to reconcile a major astronomical catastrophe of continental dimensions as recently as 12.9 ka ago with the known population of near-Earth objects (NEOs) currently being revealed by comet and asteroid search programmes such as the LINEAR and Catalina surveys. Thus, it has been estimated that a 10 Mt impact may occur anywhere on Earth with a mean recurrence time 2000–3000 yr (Stuart & Binzel 2004). An impact of energy 1000 Mt or more is expected with recurrence time ∼60 000 yr (loc. cit. and Morbidelli et al. 2002). While this would be immensely damaging, the effect would be regional rather than continental or global. On conventional reckoning, the impact of a 4 km asteroid as recently as 13 000 yr ago is an one in a thousand event, and if an active comet, the probability is a few in a million (Harris 2008). It has also been pointed out that, to ignite fires in southern Arizona by thermal radiation from a fireball over the Great Lakes, the projectile would have had to be 100 km across, and all non-microbial life on Earth would have been wiped out (Melosh 2009). The geophysical evidence for an astronomical catastrophe at this boundary has been extended to Venezuela (Mahaney et al. 2010), which exacerbates that difficulty.
However, a potential deficiency of impact assessments based on current Spaceguard data is that they assume statistical completeness. To estimate impact risks at say the ∼10−4 per annum level by extrapolating from ∼10 yr of Spaceguard surveys is analogous to forecasting 3 yr weather by extrapolating one day's observation. A cascade of comet disintegration, for example, could be a significant hazard on such time-scales (cf. the 1994 impacts on Jupiter following the disintegration of D/1993 F2 Shoemaker-Levy 9).
It has also been objected that, if the Younger Dryas event was due to an encounter with a swarm of debris 13 000 yr ago, the material would still be visible in the inner Solar system at the present time. Meteoroid data, being largely tracers of comet disintegration, do indeed have the potential to look into the history of the cometary environment for tens of thousands of years, and in this paper we discuss such evidence. We first, in Section 2, summarize evidence which indicates that a large, short-period comet entered the inner planetary system a few 104 yr ago and has since then been undergoing a hierarchy of disintegrations, yielding the Taurid Complex. We show that at least 19 of the largest NEOs have orbits significantly close to that of Comet Encke. In Section 3, we numerically model the disintegration history of the progenitor comet. This reveals that encounters with dense swarms of material, sufficient to produce a 12.9 ka cosmic event, are indeed reasonable expectations of recent Earth history. Section 4 gives a broad-brush description of the likely consequences of an encounter with such a swarm. There is satisfactory agreement with the main physical features at this boundary.
The occurrence of a major celestial catastrophe over continental dimensions as recently as 12 900 yr ago, which is quite unexpected from Spaceguard data alone, may carry implications for assessments of the impact hazard currently faced by civilization (cf Napier & Asher 2009)
