© Adapted from J. S. Pigati et al., PNAS Early Edition (2012)It's in there. The purported markers of an extraterrestrial impact found in a dark layer of sediment at Murray Springs, Arizona (left), also appear in similar yet older layers elsewhere, including Chile's Atacama Desert (right), suggesting the markers are actually formed on Earth by natural processes.
Did a comet wipe out woolly mammoths and an ancient Indian culture almost 13,000 years ago? Geologists have fiercely debated the topic since 2007. Now a new study says an extraterrestrial impact wasn't to blame, though the scientists who originally proposed the impact idea still aren't convinced.
Three unexplained phenomena happened on Earth around 12,900 years ago. An extended cold spell known as the Younger Dryas cooled the world for 1300 years. Large creatures such as mammoths, mastodons, and their predators went extinct. And the Clovis culture - a group defined by the distinctive stone and bone tools that they manufactured, and presumed by many archaeologists to be the first inhabitants of the New World - suddenly disappeared.
In 2007, a team of researchers tried to tie together these seemingly disparate events to a single cause: an extraterrestrial object, possibly a comet, exploded above eastern Canada, they speculated. Their claimed evidence, which has been
much disputed since it was first reported, included several types of "impact markers" sometimes found after an extraterrestrial object strikes Earth. These purported markers include unusual grains of a titanium-rich form of the mineral magnetite; tiny magnetic spherules; and elevated levels of iridium, a relatively rare element that's more common in extraterrestrial objects than in Earth's crust.
The researchers found all of these markers embedded within unusual layers of dark, organic-rich sediments that scientists often call "black mats." These strata are the remains of ancient marshes and swamps, and at many sites across North America, especially in the American Southwest, black mats began accumulating at the beginning of the Younger Dryas, the researchers noted. Many paleontologists have noted that black mats are often a sort of dividing line between older sediments containing fossils of ice-age megafauna, and younger sediments that don't. And many archaeologists have observed that black mats seem to mark the demise of the Clovis culture, because the distinctive spear points that they produced are common in sediments below the layers but nonexistent above.
According to the 2007 comet-strike hypothesis, large amounts of heat generated by the explosion of the comet shattered and melted much of the region's ice sheet, suddenly flooding the North Atlantic with fresh water that interrupted ocean circulation, which in turn triggered an extended cold snap that wiped out the Clovis culture and polished off the last remaining ice age megafauna.
"It's an appealing idea because it links all of these things together," says Jeffrey Pigati, a geologist with the U.S. Geological Survey in Denver, who isn't a proponent of the comet-strike idea. Unfortunately for that hypothesis, he and his colleagues report online today in the
Proceedings of the National Academy of Sciences,
not all of the purported "impact markers" are produced solely by extraterrestrial objects striking Earth.
Pigati and his team studied black mats at 13 sites in the American Southwest and in the Atacama Desert of northern Chile - including some sites where the mats began accumulating 40,000 years ago. At 10 of the 13 sites, regardless of a site's age or location, the researchers found all three of the "impact markers" described in the 2007 study, says Pigati. Although those presumed markers had also been found at a site in Belgium, he notes, they probably wouldn't have dispersed to the South American sites he and his colleagues sampled in their new study, because effects of the purported impact likely would have been limited to the Northern Hemisphere.
Also, Pigati says, he and his team found supposed markers even at sites much older than 12,900 years - indicating that the purported impact couldn't have been the source of those markers. Finally, he notes, the ratios of the concentrations of several rare-earth elements and other trace elements, including iridium, in the spherules embedded in the black mats match those found in Earth's crust, not in extraterrestrial objects. Rather than coming from an extraterrestrial impact, the spherules were formed on Earth and then trapped in the ancient wetlands by natural processes, the team concludes. The dense spherules then sank to the base of the mat because they're heavier than other windblown dust, sand, and silt. The chemical composition and location of the spherules, as well as their presence in black mats of many different ages, are more easily explained by natural processes than by extraterrestrial impacts, the team contends.
But the new study has several flaws, says Richard Firestone, an isotopic chemist at Lawrence Berkeley National Laboratory in Berkeley, California, and a member of the team that originally proposed the impact hypothesis in 2007. For one thing, he notes, Pigati and his colleagues didn't use a scanning electron microscope to scrutinize the surface of the spherules - the only way to distinguish impact generated spherules, he says, that were melted at high temperatures and had a distinctive pattern inscribed on their surface as they splashed through the air, from the spherules commonly found in wetland sediments. Also, Firestone notes, Pigati's team didn't scrutinize all of the spherules, only the ones that were truly spherical - thus discarding many of the tiny markers that might have been generated by the impact, including teardrop-shaped blobs that cooled in midair as well as misshapen blobs that formed when one near-molten droplet bumped into another.
Pigati says he accepts Firestone's criticisms but stands by his team's findings and interpretations. "We admit in our paper that we can't disprove the impact hypothesis," he notes. "Our point is that some of the spherules and other markers [cited in the 2007 report] aren't uniquely produced by impacts."
Boy is this article citing old and outdated data! Catch up folks! The Younger Dryas Impact Hypothesis is no longer the "2007 Comet strike hypothesis" There is a new paper in PNAS that provides almost conclusive evidence this time, and it stands as a complete revision, and update of the Younger Dryas Impact Hypothesis.
News flash boys and girls; work Firestone et al was confirmed many times by other teams of scientists since 2007. But that's beside the point because Firestone 2007 is no longer the flagship paper anchoring the Younger Dryas Impact Hypothesis.
In fact it has been superseded as the primary reference for the YDIH by the March 6 2012 PNAS paper titled ‘Evidence from central Mexico supporting the Younger Dryas extraterrestrial impact hypothesis’ by Isabel Israde-Alcántara et al. [Link]
Abstract:
We report the discovery in Lake Cuitzeo in central Mexico of a black, carbon-rich, lacustrine layer, containing nanodiamonds, microspherules, and other unusual materials that date to the early Younger Dryas and are interpreted to result from an extraterrestrial impact. These proxies were found in a 27-m-long core as part of an interdisciplinary effort to extract a paleoclimate record back through the previous interglacial. Our attention focused early on an anomalous, 10-cm-thick, carbon-rich layer at a depth of 2.8 m that dates to 12.9 ka and coincides with a suite of anomalous coeval environmental and biotic changes independently recognized in other regional lake sequences. Collectively, these changes have produced the most distinctive boundary layer in the late Quaternary record. This layer contains a diverse, abundant assemblage of impact-related markers, including nanodiamonds, carbon spherules, and magnetic spherules with rapid melting/quenching textures, all reaching synchronous peaks immediately beneath a layer containing the largest peak of charcoal in the core. Analyses by multiple methods demonstrate the presence of three allotropes of nanodiamond: n-diamond, i-carbon, and hexagonal nanodiamond (lonsdaleite), in order of estimated relative abundance. This nanodiamond-rich layer is consistent with the Younger Dryas boundary layer found at numerous sites across North America, Greenland, and Western Europe. We have examined multiple hypotheses to account for these observations and find the evidence cannot be explained by any known terrestrial mechanism. It is, however, consistent with the Younger Dryas boundary impact hypothesis postulating a major extraterrestrial impact involving multiple airburst(s) and and/or ground impact(s) at 12.9 ka.
Because the impact layer at sites they tested was so thin, Firestone et al.’s 2007 stratigraphic work on the Younger Dryas boundary layer was difficult to replicate. As a result the debate concerning the Younger Dryas impact hypothesis continued to go around and around for a few years; and with numerous papers coming out during that time claiming to have debunked their work.
But the work of the skeptics who published work claiming to have debunked Firestone et al. was answered by Isabel Israde-Alcántara et al. in their section titled
‘Potential Misidentification of Markers’:
Surovell et al. reported finding no YDB MSp [microspherule] peaks, although claiming to follow the protocol of Firestone et al. for quantification of MSp, and concluded that Firestone et al. misidentified and/or miscounted the MSp. Later, Lecompte et al. independently examined two YDB sites common to Firestone et al. and Surovell et al. They reported that “spherule abundances are consistent with those of Firestone et al.” and “inconsistent with the results of Surovell et al.” They also concluded that Surovell et al. altered the prescribed MSp protocol in fatal ways, particularly by not observing requirements for sample thickness, sample weight, and size sorting. We consider these discrepancies significant enough to negate the conclusions of Surovell et al.
Daulton et al. found no YDB NDs [nanodiamonds] at Arlington Canyon, California, or at Murray Springs, Arizona, as earlier reported in Kennett et al. They searched for NDs in “microcharcoal aggregates” from the Murray Springs YDB site and, finding none, claimed to refute the previous results. However, Kennett et al. never claimed to find NDs in charcoal, and instead, observed NDs at Murray Springs in acid-resistant residues from bulk sediment which Daulton et al. did not investigate. Daulton et al. further speculated that Kennett et al. misidentified YDB NDs, observing copper instead, which displays d-spacings nearly identical to n-diamond and i-carbon. In addition, Daulton et al. pointed out that graphene and/or graphane have d-spacings similar to lonsdaleite and that the lonsdaleite diffraction pattern reported from Arlington Canyon by the Kennett et al. was missing the lonsdaleite diffraction line at 1.93 Å. However, in YD-aged ice in Greenland, Kurbatov et al. identified lonsdaleite with the 1.93- Å line, which definitively demonstrates that those Greenland nanoparticles cannot be graphene or graphane.
At Lake Cuitzeo, numerous NDs have been identified with the 1.93 Å line, as shown in Fig. 8 A and B and Fig. 11B, eliminating the possibility that these crystals are graphene or graphane. SAD and all other analyses conclusively show that the Cuitzeo nanoparticles analyzed have d-spacings consistent with lonsdaleite and other NDs. In independent support of NDs in the YDB, Tian et al. and Van Hoesel‡ identified cubic NDs in the YDB layer in Europe.
Regarding CSp [carbon spherules], Scott et al. speculated that those found at YDB sites are simply charred fungal sclerotia, which are ball-like clusters of long, branching filamentous structures, common to some fungi. The CSp from Cuitzeo and other YDB sites are unmistakably different from sclerotia in numerous critical characteristics. In particular, charred and uncharred sclerotia have textured, filamentous, low-reflectivity interiors, whereas at Cuitzeo, SEM [scanning electron microscope] imaging demonstrates that CSp have smooth, glassy, highly reflective interiors with no evidence of filamentous structure observed in fungal sclerotia (or cellular structure found in charcoal) (SI Appendix, Fig. 5). Cuitzeo CSp also contain numerous noncarbon particles, including aluminosilicates, indicating that these cannot be primary biological entities, such as sclerotia. In support of this, several lines of evidence support the formation of CSp during biomass burning. For example, Firestone et al. reported the production of CSp in modern wildfires, and laboratory experiments have demonstrated the production of CSp from charred tree resin at approximately 500 °C***. These CSp are morphologically identical to those found in the YDB but contain no NDs. Also, CSp similar to those found in the YDB have been reported by Harvey et al., who observed vesicular CSp in the impact layer at the KPg, [Cretaceous–Paleogene boundary] and suggested that CSp, along with aciniform soot, formed during impact into carbon-rich target rocks.
At all of the cites Firestone et al tested the impact layer was millimeters in thicknes, making there work difficult to replicate. But at 10-cm thickness, the impact layer at Lake Cuitzeo is not difficult to identify at all. In fact the data from central Mexico is so strong and compelling that the 2012 work of Isabel Israde-Alcántara et al. effectively replaces Firestone 2007 as the flagship paper for the YD impact hypothesis.
But to imply that other independent teams haven’t found supportive evidence is ludicrous. Here’s a parcial list.
Tian et al, 2010 went searching for diamonds in the YDB (In Europe it is also referred to as the Usselo Horizon) in Belgium. They wrote that “our findings confirm, and in fact reveal more direct proof than the earlier studies, the existence of diamond nanoparticles also in this European YDB layer No such particles are found in the overlying silt and clay or in the underlying fine sands.”
Van Hoesel A, Hoek W, Braadbaart F, van der Plicht H, Drury MR. (2011) Nanodiamonds and the Usselo layer. Paper #1556, XVIII INQUA-Congress, 21-27 July 2011 in Bern, Switzerland, reported finding “carbon aggregates [consistent with] nanodiamond” in YD-aged sediments In the Netherlands.
Abstract from Marshall W, Head K, Clough R, Fisher A. (2011) Exceptional iridium concentrations found at the Allerød-Younger Dryas transition in sediments from Bodmin Moor in southwest England.
Paper #2641, XVIII INQUA-Congress, 21-27 July 2011 in Bern, Switzerland. Elevated iridium values, dated to start of the Younger Dryas cooling event, have been found in sediments deposited at a number of Late Glacial sites in North America and one in Europe. It has been proposed (e.g., Firestone et al., 2007, PNAS 104: 16016-16021) that this widespread iridium enrichment signal is the result of an explosive disintegration of a large extraterrestrial object over North America around 12,900 cal. yr BP, and it is contended that it was this event which instigated the Younger Dryas cooling. This scenario is controversial, and the ‘ET’ explanation of these geochemical signals is not universally accepted. This notwithstanding, we report here the finding of an iridium anomaly in the Allerød-Younger Dryas boundary sediments at Hawks Tor in the southwest of England. The concentration of iridium and other elements is determined in peat monoliths using ICP-MS, operated in collision-cell mode, and ICP-OES instruments. We find an increase of over 300 % in the iridium concentration measured in the bulk sediment immediately above the Younger Dryas boundary compared with the values found below the transition. The iridium-titanium ratio is used to confirm a lag between the start of the iridium enrichment and the timing of abrupt environmental disruption at the site signalled by decreases in the organic carbon content, and changes the concentrations of potassium, iron and manganese. These geochemical changes coincide with a shift from a humified peat to a minerogenic lithology. By using a new calibration of existing 14C ages, integrated with new AMS dates and optically stimulated luminescence ages, we show that the timing of this iridium enrichment found in southwest England is in agreement with the dates proposed for the iridium enrichment signals previously found in North America and Belgium.
In Germany: Wolfgang Roesler et al., Carbon Spherules With Diamonds In Soils
In South America:
Mahaney WC, et al. (2010a) Evidence from the northwestern Venezuelan Andes for extraterrestrial impact: The black mat enigma. Geomorphology, v. 116, iss. 1-2, p. 48-57.
Mahaney WC, Krinsley D, Kalm V (2010b) Evidence for a cosmogenic origin of fired glaciofluvial beds in the northwestern Andes: Correlation with experimentally heated quartz and feldspar. Sedimentary Geology, v. 231, iss. 1-2, p. 31-40.
Mahaney WC, David Krinsley, Kurt Langworthy, Kris Hart, Volli Kalm, Pierre Tricart and Stephane Schwartz. (2011a) Fired glaciofluvial sediment in the northwestern Andes: Biotic aspects of the Black Mat. Sedimentary Geology. 237, (1-2), pp73-83
Mahaney, WC, Dave Krinsley, James Dohm, Volli Kalm, Kurt Langworthy and J. Ditto. (2011b) Notes on the black mat sediment, Mucunuque Catchment, northern Mérida Andes, Venezuela.. Journal of Advanced Microscopic Research, vol. 6, no. 3.
And on the North American Continent
Firestone RB, et al. (2007) Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling. Proc Natl Acad Sci USA 104:16016–16021.
Baker DW, Miranda PJ, Gibbs KE. (2008) Montana Evidence for Extra-Terrestrial Impact Event That Caused Ice-Age Mammal Die-Off. American Geophysical Union, Spring Meeting 2008, abstract #P41A-05.
Fayek, M.; Hull, S.; Anovitz, L.; Haynes, V.; Bergen, L. (2008) Evidence of impact material and the extinction of the mega-fauna 12,900 years ago. American Geophysical Union, Fall Meeting 2008, abstract #PP13C-1469.
Tankersley K. (2009) “Evidence of the Clovis Age Comet at Sheriden Cave, Ohio.” Midwest Chapter of the Friends of Mineralogy Symposium and Field Conference (Geology Department of Miami University, Oxford, Ohio, USA), 5 September 2009.
Firestone RB. (2009) The Case for the Younger Dryas Extraterrestrial Impact Event: Mammoth, Megafauna, and Clovis Extinction, 12,900 years Ago. Journal of Cosmology (journalofcosmology.com)
Kennett DJ, et al. (2009a) Shock-synthesized hexagonal diamonds in Younger Dryas boundary Sediments, Proc Natl. Acad. Sci. USA, 106 (31): 12623-12628.
Kennett DJ, et al. (2009b) Nanodiamonds in the Younger Dryas boundary sediment layer. Science 323:94.
Sharma M, Chen C, Jackson BP, Abouchami W. (2009) High resolution Osmium isotopes in deep-sea ferromanganese crusts reveal a large meteorite impact in the Central Pacific at 12 ± 4 ka. American Geophysical Union, Fall Meeting 2009, abstract #PP33B-06.
LeCompte MA, Goodyear AC, Demitroff M, Batchelor D, Mooney C. (2010) An Independent Review of the Younger Dryas Extraterrestrial Impact Hypothesis and its Recent Re-Evaluation by Surovell et al. 21st Biennial Meeting of the American Quaternary Association (AMQUA). Laramie, Wyoming. (this was the rebuttal of Surovell et al that is cited in the Lake Cuitzeo paper)
Andrei V. Kurbatov et al. (Journal of Glaciology, Vol. 56, No. 199, 2010) reported the ‘Discovery of a nanodiamond-rich layer in the Greenland ice sheet’
Scruggs, MA, Raab LM, Murowchick JS, Stone MW, Niemi TM. (2010) Investigation of Sediment Containing Evidence of the Younger Dryas Boundary (YPB) Impact Event, El Carrizal, Baja California Sur, Mexico. Geological Society of America Abstracts with Programs, Vol. 42, No. 2, p. 101.
Wu Y. (2011) Origin and Provenance of Magnetic Spherules at the Younger Dryas Boundary. Thesis, Dartmouth College.
And finally: Isabel Israde-Alcántara et al. (2012) Evidence from central Mexico supporting the Younger Dryas extraterrestrial impact hypothesis
And it's not just the Earth sciences that are reporting evidence of cosmic catastrophe at the time of the Pleistocene/Holocene transition. But the astronomers as well.
W.M. Napier (2010), Palaeolithic extinctions and the Taurid Complex