Holliday et al.'s Gish gallop: Introduction

The Younger Dryas impact hypothesis (YDIH) is first defined in Firestone et al. (2007).

“We propose 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.”

Furthermore, in their text we read;

“A number of impact-related effects most likely contributed to the abrupt, major cooling at the onset of the YD and its maintenance for ~1,000 years. Cooling mechanisms operating on shorter time scales may have included (i) ozone depletion, causing shifts in atmospheric systems in response to cooling, with the side-effect of allowing increased deadly UV radiation to reach survivors on the surface (46); (ii) atmospheric injection of nitrogen compounds (NOx), sulfates, dust, soot, and other toxic chemicals from the impact and widespread wildfires (46), all of which may have led to cooling by blockage of sunlight, with the side-effect of diminished photosynthesis for plants and increased chemical toxicity for animals and plants (46); and (iii) injection of large amounts of water vapor and ice into the upper atmosphere to form persistent cloudiness and noctilucent clouds, leading to reduced sunlight and surface cooling (46). Although these cooling mechanisms tend to be short-lived, they can trigger longer-term consequences through feedback mechanisms. For example, noctilucent clouds can reduce solar insolation at high latitudes, increasing snow accumulation and causing further cooling in a feedback loop. The largest potential effect would have been impact-related partial destabilization and/or melting of the ice sheet. In the short term, this would have suddenly released meltwater and rafts of icebergs into the North Atlantic and Arctic Oceans, lowering surface-ocean salinity with consequent surface cooling. The longer-term cooling effects largely would have resulted from the consequent weakening of thermohaline circulation in the northern Atlantic (54), sustaining YD cooling for ~1,000 years.”

Since this initial proposal, the geochemical evidence gathered suggests a more widely distributed event than implied by the above. Israde-Alcantara et al. (2012), which has many of the same co-authors as Firestone et al. (2007), write in their section “Comets”;

“Based upon astrophysical observations and modeling, Napier (48) proposed that YDB impact markers were produced when Earth encountered a dense trail of material from a large already fragmented comet. His model predicts cluster airbursts and/or small cratering impacts that could account for the wide distribution of YD impact debris across more than 10% of the planet, including Cuitzeo.”

They also write in the section “YD Impact Model”;

“A comet or asteroid, possibly a previously fragmented object that was once greater than several hundred meters in diameter, entered the atmosphere at a relatively shallow angle (>5° and<30°).”

Later, in Wolbach et al. (2018a), which also has many of the same co-authors as Firestone et al. (2007), we read in their section “Extended YD impact theory”;

“On the basis of evidence from the YDB, known impact events, and nuclear detonations, here we update the YDB impact theory. A giant, ≥100-km-diameter comet entered an Earth-crossing orbit in the inner solar system and began a cascade of disintegrations (Napier 2010). Numerous cometary fragments from the debris stream entered Earth’s atmosphere ∼12,800 y ago and detonated above and/or collided with land, ice sheets, and oceans across at least four continents in the Northern and Southern Hemispheres (Firestone et al. 2007; Napier 2010). Vaporization of cometary materials and platinum-group element (PGE)–rich target rocks injected Pt, Ir, Os, and other heavy metals into the stratosphere (Petaev et al. 2013; Wu et al. 2013; Moore et al. 2017), accompanied by impact-related nanodiamonds (Kinzie et al. 2014), meltglass (Bunch et al. 2012), and microspherules (for all proxies, see table A3). The impact event destabilized the ice-sheet margins, causing extensive iceberg calving into the Arctic and North Atlantic Oceans (Bond and Lotti 1995; Kennett et al. 2018). The airburst/impacts collapsed multiple ice dams of proglacial lakes along the ice-sheet margins, producing extensive meltwater flooding into the Arctic and North Atlantic Oceans (Teller 2013; see Kennett et al. 2018 for summary and references). Destabilization of the ice sheet also may have triggered extensive subglacial ice-sheet flooding, leaving widespread, flood-related landforms across large parts of Canada (Shaw 2002). The massive outflow of proglacial lake waters, ice-sheet meltwater, and icebergs into the Arctic and North Atlantic Oceans caused rerouting of oceanic thermohaline circulation. Through climatic feedbacks, this, in turn, led to the YD cool episode (Broecker 1997; Teller 2013; Kennett et al. 2018). Unlike for typical warm-to cold climate transitions, global sea levels rose up to 2–4 m within a few decades or less at the YD onset, as recorded in coral reefs in the Atlantic and Pacific Oceans (Bard et al. 2010; Kennett et al. 2018). Multiple impact-related drivers caused warm interglacial temperatures to abruptly fall to cold, near-glacial levels within less than a year (Steffensen et al. 2008), possibly in as little as 3 mo (Manchester and Patterson 2008). A rapid increase in wind strength across Greenland at the YD onset deposited extensive dust, sea salt, Pt-rich impact debris, and combustion aerosols into the ice sheet (this study). The radiant and thermal energy from multiple explosions triggered wildfires that burned ∼10%of the planet’s biomass, producing charcoal peaks in lake/marine cores that are among the highest in 368,000 y (Wolbach et al. 2018). This widespread biomass burning generated large amounts of long-lived, persistent AC/soot that blocked nearly all sunlight, rapidly triggering impact winter that transitioned into the YD cool episode (Wolbach et al. 2018). This widespread biomass burning delivered combustion aerosols (e.g., NH4 and NO3) to Greenland ice at some of the highest concentrations in ∼120,000–386,000 y (this study). NH4, NO3, and other biomass-burning by-products underwent chemical reactions in the atmosphere that resulted in acid rain (Firestone et al. 2007; this study). Climate change, wildfires, and related environmental degradation contributed to the late Pleistocene megafaunal extinctions and human cultural shifts and population declines (Firestone et al. 2007;Anderson et al. 2011; Wolbach et al. 2018; this study). ”

Furthermore, in Wolbach et al. (2018b), which also has many of the same co-authors as Firestone et al. (2007), we read in their section “Impact scenario and biomass burning”’

“Studies of the YDB impact, other known impact events, and nuclear detonations allow us to update the YDB impact hypothesis, as follows. Multiple fragments from a large, disintegrating comet collided with Earth ∼12,800 y ago (Firestone et al. 2007; Napier 2010). Airburst fireballs and the ejection of molten rocks would have triggered many individual wildfires over wide areas (Firestone et al. 2007; Napier 2010; Bunch et al. 2012; this study), producing one of the largest concentrations of combustion aerosols deposited in the Greenland ice sheet during the past 120,000–368,000 y (Wolbach et al. 2018). In the higher midlatitudes, atmospheric and oceanic temperatures abruptly decreased from warm interglacial to near-glacial conditions within a few months to a year (Manchester and Patterson 2008; Steffensen et al. 2008; Kennett et al. 2018). Atmospheric and cometary dust, along with AC/soot, triggered the rapid onset of impact winter (Kennett et al. 2018; this study). This blocking of sunlight led to a die-off of vegetation (this study). Damage to the ozone layer likely led to an increase in ultraviolet-B radiation reaching Earth’s surface, damaging flora and fauna (this article; Pierazzo et al. 2010; Thomas et al. 2015; Wolbach et al. 2018). Increases in nitrogen compounds, sulfates, dust, soot, and other toxic chemicals from the impact and widespread wildfires likely led to an increase in acid rain (Firestone et al. 2007). Increased production of organic matter and burn products from environmental degradation and biomass burning contributed to algal blooms and the subsequent formation of widespread black mats (Firestone at al. 2007; Haynes 2008). The likely reduction in soil-conserving vegetation would have led to higher water runoff, ponding, and increased erosion (Firestone at al. 2007). At or close to the YDB onset, many megafaunal taxa became extinct, and some surviving species experienced population declines and/or evolutionary/population bottlenecks.”

Thus, it is clear the favoured impact model involves a wide distribution of comet fragments from a previously disrupted giant comet that was dispersed across several continents, some fragments of which may have been sufficiently large to produce ground impacts. Together, these are proposed to have triggered extensive wildfires as well as massive flooding sufficient to alter major ocean currents leading ultimately to the Younger Dryas cooling, many megafaunal extinctions, and significant changes to human populations and cultures. The giant comet scenario proposed is consistent with a process known as ‘coherent catastrophism’ which is thought to be responsible for creation of the Taurid meteor stream.

Holliday et al. know these papers very well but misrepresent the YDIH and their work is a corruption of the scientific method. We see this immediately in their Introduction with a series of false and misleading statements. To see this, consider the following. My words are in italics.

     2. Section 1: Introduction

The Younger Dryas impact hypothesis (YDIH) is a collection of ideas proposed to

explain terminal Pleistocene environmental change across North America and other continents at the onset of the Younger Dryas (YD) stadial and the beginning of the YD Chronozone (YDC)

(Section 2).

This is misleading. The YDIH is not a “collection of ideas” and no evidence is provided to justify this statement. Instead it is a coherent theory.

While the specific details of the YDIH vary from publication to publication, the

general premise is that at ~ 12.9 ka North America and other continents were subjected to some

sort of extraterrestrial ‘event’ (either supernova shockwave; meteoritic, cometary, or very low-density object - impact(s); bolide airburst(s); or some combination thereof).

This is false. It is clear from its inception in Firestone et al. (2007) that the YDIH posits an ET impact by a low-density object (see above). The update in Wolbach et al. (2018a, 2018b) makes it clear that the preferred scenario is that of a previously-fragmented comet (see above).

The term ‘impact’ in “YDIH” represents all these possible cosmic events.

This is false. See above.

That event supposedly caused climate changes that define the onset of the YD stadial (see Firestone and Topping, 2001; Firestone et al., 2006, 2007; Kennett et al., 2008a, 2009a; Bunch et al., 2012; Israde-Alcántara et al., 2012; LeCompte et al., 2012; Wittke et al., 2013a; Moore et al., 2017; Kennett et al., 2018; LeCompte et al., 2018; Sweatman 2021; Powell, 2020, 2022). More significantly, YDIH proponents claim that the proposed impact at the beginning of the Younger Dryas (i.e., the lower “Younger Dryas Boundary [YDB]”) “triggered an ‘impact winter’ and the subsequent Younger Dryas (YD) climate episode, biomass burning, late Pleistocene megafaunal extinctions, and human cultural shifts and population declines” (Wolbach et al., 2018a, abstract), among other claims.

Correct.

A comprehensive and self-consistent statement that describes the YDIH, clarifies confusing/contradictory data, arguments, and interpretations does not exist.

This is false. Wolbach et al. (2018a, 2018b) provide a consistent and comprehensive ‘update’ statement of the YDIH (see above).

This paper is an in-depth critical review of the data and interpretations used to both

promote the YDIH and counter critics of the YDIH, including recent summary reviews of the

hypothesis (Sweatman, 2021; Powell, 2020, 2022).

Use of the word “promote” is pejorative and indicates Holliday et al.’s motivation might not be limited solely to debating the science. Inclusion of the recent reviews of Sweatman (2021) and Powell (2022) in this sentence indicates they believe these works are used to “promote” the YDIH. This is false. They are peer-reviewed and focus solely on the physical evidence, unlike Holliday et al. (2023).

In the following discussion we make liberal use of direct quotes to clarify communication disconnects that seem to characterize the debate and to better make our points. 

Any potential “disconnects” are caused by Holliday et al.’s (2023) use a wide range of texts, many not intended for a scientific audience, and the least charitable interpretations to create a long series of “gotchas” which are mainly just misinterpretations, misrepresentations, misunderstandings, or various forms of fallacious argument.

We repeat some of the critiques from previous papers. The

reason is obvious, as is apparent throughout this paper. The vast majority of critiques and

contradictory data have never been directly addressed by YDIH proponents.

Mainly, this is because these previous critiques were also Gish gallops, like Holliday et al. (2023). Until now, such challenges have been largely ignored as irrelevant since they usually contain no new physical evidence.

Critiques of the YDIH were published by researchers in a broad array of fields regarding reproducibility of results, extinctions, Clovis archaeology, stratigraphy, dating methods, YDC climate change, mineralogy, geochemistry, statistical probability, and impact physics, among other topics.

As we have already seen, Holliday et al.’s understanding of these various fields is limited by their poor understanding of experimental data analysis, especially uncertainty analysis, which underpins all science.

Proponents of the YDIH have argued that such critiques have been addressed, but either provide no citations or when provided, those citations do not adequately address the critiques (see Table 1). For example, Kennett et al. (2015b, p E6723) assert that criticisms that purported YDIH “impact proxies” also occur in multiple horizons outside the YDB were “refuted in detail” (citing Kennett et al., 2015a; Bunch et al., 2012; LeCompte et al., 2012; Wittke et al., 2013b). Similarly, Sweatman (2021, p 14) falsely asserts that rebuttals to Wolbach et al. (2018a, b) “were already addressed” but provides no references regarding those claims. Holliday et al. (2020, table 2) list eleven major claims based around the YDIH that are either partially or completely unaddressed in the YDIH literature. Table 1 summarizes the limited rebuttals to critics of the YDIH. Wolbach et al. (2020) provide the only lengthy attempt to rebut criticisms, but most of those rebuttals either repeat claims regarding the YDIH previously dismissed or miss the key points raised by critics (Table 1). This review demonstrates that the YDIH is untenable in the light of research since its initial conception.

They are addressed in this blog in a series of blog posts, line-by-line, at least within the chosen sections of Holliday et al.

[Table 1 is omitted for brevity]

The YDIH has a long, checkered history that is not rooted in science (see Daulton et al., 2017a, p. 7). 

This is false, because the YDIH was not defined before 2007. The YDIH began as a scientific proposal in Firestone et al. (2007) (see above). The physical evidence supporting the YDIH is contained only in peer-reviewed articles since 2007. Earlier non-refereed publications aimed at the general public might be of interest to historians, but they are irrelevant to the scientific debate.

Ultimately, the truth of the YDIH will be settled solely by the physical evidence. All other forms of evidence and argument are irrelevant.  Holliday et al.’s interest in other texts shows they are not motivated solely by the science and is a corruption of the scientific method.

One of the earliest versions of the hypothesis is the speculative book by Donnelly

(1883), which claims a comet struck North America forming the Great Lakes. As the story goes,

the aftermath devastated human (in particular) and other faunal populations and plunged the

climate into a period of extreme cold (or a return to glacial conditions). 

Clearly, this idea is similar, but it is not the same as the YDIH which was only defined in 2007. Thus, Holliday et al.’s argument is known as a ‘strawman’ fallacy and their observation is therefore irrelevant. In any case, YDIH proponents have never (to my knowledge) cited Donnelly. Moreover, Holliday et al.'s logic is flawed. It can just as easily be argued that since the YDIH is almost certainly true, then Donnelly likely had some insight into the YD comet impact event.

This idea was resurrected by R. Firestone in a series of popular magazine comments and a popular-press book. Firestone and Topping (2001) embraced and combined earlier, long-rejected ideas predating modern understanding of impact craters to argue that the Carolina Bays are Late Pleistocene impact structures (Melton and Schriever, 1933; Sass 1944; Eyton and Parkhurst 1975) and that a supernova irradiated the Earth in the Late Quaternary (Brakenridge, 1981). Subsequently, Firestone’s focus shifted from a supernova (Firestone and Topping, 2001; Firestone 2002) to Donnelly’s (1883) comet that created the Great Lakes. This shifted focus is described in the book The Cycle of Cosmic Catastrophes: How a Stone-Age Comet Changed the Course of World Culture (Firestone et al., 2006) and a journal article (Firestone et al., 2007) (see also Section 7).

All these works, except Firestone et al. (2007), predate the initial scientific proposal (Firestone et al. (2007)) and are therefore only of relevance to historians. Only peer-reviewed articles with physical evidence since 2007 are relevant to the scientific debate. Holliday et al.’s continued attempts to extend the debate backwards before 2007 is a corruption of the scientific method. This line of argument might be of interest to Wikipedia's editors, but it is of no relevance to science.

As for the supernova, it was then claimed to have perturbed the orbit of a solar comet (Firestone et al., 2006) or ejected an exosolar comet (Firestone et al., 2006, Firestone 2009a,b) that struck the Earth.

This is entirely plausible and Holliday et al. (2023) provide no evidence to the contrary.

Firestone et al. (2006, 2007) were the first publications to gain wide attention, in part

due to an AGU symposium in 2007 that drew considerable attention from the news media. 

This observation is irrelevant to the scientific debate as it contains no physical data or arguments.

The book is based on fanciful speculation and demonstrates a remarkable lack of understanding of the archaeological and stratigraphic data discussed. It contains many examples of misleading or blatantly untrue statements (noted throughout this review) and was described by Morrison (2010) as “pseudoscience.”

Morrison’s opinion is published in a popular science magazine and is not peer-reviewed by independent scientists. It is therefore irrelevant to the scientific debate which has taken place in scientific peer-reviewed articles since the publication of Firestone et al. (2007). In fact, inclusion of this term in reference to the YDIH is blatantly abusive and a corruption of the scientific method. It should never have been published in a scientific peer-reviewed journal article. Holliday et al. should be retracted.

The 2006 book and the other papers were not an auspicious prelude to the 2007 paper by Firestone et al. 

Since these earlier publications are irrelevant to the scientific debate in peer-reviewed journals since 2007, Holliday et al.’s (2023) opinion of them is irrelevant.

Further, that 2007 paper has problems including: a) poor-to-nonexistent

numerical age control for most sites (see Section 5.3); b) no data on identification of

nanodiamonds, polycyclic aromatic hydrocarbon (PAH) molecules (see Section 9.3), and

fullerenes with extraterrestrial (ET) helium (see Section 13.2); c) highly speculative

interpretations of the origins of magnetic spherules (see Section 10) and carbon spherules (see

Section 12.4); and d) failure to publish a table of the measured concentrations of their proposed

markers that they used to generate ambiguous graphs (see Section 13.6). That publication, as

well as many subsequent papers by the YDIH proponents, contains many significant and obvious misstatements of fact, circular reasoning, and problematic age control, all reviewed here.

Most of these sections are already addressed in this series of blog posts. We saw that these issues are almost entirely due to misunderstandings, misrepresentations, misinterpretations and fallacious arguments on behalf of Holliday et al..

Misunderstanding or misstating stratigraphic and archaeological records is a common theme in

support of the YDIH, as documented in this review and elsewhere. Claiming evidence where

none exists and providing misleading citations may be accidental, but when conducted

repeatedly, it becomes negligent and undermines scientific advancement as well as the credibility of science itself.

This is nonsense. Instead these are misunderstandings on behalf of Holliday et al. (2023).

Also culpable is the failure of the peer review process to prevent such errors of fact from entering the literature. The Proceedings of the National Academy of Sciences “contributed review” system for National Academy members (e.g., Aldous, 2014), as in the case of Firestone et al. (2007) and Kennett et al. (2009a), is at least partially responsible. The “pal reviews” (as some refer to them) were significantly curtailed in 2010, in part due to the YDIH controversy.

This is highly misleading. As already stated, the truth of the YDIH rests on the physical evidence alone and all other arguments, such as this one, are irrelevant. Much of the evidence Holliday et al. (2023) use to support their case is fundamentally flawed and should be withdrawn.

We begin our review of the YDIH by examining its foundation; specifically, we probe

the enigmatic questions the YDIH attempts to answer, and the assumptions behind those

questions. To place the archeological, paleontological, and paleoclimatic questions the YDIH

attempts to answer into proper context the “Younger Dryas” is defined in Section 2. In Section

3, the assumptions that underpin the foundational questions of the YDIH are examined in detail,

showing that several are flawed or fundamentally false, any one of which would reject the

overall hypothesis.

Previous blog posts have already rebutted these claims. We found that Holliday et al. are ignorant of core scientific principles.

The remaining part of Section 1: Introduction is addressed here.