Holliday et al.'s Gish Gallop: Impact scenarios

Holliday et al.'s "comprehensive refutation" of the Younger Dryas impact hypothesis (YDIH) is a highly misleading Gish gallop.

An earlier blog post addressed the presence of impact melts and microspherules in the YDB (Younger Dryas boundary). By themselves, they confirm a cosmic impact at Abu Hureyra near the YD onset. Due to the similarity of debris at the YDB across a wide range of sites, it's likely there were cosmic impacts across a wide area within a short timespan.

Other blog posts examined the presence of nanodiamonds and platinum in the YDB. As they occur in the debris layer with microspherules at Abu Hureyra, they are also good impact indicators. They are present in many other YDB layers, along with apparent impact spherules, signalling a widely distributed cosmic impact.

The previous blog post showed this layer of impact debris distributed across four continents within a narrow timeframe suggests a single event or highly correlated series of events within a narrow period is almost certain.

The remaining topic to be discussed is, therefore, the suggested impact scenario(s). The previous blog post already discussed this to a limited extent. A popular view of impact proponents is that the event likely involved a multitude of comet fragments that impacted an entire hemisphere of Earth within a single day.

Another possibility is that some comet fragments were captured by Earth's orbit and impacted at a later date, perhaps several years layer.

While some of these comet fragments might have caused ground impacts, it remains a possibility that all the impacts were airbursts. In this case, some would need to be at low altitude to generate large pieces of melt glass that have been discovered in several locations. 

Finally, there remains the possibility of a large ground impact, with the crater yet to be confirmed. Whether Hiawatha crater, ~30 km in diameter in north east Greenland, is a YDB-age structure remains to be seen. So far, evidence for its age has been highly inconsistent.

But let's see what Holliday et al. have to say in their Section 7. My comments are in italics as always.

7. Multifarious YDB Impact Scenarios

As noted by Boslough et al. (2012, p 13) “there is not one single Younger Dryas (YD)

impact hypothesis but several that conflict with one another regarding many significant details.”

This is due to the fact that different impact scenarios are necessarily required by YDIH

proponents to explain the disjointed contradictory evidence that is purported to support an

impact. 

This is nonsense. Holliday et al. appear to misunderstand how science progresses. It is normal in science to adjust, or fine-tune, a hypothesis as new evidence arises. See the diagram above, from Wikipedia, for example. The impact scenario has simply been refined over the last 15 years as new evidence has arisen.

Firestone and Topping (2001, p 15) speculated a supernova shock wave “gouged out”

the Carolina Bays to explain purported abnormal ratios of uranium isotopes and elevated

plutonium at Clovis sites.

I agree, this is unlikely, but it is not the Younger Dryas impact hypothesis.

The impact hypothesis was published by Firestone et al. in 2007, several years later. It is important that we address the impact hypothesis itself rather than a strawman.

To explain microspherules and other purported impact markers along

with the radiogenic isotopes Firestone et al. (2006) then speculated in their book that a supernova

“can knock asteroids and comets out of orbit to collide with the Earth” (p 21) and the “supernova

may have bathed a meteorite or comet with powerful radiation that altered its chemistry to form

the 40K that was carried to Earth in an impact” (p 93). Later in the book, Firestone et al. (2006)

apparently selected what they considered a more likely scenario and wrote, “We suggest that the

comets came directly from the supernova” (p 264), presumably as exosolar objects.

These various suggestions and interpretations are contrary to common knowledge about

supernovae, impacts, comets, and related phenomena. They represent pure fiction with little

science, defying the laws of physics and logic (also noted by Morrison, 2010). For example, the

origin of the Carolina Bays is controversial, but they do not have attributes associated with

known impact structures (Section 13.1) nor are they of YDB age (Table 4). Supernova shock

waves cannot "gouge" the Earth’s surface. Any close enough to be felt would blow away the

atmosphere and create far more damage than simply create the Bays.

Agreed. But this is further elaboration of a strawman argument.

Firestone et al. (2006, 2007) shifted focus from the supernova to the comet in order to

explain a range of claimed impact markers. 

In other words, they updated their hypothesis as new evidence and arguments were generated. This is how science works (see above).

However, the lack of any known YDB-aged crater

(Section 8) presented a serious challenge to the YDIH. To explain purported impact markers in

YDB sediments and the lack of associated crater(s), Firestone et al. (2007) speculated, “one or

more large, low-density ET objects exploded over northern North America” (abstract)

Correct. This is the impact hypothesis, and this scenario remains relevant. The later discovery of further YDB sites on four continents suggests these comet fragments were distributed over an entire hemisphere of Earth.

 ... and “if multiple 2-km objects struck the 2-km-thick Laurentide Ice Sheet at <30°, they may have left

negligible traces after deglaciation” (p 16020).

I agree, this scenario seems unlikely since 2-km objects are probably too large to not leave some kind of obvious crater. However, no evidence against this possibility if offered by Holliday et al..

To explain their claimed Ir and Ni concentrations in the YDB Firestone et al. (2007, p

16019-16020) narrowed down that their proposed “ET objects” were the comets from Firestone

et al. (2006), “The relatively low Ir and Ni peaks associated with the YDB are more consistent

with the generally proposed composition of comets and inconsistent with the high-Ir content

typical of most stony, nickel-iron, or chondritic meteorites.” 

Yes, this is how science works. Models that can explain the evidence are continually updated. We choose between the models on the basis of their "explaining power". This is Occam's razor, which is the central dogma of science. A swarm of comet fragments is the favoured scenario, and Holliday offer no arguments or evidence against this scenario.

Firestone et al. (2007, p 16020) also

reported that “some megafaunal bones in the YDB are highly radioactive” and “high

concentrations of U and Th were found in the YDB sediment at six of six Clovis-age sites

analyzed and in four of four [Carolina] Bays”. 

This sounds very interesting. Is it known to be false?

They speculated (p. 16020) “elevated levels of U

and Th may result from … dispersal of ejecta from the impactor and/or the target area”. This

would require improbable scenarios where either the comet was radioactive, or it struck

relatively-rich U deposits forming a problematic yet to be discovered crater.

In what sense is it problematic? Are all comets known to not be radioactive? No argument or source is provided to substantiate this claim by Holliday et al..

Embracing the ideas of Donnelly (1883) that the Great Lakes were formed by a comet

impact, Firestone (2009a, abstract) also speculated that the “comet fragmented and exploded

over the Laurentide Ice Sheet creating numerous craters that now persist at the bottom of the

Great Lakes.”

I agree, this seems unlikely. But is it known to be wrong? Holliday et al. offer no arguments or evidence against this scenario.

This followed Firestone et al.’s (2007, p. 16020) presupposition that oblique

impacts of comet fragments on the ice sheet produced “enigmatic depressions or disturbances in

the Canadian Shield (e.g., under the Great Lakes or Hudson Bay).” 

Is this not a possibility? No evidence or arguments are provided by Holliday et al. to rule out this scenario.

Firestone (2009a, section 6)

falsely claims, “Charity Shoal, a 1 km crater in Lake Ontario, has already been identified as

dating from the time of the YD impact (Holcombe et al., 2001)”, when in fact Holcombe et al.

(2001, abstract) reported, that the “feature may be extraterrestrial impact crater, but other origins

… are not ruled out. Time of formation is not known.”

Ok, but Charity Shoal remains a potential YD impact structure.

 Subsequently Firestone et al. (2010a, pp.

57–58) suggest “deep holes” beneath four of the Great Lakes could represent impact craters and

the “Finger Lakes region of New York radiate out from the hole in Lake Ontario as if they were

formed by the force of the impact pushing water and ice to the south.” They dismiss the

possibility that these holes were the result of glacial erosion, citing only the latest edition of a

19th century book by Dawson (1891). 

Again, this scenario seems unlikely.

As summarized by Holliday et al. (2014, p 517), the

problem with that speculation is that they provide no evidence that these depressions date to

~12.9 ka and at that time only the Lake Superior basin was still under glacial ice (Dyke, 2004).

I agree, that scenario seems speculative. But what about the possibility of comet fragments dispersed over an entire hemisphere? Will Holliday et al. ever discuss this or are they intent only on critiquing the more unlikely scenarios?

Further, the Great Lakes basins are elongated, oriented parallel to local ice flow and the “deep

holes” are in the up-ice end of the respective lake basins. Thus, the “enigmatic depressions” are

probably the result of glacial erosion and not the missing YDB craters that could explain many of

the claimed impact markers purported in the YDB.

Maybe. But what about the main impact scenario? Will Holliday et al. ever get to the point?

While Firestone and many other YDIH proponents continue to propose an airburst to

explain lack of a crater and purport microspherules as evidence of the ET event, the

microspherules lack a meteoritic component expected for bolide debris (see Section 10). 

This is false. While microspherules at many YDB sites mainly have a composition similar to Earth's crust, some do contain meteoritic components. For example, some microspherules found at Abu Hureyra have a meteoritic component.

For example, Bunch et al. (2012, p E1907) concluded that YDB “SLOs and spherules are terrestrial

in origin” and Sweatman (2021, p1) claims, “Elemental analysis shows most microspherules are

consistent with a terrestrial source.” These purported markers cannot be the product of an

airburst over an ice sheet or an impact that did not penetrate an ice sheet, and so require a solid earth

crater-forming impact (Section 8; but see Section 8 and 13.7). 

This is false. Sweatman's statement  that "most microspherules are consistent with a terrestrial source" is accurate and consistent with an ET impact event. Not all microspherules need to have a meteoritic component, regardless of whether they are formed by a ground impact or an airburst. And the favoured scenario is for a wide distribution of comet fragments, not just a few over the Laurentide ice sheet. Holliday et al. provide no evidence to support their assertion that microspherules cannot be formed by airbursts. In fact, there is strong evidence they can be, from sites like Tunguska and the Trinity atomic weapon test site.

Nevertheless, to explain

YDB microspherules, Firestone et al. (2007, p 16019) speculate they “resulted from the influx of

ejecta from an unidentified, unusually Ti-rich, terrestrial source region and/or from a new and

unknown type of impactor [/bolide].” 

There is nothing wrong with this statement and Holliday et al. provide no evidence or arguments to counter it.

Since no YDB-aged crater is known, Firestone et al.

(2010a, p 56) argued the latter option of an airburst claiming, “relatively little terrestrial ejecta

were created due to the shielding of the airburst from the ground by the ice sheet” and that

“[m]icrospherules from various sites … are comparable to lunar KREEP [acronym for

composition rich in potassium (K), rare-earth elements (REE), phosphorus (P) and a component

of some lunar impact breccia and basaltic rocks] and inconsistent with other terrestrial or

meteoritic sources except for meteorite SAU-169.”

It seems unlikely that an airburst could penetrate a thick ice shield. But, again, Holliday et al. provide no evidence or arguments to counter this scenario. Clearly, microspherules were mainly produced by airbursts or ground impacts elsewhere.

Firestone et al. (2010a, p 23) further write,

“It seems unlikely to have come directly from the moon however it is coincidental that SAU-169

[Lunar meteorite] fell in Oman near the time of the YD[B] impact”. They appear to reject but

also suggest the bolide is of Lunar origin rather than a comet, “low-density object” (Firestone et

al., 2006, 2007), or “very low density and/or unusually high velocity” object (Firestone, 2009a,

conclusion) that was proposed earlier.

Holliday et al. misunderstand this statement. Firestone et al (2010a) are noting the similarities of some microspherules with KREEP. The implication is that these microspherules were generated by an ET impact, not that they came directly from the moon! 

Teller et al. (2020, p 77) supports that scenario, “We

concur with Firestone et al. (2007), who concluded that elevated concentrations of these

elements most likely resulted from processes related to cosmic impacts/airbursts including… an

influx of meteoritic material from the impactor … For an example of the later… lunar

meteorites typically enriched in Th, U, Hf, and La.” 

Again, this is fine for the same reason as above. Holliday et al. have misunderstood these statements.

However, most YDIH proponents report

microspherules have terrestrial elemental composition (see Section 10), inconsistent with an

airburst/impact not penetrating the ice sheet.

Although the YDIH proposes an impact onto the Laurentide ice sheet, it does not preclude the possibility of further impacts elsewhere. Clearly, the microspherules were generated either by a ground impact or an airburst with sufficiently low altitude to melt and/or vaporise the ground. Such impacts are expected to generated microspherules with mainly terrestrial composition, but some will have a meteoritic component, exactly as observed. Some may have occurred over the Laurentide ice sheet, many probably did not.

Proponents of the YDIH claim that purported Pt anomalies at the YDB (Section 11) are

strong evidence of meteoritic material and an ET event (e.g., Moore et al., 2017; Sweatman

2021; Powell 2020, 2022) and ardently cite measurements of Greenland ice by Petaev et al.

(2013a). 

The word "ardently" is unnecessary and pejorative. It should not be included in a scientific paper.

However, that Pt anomaly would require a surface impact not an airburst, 

This would be interesting if true, since it suggests a YD-age crater should exist, but no arguments or evidence are cited to support this assertion. I expect airbursts or meteoritic ablation are fully able to produce a platinum anomaly.

and the lack of an identified YDB crater is again a serious problem for the YDIH. 

This is false. Lack of a crater is not yet a problem because the evidence can be explained by airbursts, although some would need to be very low altitude. However, a crater remains a distinct possibility.

Petaev et al. (2013a, p

12918) observed, “the highly fractionated Pt/Ir ratio rules out mantle or chondritic sources of the

Pt anomaly”, excluding icy comets with chondritic dust. 

This is false. We do not know enough about the composition of comets to rule them out and no evidence is cited to support this assertion. Moreover, as discussed by Petaev et al. (2013a), fractionation could lead to platinum observed in excess to other elements with a higher vaporisation temperature (like iridium).

Petaev et al. (2013a), in evaluating the

YDIH calculated that if the Greenland Pt measurements are representative of a global anomaly,

then the “Pt budget at the YDB” would “require an iron meteorite… of ~0.8 km in diameter”

that is “expected to form a crater of a few kilometers in diameter.” 

This is misleading because Petaev et al. (2013a) do not consider all scenarios. A series of comet fragments rich in platinum will suffice, and not necessarily require a crater.

In attempts to explain

without any experimental or theoretical support the purported diamondoids in Lake Cuitzeo,

Mexico YDB sediments (see Section 12.8), Kinzie et al. (2014, p 487) speculates another

scenario that “an impact took place in deep, petroleum-rich offshore sediments”, yet no YDBaged

submarine craters were identified. 

Agreed, some observations are not yet fully explained. But Holliday et al. offer no explanation either.

Pino et al. (2019, p 21-22) wrote, “Cr-rich spherules are

found in the YDB layer at Pilauco [Chile], but not found at the ~50 other sites on four continents,

suggesting ... airbursts occurred in the Cr-rich basaltic terrain circa Pilauco.” However, no YDB aged

crater was identified as the source of the “Cr-rich basaltic terrain” ejecta.

No crater is needed to explain airbursts. Microspherules can be produced without forming a crater because the temperature at ground level of a large airburst can reach tens of thousands of degrees Celcius, more than adequate for surface melting and vaporisation. Holliday et al. offer no arguments or evidence against airbursts as the source of many microspherules.

In attempt to explain skulls buried with microspherules, Hagstrum et al. (2017) with

Firestone as a coauthor propose yet a different impact scenario where impacts/airbursts

repeatedly occurred from ~46 kyr to ~11 kyr BP causing the megafauna extinctions over that

time (Section 3.2). 

This is almost correct, and Holliday et al. offer no arguments or evidence against other cosmic impacts over this time period. Note this scenario would be consistent with coherent catastrophism, which is discussed later. However, there is no claim that cosmic impacts have caused all the megafaunal extinctions over this period. They might only have "contributed" to some of them. This view is consistent with Napier (2010).

This contrasts with most YDIH versions that speculate a single impact event

involving multiple fragments occurred at the YD/GS-1 onset and caused the megafauna

extinctions.

This is false. The impacts suggested by Hagstrum et al. (2017) are not directly related to the Younger Dryas impact hypothesis. They are offering another impact hypothesis, obviously. The only connection is coherent catastrophism, i.e. if the YD impact occurred, then a reasonable hypothesis is that it was caused by coherent catastrophism, and this in turn suggests other large impacts over the last 20-30 thousand years or so are much more likely.

Pino et al. (2019, p 21) proposed a prodigious number of impacts at the YD/GS-1

onset, “There is a reasonable probability of one or more encounters within the last 13,000 years

with debris swarms from the Taurid Complex or other large fragmented comets, and such an

encounter would be hemispheric in scope, lasting for only a few hours. The resulting debris field

would be a mixture of dust and larger fragments, potentially equivalent to the impact of ~1000 to

10,000 destructive airbursts, such as occurred in Tunguska, Siberia in 1908... If such an event

occurred at the YD onset, larger objects in the debris swarm could have created craters on land,

struck the world’s ice sheets, and/or impacted the world’s oceans”. 

Yes, this is the currently favoured version of the impact scenario which is consistent with current observations, as stated right at the top of this piece.

A possible motivation for

such claims might be found in the claims of nanodiamond-containing carbon spherules at 14

purported-YDB sites across the globe (Kinzie et al., 2014). Impact proponents claim that while

carbon spherules and glassy carbon formed by common wildfires, only those that contain

nanodiamonds formed within the fireball of an impact (e.g., see Kinzie et al., 2014; Wolbach et

al., 2018b).

Correct.

Their formation would require a scenario of at least 14 separate but associated

impact events across several continents.

Correct, although it remains possible some of these YDB sites are connected by larger super-Tunguska events or by ground impacts.

However, see Sections 9.3, 12.4, and 12.5 regarding the

misidentification/ misinterpretation by impact proponents of purported YDB nanodiamonds and

carbon spherules (one supplied by A. West was dated at 207 ± 87 yr BP by Boslough et al.,

2012).

See an earlier blog post that rebuts Holiday et al.'s claims against the nanodiamond evidence.

Sweatman (2021, p 17) supports scenarios of numerous impacts with the concept of

“coherent catastrophism”, “Based on 30 years prior research into the Taurid meteor stream and

comet Encke, and the theory of ‘coherent catastrophism’”, citing Asher et al. (1994), Clube and

Napier (1984), and Napier (2001), then states Napier (2010) “proposed this meteor stream as a

potential culprit, citing an encounter with the equivalent of 2000–10,000 Tunguska-like objects

over about an hour was a ‘reasonably probable event’.” Sweatman (2021, p 18) also asserts,

“Napier’s ‘coherent catastrophism’ scenario is later boosted by Hagstrum et al. (2017).”

All correct, and Holliday et al. offer no evidence or arguments against this model here.

However, as noted in Section 3.2, Hagstrum et al. (2017) is highly speculative.

No evidence against Hagstrum et al. (2017) is offered here. Section 3.2 concerns megafaunal extinctions. An earlier blog post deals with this issue.

Sweatman (2021) castigates “Holliday et al. (2014), with Boslough as co-author” for

ignoring “coherent catastrophism” (p 18). Holliday et al. (2014) ignore it because “coherent

catastrophism” is a speculative hypothesis that is unsupported by observational data and

inconsistent with the cratering record.

This is false. Let's discuss coherent catastrophism now.

The cratering record shows that impacts with a wide range of energy scales have occurred over Earth's lifetime. However, the cratering record is far from complete and does not include any records of airbursts.

The impact rate derived from Earth's cratering record and other sources, like space surveys and lunar cratering, is an average over many hundreds of millions to billions of years. Yet the impact rate is expected to fluctuate significantly on various timescales, i.e. it is not expected to be constant. This is because we expect giant comets in Earth-crossing orbits will temporarily increase the impact rate. This is coherent catastrophism. Note that Wikipedia's definition of coherent catastrophism, which is found on the page titled the "Shiva hypothesis" is false. That article focuses on the timescale of impact fluctuations being correlated with the period of the solar system's passage through the plane of the Milky Way. In fact, coherent catastrophism is much more general than that.

To be clear, coherent catastrophism was suggested first by Clube and Napier (to the best of my knowledge) and quite generally applies to all situations where a comet decays in an Earth-crossing orbit. The idea was suggested again by Rampino (who must already have known about coherent catastrophism) and re-named the Shiva hypothesis with a special focus the period of motion of the solar system through the galactic plane.

Since we know that giant comets, from the population of centaurs, exist in the outer solar system and we know they have unstable orbits because of the presence of the outer planets, it is inevitable that they will occasionally form Earth-crossing orbits (see, for example, Horner et al. (2004), Napier et al. (2015), Galliazzo et al. (2018) etc.). Simulations reveal we can expect a centaur to form an Earth-crossing orbit every ~ 1000 years. The possibility this has never occurred is essentially 0. Most of these Earth-crossers will be small and not lead to significant changes in Earth's impact risk. Nevertheless, since most of the centaur population mass is contained in the largest centaurs, a giant centaur will occasionally significantly perturb Earth's bombardment risk.

Thus, coherent catastrophism is inevitable, and not a speculative theory. In my view, its creators deserve a Nobel Prize.

Evidence this has occurred recently, within the last 20-30 thousand years or so, is provided by the existence of the Taurid meteor stream and the density of the zodiacal dust cloud, both of which point to a recent episode of coherent catastrophism.

Napier et al. (2013) already responded to Boslough et al. (2013) on this subject, but it seems they were ignored (again). Their comments are repeated below.

"Boslough et al. (1) offer no alternate explanation for ∼10 million tonnes of Younger Dryas spherules recovered from 18 sites across ∼50 million square kilometers of North America, Europe, and the Middle East (2). In addition, the authors claim that our hypothesis “demonstrates a misunderstanding of comets.” However, the misunderstanding is theirs alone, because the model they criticize is their own creation and not the one we adopt, which derives from a substantial body of comet literature (e.g., ref. 3).

Most Earth-crossing comets arrive from the Oort cloud and the transneptunian region by way of the centaur population, an unstable reservoir orbiting between the giant planets that feeds the Jupiter family and Earth-crossing populations. From the size distribution of centaurs, as revealed by the Near-Earth Object Wide-field Infrared Survey Explorer space telescope (4), we find that there should be 30 centaurs >100 km in diameter with an average semimajor axis of ≤18 astronomical units (au) at any time. About half of them become Jupiter-crossing within 500,000 y, and 1 in 10 enter Earth-crossing, short-period orbits in the ecliptic plane, usually repeatedly (5). About one such entry occurs per 25,000 y, with a mean duration 2,000 y, although comets that reach sub-Jovian orbits achieve relative stability and much longer dynamical lifetimes. It is, therefore, not surprising to find the fossil remains of a large, recent, short-period comet in the near-Earth environment. There is such a system, comprised of about 15 correlated meteor streams containing some of the larger near-Earth “asteroids” (∼2–5 km wide), as well as comet Encke at 5 km in diameter. The whole system is embedded in a broad swath of meteoroidal material dominating the zodiacal cloud. The mass and dispersion of this material indicates that the progenitor comet was initially 100 km across and began to break up at least 20,000 y ago (6). This system’s mass is far larger than the entire current near-Earth asteroid system.

Most meteor streams originate from discrete fragmentation events rather than gradual sublimation, and the substreams of the complex show that the progenitor comet also followed this route. Breakups tend to happen just after perihelion passage, yielding clusters of fragments with, at 1 au, about 10,000 times the cross-sectional area of the Earth. Hundreds of such clusters (∼10^18 g) may be temporarily created over the lifetime of the comet. Passages through one or two are reasonably probable events, and are capable of yielding Younger Dryas boundary-like phenomena (3); no “exquisite timing” is required, as claimed by Boslough et al. (1)."

Sweatman (2021) misunderstands or misrepresents the

objections by orbital dynamics and impact physicists to the extreme version of the coherent

catastrophism hypothesis, which postulates without evidence that the current impact rate is

grossly underestimated. 

See above. The evidence for coherent catastrophism is overwhelming and the "objections by orbital dynamics and impact physicists" are not described or even cited by Holliday et al.. Nor is the "extreme version" of coherent catastrophism described. Is this just Holliday et al. setting up another strawman argument?

Cometary scientists who actually work in this area use the giant comet model, i.e. coherent catastrophism, as their working model.

It is worth stating again that it is obvious the impact rate must be underestimated by the cratering record and that it must fluctuate significantly on several different timescales. No evidence is offered by Holliday et al. against this. For example, they do not specify in what sense they think coherent catastrophism is inconsistent with observational data.

As with many of the YDIH proponents’ exaggerated claims, there is a

grain of truth to dynamic arguments for resonant Taurid swarm and the possibility of transient

increases in airburst rates when it intersects with Earth. Far from ignoring coherent

catastrophism, Boslough and Brown (2018) spearheaded an effort for an observational campaign

in the summer of 2019 to conduct astronomical surveys with the aim of detecting possible

objects in the hypothetical Taurid resonant swarm, which is foundational to coherent

catastrophism. 

It seems Holliday et al. do, in fact, accept the basic idea of coherent catastrophism after all? This is highly inconsistent and suggests they are mischievous.

So, what did this observational campaign conclude?

They used computational models to show that the Tunguska airburst effects were

indeed consistent with the trajectory of a Beta Taurid. Clark et al. (2019) subsequently calculated

the observability of the postulated resonant swarm and recommended an observational campaign

to document it in the summer of 2019. There were no reports of significant discoveries of

predicted Taurid swarm objects in 2019, however. The lack of observational evidence for the

predicted high-density swarm of such objects is inconsistent with the models of Napier (2010,

2019) that were invoked by Sweatman (2021) in support of the YDIH. Coherent catastrophism is

also discussed in Section 5.2.

That's because those observations never happened, so Holliday et al.'s pretence that it did is disingenuous (since they must know it never happened as intended). It's notable that no paper is cited here, since no paper was ever written on the results of this 2019 campaign (because it never happened). However, the 2015 observational campaign, not mentioned by Holliday et al., had already confirmed the presence of the predicted Taurid resonant swarm.

Moreover, Holliday et al. seem unaware of developments since the non-event of the 2019 observations. Indeed, Boslough's co-author back in 2018, P.G. Brown, co-authored a couple of papers in 2022, i.e. 6 months before Holliday et al. submitted their work. In one paper, Egal, Wiegert and Brown summarise;

"The Taurid Meteoroid Complex (TMC) is a broad stream of meteoroids that produces several annual meteor showers on Earth. If the linkage between these showers and 2P/Encke is at the centre of most TMC models, the small size and low activity of the comet suggest that 2P/Encke is not the unique parent body of the Taurids. Here, we simulate the formation of the TMC from 2P/Encke and several NEAs. In total, we explored more than a hundred stream formation scenarios using clones of 2P/Encke. Each modelled stream was integrated and compared with present-day Taurid observations. As previously reported, we find that even slight variations of 2P/Encke’s orbit modified considerably the characteristics of the simulated showers. Most of the comet’s clones, including the nominal one, appear to reproduce the radiant structure of the Taurid meteors but do not match the observed time and duration of the showers. However, the radiants and timing of most Taurid showers are well reproduced by a particular clone of the comet. Our analysis thus suggest that with this specific dynamical history, 2P/Encke is the sole parent of the four major TMC showers that have ages from 7 to 21 ka. Our modelling also predicts that the 2022 Taurid Resonant Swarm return will be comparable in strength to the 1998, 2005, and 2015 returns. While purely dynamical models of Encke’s orbit –limited by chaos –may fail to reveal the comet’s origin, its meteor showers may provide the trail of breadcrumbs needed to backtrack our way out of the labyrinth."

In another paper, Egal, Brown, Wiegert and Kipreos summarise (Abstract);

"We provide an overview of the observational properties of the four major Taurid showers, namely the Northern and Southern Taurids (#017 NTA and #002 STA), the βTaurids (#173 BTA), and the ζPerseids (#172 ZPE). Analysing more than two decades of meteor observations from visual, optical, and radar measurements, we present the Taurids average activity, annual variations in strength, radiant drift, and orbital variations as a function of solar longitude and particle size. The Taurid showers are detected over several weeks in the spring and autumn, but their annual activity level is generally low (less than 15 visual meteors per hour). We find the STA to be pre-dominant in autumn, while its twin, the ZPE, dominates over the BTA in spring. Due to their long duration, the position of each shower’s radiant and orbital elements are variable with time. Optical measurements have previously recorded enhanced STA activity and increased fireball rates caused by the return of a swarm of meteoroids trapped in the 7:2 mean motion resonance with Jupiter. However, we find no presence of the swarm in radar data, suggesting that small meteoroids are removed from the resonance faster than fireball-producing meteoroids. We also find the STA to be enriched in smaller particles early in their activity period. The differences we identify in our analysis between the showers at different particle sizes provide strong observational constraints to future dynamical modelling of the Taurid Meteoroid Complex."

Thus, it is clear that Boslough's colleague back in 2018, P.G. Brown, who also "spearheaded" the 2019 observational non-campaign takes a different view to Holliday et al. about the Taurids and the resonant swarm. Indeed, it all seems very consistent with the view expressed by Napier et al. (2013). It is stated as a matter of fact that the Taurid resonant swarm exists.

Then we have this paper by Ferrin and Orofino (2021) (Abstract)

"Using the Secular Light Curve (SLC) formalism (Ferrín, 2010), we have catalogued 88 probable members of the Taurid Complex (TC). 51 of them have useful SLCs and 34 of these (67%) exhibit cometary activity. This high percentage of active asteroids gives support to the hypothesis of a catastrophe that took place during the Upper Paleolithic (Clube and Napier, 1984), when a large short-period comet, arriving in the inner Solar System from the Kuiper Belt, experienced, starting from 20 thousand years ago, a series of fragmentations that produced the present 2P/Encke comet, together with a large number of other members of the TC. The fragmentation of the progenitor body was facilitated by its heterogeneous structure (very similar to a rubble pile) and this also explains the current coexistence in the complex of fragments of different composition and origin. We have found that (2212) Hephaistos and 169P/NEAT are active and members of the TC with their own sub-group. Other components of the complex are groups of meteoroids, that often give rise to meteor showers when they enter the terrestrial atmosphere, and very probably also the small asteroid that in 1908 exploded in the terrestrial atmosphere over Tunguska. What we see today of the TC are the remnants of a very varied and numerous complex of objects, characterized by an intense past of collisions with the Earth which may continue to represent a danger for our planet."

and this paper by Devillepoix et al. (2021) (Abstract)

"The Desert Fireball Network observed a significant outburst of fireballs belonging to the Southern Taurid Complex of meteor showers between 2015 October 27 and November 17. At the same time, the Cameras for Allsky Meteor Surveillance project detected a distinct population of smaller meteors belonging to the irregular IAU shower #628, the s-Taurids. While this returning outburst was predicted and observed in previous work, the reason for this stream is not yet understood. 2015 was the first year that the stream was precisely observed, providing an opportunity to better understand its nature. We analyze the orbital elements of stream members and establish a size–frequency distribution from millimeter to meter size range. The stream is highly stratified with a large change of entry speed along Earth's orbit. We confirm that the meteoroids have orbital periods near the 7:2 mean motion resonance with Jupiter. The mass distribution of this population is dominated by larger meteoroids, unlike that for the regular Southern Taurid shower. The distribution index is consistent with a gentle collisional fragmentation of weak material. A population of meter-sized objects is identified from satellite observations at a rate consistent with a continuation of the size–frequency distribution established at centimeter size. The observed change of longitude of perihelion among the s-Taurids points to recent (a few centuries ago) activity from fragmentation involving surviving asteroid 2015 TX24. This supports a model for the Taurid Complex showers that involves an ongoing fragmentation cascade of comet 2P/Encke siblings following a breakup some 20,000 yr ago."

So it seems the cometary science community is united behind coherent catastrophism and the Taurid meteor complex resulting from the decay of giant comet over the last 20,000 years or so. It looks like Holliday et al. are on a limb on their own.

Multifarious and conflicting impact scenarios (airbursts or impact cratering) involving

different impactors (exosolar comet, solar comet, lunar meteorite, iron meteorite, as well as “new

and unknown type of impactor” (Firestone et al., 2007, p 16019)) that strike at the YD/GS-1

onset or over tens of thousands of years are needed to explain the collection of otherwise

disjointed indicators that impact proponents also claim can only be explained as occurring

together by some unspecified mutually-compatible impact scenario.

This is false. See above. A swarm of comet fragments is thought to be able to explain observations and no evidence is provided by Holliday et al. against this.

 Despite the diverse

scenarios, with impacts in terrains of various geologies, that are required and have been proposed

for the YDIH, Sweatman (2021, p1) in his review of the YDIH inaccurately proclaimed, “The

YDIH explicitly claims the impact event was caused by one or more low density ET objects

falling onto the Laurentide Ice Sheet” and that “Elemental analysis shows most microspherules

are consistent with a terrestrial source” (emphasis added). Together Sweatman’s statements

necessarily suggest the impact must have penetrated the ice sheet leaving a yet to be recognized

YDB-aged crater in North America.

This is misleading because Holliday et al. fail to appreciate that impacts also occurred elsewhere. Although the YDIH posits that some fragments fell on the Laurentide ice sheet, it does not, and has never, claimed they only fell on the Laurentide ice sheet.

Sweatman (2021, p 5) asserts the “Greenland platinum

abundance [of Petaev et al. (2013a)] is one of the key pieces of evidence” and he incorrectly

paraphrases Petaev et al. (2013a) that they “maintained it [the YDB impact] must have been a

massive event, likely caused by a ~ 0.8km iron-rich meteorite” (p 4). Sweatman fails to provide a

viable explanation of how the Pt anomaly in Greenland, which he misreads as attributed by

Petaev et al. (2013a) to a massive iron meteorite, could be evidence for the impact of “low

density ET objects.”

Holliday et al. are very confused. The platinum anomaly could be caused by many impact scenarios. If the impactor(s) contained abundant platinum, as proposed, then an impact resulting in the creation of atmospheric platinum-laden dust with a lifetime ~ 20 years could have occurred anywhere on Earth, including via an airburst above an ice sheet and Holliday et al. provide no evidence against this.

Petaev et al. (2013a) did not conclude a massive iron meteorite was responsible for the Pt

measured, they only estimated its size to test a scenario that assumed a global Pt distribution

commensurate with that measured in the single Greenland ice core. 

Correct.

They subsequently pointed

out that a 0.8 km iron object was unlikely to disintegrate before it struck the ground, and that no

YDB crater has been found to support that scenario. 

This is misleading. Petaev et al. were simply estimating the size of the impactor without defining an impact scenario. The platinum could be contained in more fragile impactors that did not leave an impact crater. The favoured YD impact scenario is for a swarm of comet fragments. Some of these might have been large enough to impact the ground and form craters, but this possibility is not yet required.

The lead author of that paper who is also a

coauthor of this review (MP) now attributes it to a small local event, probably the one associated

with the Cape York meteorites as suggested by Boslough (2013). 

No impact crater is associated with the Cape York meteorite, yet previously Holliday et al. claimed a crater-forming ground impact is necessary to produce the platinum signal. Thus, it seems their position is inconsistent.

Despite specifically proposing the Laurentide Ice Sheet was impacted, 

The YDB extends across four continents, so a widespread event is envisaged.

... Sweatman (2021, p 18) suggests the subglacial Hiawatha

crater in Greenland is the “YD[B]-age impact structure” (Section 8.1). However, Sweatman

(2021, p 19) later confounds the confused issue further by proclaiming, “in principle no craters

are required for the [YDIH] theory" leaving many claimed YDIH impact markers unexplained

enigmas.

The confusion is in the minds of Holliday et al only. While Hiawatha could be a YDB-age impact structure, it is not yet required to explain the impact evidence. This is really very simple.

The dating of Hiawatha crater in several sources is conflicting and more evidence is needed to confirm either a very young or old age.

Summary

Holliday et al. offer no arguments or evidence against the preferred scenarios mentioned at the top of this piece. Thus, this entire section is one long strawman argument against scenarios that have been discarded as new evidence has arrived or are not relevant.

Further, it seems they are simultaneously antagonistic towards coherent catastrophism while also accepting its basic premise. They suggest the "more extreme" versions of this concept are speculative without providing any definition of what they mean or evidence to back up their claims. In fact, coherent catastrophism including the Taurid resonant swarm are largely accepted by the cometary science community. 

They disingenuously claim the latest 2019 observational campaign failed to detect the Taurid resonant swarm. Of course, that is true because that campaign never happened as intended an no papers have been published concerning it. Holliday et al. must know this themselves. It therefore provides no evidence against the existence of the Taurid resonant swarm, whose existence was already confirmed by the previous observational campaign in 2015.

They further claim, without evidence, that microspherules can only be generated by ground impacts. All the evidence we have is to the contrary. The extreme temperatures generated by low-altitude airbursts are fully sufficient for melting and vaporizing surface sediments without creating a crater. Boslough's own research shows this. Furthermore, impact-like microspherules have been found at Tunguska and Trinity.

They also claim that ground impacts are needed to produce the platinum signal. This would be interesting if true, but no evidence or arguments are provided to back this up.