Holiday et al.'s (2023) Gish gallop: impact microspherules
Holliday et al.'s "comprehensive refutation" of the Younger Dryas impact hypothesis (YDIH) is a highly misleading Gish gallop. Their arguments against megafaunal extinctions and extensive wildfires at or near the YD onset are undermined by the recent paper by O'Keefe et al. (2023) concerning megafaunal extinctions in southern California. Even though those authors attribute the wildfires to human behaviour in a warming environment, their data is fully consistent with the YDIH and strongly supports it. See an earlier blog post for a discussion of this point.
Similar conclusions concerning the YD onset megafaunal extinctions are obtained by Stewart et al. (2021) generally for N. America and Conroy et al. (2020) specifically for the region of Alaska and the Yukon. That is, when only the most accurate radiocarbon dates and modelling are used, the data typically supports the YDIH. Remember, the YDIH only proposes the event 'contributed to' these extinction events, so this aspect of the YDIH has good support.
The last blog post addressed the timing of the Younger Dryas climate change and Greenland platinum spike. The remaining topics to be discussed are, therefore, the demise of Clovis culture, geochemical evidence for an impact, and the apparently synchronous timing of this evidence across ~ 50 sites on several continents. As the focus of my earlier review paper (Sweatman, 2021) was on the direct impact evidence and its apparently synchronous timing, I'll focus on these issues in the remainder of these posts. In any case, my main interest is in the local impact at Abu Hureyra, since it is likely recorded at Gobekli Tepe. I begin with the Younger Dryas boundary (YDB) microspherule data as it is some of the most convincing and revealing evidence for the YDIH. This corresponds to section 10 of Holliday et al..
My comments are in italics.
10. Purported YDIH evidence of impact: Spherules/Microspherules
Spherules and microspherules feature prominently in the YDIH. Determination of their
origin is experimentally challenging, prone to misinterpretation, frequently debated, and can be a
confusing topic. Impact proponents claim many different types, but they often talk about them
collectively with sweeping statements. The following discussion is a clarification of what
microspherules are and their utility as impact indicators.
Let's briefly review the micro-spherule evidence provided by YDIH proponents. I'll focus on the iron and silica-rich YDB microspherules as these were the ones I reviewed previously. These microspheres form a coherent 'assemblage' as they are found in an isolated (YDB) layer precisely where they are expected, frequently at or near the base of a conspicuous stratigraphic layer (the YD black mat).
The strongest YDB microspherule evidence is found in 3 papers, Bunch et al. (2012), Wittke et al. (2013), and Moore et al. (2020) which focusses on Abu Hureyra.
Bunch et al. (2012) examined microspherules and glassy debris (or SLOs - scoria-like objects) from the YDB layers at Abu Hureyra, Blackville and Melrose. Their sizes range from micrometres to millimetres. Apart from their size and shape they examined their ternary composition and melting diagrams.
Bunch et al. (2012) state: "Our research demonstrates that YDB spherules and SLOs have compositions similar to known high-temperature, impact-produced material, including tektites and ejecta. In addition, YDB objects are indistinguishable from high-temperature melt products formed in the Trinity atomic explosion. Furthermore, bulk compositions of YDB objects are inconsistent with known cosmic, anthropogenic, authigenic, and volcanic materials, whereas they are consistent with intense heating, mixing, and quenching of local terrestrial materials (mud, silt, clay, shale)."
Furthermore, Bunch et al. (2012) compare the surface textures of YDB microspherules with those from known impact sites; the KPg boundary (Chicxulub impact), Tunguska and meteor crater. Note the 'dendritic' surface texture.
Bunch et al. (2012) write: "Dendritic texturing of Fe-rich spherules and some SLOs resulted from rapid quenching of molten material. Requisite temperatures eliminate terrestrial explanations for the 12.9-kyr-old material (e.g., framboids and detrital magnetite), which show no evidence of melting."
Furthermore, Bunch et al. find evidence for inter-particle collisions and microcratering. They state: "Evidence for interparticle collisions is observed in YDB samples from Abu Hureyra, Blackville, and Melrose. These highly diagnostic features occur within an impact plume when melt droplets, rock particles, dust, and partially melted debris collide at widely differing relative velocities."
Bunch et al. also find evidence for extremely high melting temperatures, notably Lechatelierite with flow lines (Schlieren).
Bunch et al. (2012) state: "Lechatelierite is only known to occur as a product of impact events, nuclear detonations, and lightning strikes (15).We observed it in spherules and SLOs from Abu Hureyra, Blackville, and Melrose (Fig. 5), suggesting an origin by one of those causes." ... "Melting of SLOs, some of which are >80% SiO2 with pure SiO2 inclusions, requires temperatures from 1,700–2,200 °C to produce the distinctive flow-melt bands. These features are only consistent with a cosmic impact event and preclude all known terrestrial processes, including volcanism, bacterial activity, authigenesis, contact metamorphism, wildfires, and coal seam fires. Depths of burial to 14 m eliminate modern anthropogenic activities as potential sources, and the extremely high melting temperatures of up to 2,200 °C preclude anthropogenic activities (e.g., pottery-making, glass-making, and metal smelting) by the contemporary cultures."
Furthermore, Wittke et al. (2013) show that the same types of microspherule occur at the YDB at dozens of sites, exactly where they are expected.
Moore et al. (2020) give further details of high temperature melting of mineral inclusions in YDB microspherules from Abu Hureyra. The highest temperatures they infer (>2000 degrees C) is due to melted chromite and monazite, for which only cosmic impact and lightening strike are plausible processes.
Additionally, they find evidence for a meteoric component; "High YDB concentrations of iridium, platinum, nickel, and cobalt suggest mixing of melted local sediment with small quantities of meteoritic material."..."SEM-EDS analyses identified a melted PGE-rich grain embedded in a vesicle that contains ~20.9 wt.% Pt; ~6.9 wt.% Ir; and ~56.3 wt.% Fe (Fig. 12f). This nugget was embedded in the wall of the vesicle when the AH glass was molten. The occurrence of Pt and Ir in AH glass is consistent with neutron activation analyses which showed high Pt concentrations in the YDB layer, in which peak concentrations of 6.2 ppb Pt were measured in bulk sediment and of 8.1 ppb Pt in the magnetic fraction from sample E301, the YDB layer (Appendix, Table S7). Layers above and below showed negligible concentrations of Pt."
Finally, Moore et al. (2020) provide a diagnostic table that shows only a cosmic impact can explain the observed data, which is similar to other known impact sites.
Now, let's see how Holliday et al. explain these observations...
YDIH proponents claim that various spherules/microspherules are indicators of comic
impact or subsequent impact-generated wildfire (e.g., Firestone et al., 2007; Firestone et al.,
2010a; Kennett et al., 2009a; Bunch et al., 2012; Israde-Alcántara et al., 2012, 2018; Wittke et
al., 2013a; West et al., 2020a) and they describe different types at the YDB: carbon (see Sections
9 and 12.4), copal, magnetic (i.e., Fe-rich), Si-rich, Al-rich, Al-Si-rich, and Cr-rich. Impact
proponents often use the term spherule or microspherule (e.g., written alone and not with
adjectives such as carbon, magnetic, etc.) to discuss these different materials collectively, with
the implicit presumption they are physically similar and of common origin. However, they
exhibit compositional, mineralogical, as well as microstructural differences and, as such, should
be presumed of different origins unless proven otherwise.
This is nonsense. The YDB objects are all found within a distinct layer exactly where they are expected to be found, not above or below (within a reasonable time period/depth), dispersed across several continents within a relatively small time period. Of course we can considered them together - this is the 'hypothesis' part of the YDIH. Moreover, we can expect compositional, mineralogical and microstructural differences for the YDB objects. Even if there was only one impact site (which is generally not thought to be correct), a wide range of surface materials will have been vaporised or liquified, from surface sediments to trees, and a wide range of dynamic temperature and pressure profiles generated at different points, so there is no reason to expect the microspheres to be identical. In fact, we expect the opposite.
Possibly, Holliday et al. are thinking about a single large ground impact - they seem to be stuck in this mode of thinking. For a single large ground impact most of the ejecta will be produced from the underlying bedrock, and therefore will have a similar composition. But this is not the YD impact model proposed. Instead, YDIH proponents generally propose a wide distribution of impacts, mainly airbursts (some possibly low-altitude ones), and these will tend to mainly destroy surface features instead, not bulk bedrock.
For example, Wittke et al. (2013a, p
E2091) wrote, “Nearly all of the largest YDB spherules (maximum: 5.5 mm) are vesicular,
consistent with outgassing at high temperatures, followed by rapid cooling that preserved the gas
bubbles, and in some samples formed quench crystals within the bubbles. The prevalence of
vesicles decreases with [smaller] spherule diameter, and most small spherules <50 μm in
diameter are solid. All Fe-rich spherules and some Al-rich ones display dendritic crystals on their
surfaces, consistent with high-temperature melting and quenching [Bunch et al., 2012]. Most Al-
Si–rich spherules are smooth, but sometimes display flow marks, or schlieren, along with melted
SiO2 (lechatelierite) inclusions, both indicative of high-temperature melting at >2,200 °C [Bunch
et al., 2012].”
This passage explains the main differences between iron-rich and aluminium-rich YDB microspherules and is not evidence against the YDIH at all for the reason given above.
Even though small (mm to cm-size), often glassy impact melt bodies (“spherules”) may
be ejected from the cratering site during impact, and form (often geographically extended) ejecta
deposits, their existence as melt particles is not as such diagnostic for impact, and their
identification as impact products depend on association with other, confirmed impact-produced
features, such as shock effects.
As described above, YDIH proponents use a diagnostic process based on many characteristics to reach their conclusions that most of these microspherules are very likely impact generated. They do not simply rely on them being "small, round and glassy". Moreover, the general view is that most of the impacts were airbursts, perhaps quite low in altitude, and not often (or perhaps at all) crater-forming. Thus, the requirement that only evidence for crater-forming impacts is acceptable is not appropriate. Consider that, if a YD airburst actually did occur, according to Holliday et al.'s criteria it would be impossible to scientifically verify this. Thus, their approach can be rejected; it is not scientific.
Tektites and microtektites are the best-known and most-studied
of these ejecta deposits (e.g., Koeberl, 1994), but a variety of other glass-rich ejecta deposits,
have also been noted. The identification of such glasses as impact or non-impact products is
difficult and commonly controversial as discussed in the extensive review by French and
Koeberl (2010).
French and Koeberl (2010) is focussed on ground impacts, not airbursts, and is therefore not so relevant for this debate, except where ground impacts may have occurred. Consider that the word "airburst" is mentioned only twice in their paper, both times in the context of the YDIH. The word "crater", however, is mentioned 272 times. Clearly, their work is not really appropriate for critiquing the YDIH.
Especially, French and Koeberl (2010) do not critique the detailed, simultaneous, diagnostic features described by YDIH proponents. Simply saying their identification is "difficult" is not good enough. Therefore, they provide no good arguments against the YDIH, and there are certainly no refutation arguments.
However, several quotes from French and Koeberl (2010) are worth highlighting; "Likewise, siderophile-element anomalies may be absent in spherule layers" and "the use of microspherules alone as impact indicators requires careful and meticulous work to demonstrate conclusively that they are: (1) natural and (2) impact-produced" (my emphasis).
Clearly, French and Koeberl (2010) accept that PGEs may be absent in impact microspherules and that it IS possible to diagnose an impact from microspherule evidence alone. Regarding the latter point, French and Koeberl (2010) state; "Petrographic characteristics of impact-produced spherules include (Simonson, 2003, pp. 52–62):
(1) a restricted size range (typically 60–
2000 μm);
(2) the presence of abundant splash-form shapes (spheroids, dumbbells, teardrops, etc.) indicative of melting;
(3) crystallization textures that develop inward from the rims rather than
outward from a central core; (4) an absence of associated typical
nonspheroidal fine volcanic materials (e.g., glassy shards, glassy
filaments [“Pele's Hair”], corroded phenocrysts, or volcanic rock
fragments); and
(5) the presence of associated definite shock effects,
such as particles containing lechatelierite or grains of shocked quartz
with PDFs (but see also Marini and Rauka, 2004).
Geochemical
indicators of impact-produced spherules include (Montanari and
Koeberl, 2000, pp. 71–77, 118–123; Simonson, 2003; Simonson and
Glass, 2004):
(1) original compositions corresponding to natural
crustal target rocks or to mixtures of such rocks;
(2) compositions
unlike typical volcanic rocks;
(3) an absence of exotic compositions,
e.g., rare-metal alloys, hydrocarbons or enrichments in non-meteoritic elements such as Ba, Ti, Mn, Pb, etc., which, if present, suggest a
natural or artificial terrestrial origin; and
(4) the presence of
anomalous concentrations of projectile-related chemical elements
(e.g., Ir and the other PGEs) or isotopic signatures (e.g., Os, Cr)"
The above criteria mirror those used in the BWM papers. Lechatelierite, platinum and iridium are found in YDB objects at Abu Hureyra, confirming a ground impact or airburst there. Elevated platinum or lechatelierite are noted at ~ 10 other YDB sites, strongly suggesting impacts at those locations too. The similarity of microspherules across many more YDB sites strongly suggests they were subjected to ground or airburst impacts too, although it would be helpful to know exactly which YDB sites exhibit lechatelierite. The relative lack of craters or shock metamorphism discovered at YDB sites to date points mainly to airbursts, but this could change.
Some studies, in which such spherule layers have been carefully examined by geological,
petrographic, and geochemical techniques, provide strong evidence that they formed by
meteorite impact events. In most cases, however, the confirmation of the impact origin did not
come from the spherules themselves, but from associated minerals or geochemical anomalies.
This section appears to plagiarise French and Koeberl (2010), which states "Recent studies, in which such spherule layers have been carefully examined by geological, petrographic, and geochemical techniques, have provided strong evidence that they have been formed by meteorite impact events (Margolis et al., 1991; Montanari and Koeberl, 2000, Chs. 2,3; Simonson, 2003; Simonson and Glass, 2004). However, in most cases the confirmation of the impact origin did not come from the spherules themselves, but from associated minerals or geochemical anomalies."
“Microspherules are not, by themselves, diagnostic indicators of impact events, because similar
objects can be produced by a wide range of geological and artificial processes… Identification of
microspherule-bearing layers as impact ejecta needs additional evidence: geological context,
association with genuine quartz PDFs [planar deformation features], high-pressure minerals, or
definitely extraterrestrial siderophile-element anomalies” (French and Koeberl, 2010, p 151-
152). Only evidence based on highly siderophile elements and/or isotope ratios, such as Os or Cr
isotopes, can provide unambiguous evidence of the presence of a meteoritic component in
spherules. For example, the ratio of Ni/Fe provides no evidence for a potential meteoritic
component.
YDIH proponents have presented detailed diagnostic evidence for impact microspherules. To counter their claims, Holliday et al. need to provide equally detailed counter-evidence. They need to be specific.
It is important to recognise essential differences between airbursts and ground impacts. For a ground impact the resulting fireball will tend to effectively mix materials from the ground and the meteor. The impact shock will also be intense, sufficient to create shock processed materials. This is not necessarily the case for airbursts. Instead, the explosive fireball occurs at height in the absence of ground materials. Therefore, much of the resulting meteoric debris will be dispersed at height without mixing with the ground. Nevertheless, some of the meteoric debris will be carried downwards in the hot explosion gases to melt and/or vaporise (and thus mix with) surface materials, but without necessarily forming a crater (none has been found at Tunguska, for example). Likewise, depending on the airburst height, the shock will be much less intense at the ground and may be insufficient to cause typical (or any) shock processing of materials. Moreover, if the impactor was a comet, as suspected, then the meteoric component is likely to be a smaller fraction of the total mass (which is mainly different kinds of ice (water, CO2, CO etc.)), especially if it was low in chondritic or PGE materials. So, detection of comet airbursts using only the PGE (platinum group element) composition is more uncertain as any PGE component will more likely be concentrated within a small fraction of the debris (the nugget effect).
Nevertheless, meteoric components within YDB objects have been detected at Abu Hureyra (platinum and iridium inclusions within microspherules), and in other work (Moore et al. (2017)) a distinct platinum signal was detected in the YDB at many locations. Although Wu et al. (2013) do not detect an obvious osmium signal in YDB microspherules from 6 sites, this does not rule out an ET impact since the GISP2 evidence is that the impactor was weak in some PGEs. Remember, French and Koeberl's (2010) statement "Likewise, siderophile-element anomalies may be absent in spherule layers".
As summarized by French and Koeberl (2010, p 145-147), “There are several major
problems in attempting to use spherules as independent evidence for meteorite impact events…
Spherules alone do not provide diagnostic evidence of origin by impact.
This contradicts other statements in the same paper - see above. It all depends how 'diagnostic' your evidence is. The BWM papers set out a very convincing diagnostic case. Only equally detailed arguments can refute it. Moreover, their work meets all the criteria listed by French and Koeberl (2010) for impact microspherules at Abu Hureyra. Elevated platinum, Lechatelierite or other high temperature melts strongly suggests impacts at many other YDB sites and the similarity of microspherules across most YDB sites suggests impacts at those locations too.
Like other impact melts, droplet spherules generally preserve no evidence of shock processes or of their original ultrahigh-temperature origin….
This is false. YDIH proponents have provided plenty of good evidence for YDB microspherule production at very high temperatures, including surface textures, molten mineral inclusions and ternary composition-melting diagrams.
In many distal ejecta layers, spherules are not accompanied by
other materials that show distinctive and unambiguous shock-metamorphic effects. Exceptions
include the occurrence of coesite and shocked quartz grains with microtektites...
This still does not address the diagnostic evidence provided by BWM.
An especially
severe problem in using spherules as a unique impact criterion is that the spheroidal shape by
itself is not a unique indicator of impact or even of melting. A wide variety of nonimpact
spherical, spheroidal, or droplet-shaped bodies, both natural and artificial, are abundant in the
geological environment. Such features can easily be (and frequently have been) [mis]interpreted
as impact-produced objects… Natural glassy spheroidal objects in the same size range as impact
produced spherules include volcanic droplets and lapilli... and meteorite ablation debris… In
addition, natural non-melting processes can produce a wide variety of similar spheroidal objects.
Low-temperature sedimentary and diagenetic processes can produce spheroidal oolites, fecal
pellets, spherulites, fossils, algal structures, and other organic and inorganic constructions...
Other spheroidal objects in sediments can include organic pollen and plant spores… siliceous
plant phytoliths…, and objects produced by the alteration of hydrocarbon deposits.” In addition,
a dismayingly large variety of artificial spherules (cf. Marini, 2003), produced intentionally or
accidently by various melting and manufacturing processes, even containing the hightemperature
silica melt glass lechatelierite (Marini and Raukas, 2009), are being increasingly
recognized as contaminants in geological samples and laboratories (French and Koeberl, 2010).
Again, YDIH proponents use a diagnostic method that considers many YDB microspherule characteristics. At no point are their arguments based solely on sphericity. So the above discussion is irrelevant.
The paper by Firestone et al. (2007) was the first widely published study to claim
magnetic and carbon microspherules as evidence for a YDB impact. However, the critically
important question on the origins of the microspherules was not answered in the Firestone et al.
(2007) study. Firestone et al.’s (2007) elemental measurements of magnetic spherules were
indeterminate (p. 16019) stating, “composition of YDB magnetic microspherules and magnetic
grains … cannot be explained at this time” and potential shock effects in associated minerals
were not investigated. Firestone et al. (2007, p. 16019) wildly speculated they “most likely
resulted from influx of ejecta from an unidentified, unusually Ti-rich, terrestrial source region
and/or from a new and unknown type of impactor” (emphasis added). The latter improbable
claim can be made of nearly any mineral specimen found on the Earth’s surface. As for the
carbon spherules, Firestone et al. (2007) speculated they were products of impact-generated
wildfires based on finding them in wildfire-impacted forests (see Section 9.3). However, no
systematic control study was performed on forests not impacted by wildfire, for if they had, they
certainly would have found sclerotia (see Section 12.4). Curiously and equally revealing, no
YDIH impact proponent that has studied the YDB (including those that investigated “carbon
spherules”) has made any mention of also finding sclerotia in over 50 examined YDB sites (e.g.,
see Tables 3 and 4), despite the fact sclerotia are ubiquitous in soils and sediments (see Section
12.4). The only purported characteristic of the YDIH microspherules that could potentially
suggest an impact was Firestone et al.’s (2006, 2007) claim that their concentration spiked at the
YDB.
Since the original Firestone et al. (2007) paper, much more detailed studies have been performed on YDB objects by BWM. It is pointless challenging Firestone et al. (2007) which has long since been superseded.
Surovell et al. (2009) was the first attempt to reproduce that claim by examining two of
Firestone et al.’s (2007) sites and five additional sites. The test failed. Among sites studied by
Firestone et al. (2007), Surovell et al. found no spherules at Topper. At Blackwater Draw they
found them only above the YDB and at concentrations an order of magnitude less than Firestone
et al. purported at the YDB. LeCompte et al. (2012, E2964-E2967) responded with five
perceived issues regarding the methodology used by Surovell et al. (2009). Sweatman (2021, p
6) summarized, “They concluded that there were significant deficiencies in the analytical
methods used by Surovell et al. (2009).” Surovell et al. (2009) outline their protocols and
Surovell (2014) carefully responded to the critique. Sweatman (2021) and others ignored
Surovell’s comments and failed to consider the fundamental issues. Surovell et al. (2009)
designed their study to follow the protocols as described by Firestone et al. (2007, supplemental
materials) in an attempt to reproduce their results, a fundamental practice in scientific research.
Improving and optimizing the methodology of Firestone et al. (2007) was not their goal. It was to
see if the controversial results could be independently reproduced. However, the criticisms by
LeCompte et al. (2012) grossly mischaracterize and revise the protocols described by Firestone
et al. (2007). More problematic, following the publication of Surovell et al. (2009), A. West
dramatically revised his protocols for collection of spherules. Other researchers also note
changing criteria for collection of carbon spherules (Hardiman et al., 2012) (Section 12.4),
another problem in YDIH research not addressed by YDIH proponents. ENDNOTE 14
While visual-based quantification of the spherule abundance is subject to selection bias of
the investigators, the magnetic grains isolated from the sediments (from which the spherules
were selected for counting) are not subject to same selection bias. The concentration profiles of
the magnetic grains at Blackwater Draw and Topper (and at five additional sites) measured by
Surovell et al. (2009) showed no peak at the YDB but rather occurred throughout the layers
investigated contrary to results of Firestone et al. (2007).
Powell (2020, 2022) likewise devotes considerable space to a critique of Surovell et al.
(2009). In his book Powell (2020, p. 146) writes, “LeCompte et al. sampled [Topper and Paw
Paw Cove] the very YDB sites where Surovell et al. could find not a single microspherule and
found them in abundance. No scientist who convincingly located the YDB and used SEM and
XRS has failed to find ET microspherules”, although LeCompte et al. (2012) interpreted the
spherules as likely to be terrestrial. Powell (2022, p 12) states, “[a]t Topper … Surovell et al.
found no microspherules at all” and “Surovell et al. failed to sample the YDB and/or erred in
their procedures. When dealing with objects on the scale of tens of microns, avoiding such errors
requires punctilious care.” Powell (2022, p 14) concludes, “The simplest explanation is again
that Firestone et al. sampled the YDB at Topper while Surovell et al. did not.” The
condescending argument about procedural errors is an after-the-fact explanation of inconvenient
data. The Clovis and post-Clovis levels at Topper are mixed (Miller, 2010; see also Section 5.7),
a point Powell (2022) may be unaware of, and this mixed stratigraphy could well explain the
discrepancy in results.
Regardless of methodological details, critics fail to note that Surovell et al. (2009) did in
fact recover both microspherules and magnetic grains (confirmed by A. West; Table 1), claimed
to be two of the most reliable markers from the Firestone et al. (2007) study. The key issue is
that they were unable to recover spherules in YDB zones with purported dramatic spherule
spikes at sites studied by Firestone et al. (2007) (Blackwater Draw and Topper). At additional
sites not studied by Firestone et al. (2007), Surovell et al. (2009) recovered no spherules from
two sites (Paw Paw Cove, MD and Shawnee-Minisink, PA), but at three other sites (Agate Basin,
WY, San Jon, NM, and Lubbock Lake, TX) spherules were recovered outside the YDB at
abundances similar to those at the YDB. Lubbock Lake was selected for further study (Holliday
et al., 2016, see also Section 12.6) by Surovell et al. and J. Kennett (a leading YDIH proponent).
They analyzed splits of the same samples collected continuously across the YDB. The difficult
question on origin of the microspherules was not addressed; instead the study focused on
quantification within the sediments. They recovered similar levels of microspherules from
samples spanning ~13k to ~11.5 cal ka BP (<0.4 gm/kg), indicating that the methods used by
Surovell et al. were adequate. More significantly, Kennett’s analyses recovered very high
amounts (roughly an order of magnitude higher) in a layer dated <11.5 cal ka BP (Holliday et al.,
2014), but Surovell et al. (2009) recovered none. Powell (2022) repeatedly raises issues of
reproducibility but ignores this study. The methods used to recover spherules by Surovell et al.
(2009), following Firestone et al. (2007), works but is unable to reproduce the concentration
profiles purported by Firestone et al. (2007). In another study at Blackwater Draw, Andronikov
et al. (2016b) had mixed results. The concentration profile of magnetic grains was consistent
with Surovell et al. (2009) and showed no peak at the YDB, while the profile of the spherules
showed a peak at the YDB consistent with Firestone et al. (2007). ENDNOTE 15
This is just re-hashing ancient history, and a distraction from the diagnostic evidence presented by BWM. Squabbling over who followed the correct procedure before BWM is irrelevant. To refute the YDIH microspherule evidence, Holliday et al need to address the strongest evidence, which is in the BWM papers.
Other problems in the identification and dating of microspherules abound (see Tables 3,
4). For example, Wu et al. (2013) “analysed microspherules from a range of YDB sites in North
America and Belgium” and claim only the Melrose, PA and Newtonville, NJ sites suggest an
impact and “the impact took place near the southern margin of the Laurentide Ice Sheet” (p.
3565).
This is blatantly untrue. Wu et al. (2013) claim all the YDB sites they investigated have evidence for an impact.
Similar to the first reports of discovery of the Hiawatha Crater (Section 8.1), YDIH
proponents greeted the news about these microspherules as “unequivocal evidence for an
impact” (Jones, 2013). Compositional and Os isotopic measurements of the microspherules
yielded no evidence of a meteoritic component, however, and their possible association with
shocked materials was apparently not investigated.
This is correct, but does not refute the YDIH at all for the same reasons as given above.
Wu et al.’s (2013) identification of the
microspherules as impact products was assumed from textures and presumed upper-bound melt
temperatures.
Actually, like the BWM papers they use a wide range of impact indicators. So this is not accurate.
Since there is no age control at Melrose and Newtonville (see Table 4), the
presumed impact indicators were used by Wu et al. (2013) as stratigraphic markers to identify
the YDB, again representing circular reasoning. Further, the purported but undated YDB zone at
Newtonville produced about the same number of spherules as the deeper and dated “Late
Wisconsin” layer (Table 4).
This is also misleading. The two (upper and lower) samples at Newtonville are in contact at a stratigraphic horizon, so they could both contain YDB objects without any contradiction. In any case, it is correct that the Melrose and Newtonville sites investigated by Wu et al. (2013) have poor age control. So we should not place to much emphasis on these particular results. Nevertheless, they are consistent with other YDB sites that do have more accurate age control. Still, Holliday et al. do not critique the very strong evidence in the BWM papers.
No objective evidence is presented to support a YDB age for any
part of either the Melrose or Newtonville sites (Holliday et al., 2014; Meltzer et al., 2014).
“Thus, the conclusions of Wu et al. (2013) are drawn from two undated sections correlated to an
unknown crater” (Holliday et al., 2014, p 518).
See above.
Israde-Alcántara et al. (2018) report on YD-age black mats from five sites in northern
Mexico (Table 4). Four of the sites yielded microspherules and other claimed impact indicators,
three of those sites yielded microspherules dating younger than the YDB (Table 4). Only four sites
(including the one with no microspherules) had black mats and they produced YDC
ages with only one of them (Tocuila) that may date to the YDB.
As you might expect by now, this statement is also misleading. The five lakes are; Tocuila, Acambay, Cuitzeo, Chapala and Cedral. The dates of the claimed YDB impact debris are as follows. The Tocuila black mat is consistent with a YDB age (10,800,+- 50 BP uncalibrated radiocarbon age). The Acambay black mat is undated, but lower sediments are older than the YD onset, as expected. The Cuitzeo black mat from the main core has a single radiocarbon date inconsistent with the YD onset, but a detailed age-depth model from another core from the same lake with the same stratigraphy does have a consistent YD age (12,897 +- 187 cal BP). This is described in Kinzie et al. (2014). The Chapala microspherules are in a black mat layer that has a single radiocarbon date inconsistent with the YD onset. This core would benefit from a higher resolution age-depth model, like Cuitzeo. Cedral has no microspherules, but several black mats, although none are thought to be of YD age.
Israde-Alcántara et al. (2018) argue the microspherules
formed by an impact based on their textures and claim their impact origin “is
confirmed by comparing the geochemical composition of the microspheres to those from known
impact events, as discussed in previous studies, including Bunch et al. (2012), Wittke et al.,
[(2013a)], and references therein” (p. 76). However, Israde-Alcántara et al. (2018) did not detect
a meteoritic component in the spherules, and they apparently did not search for associated shock
features.
Once again, Holliday et al. are fixated with shock features and obvious meteoric components, but such features are not necessarily associated with microspherules generated by airbursts.
This is also the case for Bunch et al. (2012), Wittke et al. (2013a), Andronikov et al.
(2016b), Hagstrum et al. (2017), LeCompte et al. (2018), Kletetschka et al. (2018) and Pino et al.
(2019) yet they concluded an impact origin based on several or all of the following factors:
presumed melting temperatures; the similarity of the internal and surface morphologies as well
as elemental composition of YDB spherules to those from known impact sites.
At last, we get to the best evidence. Bunch et al. (2012) and Wittke et al. (2013) are two of the 3 papers with the best evidence for microspheres, the other being Moore et al. (2020). But Holliday et al. completely ignore their diagnostic evidence for YDB microspheres, instead focussing only on the issues of PGEs (meteoric components) and shock features, none of which are necessarily associated with microspheres generated by airbursts. Moreover, they ignore the presence of platinum and iridium in microspheres at Abu Hureyra in Moore et al. (2020).
Let's see what detailed arguments they make against the BWM papers...
However, Niyogi
et al. (2011) found that the “shape, size, surface features and chemistry of spherules are not
diagnostic of impact cratering process and cannot distinguish microtektites and impact spherules
from the coal fly-ash spherules produced from natural wildfires and thermal power plants.”
Let's take a quick look at Niyogi et al. (2011). In fact, the above quote does not reflect their results.
In Figure 4, Niyogi et al. (2011) show very well that silica spherules from the roadside (SS) at a location in India can be distinguished from impact spherules using multi-element composition diagrams. Moreover, Niyogi et al. (2011) have nothing to say about dendritic surface patterns or high temperatures inferred from Lechatelierite, Schlieren etc. In fact, the final sentence of Niyogi et al. (2011) is "Hence, it is necessary to adopt a multi-pronged method to evaluate the spherules before assigning their origin as they can be carried over hundreds of km in air from various sources prior to their deposition."
I think it is clear that the first quote above from Niyogi et al. (2011), emphasized by Holliday et al., should be interpreted as meaning that individual characteristics should not be used alone. Instead, a multi-characteristic approach should be used. But this is precisely the approach of BWM. So, rather than refuting the work of YDIH proponents, Niyogi et al. (2011) actually supports it.
As Jaret and Harris (2021, p1) point out, “presence of only non-diagnostic features – even if these
same features sometimes occur in shock materials – is not sufficient to claim impact.”
Jaret and Harris (2021) discusses shock (planar deformation) features and PGEs (platinum group elements) only and so, once again, does not consider the suite of diagnostic microspherule characteristics in BWM and is not appropriate for investigating microspherules from airbursts.
While
some YDB microspherules were purported to contain lechatelierite (Bunch et al., 2012, Wittke et
al., 2013a, Wu et al., 2013, LeCompte et al., 2018), lechatelierite is present in anthropogenic
spherules (Marini and Raukas, 2009), in non-impact frictionites/pseudotachylytes (Masch et al.,
1985; Lin 1994; Sanders et al., 2020; Tropper et al., 2021) and can form by lightning strikes.
This may well be true, but identification of YDB microspherules does not depend on the presence of lechatelierite alone. It depends on identification of several simultaneous characteristics (see the table above), including high temperature generation. Holliday et al. need to show evidence that non-impact microspherules can display all these characteristics simultaneously to refute the YDIH.
Through lightning discharges, lechatelierite could also be in volcanic spherules (e.g. see,
Genareau et al., 2015, 2019; Wadsworth et al., 2017; Kletetschka et al., 2017, 2018), contrary to
Bunch et al. (2012, p. E1904).
It is simply not plausible that this process can generate the assemblage of YDB objects across several continents. Moreover, BWM take great care to rule out volcanism as a possibility. Most obviously, the absence of volcanic ash or tephra at YDB sites, and analysis of volcanic tephra from many different locations shows no indication of any similar microspherules.
Various materials can be misidentified as lechatelierite if
insufficient microanalysis is performed, as is commonly the case in YDIH papers.
This sweeping sentence is not supported by any evidence.
At the
Blackwater Draw site, Andronikov et al. (2016b) failed to detect a meteoritic component in
microspherules where the “overall low platinum group elements (PGEs) concentration in the
microspherules … slightly above detection limit” (their emphasis added) precluded any
interpretation. They also did not investigate shock effects and presumed an impact origin based
only on non-diagnostic features.
Holliday et al. keep listing criteria that are not required for impact microspherules generated by airbursts.
Impact proponents invoke the presence of any kind of spherules as definite proof of
impact events.
This sweeping statement is obviously false for all the reasons given above.
In many of these studies, the characterization of the alleged spherules is
superficial and/or incomplete, and their use to support the existence of impact events is just
speculation (see Detre and Toth, 1998, for a large collection of studies of varying quality, on
natural and artificial spherules).
On the contrary, the BWM papers use diagnostic criteria identical to those suggested by French and Koeberl (2010).
A theme running through the publications using microspherules
as evidence of a YDB impact (e.g., Firestone et al., 2007; Bunch et al., 2012; LeCompte et al.,
2012, 2018; Wittke et al., 2013a; Wu et al., 2013; Israde-Alcántara et al., 2012, 2018;
Andronikov et al., 2016b; Hagstrum et al., 2017; Kletetschka et al., 2018; Pino et al., 2019;
Sweatman 2021; Powell 2020, 2022) is failure to address the accepted criteria for the
identification of impact markers (French and Short, 1968; Stöffler, 1971; Grieve et al., 1996;
Langenhorst, 2002; French and Koeberl, 2010; Ferrière and Osinski, 2013; Stöffler et al., 2018).
The comet research group is pioneering new scientific criteria for large airburst events that cannot be refuted using the standard criteria for crater-forming impacts. Indeed, the BWM papers use criteria identical to those suggested by French and Koeberl (2010) for detecting impacts based on microspherules alone.
Proponents of the YDIH tend to focus on the shape, textures, and/or presumed melting
temperatures of spherules as well as their presence in presumed YDB sediments, but none of
those are diagnostic of an impact origin.
Actually, they use the criteria suggested by French and Koeberl (2010) and their diagnostic process practically rules out non-impact processes.
Most YDIH proponents propose an airburst event to explain the lack of a YDB-aged
crater. This is despite the fact that YDB microspherules (used as evidence of an ET event) lack a
meteoritic component as expected for bolide debris.
Remember, in French and Koeberl (2010) we read "Likewise, siderophile-element anomalies may be absent in spherule layers".
Even assuming that microspherules are
from an airburst and evidence of a meteoritic component was missed there is no reason to
associate them with a large, global event. There is a non-negligible probability that sampling a
random 360-year core in a random place on Earth will turn up condensed debris from a more
frequent, small nearby bolide.
The YDB microspherule evidence (see top of this post) in the BWM papers demonstrates they are mostly of terrestrial origin. Further evidence is given in Wittke et al. (2013) below;
Figure 4 above shows YDB objects are unlike cosmic rain and micrometeorites. Figure 5 shows YDB objects are unlike anthropogenic (e.g. fly ash) and volcanic materials, and Earth's mantle. Figure 6 shows YDB objects are most similar to other surface metamorphic rocks and known impact tektites. Together, these figures show the YD impacts most likely involved ground-altering impacts, i.e. airbursts or ground impacts. Thus, calculations based on small meteorite ablation or airbursts that do not significantly alter the ground are not appropriate. Instead, we should be thinking on terms of Tunguska-sized events, or larger, for which the same kind of impact debris was generated.
The "YDB", when defined recursively this way, will always be
found in core samples of sediments with approximately the right age.
There are many small airbursts every year that leave meteoritic traces in the stratigraphic
record. Larger airbursts (e.g., Chelyabinsk) take place on time scales measured in decades or
centuries and leave larger concentrations. Correlating widely separated concentrations of
meteoritic debris is not a valid stratigraphic method because, given the power-law size/frequency
distribution of meteoroids and asteroids, they are far more likely to result from two different
airbursts.
This is all irrelevant for the reasons given above.
Spherules are common throughout the stratigraphic column and without independent
high-resolution age dating it is impossible to know if they are from the same event. The analogy
would be to see evidence of a burning event in two tree cores from widely separated forests. It
would be foolish to claim they were from the same fire, especially if they were not
dendrochronologically correlated. Even if they occurred at the same time, it is far more likely
that the trees were burned in different small fires rather than one continent-wide conflagration. A
layer of meteoritic material that is within a few hundred years of the YDB, cannot simply be
assumed to be a YDB layer. Small airbursts happen all the time, everywhere. YDIH papers
clearly do not have independent radiocarbon ages with low enough uncertainty to show that their
“YDB” layer is coeval or correlates with the actual start of the YDC (Meltzer et al., 2014)
(Section 5; Table 4). ENDNOTE 16
This is false because the YD impact events are best considered as Tunguska-like impacts, not small airbursts that do not significantly affect the ground, and this makes all the difference. Elsewhere in Holliday et al. we read that the Tunguska event was a 1 in 1,000 - 10,000 year event. Although this very likely underestimates the true impact rate of such events, we can take it as a rough estimate here. Given this probability, let's suppose that the probability of a Tunguska-like event somewhere on Earth is 0.5 in any given 1000 year period. The number of different YDB sites is ~ 50, although some are relatively close and could be recording the same airburst event and a few are poorly dated. Nevertheless, let's suppose there are 40 well-separated sites with age resolution within a 1000 year period. The probability of 40 such events occurring within the same 1000 year period within the last 10,000 years is ~ 10 x (0.5)^40 = 9 x 10^-12. This estimate is astronomically low. It is therefore far more likely that a single very large event, or a few related large events within a short timespan, took place.
As described above, it is not possible to establish the spherules formed synchronously
due to the problematic dating of the presumed YDB sediments at the various sites.
We will examine the issue of YDB dating at a wide range of sites in a later blog post.
The rest of section 10 of Holliday et al. discusses carbon microspherules which I did not review in my earlier paper, so I'll finish my rebuttal here.
Summary
It is clear that Holliday et al.'s refutation of YDIH microspherule evidence is fundamentally flawed and can be rejected utterly. It suffers from several major problems;
1. They habitually misrepresent the YDIH by considering mainly ground impact events, they make many demonstrably false statements, and they often misrepresent evidence and arguments in cited papers.
2. To refute the YDIH, Holliday et al. should tackle the strongest evidence. Instead, they focus mainly on early YDIH papers, largely ignoring the detailed, diagnostic evidence in the later BWM papers.3. They frequently demand evidence, i.e. a clear PGE abundance and shock processed materials, that is not required for microspherules from what are usually considered to mainly be airburst events caused by comet fragments. Their problem is probably that they habitually think in terms of ground impacts. The comet research group are pioneering the new science of ancient airburst detection. If we were to accept Holliday et al.'s criteria, it would be much more difficult to detect ancient airburst events, which means it is likely that the frequency of ancient airbursts has been seriously under-estimated. Thus, their view that such events are very rare is obviously a self-fulfilling prophecy, and their approach appears to be detrimental to progress in impact science.
4. They ignore contrary guidance in French and Koeberl (2010) (note that Koeberl is a co-author of Holliday et al.) for the detection of impact events using microspherule evidence alone. Evidence in the BWM papers fully satisfies this guidance for Abu Hureyra. Given the similarity of microspherules across many YDB sites, and the diagnostic process of the BWM papers, it is very likely these other sites were also subjected to ground or airburst impacts.
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