Initial thoughts on Holliday et al.'s 'comprehensive refutation' of the YDIH

Apparently coincident timing of the Greenland platinum signal, which is attributed to a cosmic impact, and the onset of YD cooling, from Petaev et al. (2013).

Recently, Holliday et al. published a paper (currently a pre-print) purporting to 'comprehensively refute' the Younger Dryas impact hypothesis (YDIH). Unsurprisingly, their claims have been greeted enthusiastically by Wikipedia's editors.

After reading through the article in some detail (it is very long), it is clear that it is not a refutation at all. In fact, its arguments are weak and most have already been rebutted by earlier YDIH papers. The dictionary definition of 'refute' is to 'disprove' an idea. This has clearly not been achieved by Holliday et al., so their title is immediately misleading. 

Essentially, the paper is a monument to obfuscation, much like their other work. It re-hashes arguments that have already been rebutted in earlier YDIH papers, and it is chock-full of logical fallacies, wordplay, straw-mans, and the most uncharitable interpretations of YDIH papers. It draws on ancient texts (like Donnelly's Ragnarok!) to make its case as well as quoting from other non-refereed texts that were never intended to play a part in the academic debate on the YDIH.

The reason for their own confusion and frustration about the YDIH is that, in my view, they are determined to defeat the idea rather that treating it with an open mind. The result is that they habitually mischaracterise and misrepresent it. In fact, much of their disagreement can be traced to their unreasonable view on (radiocarbon) dating of sediments. The key point here is that an instantaneous event (i.e. the YD impact event), due to the inherent uncertainty in radiocarbon dating, will appear as a smooth transition over several thousand years if radiocarbon dates are taken at face value. To see this, look at the figure below from Gill et al. (2009). Therefore, any papers which do not compare the archaeological context to the YD boundary layer (or YDB), which are favoured by YDIH opponents, can automatically be questioned. In other words, proponents of the YDIH are working to a different internally-consistent model for the dating of archaeological finds where the YDB (which is located by its geochemical proxies) represents a fixed moment in time.

Radiocarbon dating of sediments from around the YDB in Gill et al. (2009). Clearly, the dispersion in radiocarbon dates is far larger than the error in individual measurements. Therefore, an instantaneous event can appear to be continuous over 1000s of years if individual radiocarbon dates are taken at face value. Instead, competent age-depth models are needed to deal with this kind of data. Unfortunately, the age-depth models of opponents of the YDIH are rarely competent because they often neglect to report the uncertainty in the regression parameters (i.e. the uncertainty in the slope or vertical shift in the line of best fit). This is a basic scientific requirement for regression (line fitting) that we teach to our undergraduates, but is neglected by Gill et al. (2009) and Meltzer et al. (2014) and yet defended by Holliday et al. who claim these uncertainty coefficients are an irrelevant detail. On the contrary, they are essential for their argument. Neglecting these uncertainty coefficients can be seen as an oversight, but defending that neglect, as Holliday et al. do, can be seen as pseudoscientific because they are defending results that are clearly not scientific.

The other disagreements relate to interpretation of what are considered impact proxies by YDIH proponents, but are simply denied by opponents. However, comparing Holliday et al.'s counter-arguments against the evidence and arguments put forward by YDIH proponents is like comparing chalk and cheese. Indeed, most of their arguments have already been refuted by earlier YDIH papers. Ultimately, the geochemical evidence for the YDIH remains exceptionally strong.

But Holliday et al. go further. They seem to claim that only evidence for ground impacts is acceptable, even though the YDIH geochemical evidence is often explained in terms of airbursts. Thus, Holliday et al.'s position is often unreasonable. It begs the question; how should ancient airburst events be detected? If only evidence for ground impacts is deemed acceptable, they are ruling out the possibility of ever detecting and counting the rate of ancient airburst events, which is an unscientific position. 

Moreover, even if some of the impact proxies are correct, Holliday et al. deny they are synchronous or significant. But ~10 million tonnes of impact microspherules (mainly of terrestrial composition - indicating melting or vaporisation of surface sediments) from sites across multiple continents, often dated with good accuracy, occurring apparently synchronously in sediments with nanodiamonds, platinum and very high temperature impact melts instead overwhelmingly suggest a large synchronous impact event.

To show this, I present below their list of arguments in their introduction that claim to debunk the YDIH. In each case, I show how they are mistaken. These points serve to illustrate the basic sources of disagreement and act as a guide to how Holliday et al. operate.

My response is in italics.

Basic misunderstandings of Holliday et al. in their introduction

a) The environmental changes at the beginning of the YDC are synchronous around the world. This assumption is probably true, and is supported by high-resolution, independently dated speleothem and lake records (Section 3.3), but synchroneity is not unique to the YDIH.

The phrase “synchroneity is not unique to the YDIH” is unclear. Perhaps Holliday et al. (2023) refer to the dating of YDB sites? While we would naturally prefer the dates of all YDB sites to be both consistent and precise, we must instead interpret the evidence as it is found rather than as we would like it to be. In other words, we can have no expectation a priori that the dates of YDB sites will always be precise, but we can have an expectation that they are not inconsistent with synchroneity. Holliday et al.’s (2023) view that the dating of all YDB sites must be both consistent and precise is an unrealistic demand. So far, no YDB site has been found to be obviously inconsistent with the proposed date of the YD impact.

b) The direct effects of the hypothesized impact were synchronous around the globe and date precisely to the YDB. This is clearly contradicted in archaeological, paleontological, and paleoenvironmental records (Sections 3, 5, 13.1, 13.7).

Again, it is not entirely clear to what this statement refers. If Holliday et al. (2023) are referring

to the onset of the Younger Dryas cooling, then evidence from the GISP2 ice core shows that

a clear platinum signal is synchronous with the onset of YD cooling in central Greenland to within experimental

uncertainty. If they refer to the demise of the Clovis culture then recall that no studies have yet

shown any Clovis artefacts above the YDB. If they refer to the end-Pleistocene megafaunal

extinctions, then remember that independent studies have shown that the extinction of many

megafaunal species are synchronous with the YD impact to within experimental error. 

This issue is really concerned with accurate dating of these “archaeological, paleontological,

and paleoenvironmental records”. As indicated above, Holliday et al. (2023) misinterpret the

uncertainty in such data. Probably, Holliday et al. (2023) are placing too much confidence in

individual radiocarbon dates.

c) The direct and indirect effects of the hypothesized impact were consistent in sign, pattern and magnitude with the “Impact Winter” scenario (or with nuclear winter or exceptional volcanism scenarios). This is contradicted by the spatial pattern of YD climates (Section 3.3).

The expected climate response of the YDIH is unknown, yet Holliday et al. (2023) presume it

must be dominated by an “impact winter” scenario. In fact, the YDIH does not claim to replace

the traditional explanation for the YD cooling, i.e. massive changes in ocean circulation currents

(AMOC). Instead, the YDIH provides the possibility that this mechanism was augmented by

other processes, like a brief impact winter. This is stated explicitly in Firestone et al. (2007) and

Wolbach et al. (2018a,b). The interaction between any putative impact winter and an altered

AMOC on the climate is unknown and remains to be investigated.

Moreover, the scenario of a brief impact winter cannot be refuted using the low-resolution data

cited by Holliday et al. (2023) in their section 3.3. Instead, detailed sub-annual paleoclimate

records must be used, and Holliday et al. (2023) do not use them.

d) The YD (and its accompanying climate reversals) was a unique episode during the Quaternary and requires a special explanation. This is contradicted by numerous long terrestrial, marine and ice-core records, which demonstrate that hundreds of such episodes occurred during the Quaternary (Section 3.3).

The mere existence of numerous climate ‘episodes’ during the Quaternary says nothing about how they occur. Even if they are generally consistent with the AMOC-halting hypothesis, there could be some that are not or some where the AMOC-halting mechanism is triggered by a catastrophic event. The mechanism for each episode should be investigated separately. There could be many different kinds of trigger, or just a few. A cosmic impact trigger might be common or limited to just the YD onset. The YDIH makes no claims about these other episodes. Indeed, the assumption that the trigger for any such episodes cannot be related to cosmic impacts is patently unsafe and no evidence is presented by opponents of the YDIH to support this view.

e) Clovis Paleoindians disappeared immediately after the impact. The ‘disappearance’ of Clovis was no more than an instance of cultural change, technological change and/or a change in settlement strategy (Section 3.1).

The extent to which the Clovis population was reduced by the YD impact is an open question. Holliday et al.’s (2023) assertion that it involved only “cultural change, technological change and/or a change in settlement strategy” is overconfident. As with many other issues in this area, interpretation of the “archaeological, paleontological, and paleoenvironmental records” depends strongly on the dating of such sites. This is, therefore, just another argument like b).

f) Megafauna extinctions began immediately following the impact (although extinctions are also claimed by some YDIH proponents to have occurred from multiple impacts over tens-of thousands of years). Many genera have last appearance ages that predate the YDC by millennia, and others survived to the end of the YDC or into the Holocene (Section 3.2).

To which specific extinction events does this phrase refer? The YDIH proposes the extinctions

clustered around the YD onset, sometimes called ‘Rancholabrean’ or end-Pleistocene, were

triggered by the YD impact. As there is good independent evidence these specific extinctions

are linked to the YD onset (e.g. Stewart et al. 2021, O'Keefe et al. 2023), there is naturally a

good case they were actually triggered by the YD impact. No claims are made about other

extinctions by the YDIH, but it is an open question as to whether cosmic impacts played a role

more generally (e.g. see Napier 2010). Holliday et al.’s (2023) assumption that cosmic impacts

cannot have played a role in any of these extinction events is patently unsafe and Holliday

et al. (2023) provide no evidence to support this view.

g) The demise of Clovis technology, and megafauna extinctions were unique, discretely dated events and require special explanation. These are baseless interpretations or assertions that contradict extensive data sets (Sections 2, 3, 5, 13.1, 13.7).

The apparently synchronous timing of YD climate change, a cluster of Rancholabrean megafaunal extinctions and cultural changes (including Clovis) with widely dispersed impact proxies at the YDB suggests otherwise. Again, this argument is really about how the YDB and archaeological and paleontological records are dated and is therefore an extension of b).

 

h) The sampling for data from sections spanning only hundreds or a few thousand years is sufficient to categorize an event as unique and unprecedented within many millennia. Long, well-dated sections with records of uninterrupted deposition must be subjected to discrete, continuous sampling and analysis to demonstrate the uniqueness of any claimed event of suite of purported impact indicators (Section 4). No such sections and data sets have been reported.

This presumably refers to the geochemical impact proxies found at > 50 YDB sites and is an

important point. Large, isolated airburst events, like Tunguska, are capable of generating impact debris similar to that observed at many YDB sites. Smaller airburst events that do not significantly affect the ground, like Chelyabinsk, and the background rain of cosmic dust are incapable of doing this.

Holliday et al. (2023) claim events like Tunguska occur on a timescale 1,000-10,000 yrs. So

far, 18 YDB sites have been discovered with such debris dated to within a 1,000 year period

around the estimated date of the YD impact. The probability that at least 18 Tunguska-like

impacts within a 1,000 year period can be estimated using a Poisson statistic. It is around

10^-17 and therefore negligible. Since YDB sites are widespread on four continents and easy

to find, the ultimate number of such sites is expected to number in the hundreds or thousands.

The vanishing probability that hundreds of such sites are dated to within 1,000 years

effectively proves that a single, large event, or series of highly correlated events over a short

timespan, took place. However, the probability of a single ~10,000 Mtonne event, which is the

minimum size currently thought able to explain the observed data, is quite reasonable. This

would correspond to a fragmented comet in the region of 500-1,000 m in diameter.

Essentially, the only reasonable explanation for a highly correlated layer of impact-like debris

on four continents is a major ET impact event.

The GISP2 ice core reveals a platinum spike within ~ 20 year period followed immediately by

significant cooling. This suggests a single large event is almost certain if this platinum signal

is the same as the one found at the YDB on several continents, as seems highly likely.

i) The beginning of the YDC must be determined using terrestrial age control. The YDC is defined as a component of the geologic time scale and its lower and upper boundaries are defined by Greenland ice-cores, supplemented by speleothem and other annual-resolution records (Section 5.1).

Proponents of the YDIH are working to a different model where the YD onset was triggered by

an ET impact. According to this model, the GISP2 platinum spike defines the onset of the YD

and coincides with a significant reduction in temperature in central Greenland. This platinum

abundance was airborne for around 20 years and therefore should be global, as remarked by

Petaev et al. (2013). A platinum anomaly has been detected at many YDB sites on several

continents coincident with other indicators of an ET impact, such as microspherules and

nanodiamonds.

Holiday et al. (2023) instead claim the onset of the YD climate anomaly precedes this platinum

spike. But close scrutiny of the ice core evidence from Steffenson et al. (2008), Seierstad et al.

(2014), Rasmussen et al. (2014) and Svensson et al. (2020) suggests the onset of the YD in

Greenland is highly uncertain with many questions unanswered.

The environmental response to the impact might not be synchronous across the whole globe. While an impact winter would affect the whole globe synchronously to varying extents, the YD climate change which followed also likely also involved disruption of major ocean currents (the AMOC hypothesis) and thus its effect might not be synchronous across the whole globe.

 

j) Numerical age control is accurate and precise at most sites with impact indicators and statistically conforms to a singular geologic event. Most sites lack directly dated samples from within their purported YDB layers and on adjacent layers, and even among those that have such samples, their dates vary between sites and many dates lack precision (Section 5). Age-depth models provide only an estimated age, typically with large statistical errors.

Currently, the YDB at 18 sites (and counting) is dated to within a 1,000 year period around the

expected date of the YD impact. As argued above, the possibility this debris resulted from

anything other than a single ET impact event or a highly correlated series of events within a

short timespan is negligible. 

YDIH opponents claim that only dates from within the YDB layer can be used for “age control”,

and that these must be consistent and precise. This is an unreasonable demand considering

the uncertainty inherent in radiocarbon dating and construction of age-depth models. Again,

this is an argument about dating, like b).

k) So-called “black mat” deposits and the Usselo/Finow soils are unique, date to the YDB (or YDC, depending on the version of the YDIH), and are a consequence of the impact. These organic-rich soils and sediments comprise a major source of confusion and contradictions surrounding YDIH. They are not linked to the YDB, and few examples are unique to the YDC (Sections 5.6, 6).

Proponents of the YDIH are working to a different internally-consistent model. No evidence is provided by Holliday et al. (2023) that these deposits cannot be related to the YD impact, and the 'confusions and contradictions' mentioned are word-play by YDIH opponents. Their assertion that “they are not linked to the YDB” is not supported by the evidence, which shows that a distinct layer of impact-like debris containing platinum, nanodiamonds and microspherules often occurs at the base of these “black mat” deposits. Again, this is an argument about dating, like b).

l) There is a simple YDB impact scenario consistent with known physics and all the purported evidence. Various (often conflicting and disjointed) impact scenarios have been proposed and are necessary to explain the wide range of physical sediment constituents offered in support of an “impact event”, i.e., supernova event, surface impact(s), and/or aerial bolide(s) (Section 7). The YDIH is a collection of different variant hypotheses (and impact scenarios) that attempt to use the same purported set of evidence with unavoidable conflicts and contradictions.

It is the very nature of science that explanations of observations will change over time as new evidence emerges. Here, opponents of the YDIH seem to be criticising the scientific method itself. We are still a long way from comprehensively understanding the precise nature of the YD impact event in all its aspects. Nevertheless, a coherent and concise impact model was described in Wolbach et al. (2018a,b).

 

A broad array of physical evidence is claimed by YDIH proponents to support the various impact scenarios. Proponents of the YDIH make a number of assumptions in their interpretation of the physical evidence and these include:

m) Craters that date to the YDB may or may not exist regardless of the purported evidence (to the contrary see Sections 7, 8, 13.1). Craters provide the strongest evidence of an impact and those dating to ~12.9 ka should be well preserved, but none are known (Section 8).

Impact craters are not yet a requirement of the YDIH, although the geochemical evidence at some sites suggests at least one is likely. Clearly, discovery of a YDB-age crater will tend to confirm the YDIH. This position has been consistent throughout. However, large amounts of impact microspherules with mainly terrestrial composition within a narrow timeframe on several continents does require some kind of surface alteration (from airbursts) or cratering (from ground impacts). But craters can be elusive, e.g. if they are under water, and surface scarring generated by airbursts can easily be missed today, and so their apparent absence to date does not negate the YDIH, much like the initial absence of a crater did not negate the Chicxulub impact.

n) The charcoal record of fire has been interpreted correctly and shows “the entire continent was on fire” (J. Kennett in Pringle 2007). The data on wildfires cannot be used to unambiguously indicate the extent, type, intensity or temperatures of fire (Section 9). The global charcoal record has been subject to various misapprehensions and misinterpretations (Section 9.1) and when reanalyzed by YIDH proponents yields results similar to that in the literature. Multiple peaks in charcoal abundance are documented through late Quaternary sections, but none have been shown to be uniquely associated with an impact.

It is not possible to show that a charcoal abundance is uniquely associated with the YD impact since charcoal is common in soils and usually related to non-impact wildfires. So, this complaint is unfair. Nevertheless, we can expect a cosmic impact to generate significant wildfires, and so a charcoal abundance, or an abundance of other forms of carbon/soot, are expected around the YD onset. Clearly, this issue is also related to dating, and so is just another form of b).

o) The ice-core record of fire was interpreted correctly and shows a big peak in fire at the YDB. YDIH proponents have badly misinterpreted the ice-core record (Section 9.2). The ice core and charcoal records are in agreement that the YDC (and the YDB in particular) was a time of low incidence of fire (Section 9).

Again, charcoal alone cannot be used to distinguish normal wildfires from cosmic impact triggered wildfires. Moreover, the charcoal database relies on radiocarbon dating alone and so cannot be used to draw reliable conclusions on the timing of charcoal spikes relative to the YDB. Instead, all evidence for wildfires and burning must be related to the YDB. This is another argument like b).

p) Spherules and microspherules are unambiguous indicators of an extraterrestrial impact and/or impact-generated wildfire. Microspherules can have various origins other than impact and cannot be used as impact indicators unless they are shown to be of meteoritic origin, which is not the case for most purported YDB microspherules (Section 10). The YDB carbon spherules are not impact-generated wildfire products but rather are fungal sclerotia that are ubiquitous in sediments (Sections 9.3, 12.4).

French and Koeberl (2010) list criteria by which cosmic impacts can be determined from microspherule evidence alone. The microspherules recovered from the YDB at Abu Hureyra appear to satisfy these criteria. The microspherules recovered from the YDB at many other sites seem to be very similar. Mostly they have a terrestrial composition, but meteoric components are also present in some cases. The carbon microspherules containing nanodiamonds very likely result from the impact while others may derive from other wildfires. 

q) Platinum-group element measurements of YDB sediments and ice provide support for an impact (to the contrary see Section 11). Platinum anomalies can arise from terrestrial sources and those reported by YDIH proponents are not uniquely associated with the YDB (Section 11).

It is true that platinum anomalies can also arise from volcanism. But volcanism produces other obvious signals that appear to be absent at the YDB. Given the platinum is coincident with other impact proxies in the YDB, it is most likely extra-terrestrial. It therefore is very likely the global signal expected after discovery of the GISP2 platinum spike.

r) Techniques and methods used to measure nanodiamond abundances are correct and accurate, nanodiamond identification is also correct, and nanodiamonds are reliable impact indicators. In most cases nanodiamond identification is suspect. All measurements of nanodiamond concentration in sediments/ice is scientifically meaningless, and in several cases irreproducible by YDIH proponents (Section 12).

Abundance peaks of nanodiamonds are not scientifically meaningless, especially if they are

synchronous with other impact proxies on at least four continents. Nanodiamond identification

is not suspect in general, although there are issues related to identifying the precise

nanocrystalline form of diamond. This is a rapidly-evolving research area in itself.

Nanodiamonds at the YDB have been identified by multiple independent research groups,

including opponents of the YDIH. This is the gold standard in science.

Daulton et al. (2010) failed to detect nanodiamonds in the YDB because they did not sample

materials likely to contain nanodiamonds nor did they sample the YDB at the site they claim.

This is very clear from the coordinates provided by Scott et al. (2010).

Holliday et al.'s claims of coincidence are preposterous

Firestone et al. (2007) proposed a cosmic impact onto northern ice sheets (and since found to extend beyond this) triggered the onset of the YD. The YD impact was therefore also potentially implicated in the decline of Clovis and the extinction of many megafauna. Several detailed forms of geochemical evidence supported the impact hypothesis, including nanodiamonds, microspherules and other high-temperature melts. All this evidence was found at the YDB at many locations on several continents, which is sometimes located indirectly via the termination of Clovis artefacts and megafaunal bones at the base of a black mat layer. Holliday et al. argue that this is all a coincidence. That is, it's just a coincidence that all this geochemical evidence was found precisely where it's expected at dozens of sites on several continents. 

Broecker, a key architect of the AMOC-halting cooling mechanism, proposed that, if the YDIH is correct, a PGE signal should be evident in Greenland ice cores at the onset of significant cooling. A platinum spike in the GISP2 ice core was duly found precisely where expected by Petaev et al. (2013). Holliday et al. (including Petaev now) argue this is also a coincidence, despite concluding the platinum signal was likely generated by an extra-terrestrial impact.

After the GISP2 platinum discovery Moore et al. (2017) raced to find a platinum anomaly in the YDB. They found it precisely where it was expected at many locations on several continents in the same place as the microspherules and nanodiamonds. Holliday et al. argue this is also a coincidence.

They also argue this evidence can be explained by other mechanisms. It can't.

It can't be explained by volcanism, for example, as obvious volcanic signatures are not found at the the YDB. Nor does any volcanic dust analysed by YDIH proponents contain any of the microspherules, nanodiamonds or high-temperature melts found at the YDB. And, moreover, several YDB sites are too far from any known volcanoes. Neither can the geochemical evidence be explained by small high-altitude airbursts that do not significantly affect the ground or by the background rain of cosmic dust.

They also argue that the YDIH can be refuted by the existence of other black mats at other time horizons in sediments. But it can't, because the origin of these other black mat sites is unknown. In many cases, the other black mats might have ordinary terrestrial triggers, including volcanism. Essentially, each black mat layer must be investigated in detail on its own terms.

No, all the evidence together at the YDB can only reasonably be explained by cosmic impacts that significantly affect the ground. In fact, Holliday et al. show this themselves. That is, they list many kinds of process that might generate the microspherules, high-temperature melts, nanodiamonds and platinum at the YDB. However, the only consistent mechanism among them all is cosmic impact. They therefore refute themselves.

They also argue that many Tunguska-like airburst impacts (potentially including some ground impacts, but this is not yet certain) that occurred in many locations on several continents within a small time window (potentially within 20 years according to the GISP2 platinum signal) are just a coincidence, i.e. they are unrelated. This is absurd.

No, Holliday et al.'s claims of coincidence are implausible. 

Irrelevant arguments

Holliday et al. (2023) also includes many irrelevant non-scientific details, such as TV shows, and much more that play no role in the truth of the YDIH. In science, all that matters is the scientific evidence and the arguments that link that evidence to the conclusions. Everything else is a distraction. One side of the debate (pro YDIH) keeps to a discussion of the scientific evidence, while the other (Holliday et al.) does not. This pattern is consistent (e.g. see Powell in Research Ethics (2022)). It should be clear enough from this alone who is debating in good faith. By inclusion of such irrelevant details, instead of casting doubt on the YDIH, Holliday et al. cast doubt only on their own motivations, the good name of the journal and the scientific method itself.

This is extremely important, as the inclusion of irrelevant details in scientific debate can lead to a chilling effect on scientific progress. This is because they can be perceived as forms of intimidation and bullying which, in turn, can prevent others from joining the debate or even from pursuing a career in science.