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| Maybe. Maybe not. Not in 2012, at any rate! |
After my [you'll pardon the expression] 'crash and burn' on the matter of the Younger Dryas Boundary Impact Hypothesis, I'm very pleased that Dr. Marco Langbroek will contribute his thoughts on the matter. Some of you may remember several exchanges he and I had in the early days of the Subversive Archaeologist. [Can I really say 'early days' if it's only 6 months ago? Somehow it seems a lot longer...].
If you're curious, you can call up those old posts using the search window lower down on the left sidebar. Given that history, I'm doubly pleased to have him here today, simply because he has stuck with this blog despite any social imperfections I may have demonstrated in our earlier conversations.
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| Dr. Marco Langbroek |
Marco
Langbroek (b. 1970) is a Dutch palaeolithic archaeologist. He obtained his MA and
PhD in palaeolithic archaeology at Leiden University in 1998 and 2003, worked
as a post-doc researcher at the Institute for Geo- and Bioarchaeology of the VU
University Amsterdam from 2008-2012 and is currently affiliated as a guest
researcher with the same University.
Impacts and
human evolution are a long-standing research interest of his. In an appendix to
his 2003 PhD dissertation he pointed to the contemporaneity of a known large cosmic impact in SE
Asia, and Homo erectus presence in Asia (published in 2004 as BAR International Series nr. 1244) .
He's involved in small solar-system body research as
a high-end amateur. He has authored/co-authored a number of peer-reviewed papers
in the field of meteor research, and has discovered several asteroids, including
the NEA 2005 GG81.
The International Astronomical Union has named asteroid (183294) "Langbroek" after him in honour of these activities.
I received this from Marco this morning. Dr. Langbroek pulled an all-nighter to get this ready. We're in his debt.
"Impact and human evolution" is a pet subject of mine. I have a somewhat
unique position with regard to archaeology and cosmic impacts, as I have a background in both
the fields of archaeology (my PhD) and in small-solar system-body research. In
addition to my archaeological work I have authored/co-authored a number of
peer-reviewed papers on meteor research, and I've discovered a number of asteroids, including a NEA.
These latter activities were conducted as a high-end amateur; I would certainly
not claim to be an impact scientist. But I believe I do have more knowledge of
that field than the regular archaeologist does.
So
naturally, when the Younger Dryas (YD) impact hypothesis was proposed
(Firestone, West et al. 2007, PNAS 104, 16016), I monitored the debates that followed with interest.
When Rob Gargett brought the subject up on this blog, I had to share some of my
thoughts, which led to Rob's invitation to write this guest post.
The Younger
Dryas (YD) Impact Hypothesis does not sit well with both the mainstream impact
science community, and the mainstream archaeological and paleontological
communities. There are clear reasons for this negative attitude. Among the
proposed impact events, the YD "impact" hypothesis is a decidedly odd
duck in the pond.
The problem with the YD impact hypothesis is that the initial evidence for it
(and indeed, still almost all of the current evidence) is based on
"novelties". In essence, almost none of the proposed "impact markers"
were recognized as such (as markers pointing to impact) before Firestone, West et
al. brought them up
as possible evidence for cosmic impact. This, while the professional impact
research community has used a set of unambiguous markers to recognize such
events for years. These include the presence of glassy impact ejecta [see below];
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| True impact evidence: tektites and impact melt glasses from various impact events (collection M. Langbroek). (a) Lybian desert glass, 28 Ma impact over North Africa; (b) Moldavite tectite, 15.1 Ma Riess crater impact; (c) Irghizites, 0.9 Ma Zhamanshin crater impact; (d) Darwin glass, 0.8 Ma Darwin crater impact; (e) Bediasite tektite, 34.5 Ma Chesapeake Bay impact; (f) Australasian tektites, 0.8 Ma Australasian impact. |
shocked
quartz grains; horizons with anomalous concentrations of Iridium and other
elements in deep sea cores, terrestrial deposits [see below] and ice cores;
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| True impact evidence: Iridium rich clays from the K-T boundary layer, Stevns Klint, Denmark (collection M. Langbroek). |
presence of shatter
cones in target rock; ejecta blankets; a crater with breccia fill [see below], melts and
overturned rim strata etcetera.
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True impact evidence: impact suevites and melt breccias from various impact craters (collection M. Langbroek). (a) Riess crater suevite breccia, Germany; (b) Söderfjärden crater impact breccia, Finland; (c) Paasselkä crater impact melt breccia, Finland; (d) Paasselkä crater impact breccia, Finland; (e) Garndos crater impact breccia, Norway; (f) Glover Bluff crater impact melt breccia, USA; (g) Dhala crater impact breccia, India; (h) Dellen crater suevite, Norway; (i) Sudbury crater metal rich impact melt breccia, Canada; (j) Sääksjärvi crater suevite impact melt, Finland.
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While these need not be present as a full set,
at least some of
them should be found before one could posit an acceptable theory that a cosmic impact
occurred.
And here is
the ringer. None of these established criteria are met by the YD impact
hypothesis. The evidence that proponents of the hypothesis have brought up so far mostly
concerns the mentioned "novelty" markers (with the exception of a
contested Iridium enrichment and a likewise disputed presence of impact
spherules, see below). And in addition, the evidence is ominously negative on a
number of important impact markers widely recognized by the impact research
community that really should be there. This should sound warning bells.
At the same time, the archaeological and paleontological aspects of the
hypothesis are problematic as well: many paleontologists would take issue for
example with presenting the disappearance of mega-fauna from the America's (or
the Late Pleistocene worldwide in general) as a short punctuated event near
12.9 Ka. It has been argued that megafauna extinctions already set in in North
America before the Younger Dryas, during the Bølling-Allerød warm climatic
oscillations (e.g. Gill et al. 2009, Science 326, 1100-1103). Likewise, elsewhere in the world Late Pleistocene
megafauna extinctions were not a punctuated event either.
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| Rancholabrean fauna (Illustration credit) |
The same
goes for the archaeological side of the story: is the end of Clovis really a
punctuated event at 12.9 ka? And if it is, does this point to extinction,
rather than cultural change? In other words, is there really evidence of a
dramatic demographic break/discontinuity at the Younger Dryas boundary,
archaeologically? Where Clovis is concerned, it should be taken into account
that evidence suggest that the appearance of Clovis was rather rapid as well (Waters and
Stafford 2007, Science 315, 1126). With a rather sudden onset, a rather sudden end seems
less odd.
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| Credit: National Geographic |
Clovis could be a response to the shortlived, warm Allerød climate
oscillation, explaining both the rapid onset and disappearance without the need
to invoke an impact at the start of the Younger Dryas. We should realize that what we call
Clovis is a techno-typological concept only, not a demographic entity. The end
of Clovis does not necessarily equal the end of human occupation of N-America (note:
I am not a N-American paleoindian archaeologist, so I have no vested opinion on
these questions and refrain from definite judgement on how "sudden"
the end of Clovis was, and matters of demographic continuity following Clovis.
Others are more qualified to judge these issues).
That the
Younger Dryas (certainly in conjunction with the preceding unusually warm and
moist Bølling-Allerød warm climatic oscillations) represents a clear and severe
climatological fluctuation, enough to potentially disrupt human presence, is
beyond doubt. The central issue is therefore the cause, so even if there was a break in
human presence, that does not prove a cosmic impact itself, as there are
alternatives to explain the onset of the Younger Dryas. Notably, the effects of
an influx of fresh meltwater from the Laurentide icesheets on the Oceanic
thermohaline circulation (see for example Broecker 2006, Science 312, 1146). It therefore really
boils down to the question whether there really is good, acceptable evidence
for a cosmic impact, other than the faunal extinctions and archaeological
change happening near (note the near) the Younger Dryas.
The absence of any clear impact crater or other impact markers dating to the YD
does present a problem in this regard. With an event of the proposed size (4-5
km body impacting), and this young an age, impact marks should be visible.
Impact in the ice sheet going "unnoticed" is not a viable explanation.
That argument does not seem to appreciate the size of the event necessary to
create extinction on a continental scale. An object large enough to have such
consequences would penetrate through the ice sheet and excavate a crater in
the bedrock below the ice: we are talking about craters with a diameter of 50
km or more and a depth of 10 or more kilometers here (see Hills & Goda 1993, Astronomical Journal 105, 1114). An air-burst (a fragile, e.g.
carbonaceous or cometary cosmic body exploding and annihilating in mid-air)
does not really explain the absence of physical impact evidence either. With an
airburst of this magnitude, one would expect geologically traceable results,
such as the presence of glassy melt-sheets (impact melt glasses), deposition of
meteoritic evaporation products, massive blast damage and other geologically
visible traces under the airburst location and a wide area around it.
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Radar: Incoming!
Hawkeye: Radar I don't hear anything.
|
As Melott et al.
recently pointed out, an impact this size should leave a notable marker in the
Greenland ice cap in terms of (a.o.) enhanced nitrate concentrations. While
some nitrate enhancements were indeed presented as evidence by the original YD
impact hypothesis authors, the signal that is present in Greenland ice core
data however falls well short of what an impact of this size should create (Melott et al. 2010, Geology 38, 355-358). The small signal
that is there points to forest fires during the YD, which however can have all
kinds of causes (for example, climate change and ecological change, e.g. in the
herbivore community, creating an environment more susceptible to forest fire as
a results of the wild climate oscillations at the Allerød-Bølling-Younger Dryas
interface).
Likewise a
reported "Iridium enhancement" at YD levels (Firestone et al. 2007), is not corroborated by other
researchers looking for it, including the same sites (and even the same sediment
samples) that
Firestone, West et al. sampled (Paquay et al. 2009, PNAS 106, 21505; Haynes et al. 2010, PNAS 107, 4010). Paquay et al. also found no trace of other
geochemical impact markers that should be there, for example in the 187Os/188Os
isotopic ratios of YD sediments (introduction into the atmosphere of
extra-terrestrial materials from the impactor should off-set these Osmium
ratios. There is no evidence for such an off-set: 187Os/188Os
isotopic ratios of YD sediments are completely terrestrial).
The YD impact
hypothesis proponents have claimed elevated, concentrated levels of magnetic
and carbon spherules in YD sediments (Firestone et al. 2007). Several researchers have
however again failed to replicate these observations (including on the very
same sampling localities as those of Firestone, West et al.) and point out that what number of metallic spherules are present in
these sediments, is consistent with the natural annual background influx of
such micro-meteoroid materials and that presence of these is not restricted to
the YD levels (Surovell et al. 2009, PNAS 106, 18155; Haynes et al. 2010, PNAS 107, 4010). I can attest myself that you can
find metallic spherules in any archaeological sediment, if you look for them (I found them in Holstein
age sediments from Schöningen for example), and geologists have been recovering
spherules from geological deposits of widely varying ages for years already.
Others have pointed out that the carbon spherules need not be of cosmic origin
at all (they can be fungal in origin, or even be insect coprolites) and indeed
Scott et al.
(2010, Geophys. Research Letters 37, L14302)) feel the reported carbon
spherules are consistent with a biological origin, rather than impact-generated.
The
"novelty" impact markers proposed are heavily disputed as well. For
example, the "nanodiamonds" that feature heavily as evidence in the
discussion (Kennett et al. 2009, Science 323, 94), might not be nanodiamonds but graphene (which is present
in non-YD sediments as well) according to Dalton et al. (2010, PNAS 107, 16043). Dalton et al. did not find any true nanodiamonds
in YD age deposits. The distinct lack of other meteorite-produced impact
elements together with these "nanodiamonds" remains a problem as well
for the interpretation of these "nanodiamonds" as impact markers. It
should be noted that nanodiamonds were never considered to be clear impact
markers (although it should be noted that nanodiamonds do occur in some
meteorites, notably in Ureilites [see below], and can form in carbon-rich impact target
rock due to shock pressure) before these were presented as such by the YD
impact proponents.
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| A meteorite that does contain nanodiamonds: a small fragment of the NWA 2625 meteorite, an Ureilite found in the Sahara in 2004 (collection M. Langbroek). |
It gets
even more dubious, when it concerns some of the very first evidence presented
for the YD impact hypothesis. These consisted of what was claimed to be
microscopic impact damage on flint tools, plus what were claimed to be small metallic
meteoritic fragments embedded in some mammoth tusks and megafauna bones.
Radiometric dates on these tusks and bones however have shown that they do not
date to the Younger Dryas at all, but have various ages (up to 10,000 years
older than the YD – for a review, see Pinter et al. (2011, Earth Science Reviews 106, 247-264). The
"microscopic damage" on flint tools is highly dubious (and downright
pseudo-science in my opinion): such small meteoric particles as were proposed
to have inflicted this damage, should not retain cosmic velocities and as a
result would impact with such low velocities that they could not have created the
claimed damage on hard flint surfaces (see also Pinter et al. 2011, Earth Science Reviews 106, 247-264, who remark the same).
Similarly
dubious are the claims for notably elevated radioactivity levels in YD faunal
bones and elevated U and Th levels in YD sediments (Firestone et al. 2007). Radioactivity has never been associated with cosmic impact
before. The natural radioactivity of meteorites is considerably lower than that
of the average terrestrial rock and mostly due to shortlived cosmogenic isotopes created by
cosmic irradiation in space, i.e. isotopes that should have completely decayed after
12,900 years. Radioactive meteorites (or comets) belong to the realm of comic
books and bad Hollywood movies, not science. Again, this part of the hypothesis
heavily smacks of pseudo-science. Moreover, these findings could again not be reproduced by other
researchers (Haynes et al. 2010, PNAS 107, 4010).
As the
reader will have noted, there is a string of "non-reproduceable
observations" involved by now. That is ominous. It points out that much of
the hypothesis is on very shaky grounds. Add to this the string of negative
observations of established impact markers that should be there, but are not,
plus the decidely odd "observations" such as claimed elevated
radioactivity levels that have hitherto never been associated with cosmic
impact (and again, cannot be reproduced by other researchers).
Taken all
together, it doesn't add up.
To
summarize: widely established impact markers that should be there are either
not observed at all, or when initially reported by Firestone et al. turn out to be non-reproducable
observations when other researchers investigate what in several occasions are the
same deposits (and
sometimes even the same samples). The "novelty" impact markers presented by Firestone et
al. are not only
unusual but also highly contentious, with "nanodiamonds" that might
not be nanodiamonds and iron-particle peppered mammoth tusks that turn out to
be from widely varying ages instead of dating to the Younger Dryas. In
addition, paleontologists dispute that a worldwide or even regional punctuated
extinction event happened 12,900 ka ago (pointing out that extinctions were
already in progress before that date in North America), and evidence for a
clear demographic break in human presence in North America at this moment in
time is ambiguous as well.
This leaves
virtually no firm evidence for either:
(a) the occurrence
of an impact at 12,900 ka, or
(b) the
"punctuated" phenomena at 12,900 ka purported to be explained by this
impact hypothesis itself.
After Alvarez et al.'s famous 1980 Science paper on cosmic impact as a cause for the K-T
extinction event (including the dinosaur extinction), impacts have entered the scientific
main stream as a recognized force in the geological and biological history of
our planet. As a personal opinion, I feel the YD impact hypothesis is an
example of how cosmic impact in the wake of this rising acceptance is now used and
abused as a "deus ex machinae" by some, to explain all that seems remotely
unusual in the geological, climatological and archaeological record.
That is
however not the way cosmic impact hypothesis should be evoked, and it detracts
from cases where well-documented cosmic impacts were truly occurring and
possibly meddling with human presence. For it is a fact, that large cosmic
impacts did
occur solidly within the timespan of human evolution. The formation of the
Australasian tektite strewnfield 0.8 Ma ago in SE Asia for example is a
genuine, well-accepted, large cosmic impact event on the global effects threshold--and Homo erectus
was already plodding through Asia at that time (see appendix to M. Langbroek 2004, Out of Africa, a study into the earliest occupation of the Old World. BAR Int. series 1244,
Archaeopress, Oxford).
Like Rob Gargett,
I however smell that "this one isn't going away for a while".
Perceived cosmic armageddon at the eve of human occupation of the Americas is
too spectacular to be dropped easily.
Marco Langbroek, April 10, 2012
Wow! Nice essay. Rob, I think Marco deserves an 'A' for that.
ReplyDeleteI s'pose you think that's funny, Wonderdog! Lovely to hear from you.
ReplyDeleteFirst of all, nobody clings to the 4km impacting bolide hypothesis anymore, the fact that you bring it up, and all of the misidentified markers indicates to me that you haven't really looked at the Younger Dryas, neither the hydrogeological hypothesis nor the impact scenario.
ReplyDeleteNice try, I give you a D, though.
The real action is in the Arctic verses the Lake Superior rerouting hypothesis and controversies. The fact that you brought none of that up indicates your unfamiliarity with the subject matter as well.
Yes, I have spoken to Melott and Boslough, so you can't fool me.
Sorry, seriously, Dr. Langbroek, it's that bad. I mark you down for an F for your essay, not quite an epic fail, but a failure nevertheless. Clearly any YDB impact does not explain extinctions and climate reversal at the Younger Dryas Chronozone, but 'virtually no firm evidence' of an impact at the YDB fails completely when workers have already identified several putative Holocene and YDB impact candidates (albeit small and not continent busters), and only puts your entire essay in a bad light.
ReplyDeletePlay nicely, kT. You might want to elaborate, for us benighted archaeologists, just what you mean by the 'rerouting' and 'hydrogeological' hypotheses, and point us in the direction of those impact candidates you mention, along with a reference or two to enable us to follow up on your claims. Moreover, I think that no one here is trying to 'fool' anyone. So, again, I would ask that you do more than point and sneer at a guest of the Subversive Archaeologist and try to be just a little constructive. Thanks!
ReplyDeleteI'm sneering at it because it is a poor essay, something I would expect from an undergraduate.
ReplyDeleteCorossol Crater
The Younger Dryas itself boils down between a controvery between the Moorehead Phase dischage of Glacial Lake Agassiz, either to the Arctic as per Murton et al. and other coworkers, and somehow into Lake Superior and eventually out through the Champlain Sea and the St. Lawrence as per Rayburn and Carlson et al. and coworkers.
You'll just have to dig the references up yourself, I don't have time to list them, but it is a fundamental prerequisite for auditing Younger Dryas 905.
Commenting on this subject without understanding the broad and interdisciplinary implications of the various interacting forces results in essays of the type I have just brutally critiqued.
Sorry if that offends you.
Anyone who wants to follow kT's accomplishments can do so at cosmic.lifeform.org, the web site of one Thomas "Tommy" Lee Elifritz, rocket scientist. Scuze me while I kiss the sky.
ReplyDeleteAnd if you want to credibly research the Younger Dryas and the Younger Dryas Impact Hypothesis, google and wikipedia are your friends.
ReplyDeleteAll I see is somebody who threw their hands up in exasperation after the first hint of trouble, and yet another academic commenting on something that he clearly is unfamiliar with. When you guys can show me that you are willing to put in the least amount of effort on this subject, do get back to me. I'm on the tubes.
Thanks in advance.
Indeed, before I close the book on this chapter in the life of the Subversive Archaeologist, I'd encourage any and all of you to have a look at this, the only work attributable to kT that made it into a bona fide publication--though, how that happened is a mystery to me. Have at it: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19960000272_1996900272.pdf
ReplyDeleteKt: I would be interested to hear about the "several putative Holocene and YDB impact candidates" you claim. I know the Carolina Bays have been proposed, but really, nobody in the impact science community takes that seriously. The Carolina Bays have a long history of attempts to explain them as impact craters, none of which has been convincing. Pinter et al (see main post) explicitly comment on that as well - and among the authors of that particular paper is Christian Koeberl, one of the nestors of impact science, so you can't say those are people that don't know their stuff. In addition, there is plenty of dating evidence to suggest that the Carolina Bays do not date to the Younger Dryas at all.
ReplyDeleteThe Corossal structure in Canada you bring up in one of your comments is interesting, but the structure is basically undated. The structure is also unusually shallow for a 4-km diameter impact structure - pointing to either an origin other than impact, or it being an old, strongly eroded feature like the other Canadian impact structures. That the structure is not (yet) listed on PASSC's Earth Impact Database points out that it is certainly not yet accepted as an impact structure. Linking this to the Younger Dryas impact hypothesis is completely premature at this moment.
As for the controversies around the draining of Lake Agassiz: I don't see how controversies around this "prove" the YD impact hypothesis. You are setting up a straw man's argument here.
(for the benefits of the readers: Lake Agassiz was a hughe glacial melt water lake south of the Laurentide ice sheet. One proposed cause for the Younger Dryas, is that the lake broke through to the Atlantic ocean, discharging large amounts of fresh water into the ocean leading to changes into the thermohaline circulation. There is some controversy about the exact "route" this breakthrough took)
Draining of Lake Agassiz is one of the alternative explanations for the onset of the YD, as I noted in my essay. That there is some controversy around this as well is granted, but not the point. The essay is about the YD impact hypothesis, not these alternative causes for the Younger Dryas. If one of these alternatives is wrong or in need of amendment on details, that does not automatically prove the YD impact hypothesis is right. Nor does it make all that is wrong with the YD impact hypothesis, somehow magically right.
Looking at how the argument about the YD impact hypothesis has evolved, much of what "believers" in the hypothesis feel is "strengthening" the hypothesis, are what Binford would have called "post-hoc accomodative arguments". Meaning that when the original arguments on base of which the hypothesis was proposed are falsified, proponents come up with new, slightly altered stories to cling to their original, disproved hypothesis.