The Younger Dryas Impact Theory
Originally recorded 12/08/22 for Episode 36 — Dr. Martin Sweatman, Ph.D.: Göbekli Tepe & the Younger Dryas Impact Theory
In this episode, Martin and I discussed the Younger Dryas (“YD”) period and the evidence supporting the impact hypothesis. Here is the abstract of Dr. Sweatman’s 2021 Earth-Science Review article reviewing the impact evidence[1]. In 2007, a team of scientists:
Firestone et. al., 2007, PNAS 104(41): 16,016-16,021, proposed that a major cosmic impact, circa 10,835 cal. BCE, triggered the Younger Dryas (YD) climate shift along with changes in human cultures and megafaunal extinctions. Fourteen years after this initial work the overwhelming consensus of research undertaken by many independent groups, reviewed here, suggests their claims of a major cosmic impact at this time should be accepted. Evidence is mainly in the form of geochemical signals at what is known as the YD boundary found across at least four continents, especially North America and Greenland, such as excess platinum, quench-melted materials, and nanodiamonds. Their other claims are not yet confirmed, but the scale of the event, including extensive wildfires, and its very close timing with the onset of dramatic YD cooling suggest they are plausible and should be researched further. Notably, arguments by a small cohort of researchers against their claims of a major impact are, in general, poorly constructed, and under close scrutiny most of their evidence can actually be interpreted as supporting the impact hypothesis.
The 2007 Firestone et. al. hypothesis proposed[2]:
That one or more large, low-density ET objects exploded over northern North America, partially destabilizing the Laurentide Ice Sheet and triggering YD cooling. The shock wave, thermal pulse, and event-related environmental effects (e.g., extensive biomass burning and food limitations) contributed to end-Pleistocene megafaunal extinctions and adaptive shifts among PaleoAmericans in North America.
Earlier this year, JL Powell published another paper highlighting the YD impact evidence and arguing that it had been prematurely rejected. On Substack, I’ve included Table 4 from Powell’s paper[3] which shows the various impact markers at 56 Younger Dryas boundary sites across five continents. Powell highlighted the following impact markers:
Black mat at 30 sites. Black mat refers to the stratigraphic marker covering the Clovis-age landscape of N. America during the YD period. No Clovis sites are found above the black mat layer, and many extinct megafaunal species are found below the black mat, but not within or above.
Impact spherules at 34 sites. Microspherules can be created by cosmic impacts, airbursts, and the non-catastrophic atmospheric burn-up of small meteorites.
Melt glass at 10 sites. Melted minerals above 1,500 degrees Celsius strongly indicates production by a cosmic impact, especially in the absence of lighting and volcanism.
Nanodiamonds at 26 sites. Nanodiamonds are nanometre to sub-micron sized carbon crystals that are known to occur within meteorites and cosmic impact structures.
PGEs at 38 sites. PGEs refer to platinum group elements like platinum, iridium and osmium which are much more abundant in meteorites than terrestrial upper crust rocks.
and fire markers at 39 sites. Intense thermal radiation would be expected to ignite wildfires on the ground following a cosmic impact.
Powell further highlights six major events that all occurred at or soon after the onset of the YD, approximately 12,850 years ago[4]:
1. The “great plumbing shift” of Glacial Lake Agassiz, a proglacial North American lake larger than the present Great Lakes combined. This shift occurred alongside destabilization of the Laurentide ice sheet. Lake Agassiz shifted from draining south down the Mississippi River eastward through the St. Lawrence system and northward down the Mackenzie River.
2. The flooding of the Mackenzie River.
3. Destabilization of the Fennoscandian ice sheet in Europe, resulting in a catastrophic flood of the Baltic Ice Lake.
4. The destabilization of the Greenland Ice Shelf. As geologist James Kennett notes[5]:
“It is difficult to explain the triggering of such widespread synchronous changes at the margins of three relatively isolated Northern Hemisphere ice sheets: Laurentide, Fennoscandian, and Greenland, and their related proglacial lakes by invoking conventional climatic and/or paleoceanographic processes. Instead, this broad range of evidence is more readily explained by catastrophic processes triggered by a cosmic impact with Earth: the YDB cosmic impact theory.”
5. In North and South America, about three-fourths of megafaunal mammals became extinct at or near the YD boundary.
6. The projectile points crafted by the Clovis culture disappear from the archaeological record. The population of the Clovis people also underwent a major decline at the YD boundary.
In addition to presenting the mounds of evidence supporting the YD impact hypothesis, both Sweatman and Powell took time to address the critics of the idea. They questioned why a small cohort of researchers have been so outspokenly critical of the hypothesis, and why the broader scientific establishment latched on to their refutations despite the relatively weak supporting arguments. The abstract to Powell’s paper reads[6]:
The progress of science has sometimes been unjustly delayed by the premature rejection of a hypothesis for which substantial evidence existed and which later achieved consensus. Continental drift, meteorite impact cratering, and anthropogenic global warming are examples from the first half of the twentieth century. This article presents evidence that the Younger Dryas Impact Hypothesis (YDIH) is a twenty-first century case.
The hypothesis proposes that the airburst or impact of a comet ~12,850 years ago caused the ensuing ~1,200 year-long Younger dryas (YD) cool period and contributed to the extinction of the Pleistocene megafauna in the Western Hemisphere and the disappearance of the Clovis Paleo-Indian culture. Soon after publication, a few scientists reported that they were unable to replicate the critical evidence and the scientific community at large came to reject the hypothesis. By today, however, many independent studies have reproduced that evidence at dozens of YD sites. This article examines why scientists so readily accepted the early false claims of irreproducibility and what lessons the premature rejection of the YDIH holds for science.
The “scientists reporting that they were unable to replicate” refers to an article by Nicholas Pinter and Scott Ishman titled, “Impacts, mega-tsunami, and other extraordinary claims”. This article was published in GSA Today, a magazine for members of the Geological Society of America which appeared just four months after the Firstone et. al paper, in January 2008. Their article starts[7]:
Recognition of the importance of impact cratering ranks among the most significant advances in earth and planetary sciences of the twentieth century, but recently there has been a proliferation of reports of impact events and sites that eschew simple, less spectacular alternative explanations. Here we focus on (1) Holocene-age ocean impacts and associated ‘mega-tsunami,’ and (2) a catastrophic impact event suggested at 12.9 ka. Carl Sagan once said that “extraordinary claims require extraordinary evidence”, we argue that these impacts do not meet that standard.
Powell goes on to sum up and dissect Pinter & Ishman’s article, stating they[8]:
Presented no new evidence, appealed to irrelevant arguments (the mega-tsunami and extraordinary evidence), and suggested processes to explain the abundance peaks that would have taken far longer than the decade or less that Hayes et al. said was possible. A reasonable conclusion would have been to call for scientists to reserve judgement on the YDIH and to seek additional evidence.
Instead of doing as Powell suggests, Pinter and Ishman had instead dismissively concluded their paper as follows[9]:
Both the 12.9ka impact and the Holocene megatsunami appear to be spectacular explanations on long fishing expeditions for shreds of support. Both stories have played out primarily in the popular press, highlighting how successful impact events can be in attracting attention. The desire for such attention is understandable in an environment when science and scientific funds are increasingly competitive. The National Science Foundation now emphasizes “transformative” research, and few events are as transformative as an impact. In an ere when evolution, geologic deep time, and global warming are under assault, this type of “science by press release” and spectacular stories to explain unspectacular evidence consume the finite commodity of scientific credibility.
Consume the finite commodity of scientific credibility. Translation – if you have a theory that goes against the grain, you should shut up because proposing it somehow hurts the credibility of science as a whole. Of all the hyperbole in this closing paragraph, this statement grinds my gears the most. Scientific exploration and the capabilities of the human imagination are infinite, and anyone who says otherwise is no true scientist.
In October 2009, a subsequent critical article by Surovell et al. was published in PNAS. Surovell et. al reported that they were unable to reproduce magnetic minerals and microspherules from seven sites of similar age, including two examined by Firestone et. al. Instead of recognizing that absence of evidence does not indicate evidence of absence, the authors concluded their report by writing[10]:
Reproducibility is fundamental to the scientific method and hypothesis testing; results that are not reproducible cannot be considered reliable or supportive of a hypothesis.
In short, we find no support for the extraterrestrial impact hypothesis as proposed by Firestone et. al.
Powell responded to the Surovell et al. paper by highlighting the flawed conclusion and ensuing damage this paper did to those supporting the impact hypothesis[11]:
It should have been obvious that Surovell et. al. had not established that [the YDIH microspherule evidence is irreproducible], only that they were unable to reproduce the evidence, for whatever reason. The possibility if not the near certainty that it was Surovell et. al. who had erred should have caused scientists to reserve judgement about the YDIH. Instead, as shown in Table 2, right up to present day many scientists have embraced the results of Surovell et. al. to cast doubt on the hypothesis.
On Substack, I’ve included Powell’s Table 2[12]. This table lists 17 scientific publications between 2010 and 2021 that have cited the 2009 Surovell et al. paper as justification for dismissing the YDIH. For example, a 2011 paper by Holliday et al. extended these flawed conclusions by stating[13], “All attempts to test and replicate this claim or to confirm aspects of this hypothesis have not been successful, raising serious concerns about the veracity of the claim”.
Now, I understand the merits of peer review, and that everyone should exercise healthy skepticism. I also agree with Carl Sagan’s argument that, “extraordinary claims require extraordinary evidence”. Yet I also question - at what point do we see a trade-off? Does forcing our thinking into the rigid models of our predecessors limit the unbounded nature of our imagination? Could fearing the ridicule of our peers after proposing something novel cause us to self-censor the next great idea?
The YD impact hypothesis – like the Alvarez theory on dinosaur extinction 40 years before it - has taught us that we barely know anything about the nuanced, mysterious history of our planet Earth. But isn’t that also the fun of it? Science is meant to be an exploration, an expression of the human pleasure in figuring things out. When scientism instead becomes a tool of orthodoxy, groupthink and condescension against free thought, it has devolved into a different phenomenon altogether.
But putting those philosophical questions to the side, researchers like Sweatman, Powell, Firestone and others have done an excellent job compiling the evidence in support of the Younger Dryas impact hypothesis. Helping us to dust off the lingering remnants of gradualism and embracing catastrophism. As Dr. Sweatman highlights, “Probably, with the YD impact event essentially confirmed, the YD impact hypothesis should now be called a ‘theory’” [14]. Powell agreed, noting that[15]:
“A scientific theory is a well-substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experiment. It refers to a comprehensive explanation of some aspect of nature that is supported by a vast body of evidence. One of the most useful properties of scientific theories is that they can be used to make predictions about natural events or phenomena that have not yet been observed.
Those who have read this article and Sweatman’s have the information to decide whether the YDIH meets this definition. In this author’s opinion, there is a strong case that it does. Moreover, it should not be forgotten that no other single theory can explain the YD and its associated effects.”
The Younger Dryas impact theory folks. That has a nice ring to it, what do you think?
[1] Sweatman, Martin B. The Younger Dyas impact hypothesis: Review of the impact evidence. Earth-Science Reviews. 218 (2021) 103677. May 19, 2021. Accessed Dec 20, 2022. https://doi.org/10.1016/j.earscirev.2021.103677.
[2] Firestone, R.B. et. al. Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling. PNAS. 104 (41) 16016-16021. Oct 9, 2007. Accessed Dec 20, 2022. https://doi.org/10.1073/pnas.0706977104.
[3] Powell, James Lawrence. Premature rejection in science: The case of the Younger Dryas Impact Hypothesis. National Library of Medicine. 2022 Jan-Mar;105(1):368504211064272. Jan 5, 2022. Accessed Dec 20, 2022. https://journals.sagepub.com/doi/10.1177/00368504211064272.
[4] Ibid.
[5] Kennett, J. SYNCHRONOUS ICE-DAM COLLAPSES AND OUTBURST FLOODING FROM NORTHERN HEMISPHERE PROGLACIAL LAKES AT YOUNGER DRYAS ONSET (12.8 KA) IMPLIES COSMIC IMPACT TRIGGER. Gol Soc Amabst program; 29:51. Accessed Dec 21, 2021. 10.1130/abs/2019AM-340492.
[6] Powell, James Lawrence. Premature rejection in science: The case of the Younger Dryas Impact Hypothesis. National Library of Medicine. 2022 Jan-Mar;105(1):368504211064272. Jan 5, 2022. Accessed Dec 20, 2022. https://journals.sagepub.com/doi/10.1177/00368504211064272.
[7] Pinter, N. and S. Ishman. Impacts, mega-tsunami, and other extraordinary claims. GSA Magazine, January 2008. Accessed Dec 21, 2022. https://www.geosociety.org/gsatoday/archive/18/1/pdf/i1052-5173-18-1-37.pdf.
[8] Powell, James Lawrence. Premature rejection in science: The case of the Younger Dryas Impact Hypothesis. National Library of Medicine. 2022 Jan-Mar;105(1):368504211064272. Jan 5, 2022. Accessed Dec 20, 2022. https://journals.sagepub.com/doi/10.1177/00368504211064272.
[9] Pinter, N. and S. Ishman. Impacts, mega-tsunami, and other extraordinary claims. GSA Magazine, January 2008. Accessed Dec 21, 2022. https://www.geosociety.org/gsatoday/archive/18/1/pdf/i1052-5173-18-1-37.pdf.
[10] Surovell TA, Holliday VT, Gingerich JAM, et al. An independent evaluation of the Younger Dryas extraterrestrial impact hypothesis. Proc Natl Acad Sci 2009; 106: 18155–18158. Accessed Dec 21, 2022. https://www.pnas.org/doi/full/10.1073/pnas.0907857106.
[11] Powell, James Lawrence. Premature rejection in science: The case of the Younger Dryas Impact Hypothesis. National Library of Medicine. 2022 Jan-Mar;105(1):368504211064272. Jan 5, 2022. Accessed Dec 20, 2022. https://journals.sagepub.com/doi/10.1177/00368504211064272.
[12] Ibid.
[13] Holliday VT, Meltzer DJ and Mandel R. Stratigraphy of the Younger Dryas Chronozone and paleoenvironmental
implications: Central and Southern Great Plains. Quat Int 2011; 242; 520-533. Accessed Dec 21, 2022. https://www.smu.edu/~/media/Site/Dedman/Departments/Anthropology/pdf/Meltzer/Holliday%20Meltzer%20Mandel%202011%20QI%20on%20YDC.ashx.
[14] Sweatman, Martin B. The Younger Dyas impact hypothesis: Review of the impact evidence. Earth-Science Reviews. 218 (2021) 103677. 19 May 2021. Accessed Dec 20, 2022. https://doi.org/10.1016/j.earscirev.2021.103677.
[15] Powell, James Lawrence. Premature rejection in science: The case of the Younger Dryas Impact Hypothesis. National Library of Medicine. 2022 Jan-Mar;105(1):368504211064272. Jan 5, 2022. Accessed Dec 20, 2022. https://journals.sagepub.com/doi/10.1177/00368504211064272.