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In recent years a theory has emerged that seeks to explain three mysterious events that took place at around 13,000 years ago (kya) – the sudden cooling phase known as the Younger Dryas, at the end of the Bølling Allerød warm interstadial,  the sudden termination of the Clovis culture in North America, along with the mass extinction event there that saw the demise of over 30 genera of Pleistocene fauna, although this last event had been in progress from c.40 kya, and also affected further flung parts of the world such as Eurasia and Australia. (I’ll address this last point more fully in a forthcoming post ).

Previous theories postulated that a complex mix of terrestrial – albeit not clearly defined – factors had been responsible for these catastrophic events, which were viewed as having had separate causes, but were especially devastating because of their combined synchronicity.

Following research at numerous archaeological sites, notably by Firestone, West and Kennett, (open access paper), it was proposed that around this time a comet either hit the planetary surface, possibly on the Laurentide ice sheet, or exploded in mid air, initially causing widespread and ferocious burning, followed by a prolonged period of plunging temperatures which laid waste to the North American landscape, killing off much of the fauna, including humans, who had survived the primary fireball, in what has been proposed to have been a period akin to a nuclear winter.

Evidence cited in support of this idea included raised levels of iridium and the presence of nano-diamonds on the ground, both of which are identified as markers for an extraterrestrial impact, as well as a so-called ‘black mat’, a carbon rich layer plainly visible in various archaeological trenches across numerous sites contemporaneous with these events.

More recently, two teams of researchers set out to evaluate the evidence, both of whose findings cast serious doubt on the comet theory, but leave unexplained the presence of nano-diamonds,with one team suggesting that if a cometary impact was responsible, the comet itself must have been radically different from anything currently known to science.

Two papers have been published at PNAS, and I’ll attempt to convey their import here – my thanks go to both teams for very kindly forwarding me copies of the full texts, (which currently reside behind comet-proof paywalls) so without further ado, here they are. First we have ‘Absence of Geochemical Evidence for an Impact Event at the Bølling Allerød/Younger Dryas Transition’, by François Paquay, a Doctoral graduate student in the Department of Geology and Geophysics at the University of Hawaii at Manoa (UHM), and his team, from which this is the abstract:

High concentrations of iridium have been reported in terrestrial sediments dated at 12.9 ka and are interpreted to support an extraterrestrial impact event as the cause of the observed extinction in the Rancholabren fauna, changes in the Paleoindian cultures, and the onset of the Younger Dryas cooling [Firestone RB, et al. (2007) Proc Natl Acad Sci USA 104:16016–16021]. Here, we report a platinum group element (PGE: Os, Ir, Ru, Rh, Pt, Pd), gold (Au) concentrations, and 187Os/188Os ratios in time-equivalent terrestrial, lacustrine, and marine sections to seek robust evidence of an extraterrestrial contribution.

First, our results do not reproduce the previously reported elevated Ir concentrations. Second, 187Os/ 188Os isotopic ratios in the sediment layers investigated are similar to average crustal values, indicating the absence of a significant meteoritic Os contribution to these sediments. Third, no PGE anomalies distinct from crustal signatures are present in the marine record in either the Gulf of California (DSDP 480, Guaymas Basin) or the Cariaco Basin (ODP 1002C). Our data show no evidence of an extraterrestrial (ET)-PGE enrichment anomaly in any of the investigated depositional settings investigated across North America and in one section in Belgium. The lack of a clear ET-PGE signature in this sample suite is inconsistent with the impact of a large chondritic projectile at the Bølling–Allerød/Younger Dryas transition.

As we can see, the original intention of this research was to confirm the comet theory by looking again at the evidence, but as is made clear throughout the paper, that confirmation simply failed to materialise. They examined the layer beneath the ‘black mat’, noting a lack of characteristic markers such as ‘shocked minerals, spherules composed of glass (or its alteration products), or Ni-rich spinels.

Observing further that anomalously elevated iridium levels alone don’t necessarily confirm an extraterrestrial impact, whereas other elements present stronger evidence. Analysis of Platinum Group Elements (PGEs) combined with Osmium (Os) isotopic samples revealed that the high levels of iridium (Ir) reported at sites were not confirmed by the authors’ own analytical process. This from the paper:

The PGE abundances for Murray Springs (AZ), Blackwater Draw (NM), Howard Bay (NC), Lake Lubbock (TX), Topper (SC), and Lommel (Belgium) sections are presented in Table S1. From these same sections, but on a different powder split, a comparison of 187Os/188Os, Os concentrations and Os/Ir ratios in different depositional settings is also given (Table S1; see also Table S2).

Firestone et al. (22) have reported 169 measurements of Ir at 14 sites up to 9,200 km apart in the 14C-dated BA/YD (see Fig.S1) bulk sediments, magnetic fraction, and the black mat, but not in the over- and underlying layers. In 5 of the 12 sites reported in table 1 in ref. 22, the Ir concentrations in the bulk sediment are 0.5 ng/g, and the concentrations in four sites (Murray Springs, Blackwater Draw, Lake Hind, and Carolina Bays, Max) reach 2.3–3.8 ng/g Ir. These values are comparable to those obtained at a number of KT boundary sites (e.g., 3.7 ng/g in Petriccio, Italy; 0.85 ng/g in Brazos River, TX; 5.7 ng/g in Beloc, Haiti; 2.9 ng/g in Frenchman River, Canada; and 1.4 ng/g in Hell Creek, MT (37). The Ir concentrations in the magnetic fractions of four samples (22) (Blackwater Draw, Wally’s Beach, Lommel, and Carolina Bay, Max) show concentrations between 15 and 117 ng/g [i.e., 25% of CI chondrites, Ir  460–472 ng/g; (38, 39)].

The data presented by Firestone et al. (24) also show a clear inverse correlation between Ni and Ir (Fig. S2A). This is rather unusual as Ni and Ir are both siderophile elements, enriched in meteorites compared to the terrestrial crust. Both elements typically show the same geochemical behavior during a meteorite impact on the terrestrial crust. In numerous crater impact sites such as Popigai (40) Lapparja¨vi (41), Morokweng (42), Clearwater East (43) and at the KT boundary (25), Ir and Ni display a positive correlation (Fig. S2B).

Table S1 shows that the new analyses do not confirm the elevated Ir values published by Firestone et al. (24). The PGE concentrations measured in bulk sediments are lower by at least an order of magnitude or more. Nugget effects, which cause small-scale inhomogeneities in PGE distribution and account for a variability of concentrations in geological samples (44–46), can be fully discarded here. The use of samples 10 g precludes any nugget effect and has been demonstrated to lead to good sample reproducibility (40), confirmed by repeated analyses in this study.

The authors further claim that all samples from the Younger Dryas black mat exhibit levels considered average for continental crust, whilst the sites at Murray Springs and Blackwater Draw may have shown higher levels of osmium and iridium as a direct result of the black mat’s formation process.

Moving next from land to sea, the team examined levels of osmium isotopes present in the ocean – previous research at Chicxulub  at the KT boundary (the end of the Cretaceous 65 mya – (but check this)) showed that when high levels of osmium borne on an extraterrestrial impactor entered the ocean, the Os isotopic ratios fell. To test the comet theory, two marine environments were analysed, the Gulf of California and the Cariaco Basin, located off Venezuela. The osmium isotope values in the Gulf of California were significantly higher than those in the Cariaco Basin, indicating the lack of cometary input, whilst the lower values detected in the latter did not correspond with the timing Younger Dryas, prompting the conclusion that there was no evidence for a ‘large chondritic impact event’ that had affected the ocean in the predicted manner.

Before heading back on to dry land for the discussion, it’s worth taking a quick look at the Cariaco Basin, as its unusual properties mark it apart from the average stretch of ocean, presumably making it an ideal model for this study, as this link to the CARIACO project explains.

Broadly speaking, the first part of the discussion reiterates and expands upon the osmium isotope studies in seawater, the lack of increased Platinum Group Elements that should be present on land in the wake of an impact, as well as normal levels of iridium found in Greenland ice cores. From there we move on to those mysterious nano-diamonds, an integral component in the comet theory, but which in the context of this paper, appear to be the true anomaly in this saga.

As we have seen, a cometary impact of the types with which we are familiar, should have left both a nano-diamond and PGE signature, but all we have are the nano-diamonds. This in turn causes the authors to speculate upon the properties of a putative comet, as we see:

Results presented here show that it is unlikely that the nanodiamonds were derived from any known meteoritic projectile, in contrast to suggestions of a swarm of comets or carbonaceous chondrite (26). Specifically, mass balance calculations show that if all of the nanodiamonds in the BA/YD sections are primary meteoritic material (1,500 ppm of the bulk; (75) (covering a fluence of 30% of Earth’s surface) (Fig. S1), and were not formed upon surface impact, a chondritic projectile of 1.2 km in diameter should produce an Ir fluence of 1 ng/cm2 that is clearly missing based on our results. For clarity, we emphasize that PGE and Os isotope data are sensitive indicators of undifferentiated meteoritic material.

Alternatively, to avoid PGE enrichment, it is possible to invoke the impact of one or several differentiated, PGEs-poor, possibly still unknown type of achondritic meteorites that vaporized in Earth’s atmosphere at the BA/YD transition is one possibility. However, the probabilities of the arrival of this type of projectile to Earth is low (www.unb.ca/passc/ImpactDatabase/), and this type of bolide lacks all allotropes of nanodiamonds.

In the case of one or several surface impact(s) of achondritic projectile(s), which could explain the absence of PGEs in the studied BA/YD sections, it becomes difficult to explain the formation of nanodiamonds without a well-dated surface expression of one or several craters. So far, no BA/YD craters are yet known. Based on the existing distribution of terrestrial craters (76), one or several large craters of this very young age cannot be eroded, filled with younger sediments or erased by subduction.

Most likely such fresh impact crater(s) would have been found and confirmed many years ago. In the scenario of a 1- to 2-km diameter achondritic, PGEs-poor projectile exploding into the atmosphere or hitting only the Laurentide Ice Sheet, a crater would possibly not have been formed. However, in this case, a source of carbon is required to form the recovered lonsdaleite crystals, and the concentration of carbon in this type of meteorites is extremely low. It is possible that the nanodiamonds were formed during an airburst, but it does not explain the absence of a geochemical anomaly.

Therefore, a strong decoupling of PGEs and nanodiamonds exists which differs from other known impact events. The occurrence of high concentrations of cubic diamonds and nanodiamonds (1,340 ppb) (24) in multiple BA/YD sections, found within carbon spherules without an associated defined geochemical anomaly, is therefore not a robust diagnostic of an impact event.

Thus we have something of a mystery  – although other research has called the impact event into question, none as far as I’m aware appear so thoroughly to confound the idea as this paper. There remains however the question of what exactly created the nano-diamonds, and whether that agency was from outer space or terrestrial in origin, plus the vague possibility that the Earth was indeed hit by something whose precise nature and identity continue to elude us.

I had intended to cover another, related paper in this post, but rather than relegate it to the end of this one, I think it best to write it up separately, especially as it focusses more on the micro-spherules, or lack thereof, alluded to earlier in this post. And suffice it to say, there is a great deal more information in the reviewed paper, especially in the guise of graphs and graphics which I haven’t included here, and I’d be happy to forward a copy to anyone interested in looking at this research in greater detail.

François S. Paquay will be appearing next week at the Fall 2009 meeting of the American Geophysical Union

image from: Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling – Firestone et al, PNAS

see also: Press Release

Reassessing the Source of Long-Period Comets – by Nathan A. Kaib, Thomas Quinn

Topper Site

The Berkeley Laboratory Isotopes Project

Periodic Table

Reference: Absence of Geochemical Evidence for an Impact Event at the Bølling Allerød/Younger Dryas Transition, by François Paquay, Greg Ravizza (also from UHM), Steven Goderis and Philippe Claeys from Vrije Universiteit Brussel, Frank Vanhaeck from the Universiteit Ghent, Matthew Boyd from Lakehead University, Todd A. Surovell from the University of Wyoming at Laramie, and Vance T. Holliday and C. Vance Haynes, Jr. from the University of Arizona at Tucson.

http://www.pnas.org/cgi/doi/10.1073/pnas.0908874106 (correct link will be posted when available).