There was a paper in Science last week that has gotten quite a bit of press. It reports further evidence in support of the idea that the Younger Dryas — a distinct period towards the end of the last ice age when the deglaciation in the Northern Hemisphere was interrupted for a period of about 1300 years — was caused by a barrage of comets hitting North America.
When the first papers on this came out last year, we expressed skepticism. We remain skeptical and our reasons remain unchanged. But we think it is worth saying a bit more on this, because the reporting on this issue has largely ignored just how big an idea this is, and therefore how much more work would need to be done before it could be taken very seriously.
For background, see the good article by Kenneth Chang in the New York Times, which, however, does not address our main concerns with the hypothesis.
The brief history is that in 2007, Firestone and others published an article in PNAS showing evidence of various materials that may be diagnostic of extraterrestrial origin (and hence an impact) in layers of sediment dating to 12.9 thousand years ago, just before the beginning of the Younger Dryas cold event. Now, in a Brevia piece in Science Kennett and others show further evidence: “abundant nanodiamonds in sediments dating to 12.9 ± 0.1 thousand calendar years before the present at multiple locations across North America.”
According to Richard Kerr’s news item that accompanies the article in Science, at least some experts are skeptical that Kennett and others have really found nanodiamonds, or that, even if they have, they are necessarily evidence of an impact. But we don’t claim enough expertise in nanodiamond detection or interpretation to have an opinion on this aspect, so let’s give them the benefit of the doubt. Suppose there really was an impact (or impacts) at the right time in the right place. We’d still be skeptical that this was a trigger for the Younger Dryas.
Among our reasons for skepticism (again, see our earlier post on this) there is a basic statistical problem. The problem is — and this context is missing from most if not all of the articles we’ve seen on this — that explaining the Younger Dryas in terms of an impact leaves all the other rapid climate change events (the so-called “Dansgaard-Oeschger events”) of the last glacial period unexplained.* One would have to either accept the conventional ideas for the causes of these events, or, alternatively, one would have to propose that there was an impact not only before the Younger Dryas, but before each of the earlier events.
We recognize that it isn’t entirely an either/or situation. Indeed, the suggestion appears to be that a cometary barrage causes various kinds of havoc, including the ice sheet collapse that led to ocean circulation change (the most well-evidenced proximal cause of rapid climate change). But the point is that if these events can happen as part of the inherent variability of the ocean-atmosphere-ice-sheet system, then there is no need to invoke the impact hypothesis in the first place. And indeed it would be virtually impossible to show it was other than mere chance that comet impacts occurred at the right time, especially given that it would still be necessary to show that the ice sheet would care about comets, which we also consider unlikely (see the good discussion — particularly Mauri Pelto’s comments — on this over at the Open Mind Blog). On the other hand, if abrupt climate changes don’t happen on their own — if they only happen due to extraterrestrial causes — then one would want to see evidence of impacts for at least a few more of them, not just one. That would be a truly exceptional paradigm-breaking discovery, going against just about everything we think we know about the system.
We emphasize that we are not saying “the Younger Dryas can’t have been caused by a comet, unless all the Dansgaard-Oeschger events were caused by comets”. We’re saying that we see no need to invoke such an hypothesis, so the level of proof required for this extraordinary idea will need to be extraordinarily strong. So far, it doesn’t appear that that is the case.
Think about it. If it turned out that rapid climate change events are caused by comets, it would imply the climate system is far more stable than we thought, that abrupt climate change events are not part of the inherent variability of climate during glacial periods. That would perhaps allay fears that we could be pushing the system towards an abrupt climate change in the future. On the other hand, it would also suggest that cometary impacts are far far more common than we thought. Now that would be news. Perhaps further research by Kennett, Firestone and others will indeed show that to be the case. We’re not, however, holding our breath.
———-
*Not to mention that there is an event similar to the Younger Dryas at the end of at least one other glacial period, “termination III” (see e.g. Carlson et al., 2008).
Jim (149) — With all respect, from
http://en.wikipedia.org/wiki/Dansgaard-Oeschger_event
we have “In the Northern Hemisphere, they take the form of rapid warming episodes, typically in a matter of decades, each followed by gradual cooling over a longer period.” and “The course of a D-O event sees a rapid warming of temperature, followed by a cool period lasting a few hundred years (Bond et al. 1999).” These agrees with every text I have studied or even glanced at. Younger Dryas begins with a fairly rapid cooling (of dramatic amplitude in Greenland) followed about 1300 years later by a most dramatic warming.
For a view of THC hysterisis, etc., to partily explain this, see the link in comment #112 by SteveF.
Regarding my #150, perhaps somewhere between Figures 8 & 9 of the linked pdf paper is appropriate. Quoting from that paper, “The ice front continued its retreat in the Upper Great Lakes area ultimately moving north of and freeing the Straits of Mackinac and also uncovering a lower outlet at Kirkfield, Ontario, leading into the Trent Valley, across southwestern Ontario from Georgian Bay to Lake Ontario.”
The timing is a rather delicate matter or else one needs posit a very much larger ET object explosions and impacts. However, for this timing see
Louis J. Maher, Jr. and David M. Mickelson, “Palynological and Radiocarbon Evidence for Deglaciation Events in the Green Bay Lobe, Wisconsin”, Quaternary Research, Volume 46, Issue 3, November 1996, Pages 251-259:
http://linkinghub.elsevier.com/retrieve/pii/S0033589496900642
with a quotation “… the Straits of Mackinac were free of ice about 13000 yr …”
Here is termination 3 methane
238199 650
238623 500
238943 554
239249 538
239545 522
240205 498
240576 484
240966 470
242067 509
243657 402
244198 437
244861 401
245481 380
246709 405
247436 399
248083 424
248977 447
249751 465
250460 467
251519 421
252183 410
252957 511
253889 486
255230 398
256038 408
256489 385
from
Petit, J.R., et al., 2001,
Vostok Ice Core Data for 420,000 Years, IGBP PAGES/World Data Center
for Paleoclimatology Data Contribution Series #2001-076.
NOAA/NGDC Paleoclimatology Program, Boulder CO, USA.
ftp://ftp.ncdc.noaa.gov/pub/data/paleo/icecore/antarctica/vostok/ch4nat.txt
in which
240205 498
240576 484
240966 470
242067 509
might be considered like Younger Dryas, although only -79 ppbv, but note that is preceeded by
242067 509
243657 402
244198 437
which is preceeded by
244198 437
244861 401
245481 380
246709 405
which might also be thought of as smaller Younger Dryas-like. Using opposite phase, some of these could be considered represetative of DO events, I suppose. By way of comparison, here is termination 1, from LGM to Holocene:
10189 667
11013 621
11143 511
11329 477
11719 501
12626 662
13412 610
13457 642
14241 445
16417 395
17695 363
since we now where to look, Younger Dryas is within
11013 621
11143 511
11329 477
11719 501
12626 662
(providing another reason to doubt the Petit et al. attempt at dating Vostok ice core), where the delta methane agrees moderately well with that obtained from GISP2.
To sumarize, termination 3 appears, in this record, rather more complex, with many more muted events, that termination 1. This amateur opines that termination 3 did not actually have a true Younger Dryas like event according to just this data.
I was trying to determine whether the increased ammonia concentration could have been due to a change in the nitrogen cycle. Ammonia has a low residence time (
The hydrogen peroxide data from GRIP during the YD period and beyond looks most interesting. H2O2 rapidly decreases at 11.9kya:
11965 4.53
11982 2.5
11998 2.09
12013.5 1.77
12029 0
12044 0
12062 0
… (these must be low-detects because missing data is denoted by 9999)
12622.5 0
12638.5 0
12651.5 0
12665 1.23
12677.5 5.48
12689.5 9.2
12702.5 16.16
12713.5 14.97
12724 6.57
12735 29.82
12746 5.38
12758 7.48
It stays at a low level, with fluctuations 14.5 kya. The question is whether there was high SOx in the atmosphere to wash off any H2O2 in the atmosphere, or was this due to change in the ozone cycle?
#151
I am not disputing that the DO event is warming but it is followed by cooling. My original post is somewhat confusing because I speak of a DO event as if it were something that happens in a year or two. I really meant that the transition from warming to cooling would have had to take place coincidentally within a few years or decades after some sort of ET event because the black mat, which is caused by cold and precipitation (see my earlier post), is directly on top of the layer containing the ET evidence.
The same would go for any other explanation – freshwater melt, some cyclic change in the N. Atlantic.
In other words it would be incredible coincidence that almost immediately after the ET event the climate cools for some other reason unrelated to the event.
What’s more, as many posts here have pointed out, The YD looks different from other climate episodes of this sort (although Eric disputes this). It seems to have started more abruptly and has the odd methane behavior. There is also the black mat presence which is suggestive of unusually high precipitation along with cold. Can the black mat be found at any of the other cooling/warmings in the Holocene? This suggests that the usual explanations – DO, freshwater in the NA, etc-. – which may be correct for other episodes at least isn’t the complete story here.
My #151 post didn’t fully get through. If you examine the ammonium ion concentration, it reaches a max. value of 76 ppbw about 500 years after the proposed 12.9 event. Doubtful that it would take that long for any “ammonia” related event to precipitate out in the ice record.
Jim Cross (155) — Younger Drays and, to a lesser extent, the 8.2 kya event both begin by abrupt cooling followed by warming. This is ‘upside down’ from the definition of a DO event. I’ll agree that both are most likely examples of THC transition events.
Both YD and the 8.2 kya event have methane downturns; that for YD is much larger and indeed much larger than any I can find in methane records. Regarding black mats, it seems these have only been located during archaeological digs (although I am unsure about the ones in Europe).
The usual explanation of freshwater into the North Atlantic causing the THC transition seems perfectly reasonable since this occurred at times other than YDB. The magnitude of YD appears to be unusual, not to mention the other YDB evidence.
Pt (156) — Interesting What about the other trace substances mentioned by Firestone et al.?
James T. Teller, Matthew Boyd, Zhirong Yang, Phillip S.G. Kor and Amir Mokhtari Fard, “Alternative routing of Lake Agassiz overflow during the Younger Dryas: new dates, paleotopography, and a re-evaluation”, Quaternary Science Reviews, Volume 24, Issues 16-17, September 2005, Pages 1890-1905 states “This research raises questions about whether catastrophic overflow from Lake Agassiz ever spilled into the Superior basin near Thunder Bay, and even whether overflow from Lake Agassiz ever was routed into the Superior basin through the Thunder Bay area. The geomorphic form of channels west of Thunder Bay does not suggest that large overflows from Lake Agassiz sculpted that region, although the topography along the lower, more northern route over granite to Thunder Bay, via Dog Lake and the Kam Valley, is a possibility. As well, the absence of coarse-grained fluvial sediment at the surface in valleys west of Thunder Bay, previously interpreted as Agassiz spillways, indicates that if strong overflow ever did occur, that record must lie buried, perhaps by till or glaciofluvial and glaciolacustrine sediment associated with the Marquette ice advance about 10 14C ka; alternatively, this overflow may not have been catastropic.” and suggests a routing to the northwest during Younger Dryas.
John A. Rayburn and James T. Teller, “Isostatic rebound in the northwestern part of the Lake Agassiz basin: Isobase changes and overflow”, Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 246, Issue 1, 23 March 2007, Pages 23-30 offer new evidence for the possiblity of flow thrugh the Clearwater-Athabaska route to the arctic.
The first article makes clear that from the Lakehurst stage to the early Morehead stage, glacial Lake Agassiz dropped 100 meters; that is a lot of water. However, it seems that Younger Drays took about 150 years to fully form and so flow may not have been that catastropically large. Another possiblity, suggested by the double outflows of General Carrera/Buenos Aires Lake
http://en.wikipedia.org/wiki/Lake_Buenos_Aires
that flow to the east and to the northwest existed simultaneously.
Both exits require that either the ice sheet melts back, or else the ET object impacts breached the ice daming the exits. Nothing is known about the ET impacts in the Clearwater area; for the Thunder Bay area the evidence suggests the possiblity of such breaching.
#157
I am not arguing that the YD is the cooling phase of some variant of a DO-like event. But I don’t think it is a simple freshwater into the N. Atlantic event either whether caused by a comet or some other mechanism.
I think it is more likely a variant of Ike’s post #126 suggesting a nuclear winter type event.
The black mat is found over a wide range in N. America including areas including the Western US, the Southwestern US, and Texas. Before the event, there were clovis people and megafauna. After it, there are none. I don’t see how some other hypotheses would explain that.
The original article on the relationship to an ET event and the YD suggests a 4km comet nucleus (comets are frequently between 1-6Km in size), possibly split in two, and hitting at an oblique angle or a complete vaporization of a smaller comet at an altitude that could impact all of N. America.
Isn’t it plausible that events were leading up to the YD, and that a comet strike simply accelerated something that would have happened later anyway?
In other words, not all such events would be triggered by comets (indeed, that could be quite rare), but this particular event might have had it’s timing influenced by comet.
Not an expert, just musing…
David:
Another way to interpret the H2O2 record is that during melting the H2O2 is released back into the atmosphere; it still leaves open the question of how the H2O2 is created, and stored in the ice record. If you take the H2O2 release scenario during melting, then during the YD initiation period there isn’t much melting going on. It in fact about 1,000 years after that we see melting — major melting due to the steepness of the curve. H2O2 has been steadily going up since 12.0kya, and infact, looks it’s stabilizing.
Not sure how Firestone et al explain the ammonia and H2O2 observation.
One other thing that the impact folks miss: it’s much faster to melt an ice pack from underneath, than from above (blow torch vs. a hot pan).
Interesting poster here suggesting something happended up that way about that time:
http://ees.acadiau.ca/content/accomplish07.html
Jim Cross (159) & James McDonald (160) — Thgere are several other ‘cold reversals’ (CR) during terminations. The sharpest evidence is from termination 1, which includes Younger Dryas (YD). Only YD seems to show such a great drop in methane concentration and, for termination 1, such a great drop in temperature in Greenland. It is reasonable to assume that the YDB imapcts enhanced what would have been a more ordinary size CR. See for others
http://en.wikipedia.org/wiki/Oldest_Dryas
http://en.wikipedia.org/wiki/Older_Dryas
http://en.wikipedia.org/wiki/8.2_kiloyear_event
Pt (161) — Firstone et al. don’t explain the unusual chemistry, just note the correlation to the 12.9 kya YDB events. If that is when the anomalous ammonia begins, then I’ll offer the conjecture of repeated Great Plains grass fires for the 500 years it took for B. bison to completely recover from YDB. As for hydrogen peroxide,
http://en.wikipedia.org/wiki/Hydrogen_peroxide
“Hydrogen peroxide is naturally produced as a byproduct of oxygen metabolism …” but otherwise I have not even conjectures.
The YDB impacts I take to be shattering ice, not particularly melting it (except in the impact crater).
Ray Percat (162) — Thank you for the notification regarding the southwest Nova Scotia impact! So from the Firestone et al. Fall 2008 AGU abstract we have a potential crater in eastern Lake Ontario (and maybe even Oman). Now we have another. (Unfortunately, I am unable to actually read the poster. Software limitations here.)
David, The melting point of H2O2 (-0.41 deg C) is very close to that of water. What’s unmistakable is that there are very low H2O2 concentrations (near 0) from about 12.6 to 11.6 kya (sorry, earlier I said 12.0kya). You can check yourself for GRIP on the noaa/paleodata site. From 11.6 kya – 8 kya it increases steadily.
Pt (164) — Thank you, but I fear I am such a poor chemist that I can draw no sound inferences whatsoever from the hydrogen peroxide data.
I do conjecture that the far North Atlantic was filled with ice during Younger Dryas. Does the H@)@ data tend to support this?
Oops. H2O2 data.
“Two drainage events of the [Baltic] Ice Lake occurred at Mt. Billigen in south-central Sweden and are attributed to the melting and later re-advance of glacial barriers there (Bjeorck 1995). The first event at 10,900 [radiocarbon] years, lowering the water level by 5 to as much as 15 meters. The final drainage triggered by the weakening ice giving away to 25 meters of water level, occurred in distanct steps around 10,300 [radiocarbon] years over a few year …”
from
K.-C. Emeis et al., “Post=lacial Evolution of the Baltic Sea”, in G. Wefer et al. (eds), “Climate Development and History of the North Atlantic Realm”, Springer, 1999, page 210.
Since 10,900 radiocarbon years calibrates to 12,900 calendar years, it seems probable that the Baltic Ice Lake contributed somewhat to Younger Dryas initiation.
YD does not get a specific mention but a billionaire had/has investment in finding out (?) http://www.happinessonline.org/MoralCode/SafeguardAndImproveYourEnvironment/p14.htm
The search command was “sex + cause of global cooling”, Lol
A gold prospector would see a flake of bright shiny gold colored stuff and could plausibly assume it is gold. But this is a likely a false assumption and could lead to bad investment. It would be a plausible assumption, that needs to be tested before it could be a reasonable assumption. What we have seen is that there several sources of meltwater pulses near the start of the YD most are reported as progessive changes, none seem to have been catastrophic in nature. The releases are from general ice retreat preceding the YD, all of these were not due to an impact certainly , so why should any of them be? Where is the evidence? We have ice sheet retreat that makes sense without invoking an impact, does it make sense with an impact? A reasonable assumption needs evidence to back it up, a plausible or possible assumption still needs basic investigation.
#169
Are you disputing the evidence of any sort of ET impact near the beginning of YD? How would explain the geologic record – the ET evidence overlaid by the black mat?
I agree that we don’t need an ET event to explain the the freshwater releases. But do we need something more than the freshwater releases to explain the YD?
#161
Firestone et al explain the ammonia from biomass burning. I don’t see an explanation of the H2O2 in their article.
If there was a sudden massive drainage, then there ought to be sites, lots of them, in which there is extensive lake bottom covered with the black mat. Right?
If there was an “ordinary” slow event, even one that happened to be pushed over the brink by an impact, like say one wave cresting a longterm ice/gravel dam that had been brimfull, got a notch cut in it, and then started to drain faster and cut a drainage channel so it continued to drain faster — then whatever the black fallout material is would mostly have covered either dry land or current lake water, and we would not find extensive lake bottom areas covered with black mat.
That’s logic. Silly stuff when applied to the utterly unknown by the completely uneducated. But still, does it make any sense as a thought about where to look to distinguish a slow from fast event?
If it were possible to find and use the black layer — heck, it ought to show up in every water and gas and oil well drilled, and in most road cuts, if anyone’s looking, shouldn’t it?
If you look at the map in this:
http://www.pnas.org/content/105/18/6520.full.pdf+html
You will see that the black mat sites are all over the western and southwestern US. They are not in the path of a drainage event into the N. Atlantic.
If they are caused by cold and high precipitation, then possibly the YD itself caused high precipitation in these areas. That’s why I have been asking about the other cooling events leaving black mats. Is high precipitation in the western US associated with these other events?
The other possibility is that a comet vaporized a large part of the ice shelf sending an enormous amount of water into the atmosphere over these areas. Another possibility is a very large comet actually grazed the atmosphere and shed a large amount of water from itself. That’s a very remote possibility but it would leave no impact crater.
Correction: the Copyright date of the book mentioned in comment #167 in 2002.
Jim Cross (172) — The conditions for forming black mats may involve a combination of temperatures and precipitation such that other cold reversals (CR) don’t form such. For example, the global temperatures were higher during the 8.2 kya CR.
As I see the evidence reported on this thread so far, the ET object was most likely not a comet; there is evidence of materials highly similar to those picked up from the lunar surface, for example.
All — Just to set in perspective, the Baltic Ice Lake final discharge, here given as 25 m drop at 11,600 years BP, was about 7,800 km^3 discharging 0.12 to 0.25 Sv:
http://adsabs.harvard.edu/abs/2007GPC….57..355J
and, interestingly, about 300 years before the end of Younger Dryas,
with some helpful maps:
http://www.geol.lu.se/personal/seb/Maps%20of%20the%20Baltic.htm
From Hui Jiang, Nils-Olof Svensson and Svante Björck, “Meltwater Discharge to the Skagerrak–Kattegat from the Baltic Ice Lake during the Younger Dryas Interval”, Quaternary Research, Volume 49, Issue 3, May 1998, Pages 264-270:
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WPN-45M319N-1B&_user=137179&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000011439&_version=1&_urlVersion=0&_userid=137179&md5=8c82be05767fcb1c635d183c7f5c756b
the flow was directed into the Nordic Sea.
I estimate the initial volume, at the time of YD initiation, at about 1,400—4200 km^3. I take this as equally catastropic, so a flow of about 0.02 Sv to 0.13 Sv.
This is to be compared to the 9500 km^3 from the 100 m drop of Lake Agassiz. If this took 100 years, the flow was but 0.003 Sv; if 10 years, then 0.03 Sv. I tentatively conclude the contribution from the first release of the Baltic Ice Lake was important, expecially as it likely all went directly to the Nordic Sea.
David – Assuming that increases to H2O2 concentrations are indicative of melting (I’m ~80% sure), and that decreases to H2O2 concentrations are indicative of freezing (about ~20 % sure), I counted 4 melting/freezing cycles before YD initiation. The number of cycles corresponds to the d18O record. At 12.651 kya H2O2 is zero (no melting) and stays at a very low value (near 0) for another 1,000 years until 11.587 kya, after which time it shoots up to 28.25 ppb at 11.552 kya. The H2O2 has been on steady increase to present with melt-freeze fluctuations much stronger than before YD. Implication: whatever phenomena caused the plunge into YD, isn’t the one that caused the northern hemisphere to get out of the freeze and keep the current warming trend going.
Hopefully someone else will shed light on how melting of sea ice can be distinguished from land ice. I see lots of ice in the NA and no melting at all during YD.
Pt (174) — Data from
ftp://ftp.ncdc.noaa.gov/pub/data/paleo/icecore/greenland/summit/gisp2/isotopes/gisp2_temp_accum_alley2000.txt
(previously cited) shows a local maximum temperature at 13.588 kya
13.588 -38.4303
followed by a declining trend to
12.92 -45.9021
where it falls off a cliff
12.887 -48.0428
with the next local minimum being about 182 years later:
12.705 -50.13
I hold the view that YD cold was caused by the abrupt change in THC caused by all the fresh water being dumped into the North Atlantic by drawdown of proglacial Lake Agassiz (LA) and the Baltic Ice Lake (BIL). This is hardly original with me and agrees with what is thought to have occurred at the later 8.2 kya CR.
One unique aspect of YD, IMO, is the depth of the cold. Via methane concentrations, it was the deepest draw down (of methane) in three terminations by a factor of about 2.5 greater than the next, during termination 3.
Also, rather remarkably, both LA and BIL began dumping water at very close to the same time. While having the two ice dammed lakes fail with a few centuries of another another might not be very surprising, the synchrony of the failures and at the same time as the YDB event is, at the least, suspicious.
Be that as it may, do say more about “I see lots of ice in the NA … during YD.” Is this more than just hydrogen peroxide evidence?
Eric;
It is curious that one of the YD impact authors, Allen West , has also claimed that an assortment of mammoth tusks and other fossils from Alaska and Siberia incorporate embedded iron micrometeorites blasted into the upper surfaces of the unfortunate megafauna like so many shotgun pellets.
This is not exactly consonant with a comet rich in carbonaceous chondrites, but i have yet to see the PNAS paper and their arguments for nanodiamond identification . Both the evidence from Tunguska, and elaborate modeling of Rayleigh-Taylor instability at hypersonic interfaces on atmospheric entry suggests that such strikes can result in complete disruption of comets at ~100 kilometer elevations, but that does not explain the geographic spread of the hypothesized swarm, leaving tidal disruption as with Hale-Bopp as the obvious alternative .
I confess to having a small dog on the sidelines of this catfight- see Naturwissenschaften . Feb. 2006:
http://adsabs.harvard.edu/abs/2006NW…..93…88S
I also commented on the Mammoth Headache issue:
http://adamant.typepad.com/seitz/2007/12/palaeometeorics.html
Russell Seitz (176) — Thanks for commenting and I’ll hope you will continue to contribute here. My reading of the evidence is that the ET object was an ‘asteroid’, of sorts, not a comet. I could well be wrong about this.
All — Here is a suggested central line of the path of the ET object, Edmonton (YEG) to Berlin (TXL):
http://gc.kls2.com/cgi-bin/gc?PATH=yeg-txl&RANGE=&PATH-COLOR=&PATH-UNITS=mi&PATH-MINIMUM=&MARKER=1&SPEED-GROUND=&SPEED-UNITS=kts&RANGE-STYLE=best&RANGE-COLOR=&MAP-STYLE=
I conjecture that along this path there were repeated air explosions, sending impactors of many different sizes in, roughly, an ellipse along the route. In any case, nandodiamonds are found in Alberta and black mats in England, Belgium, Germany and Poland; hence my suggested routing. Using the base site
http://gc.kls2.com/
you can choose your own great circle route.
Regarding this one, while I could not find a suitable prarie provinces map, the great circle route goes close to the Clearwater-Athabasca proglacial Lake Agassiz overflow route; possibly the ice dam heare was seriously damaged. Note it also goes close enough to south-central Sweden that it could be the Baltic Ice Lake dam was also damaged. But more, note that various edges of LIS, GIS, the Iceland Ice Fields and the Fennoscandinavian Ice Sheet could well be so damaged that substantial quantities of ice quickly (within 180 years) made their way to the sea.
R. Seitz, I have been told that any mammoth so pelted with micro-meteorites would have been burned/blown to smithereens by a shock wave by the time these micrometeorites arrived. Mr. West proposes a highly complicated theory of why this would not be the case which involves “tunneling effects” of impactors.
Also, the dates on the tusks don’t match up, some going back to well before the alleged impact. Therefore it is proposed that the animals where this has been seen might have been died and been buried in ice 1,000s of years earlier, but been partially exposed again at the time of impact.
bigcitylib (178) — That would agree with the comments above suggesting a ‘comet’ impact around 38,000 years ago.
I believe I can show what mechanism caused the Carolina Bay (older than 38,000 BP) and the Younger Dryas (12900 PB) burn marks. The evidence supports a semiperiodic mechanism which rules out comets.
This article in Wikipedia discusses the dating of the analysis of the Carolina Bay burn marks and notes they are roughly 38,000 BP.
An impact hypothesis can not explain the geographic dispersion of the burn marks (geographically dispersed in the Younger Dryas case, i.e. Throughout North America and in multiple sites in Europe) and concentrated in along New Jersey to North Carolina coast, for the Carolina Bay marks.
Look at the Carolina Bay marks. (See link below.) How can impacts cause overlapping burn marks? How can impacts create burn marks that are elliptical up to 10 km long with no impact craters? Why do the Carolina Bay marks have an axis that points in the North West direction?
The impact hypothesis also can not explain the observed climate changes of the Younger Dryas. Abrupt cooling. Stays cold for around 1000 years and then abruptly warms and returns to interglacial.
http://en.wikipedia.org/wiki/Carolina_bays
“Many radiocarbon dates, which were obtained from organic matter preserved within undisturbed sediments, which fill Carolina Bays, are greater than 14,000 BP radiocarbon in age. The finite radiocarbon dates range in age from 440 ± 50 to 27,700 ±2,600 BP radiocarbon in age (Whitehead 1981, Gaiser et al. 2001). Some samples are so old that they contained insufficient radiocarbon for dating, which results in “greater than dates”. For example, samples from sediments filling Carolina Bays have been dated at greater than 38,000 to 49,550 BP radiocarbon years (Frey 1955, Brooks et al. 2001).”
“The cometary theory, on the other hand, popular among earth scientists of the 1930s and 40’s, is that the Bays are the result of an encounter between North America and a low density comet exploding above or impacting with the Laurentide Ice Sheet ~12,900 years ago [4]. Supporting evidence includes the failure of “wind and wave” theories to satisfactorily account for a number of the peculiar features of Carolina Bays, including the recent identification of markers suggestive of an extraterrestrial connection, the alignment of bays with points over the Great Lakes, and their tendency to overlap one another from east to west. Extraterrestrial markers include microspherules, magnetic grains with extraterrestrial chemistry, carbon spherules suffused with nanodiamonds, and levels of iridium sixty times background levels.”
Look closely at the Carolina Bay marks, figure 7 in this link. Could a impact create those marks?
http://www.pnas.org/content/104/41/16016/suppl/DC1#F7
Quote:
Carolina Bays. The Carolina Bays are a group of »500,000 highly elliptical and often overlapping depressions scattered throughout the Atlantic Coastal Plain from New Jersey to Alabama (see SI Fig. 7). They range from ≈50 m to ≈10 km in length (10) and are up to ≈15 m deep with their parallel long axes oriented predominately to the northwest. ….All of the Bay rims examined were found to have, throughout their entire 1.5- to 5-m sandy rims, a typical assemblage of YDB markers (magnetic grains, magnetic microspherules, Ir, charcoal, soot, glass-like carbon, nanodiamonds, carbon spherules, and fullerenes with 3He). …
Summary, imagery, and links in a good blog post here:
http://2bornot2b44.blogspot.com/2008/06/carolina-bays-and-younger-dryas-impact_03.html
Don’t miss the great excerpt quoted there introduced with:
“In 1933, the photo above kicked off the Carolina Bay debate. Edna Muldrow wrote The Comet that Struck the Carolinas in Harpers Monthly that year…..”
I retract the nonsense I mistakenly posted in comment #179. Please ignore that bit of muddled thinking.
I am ignoring the Carolina Bays simply because of the enigmatic nature of the evidence. There is ample decent evidence for YDB without even looking at the Bays.
Here is what might be found, if looked for thoroughly enough: “Characterisation of a Small Crater-like Structure in SE Bavaria, Germany”
http://www.rssd.esa.int/SYS/include/pubs_display.php?project=MARSEXPRESS&id=1321271
which is short, well illustrated and altogether nicely done. Unfortunately, it is surely too recent to be one of the YDB impacts; it is rather the size (11 m diameter) is suggestive.
Brushy Creek, LA: “Possible Meteorite Impact Crater in St. Helena Parish, Louisiana”
http://www.searchanddiscovery.net/documents/2003/heinrich/index.htm
might just possibly be the right age for association with YDB. The article is moteworthy in explaining the Carolina Bays as siliciclastic karst.
Carolina bays:
http://picasaweb.google.com/Swampmerchant/LIDARElevationImagesOfBays#
and here:
http://picasaweb.google.com/Swampmerchant/CarolinaBaysInCumberlandAndSampsonCounties#
Enjoy.
According to
http://www.deq.state.mi.us/documents/deq-ogs-gimdl-GGAGLAC.pdf
(with well done sketch maps), YDB occurred when Lake Ontario was covered by an ice lobe of LIS. So the Charity Shoal Impact Crater in eastern Lake Ontario
http://www.glerl.noaa.gov/pubs/fulltext/2001/20010012.pdf
is a potentially a YDB impact.
For much more speculative coincidence, note the German volcano
http://en.wikipedia.org/wiki/Laacher_See
located in what seems to be Germany’s only active volcanic zone, is dated by one source as erupting in 12,930 BP.
A few belated thoughts… I’m still with Eric’s original post in maintaining some skepticism, although the debate has been terrific and I learned a lot just by reading through this. In the end, I think the Science paper is getting a lot of press mainly because it is being sold as “the cause” for both the YD and the megafauna extinction. And parts of that all-encompassing explanation don’t fit.
To me, the most interesting part of the megafauna debate is its space- and time-transgressive nature. That is, the decline of megafauna on several (but not all) continents and its coincidence with the arrival of humans at various times on the various continents. The YD coincides roughly with the extinction and the end of Clovis in N.America, and thus with the proposed comet imapct. But it doesn’t correspond well with the main extinctions in Australia, Eurasia, or South America.
So if you want an explanation for Pleistocene-Holocene megafauna extinctions you don’t need or even really want an “event” isolated in time or space. That’s why human impact, whether hunting or introducing pests/diseases, is such an attractive explanation – people can move around and spread the cause to match the extinction pattern. Natural environmental change can do that too but you get into some fairly suspicious coincidences with the human history.
This theme is really similar to Eric’s original point about the Younger Dryas being similar to the DO events and to reversals during earlier terminations. You don’t *need* an isolated, unique cause to explain the YD, so if you propose one you are really making the story more complicated, not less.
Same goes for the Agassiz flood theme. Yes there is a big flood coinciding with the start of the YD and therefore with the proposed impact. But what about the other Agassiz floods, not to mention the other similar proglacial outburst floods that have been well-documented elsewhere.
Now, none of this proves that there was no impact or it couldn’t have been a factor. I just feel more comfortable not invoking an extraterrestrial event where one isn’t needed.
The following is a comparison of the Greenland Ice Temperatures to the Antarctic Ice Temperatures for the last 140 kyrs.
The warm interstadial periods are evident in the Greenland data and are followed by a return to the very cold glacial period. Note the abrupt end to the last interglacial period the “Eemian” Interglacial. This what we are trying to explain. There is a cyclic powerful forcing mechanism. The Younger Dryas burn marks were not created by an asteroid or comet impact.
http://upload.wikimedia.org/wikipedia/commons/9/9d/Epica-vostok-grip-140kyr.png
The same forcing mechanism that created the Younger Dryas cooling is evident through out the last glacial period. The effect of the mechanism changes due to specific variables which are not apparent unless you think about what the factors which affect the specific mechanism. (The mechanism is not ocean currents.)
A hint is some thing external is forcing the geomagnetic field. The effect of the forcing is dependent on the seasonal timing of aphelion (whether the forcing impacts the Northern or Southern Hemisphere) and the amount and location of ice on the planet’s surface. (Ice is an insulator. The continental crust is slight conductive. The effect is diffused if the event occurs over ocean.)
This paper discusses the analysis that shows there is geomagnetic field variance. The geomagnetic field intensity is 7 to 10 times greater during the interglacial than the glacial period. (The inclination does effect the forcing but the forcing is external not internally driven.)
http://www.sciencemag.org/cgi/content/abstract/295/5564/2435
Remember there is a very large change in cosmogenic isotopes at the start of the Younger Dryas which is consistent with an abrupt change in the geomagnetic field. (There are cosmogenic isotope changes at the other abrupt climate changes.)
This paper is also hint. Something is causing a change in nuclear decay time between aphelion and perihelion. Note there is an observed lag in the change in nuclear decay rates, which rules out neutrino variance as the cause of the nuclear decay rate change, as neutrinos travel at the speed of light.
http://lanl.arxiv.org/abs/0808.3283v1
The geomagnetic field (internal) resists a step change. When equilibrium is reached the effect (whether reinforcing or suppressing is dependent on the geomagnetic field’s polarity at the time of the event and the hemisphere where the event occurs.)
The complication in discovering the mechanism is connecting the evidence the geomagnetic field changes cyclically if one does not look for an external forcing mechanism (you must see and think of the burn marks at look at geomagnetic field variance data and think of what periodicity is required to explain the abrupt climate changes and adjust for orbital precision which changes the hemisphere where the event occurs.)
Curiously (from a standpoint of why didn’t someone point out this phenomenon before) or as would be expected, there is astronomical evidence(it is a fundamental stellar process) to support what is happening and the specific forcing mechanism was considered as one of the possible solar nuclear reactions.
#187
You say:
“Now, none of this proves that there was no impact or it couldn’t have been a factor. I just feel more comfortable not invoking an extraterrestrial event where one isn’t needed.”
Me either but the question is whether something else is needed to explain the length and severity of the YD. You seem to be ruling out freshwater draining into the N. Atlantic and an ET event. What cause do you have?
Regarding the megafauna. No doubt the megafauna were in decline at the start of the YD due to human hunting. However, you miss one critical point made by Firestone et al. It is not just that the megafauna go extinct roughly around the time of the YD. Megafauna and clovis evidence exists in the strata below the ET boundary but is completely absent in the strata above. The layer is a sharp divide.
The layer is a sharp divide.
Indeed it is, which is all the more important reason not to ignore something which now demonstrably did occur.
We now know that hemispherically catastrophic impact events occur on roughly millennial timescales, If it further is indicated that catastrophic and explosive hypervelocity impacts can and do occur, which then can evenly distribute millions of smallish hypersonic impactors over a very wide area (both hemispherically and regionally) on the order of tens of millennia, that is further cause for concern for the civilization and universe you reside in.
What we need to be doing is simulating the effects of high incidence, low grazing angle, explosive impacts of large carbonaceaous bodies on the surface of the Earth, since we now have evidence it occurs regularly.
Rahmstorf, S., 1995: Bifurcations of the Atlantic thermohaline circulation in response to changes in the hydrological cycle. Nature, 378, 145-149, found that a fresh water flux of as little as 0.06 Sv sufficed, in model studies, to induce a THC transition event.
From comment #173, Early BIL flux was about 0.02 Sv to 0.13 Sv directly into the Nordic Sea. Possibly this sufficed. From the same comment, drop from Lockhart phase LA to early Morehead phase LA added anywhere between 0.3 Sv and only 0.003 Sv, presumably mostly into the Arctic (Teller et al., 2005, full citation in comment #158). Additional fresh water flux came from melt of the southeastern portion of LIS and probably the southwestern portion of FIS, but no estimate of such fluxes is suggested.
This suggests that between the two (nearly) simultaneous dischanges there was sufficient flux to initiate Younger Dryas by a THC transition.
Summarizing, at 12,9 ka five events occurred nearly simultaneously:
(1) ET objects(s) air explode or impact (YDB);
(2) Early BIL discharges into the Sagerrak and on to the Nordic Sea;
(3) Lakehurst phase LA stops discharging into the Mississippi River basin and begins dropping about 100 meters to the early Morehead phase;
(4) Abrupt temperature drop of over 2 K begins in central Greenland, initiating Younger Dryas (YD) cold reversal (CR);
(5) Laacher See volcano erupted.
In the prior post, we see that (2) & (3) likely explain (4). What remains to be explained is the unusual duration and intensity of the YD CR. The interval of the YD CR is the second longest in three terminations (from comment #152, termination 3 had a 1900 year YD-like CR). It is over twice as intense as that event, the second most intense in three terminations, as measured by decreases in methane. Other indications of the extreme nature of the YD CR are given in prior comments.
The YDB hypothesis (YDBH) is that (1) initiated (2) and (3) so that these two drainages occurred nearly simultaneously, not seveeral centuries apart as might otherwise be the case, as I suppose in somewhat the similar termination 3 event. This simultaneous influx of fresh water in the far North Atlantic and Arctic Ocean caused the extreme cold in that region (and thus in Europe).
But what about (5)? Checking
http://en.wikipedia.org/wiki/Eifel#Geology
mostly likely just coincidence, although I suppose a bolide from YDB might have impacted just there, setting off the eruption. One might reserch this small point.
Other possible indicators tending to confirm YDBH would be an impact crater of the right age in Lake Vattern, Sweden, and similarly for lakes, large and small, across most of Canada. Modeling of large fluxes from Early BIL and first LA transition would also be helpful to better constrain the flux requirement for THC trnasition.
Whether YDBH is more firmly supportd by additional evidence or not, it is a productive hypothesis in that it suggests lines of further reaearch, a few of which are mentioned in the previous paragraph.
Astley, I don’t think the magnetic intensity is as significant as the change in direction of the geo-magnetic field. The Be-10 signal gets larger during the YD cold period in the GRIP ice record, and more pronounced with much large cycling prior to YD in the antartic ice record. Is this due to solar activity, geomagnetic, or a transition to a fall out mechanism?
[Response: It may well be due to changes in the deposition of the 10Be itself – which can be affected by rainfall and circulation changes independently of any production change (Field et al, 2006). – gavin]
In reply to PT 193#
The observed Younger Dryas change can not be explained by a simple increase or decrease of the solar heliosphere (i.e. Change in the solar magnetic cycle). The GCR modulation by the sun directly is not capable based on the hypothesized mechanism of the extreme affect and long term affect on planetary temperature that is observed for the Younger Dryas. (1000 years of cooling.)
There is evidence to support the hypothesis that a semiperiodic external event forces the geomagnetic field. A long term large change in geomagnetic field would based on the hypothesized mechanisms significantly force planetary temperature. For example:
Is the geodynamo unstable?
http://eprints.whiterose.ac.uk/archive/00000416/01/gubbinsd4.pdf
“Recent palaeomagnetic studies suggest that excursions of the geomagnetic field, during which the intensity drops suddenly by a factor of 5 to 10 and the local direction changes dramatically, are more common than previously expected. The `normal’ state of the geomagnetic field, dominated by an axial dipole, seems to be interrupted every 30 to 100 kyr; it may not therefore be as stable as we thought.”
“Recent studies suggest that the Earth’s magnetic field has fallen dramatically in magnitude and changed direction repeatedly since the last 700,000 years ago (Langereis et al. 1997; Lund et al. 1998). These important results paint a rather different picture of the long term behavior of the field from the conventional one of a steady dipole reversing at random intervals; instead the field appears to spend up to 20% of its time in a weak non-dipole state (Lund et al. 1998).”
As I said I believe the forcing mechanism is the sun can provide data to support that hypothesis. (See my comment above.) The resultant effect depends on the hemisphere were the event occurs which is controlled by precision (21 kyr cycle. Perihelion of the earth orbit controls the hemisphere where the event occurs.) and the planet’s inclination (41 kyr cycle. Greater the inclination at the time of the event the greater the effect.). It appears the effect is greater for greater orbital eccentricity (100 kyr cycle).
On a long term basis the position of the continents also affects the resultant which would explain the long term period when there were no magnetic field reversals. (i.e. All the continental crust is either north or south of the equator and the effect is greatest over continental crust as the conductive ocean dissipate the event. As the event is periodic the planet is similar to a capacitor with a charge difference core to surface. This explains anomalous charge releases during earth quakes and other peculiar electric phenomena which is observed when the solar charge forcing is stronger. The phenomenal is what is causing sprites which are electrical discharges from the top the earth’s atmosphere to space.
Based on this hypothesis the North Carolina burn marks would be due to restrike.
William and David,
This paper is referenced through one of postings linked to by this posting. Have either of you looked at it?
http://www.pik-potsdam.de/~stefan/Publications/Nature/Braun_etal_Nature_2005.pdf
Basically, it says you don’t need a comet to explain freshwater draining into the N. Atlantic or exotic geomagnetic theories to explain the YD.
It is, however, just a model but does correlate with the climate record,
http://en.wikipedia.org/wiki/1500-year_climate_cycle
David B. Benson (DBB) –
You should define (YSD) your acronyms (YA)or avoid them altogether (ATA) when they are not necessarily readily apparent (NNRA). It would help the average reader (AR) follow your train of thought (ToT) more readily.
BTW, this has been one of the more interesting posts in a quite a while and I do appreciate the many references and other useful information that you and many others have brought to the table.
Bob
Bob North (195) — In some comment or other, I did. Missing a repeat in comments #191 and #192 are
LIS: Laurentide Ice Sheet
FIS: Fennoscandinavian Ice Sheet
LA: Lake Agassiz
BIL: Baltic Ice Lake
Jim Cross (196) — I still don’t see the YD CR as being at all like a DO event, which is warming followed later by cooling in Greenland. Do note that the corresponding event in termination 3 methane records lasted 1900 years, which rather spoils the supposed 1500 year ‘cycle’. What I see in both cases is a THC transition event, to cooler conditions, followed later by a THC transition event to warmer conditions.
I am uncertain about the other evidencve to be found during generally cold conditions, but YD CR is clearly from terrestrial evidence an interruption in the general northern hemisphere pattern of following orbital forcing to warmer, interglacial conditons. The main cause, as I see it, is glacial meltwater finally triggering the THC down transition. The North Atlantic finally recover enough to trigger the THC up transition. The only role for the YDB extraterrestrial object encounter is to cause both ice dams, one for BIL, the other for LA, to fail simultanteously, a coindcidence for which there is evidence. I infer from the length of the corresponding event in termination3 tha such simultanaity did not occur for the drainage of large proglacial lakes at that deep time.
Going on to briefly consider the 8.2 kya event, the same THC down transition followed later by recovery matches well with the final drainage of proglacial Lake Agassiz-Objibway. Sorry, but I just don’t see any reality in the Bond 1500 year ‘cycle’.
> hemispherically catastrophic impact events
> occur on roughly millennial timescales
That’d be one hemisphere catastrophically damaged every how often, on average? With what kind of error bar?
Why Younger Dryas (YD) is not like DO events.
DO events, occurring during a ‘glacial’ between interglacials, consist of
(1) a sharp down transition in temperature;
(2) an interval of cold temperature;
(3) a sharp up transition in temperature;
(4) a period of warm, but gradually declining temperatures.
These are supposed to occur quasi-periodically with a period of about 1500 years.
Ordinarily, the sequence (3), (4), (1) is said to be the DO event; this makes no difference for quasi-periodic phenomena.
The sharp down transition of YD occurred at 12.9 ka. This was followed by a period of cold, but generally warming temperatures. The sharp up transition occured at 11.6 ka as I read it from the GISP2 temperature reconstructions by Alley (previously cited). Note that is 1600 years, already longer than the presumed DO total period. Thereafter the temperatures continue, on average, to go up, without cold reversals for some time.
In particular, these go up until the 8.2 ya sharp down transition in temperature. Time elapsed from YD down transition to 8.2 ya down transition is 4,700 years, not in accordance with the DO event period.
#197
David,
I don’t understand why you are having such a hard time grasping this warming/cooling thing?
The idea is that during glacial and early Holocene times, there are rapid warmings. It is the basic Milankovitch forcing augmented by CO2 increases. The warmings lead to meltings of ice which flush into the N. Atlantic causing the shutdown of the THC. That leads to coolings. These things go together.
My only question is whether this cycle explains completely the YD or whether some ET event augmented the YD.