|Maybe. Maybe not. Not in 2012, at any rate!|
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.
|Dr. Marco Langbroek|
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];
|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;
|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.
|True impact evidence: impact suevites and melt breccias from various impact craters (collection M. Langbroek). (a) Riess crater suevite breccia, Germany; (b) |
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.
|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.
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).
Credit: National Geographic
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.
Hawkeye: Radar I don't hear anything.
Radar: Wait for it... (Photo credit)
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.
|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
Institute for Geo- & Bioarchaeology (IGBA)
Faculty of Earth and Life Sciences
VU University Amsterdam
De Boelelaan 1085
1081 HV Amsterdam, the Netherlands
e-mail: email@example.com; twitter: @Marco_Langbroek; http://www.palaeolithic.nl; http://www.falw.vu.nl/igba; http://www.clue.nu
I'm sure that Dr. Langbroek would welcome your comments. [clears throat] Fire away!