Accumulation of impact markers in desert wetlands and implications for the Younger Dryas impact hypothesis

Firestone et al. sampled sedimentary sequences at many sites across North America, Europe, and Asia [Firestone RB, et al. (2007) Proc Natl Acad Sci USA 106:16016–16021]. In sediments dated to the Younger Dryas onset or Boundary (YDB) approximately 12,900 calendar years ago, Firestone et al. reported discovery of markers, including nanodiamonds, aciniform soot, high-temperature melt-glass, and magnetic microspherules attributed to cosmic impacts/airbursts. The microspherules were explained as either cosmic material ablation or terrestrial ejecta from a hypothesized North American impact that initiated the abrupt Younger Dryas cooling, contributed to megafaunal extinctions, and triggered human cultural shifts and population declines. A number of independent groups have confirmed the presence of YDB spherules, but two have not. One of them [Surovell TA, et al. (2009) Proc Natl Acad Sci USA 104:18155–18158] collected and analyzed samples from seven YDB sites, purportedly using the same protocol as Firestone et al., but did not find a single spherule in YDB sediments at two previously reported sites. To examine this discrepancy, we conducted an independent blind investigation of two sites common to both studies, and a third site investigated only by Surovell et al. We found abundant YDB microspherules at all three widely separated sites consistent with the results of Firestone et al. and conclude that the analytical protocol employed by Surovell et al. deviated significantly from that of Firestone et al. Morphological and geochemical analyses of YDB spherules suggest they are not cosmic, volcanic, authigenic, or anthropogenic in origin. Instead, they appear to have formed from abrupt melting and quenching of terrestrial materials.

Section: Resultsmentioning

confidence: 99%

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confidence: 87%

Independent evaluation of conflicting microspherule results from different investigations of the Younger Dryas impact hypothesis

LeCompte

Goodyear

Demitroff

et al. 2012

“…Subsequent studies (13,15,16) questioned the origins of many impact markers cited by ref. 11, but the discovery of the SLOs alongside abundant, compositionally similar microspherules in three YDB sites in North America and Asia is difficult to explain by anything other than a cosmic impact.…”

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confidence: 99%

“…The Pt peak ( Fig. 1) is unlikely to be a mass-spectrometry artifact because (a) it is smoothly defined by seven ice samples; (b) the HfO interferences ( 178 Hf 16 194,195,196 Pt isotopes, respectively, were carefully assessed; (c) the 178,179,180 Hf signals in sample 63 with the highest Pt concentration are at least a factor of 10 lower than those of 194,195,196 Pt; and (d) there is no linear correlation between Hf concentrations and Pt concentrations in the ice samples.…”

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confidence: 99%

Large Pt anomaly in the Greenland ice core points to a cataclysm at the onset of Younger Dryas

Petaev

Huang

Jacobsen

et al. 2013

112

One explanation of the abrupt cooling episode known as the Younger Dryas (YD) is a cosmic impact or airburst at the YD boundary (YDB) that triggered cooling and resulted in other calamities, including the disappearance of the Clovis culture and the extinction of many large mammal species. We tested the YDB impact hypothesis by analyzing ice samples from the Greenland Ice Sheet Project 2 (GISP2) ice core across the Bølling-Allerød/YD boundary for major and trace elements. We found a large Pt anomaly at the YDB, not accompanied by a prominent Ir anomaly, with the Pt/Ir ratios at the Pt peak exceeding those in known terrestrial and extraterrestrial materials. Whereas the highly fractionated Pt/Ir ratio rules out mantle or chondritic sources of the Pt anomaly, it does not allow positive identification of the source. Circumstantial evidence such as very high, superchondritic Pt/Al ratios associated with the Pt anomaly and its timing, different from other major events recorded on the GISP2 ice core such as well-understood sulfate spikes caused by volcanic activity and the ammonium and nitrate spike due to the biomass destruction, hints for an extraterrestrial source of Pt. Such a source could have been a highly differentiated object like an Ir-poor iron meteorite that is unlikely to result in an airburst or trigger wide wildfires proposed by the YDB impact hypothesis. meteorite impact | climate change | ICP-MS analysis | PGE

“…The reproducibility, reliability, and validity of the impact indicators have been challenged, not least because many of these may be terrestrial in origin (possibly volcanic, organic, or detrital) and have been found in deposits younger and older than the YD (14,15,(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27). Others have questioned the physics of the supposed impact and whether it could or did have consequences for Late Pleistocene environments, animals, and people (26,(28)(29)(30)(31)(32).…”

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confidence: 99%

Chronological evidence fails to support claim of an isochronous widespread layer of cosmic impact indicators dated to 12,800 years ago

Meltzer

Holliday

Cannon³

et al. 2014

Significance A key element underpinning the controversial hypothesis of a widely destructive extraterrestrial impact at the onset of the Younger Dryas is the claim that 29 sites across four continents yield impact indicators all dated to 12,800 ± 150 years ago. This claim can be rejected: only three of those sites are dated to this window of time. At the remainder, the supposed impact markers are undated or significantly older or younger than 12,800 years ago. Either there were many more impacts than supposed, including one as recently as 5 centuries ago, or, far more likely, these are not extraterrestrial impact markers.