The Younger Dryas is so called because it corresponds, in the pollen record from Europe, to the latest (i.e. youngest) appearance of the Dryas octopetala pollen, an alpine flower in regions that are now far from alpine. It marks a clear period towards the end of the last ice age when the warming trend of the deglaciation in Europe particularly was interrupted for a period of about 1300 years before it got going again. There were clear glacier advances during this time and the moraines can be seen very clearly all around Europe and Scandinavia.
The clues to what caused this remarkable, if temporary, turnaround have always lain in assessing its spatial extent, the exact timing and correspondence with other events. Two recent papers have shed some welcome and potentially controversial light on the subject.
To appreciate those papers though, you need a little background. It is clear from the Greenland ice cores that the Younger Dryas was a huge event in that region – 10 to 15ºC cooling at Summit – and this is confirmed by studies of ocean sediments in the North Atlantic which also show large temperature drops (a few degrees) over this period. Particularly clear records of climate impacts are seen off Portugal and as far south as the Cariaco Basin off Venezuela. New evidence from proxy circulation tracers suggest that the North Atlantic overturning decreased significantly during the YD, possibly shutting down completely. This has all lent support to the theory, first suggested over a decade ago, that glacial meltwater interfered with the N. Atl. circulation causing an interruption of the ocean heat flow to the North. This is of course the prototype of the “ocean circulation changes imply a new ice age” meme which has been so hard to get rid of in recent years.
But how far afield did this climate change reach? The event is been clearly seen in sediments off Santa Barbara (California) and in cave records from China, but both of these areas are still in the Northern Hemisphere, and exactly what is recorded (wind speed change and precipitation amounts?) is still a little ambiguous. But what about the south?
The initial results from Antarctic ice cores at first seemed to show something very similar – the long warming through the deglaciation was interrupted by a cold reversal half way along. The relative dating was not very good at that time and it was quite plausible that the two events in Greenland and Antarctica were one and the same. When glacial advances in New Zealand were found to be around the same time, it seemed clear that the YD cooling had extended the entire length of the Atlantic! The only problem was that the favored mechanism, an ocean circulation change, no longer matched the data. Models of these shutdowns found it very hard (actually impossible) to get a cooling in the North and South at the same time. Lots of other ideas were suggested, but none that were really convincing. So scientists went on thinking that it probably was the ocean, but always with a bit of unease about the southern hemisphere results or the models.
Clarity started to emerge when new techniques for lining up the ice cores in Antarctica and Greenland were developed. One technique used the very rapid changes in methane (which could be measured in both poles) to synchronise the chronologies. The thought being that methane changes are well mixed and so large changes in one hemisphere get transmitted very quickly to the other. With this came a big surprise – the Antarctic Cold Reversal started hundreds of years before the Younger Dryas! In fact, Antarctica stopped cooling just as the YD was getting started. This was evidence of a bi-polar see-saw in the ocean – something the models did seem happy to show.
But what about the New Zealand glaciers? How did they fit in? There had been some loosely constrained pollen data that didn’t show much cooling reported in 1999, but the result was still ambiguous. This is where the first of the new papers comes in. In it, Barrows et al show with improved dating that the New Zealand peak glacial advances actually were significantly younger than the YD. These dates seem more solid that the previous estimates and are supported by nearby ocean sediment evidence for a continued warming through the YD.
So now that the southern hemisphere oddities have left the scene, does that mean we now have a full understanding of the event? Not quite. The ocean circulation theory has indeed been strengthened in recent years, but the search for a trigger continues – why did it happen when it did? As always, many ideas have been put forward – a shift in drainage pathways for Lake Agassiz from the Mississippi to the St. Lawrence, a solar trigger or a tropical Pacific trigger and now we have a brand new idea – a cometary impact.
This has been suggested by a large group of researchers who have collectively been working on archaeological sites (Clovis) in North America and who noticed a layer of charcoal at about the same time as the YD at a number of disparate sites. They claim too that within this charcoal there is significant evidence of impact ejecta, and from this they suggest that the trigger for the YD was in fact an extraterrestrial impact. This doesn’t really undermine the ocean mechanism – the comet is hypothesised to have caused significant meltwater to flow into the Atlantic and the ocean circulation changed as would be expected in the standard view. Some suggestions were made at meetings that the direct impact due to dust and smoke forcing from fires actually caused the initial YD cooling, but this doesn’t seem quite as plausible (dust falls out of the air quickly).
The researchers have however tried to link the impact with everything that was previously linked in time to the Younger Dryas – mammoth extinctions, the disappearance of the Clovis culture etc. – but it is very difficult to disentangle a direct consequence of an impact from the indirect consequence of the subsequent climate change. But these ideas are quite intriguing and they made quite a splash when announced in a coordinated session at AGU in the spring.
There are three aspects of this work that will require independent confirmation to determine whether or not this is a viable explanation. Firstly, it should be possible to find the ejecta layer almost anywhere – peat bogs, lake sediment, ice cores etc. – wherever there is material of the right age. If that is indeed found (big if), then the first part of the hypothesis might be confirmed – that there was an impact at this time. The subsequent parts are much harder: depending on where any object landed or was centered, how can one show that it produced the meltwater that presumably caused the ocean circulation change? The source to the ocean of the meltwater (be it the Arctic, St Lawrence or Hudson Bay) has been unclear for many years. Finally, how can you show that the direct effect of the hypothesised comet was responsible for any impacts, rather than the indirect effect of the ocean change? These issues will, I suspect, take a long time to resolve.
There are still some more YD mysteries though. The ocean models might have won the Southern Hemisphere round, but they still have a hard time explaining why it lasted so long, and how the rapid warming (10 or so degrees in the space of a few decades in Greenland) at the end occurred. The fact that similar events occurred all through the glacial period (Dansgaard-Oscheger events) implies that they must be fundamental to the climate system rather than a one off. An impact event doesn’t impact those mysteries at all.
Gavin….Thanks for publishing this. I have been following the “comet impact”
theory for a couple of years, wondering what confirmation would be needed tie the theory to the Younger Dryas. The duration of the DY always seems far longer to me than any transient
effects from an impact could explain. I guess we will have to wait and see what science and scientists can reveal about the side-effects that occur and their associated time-constants when “the Earth has a really, really bad day.”
And where could this impact have happened? Lake Agassiz?
How far from this location can one expect such an ejecta layer?
All this seems prove to me is that impacts are much more common than we first believed (or want to believe) and so evidence of one more impact is not that surprising. It is old and tested, but still true, that extraordinary claims require extraordinary proof and I wait with interest to see.
I do have one question however. If an impact were to take place in (say) the Atlantic causing considerable mixing, even homogenization (?) over the entire depth of the water column and wide lateral area, how long would it take for the system to return to “normal” and what would the impact on climate be during the transition? I have no feeling for the rates involved.
Is the argument that the meteor strike melted the melt-water? or that it
“destabilized the ice sheet” and perhaps fractured an existing ice dam? I’d
think the second the more likely, as the strike would then only have served to
alter the timing of an event that was bound to occur anyway.
For example, the Intra-Allerod Cold Period has been associated with a melt-water discharge, and some of the geography of the Columbia river was apparently carved by repeated
ice dam collapses releasing water from Lake Missoula. The Younger Dryas
seems to have been one of a series of similar events.
I would be more receptive to their YD explanation, if these researchers had not been casting about for something to explain with their hypothesis for so long. They have ignored other evidence for rapid changes in the carbon cycle which have more immediate and terrestrial explanations. We cannot explain every D-O cycle blip in the Quaternary using extraterrestrial causes.
Gavin that is an excellent post. I go to all AGU meetings and listen to lots of talks, but finally the interconnectedness of all things Younder Dryas is making sense to me. Very well written!
Gavin,
There are also additional aspects of interest. Over here in the UK, the Younger Dryas isn’t actually too well dated, at least not in terms of glacial activity. We’ve got a pretty good handle on the cooling event itself from various reasonably well dated multiproxy studies and in broad terms these confirm the basic story (i.e. a bloody cold Younger Dryas!).
To a certain extent, I think this encourages us to think in these neat, comfortable little packages and assign things that are less well dated to the appropriate category. As a result, the Loch Lomond readvance has always been nicely tucked into the YD event. However, recent cosmogenic dating evidence (and, to be fair, some of the older radiocarbon dates) suggests that the Loch Lomond ice sheet actually reached its maximum extent pretty early in the YD and was most probably building up during the previous interstadial. Given that temperatures were so cold during the YD, an early start was probably necessary in order to get the required moisture.
In this sense, a bit of caution is probably required when thinking about the Kiwi glacial evidence. Obviously no one would say that the Younger Dryas didn’t exist in the Northern Hemisphere, on the basis of a pre-YD ice build up in the UK. In the same way, we probably shouldn’t use New Zealand ice to attempt to falsify a cold southern Hemisphere during the YD (although having said that the ice in New Zealand appears to have retreated a fair while before the YD, whereas in Scotland there is overlap and ice present during the cood event). The ocean core is more useful for testing the occurrence of a cooling event.
A minor typo: Dansgaard-Oeschger, isn’t it?
Re #2
“Mathis Says:
And where could this impact have happened? Lake Agassiz?
How far from this location can one expect such an ejecta layer?”
“…suggests that an extraterrestrial object exploded in Earth’s atmosphere above Canada…”
http://www.sciencenews.org/articles/20070602/fob1.asp
Mathis (2) — Somewhere I read a claim that ejecta were also found in Belgium.
What would be the effect of the comet striking deep water? I would guess that would greatly lesson the ejecta. Couldn’t a comet strike easily ignite fires from the radiation and/or stray bits coming off while the main body could have struck deep water, causing tidal waves and minimizing ejecta?
If a comet collision were to prove the catalyst of a massive North American freshwater flow into the Atlantic that results in the shut down of the THC, how would this explain the “see-saw” of cooling starting in the southern hemisphere?
Why is there an assumption that it was just one object? Why not a cloud of smaller objects over a period of a few years? Some would be large enough to produce ejecta, some would start fires on the ground, and some would just leave dust in the atmosphere.
As the dust settled on the ice, it would change the albedo of the ice as the atmosphere cleared. Thus, rapid cooling by atmospheric dust followed by a rapid rewarming/ice melt without a (big recent) crater.
It would be the kind of event that could leave a culture very much in awe of “shooting stars.” We see the Perseids and the Leonids every year. Anyone that watches the night sky has seen thousands and thousands of shooting stars. Why should so many cultures have so much fear of a common event? Maybe because, during the YD, those events were more exciting??
So, here we go again. When there is no other explanation, one can always turn to the old fire ball from the sky… Humans have done this for millennia. It was not convincing then, and it is not convincing now.
Looking it up, not likely Hudson’s bay (an impact that big in the last 10,000 years would never have been noticed by contemporary human beings — anthropic principle, in absentia; there wouldn’t be anybody home here now if that were an impact crater.
http://www.uwgb.edu/dutchs/planets/impact-No.htm
I’m not sure how this relates to GW today, aside from the debate over whether the THC might shut down, which anyway is a side or additional issue to all the more certain harms facing us. (And since I’m pretty far south — the southernmost tip of Texas — I’m thinking either my place would be staying the same, or maybe even getting warmer.)
I guess some denialists might claim that the warming now is due to a lack of meteor strikes, which have been common throughout history, pushing the world into cooler conditions, and now since we haven’t had one in a long time the natural warming is being allowed to keep increasing to its natural level.
Uh-oh, I may have just given them another argument.
I think the extraterrestrial-ites have a wee way to go to tie cause and effect together here. A cursory Google-Earth examination of the land east of Hudson Bay shows the lovely impact craters of Couture and Clearwater East and West, among others. While these impacts are dated at around 430 and 290 million years ago, they remind us that Earth is always receiving impacts and glancing blows from extraterrestrial items. It is quite conceivable that an impact occurred around the time of the Younger Dryas 12,000 years ago but what is much harder is linking that as a sufficient cause to the temperature swings observed.
This post is a worrying reminder of the rate at which climate has changed in the past. When viewed in the context of the current status of the climate it seems possible (and its hard not to be alarmed) that we are on the starting ramp of another rapid heating phase.
The Arctic Ice Area Anomaly chart is continuing to trend down in a worrying manner, http://arctic.atmos.uiuc.edu/cryosphere/IMAGES/sea.ice.anomaly.timeseries.jpg
and the droughts and subsequent fires besetting numerous areas of the world are combining to paint a very gloomy picture.
The diligently worked predictions of IPCC et al in relation to the warming likely in the next hundred years or so (around 3C) is based on the response to known forcings – in particular anthropogenic GHGs. I suspect that its because we don’t know what initiates them that this work does not suggest we are the onset of a Dansgaard-Oeschger event
What this post confirms is that there are intrinsic mechanisms in the climate-earth system that regularly give rise to temperature increase of 10 to 15C in a few decades. We obviously do not have any explanation of the way this happens, but the climate record is unambiguous. So we have good reason to be very concerned indeed. These D-O events seem to be able to commence from quite a range of initial conditions, and likewise end at a similarly disparate selection of temperatures:-
http://en.wikipedia.org/wiki/Image:Grip-ngrip-do18-closeup.png
If the delta-O-18 data is an indication of the temperature over the events then there is obviously some other mechanism that finally triggers them which is to a large degree insensitive to the starting temperature. This may be a good thing – if our own interference with climate is not awakening this dragon, but on the other hand…!!!
So – A butterfly flap around 1900 has started a warming trend; Mankind is in the process of doubling CO2 concentrations in the space of a couple of hundred years, and there is this sleeping dragon – the well-tested mechanism that kicks off D-O events which push up global temperature by over 10C in a few decades. I hope we haven’t woken the dragon!
David Benson — “investigations of a buried layer at sites from California to Belgium” — from the AGU link, in the main post at top.
I wonder what would happen if a big asteroid hits Antarctica. The kinetic energy of KT impactor that killed the dinosaurs was of the same order as the energy you need to melt all of the ice in Antarctica…
Older articles about a much older impact, for comparison of effects and energy.
http://www.lpl.arizona.edu/SIC/news/chicxulub2.html
http://www.sciencedaily.com/releases/2002/09/020910081734.htm
Following on from my previous post, if I may: Looking briefly at the relationship between delta-O-18 and temperature..
http://en.wikipedia.org/wiki/Image:Five_Myr_Climate_Change.png
gives a rough correlation between delta-O-18 and temperature. This suggests a -10C change for a +2.2 change in dO18, giving a correlation of temperature/dO18 of -4.5C per unit dO18. Note that this correlation is derived from Vostok cores, and thus intuitively it would be likely that the global mean temperature change will be less – say about -3C/unit dO18.
This then gives us initiation of the D-O event at dO18 over a range of about -36.2 to -40.1 dO18 units i.e. over 3.9 units dO18. This implies a range in global mean temperatures of around 11.7C for the temperatures at initiation of D-O events over the last 30,000 to 50,000 years.
This reinforces the impression that the D-O events are initiated by events that are insensitive to starting temperature. Any thoughts on what the initiator is? Does CO2 spike around then, by any chance?
I have a couple of problems in understanding. If “Younger” means more recent, then this statement confuses me: “Barrows et al show with improved dating that the New Zealand peak glacial advances actually were significantly younger than the YD.” If the south stopped cooling than before the YD, then the NZ glacial advances should be older than the YD rather than more recent. Uh, actually after re-reading it the other misunderstanding I had disappeared and the issue I’ve noted seems to be a minor error by Gavin. I’ll just ask quickly though, What are hypotheses for the earlier reversal in glaciation in the southern hemisphere? Maybe I need to read a third time….
[Response: It’s just reading from the paper. I had to check it twice as well. – gavin]
What would be the effect of the comet striking deep water?
A biblical flood, complete with rain for forty days and forty nights.
http://www.nytimes.com/2006/11/14/science/14WAVE.html?pagewanted=all
http://www.earth2class.org/k12/w8_s2004/content.htm
Re #23: [What would be the effect of the comet striking deep water?]
IIRC, “Lucifer’s Hammer”, Niven & Pournelle.
But as to the effects of this comet, suppose an air burst sets off widespread forest fires. That produces lots of smoke & soot, which might produce general atmospheric cooling, but fast melting of soot-covered glaciers, especially if the impact was in late spring… How much melting would covering a glacier with soot cause? Would there be a telltale layer in Greenland cores?
Nigel Williams (#17) wrote:
Nigel, where exactly are you getting the “global”?
Not saying that there aren’t any D-O events that affect all the world the same way at the same time, but the post at least points out that at this time the cummulative evidence is for the very same bipolarity which the models show.
From the post:
Not that this is something to be entirely happy about — as it suggests that the ocean bistable, bimodal, or bipolar — or at the very least, a rather fickle beast.
Aaron Lewis posts:
[[It would be the kind of event that could leave a culture very much in awe of “shooting stars.” We see the Perseids and the Leonids every year. Anyone that watches the night sky has seen thousands and thousands of shooting stars. Why should so many cultures have so much fear of a common event?]]
Shades of Velikovsky.
Before you speculate on the cause of the fear, first, demonstrate that many cultures, or even one culture, actually fears this particular event.
Rahmstorf 2003 (GRL) pointed out the approx. 1500 yr cyclicity of the D-O events, whose Younger Dryas is probably the most recent signature. He wrote that the highly precise clock excludes an internal oscillatory modes in terrestrial climate – rather it suggests an outer space origin (but I guess meteor impact is not the kind of outer space regular timing he had in mind). I don’t know if the precision of the clock Rahmstorf alluded to in his paper is still correct with new datation from ice cores.
RE: 13
This isn’t a Perseid- or Leonid-like impact… This isn’t even close in comparison to a Krakatau-like explosion. Those pale in comparison to what we’re talking about.
Most ‘shooting stars’ are grains of dust that burn up in the atmosphere.
RE: 14
It sure does seem that “catastrophic destruction from the outer solar system” is the favorite way to explain a lot of things lately.
Maybe they’ll make a movie about it… hey, i think Bruce Willis would be great in the leading role!
In a previous professional incarnation, I worked as an intern at the Lunar and Planetary Institute studying cratering on Jupiter’s icy satellites. The researcher I worked under had taken old DOD nuclear code and scaled the energies up ~10-20 orders of magnitude, thrown in a bit of new physics, and voila, the makings of a truly cataclysmic event. A 1 km ball of ice traveling at roughly 7 km per second (Earth escape velocity) has a kinetic energy of about 10^20 Joules. A comet would likely break up in the atmosphere, but that would just increase the rate of energy depostion in the atmosphere and increase the pressure wave at the surface. It would be a really bad day at ground zero.
It sure does seem that “catastrophic destruction from the outer solar system” is the favorite way to explain a lot of things lately.
Nucleosynthesis is catastrophic too, do you deny that as well as cosmic impacts?
There are a few problems with the Firestone hypothesis about the Younger Dryas, explained here:
http://www.physicsforums.com/showthread.php?t=192583
The problems of interpreting the Younger Dryas are twofold, the historical mix up of carbon dates with calendar dates and the assumption that cumulative isotope values of the Greenland ice cores represent average temperatures rather than seasonal shifting of precipitation (Jouzel et al 1997). The aridness of the Younger Dryas should have rung alert bells.
RE 17 and 21
For what it is worth, I think one theory (The binge-purge cycle) about DO events is that they due to internal instability in the Laurentide and Euroasian Ice sheets (culminating in Heinrich Events over Bond cycles), another is that it was due to variable input of meltwater changing the location of NADW formation from below Iceland to above it in the Greenland/Norweigian sea (apologies for lack of references, if this is disproven now someone please say). Atmospheric sources were suggested by Bond and Lotti (1995 – Science 267 (5200) p1005) due to the cyclicity of the events, but not sure that explains why they are then in lead/lag with the southern hemisphere (Steig and Alley, 2002, Annals of Glaciology 35, p451-456). Apologies references are shoddy – don’t have any notes to hand.
I believe that the scientific community is running out of plausible explanations of a single kind for explaining why the YD occured. It could simply have been a sequence of several factors resonating together where no single factor is responsible overall.
I don’t have any catastrophist axe to grind and certainly am not a Velikovsky freak, but I happen to have a second hand copy of “History of the Conquest of Mexico” by William H Prescott, originally published in 1842.
I read it about 20 years ago and have always been intrigued by a particular passage describing an Aztec Deluge myth.
“They believe that two persons survived the Deluge, – a man named CoxCox , and his wife. Their heads are represented in ancient paintings, together with a boat floating on the waters, at the foot of a mountain.
A dove is also depicted, with a hieroglyphical emblem of languages in his mouth, which he is distributing to the children of CoxCox, who were born dumb.”
Prescott, Swan Sonnenschein, Paternoster Square, London. page 639
Of course it’s quite possible that this account was given a particular slant by the Spanish priests who reported it.
Even Prescott has doubts about its provenance, as well as a similar tradition amongst the Michoacan.
However, Alexander Humboldt regarded it as authentic (see footnotes 16 & 17)
Just an idea – couldn’t a comet of such dimension influence the earth’s orbit around the sun?
Alex Nichols on myths of “The Flood” among pre-Columbian natives:
“Of course it’s quite possible that this account was given a particular slant by the Spanish priests who reported it.”
Gee, do ya think?
Re 28
The Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling talks about “ejecta”, so if they got that right, then the impacting objects had to be large enough to hit the ground.
For that effect, it had to be a fairly large event. If it was a single object, that would have left a large, recent crater – that would be easy to find, but there are no obvious candidates.
That leaves the possibility of many smaller impact objects over a few years. I take modern meteorite showers to be evidence that clouds of debris can still intrude into Earth orbit, and then impact the Earth. You would have to explain to me why a few large impacts are more likely than thousands or millions of smaller impacts. Just because we have not seen such an event recently, does not mean that they do not occur every so often. What is the size distribution of craters on the moon? Are the moon craters all the result of big events, or are there more small craters than large craters?
However interesting such a historical event is, I am not going to waste much time on it because we have a global warming crisis. Atmospheric aerosols are not a viable solution to AGW, too many health side effects. The best solutions focus on greenhouse gases.
Re 30
YES! My point exactly. We would still see effects from such a large, recent single impact.
Re: 31
For the record, I’ve not denied anything about this subject. It is entirely possible, and may be even probable that cometary and/or meteoric impacts have caused the mass extinctions and abrupt climate changes.
I just find it interesting/curious that 30 years ago, no one thought it was probable. Now it seems to be the preferred explanation.
And I also sleep well at night knowing that if we do find that we’re about to die due to a comet impact, Bruce Willis is standing by to save us! :-)
(30, 34) Ray, Pete, thanks, you helped me answer a question, how much energy would be needed to melt a good sized bit of the Laurentide ice sheet…
an amateur (me) looks at the problem:
The back-of-the envelope calculation, starting with Ray’s 1 km ice ball: 3.34×10^5 Joules per kg is the heat of fusion of 0 degree C water ice, so 10^20 joules would melt 3×10^14 kg of ice, or 300 GT… 1 km^3 of ice is 10^9 kg, so it would melt an ice cube on the order of 3 x 10^5 km^3, or 1000km by 300 km by 1km thick.
I hope I kept all the exponents straight. That volume translate into the area of Kansas and Nebraska put together, and ice 0.75km thick.
For reference (source EPA http://www.epa.gov/glnpo/factsheet.html), the volume of fresh water in the Great Lakes is 2.26 x 10^4 km^3, which is about 10% of the volume of ice that I calculate here. The area of the Great Lakes is 2.44 x 10^5 km^2…
I’m assuming 100% of the kinetic energy of the hypothetical 1 km ice ball goes into melting; pick any reasonable fraction. Maybe the ice ball is smaller, only 0.5 km. That reduced the available energy by a factor of 8 (mass ~ r^3)… still not unreasonable. The resulting meltwater is between 1 and 10x the volume of the Great lakes, just as a test of reasonableness.
Maybe not all the extinctions happened docudrama style, but that amount of heat and subsequent fires, etc. would certainly put a dent into the local ecology, and take species which were “endangered” and more rapidly push them over the edge.
So, my question: if the approximate volume of fresh water contained in the Great Lakes were introduced into the North Atlantic over a year, or a decade, what effect would that have on North Atlantic circulation? Is it enough to affect North Atlantic heat transport to Europe, or shut down the conveyor? Currently, less than 1% of the Great Lakes volume exits via the St Lawrence. If there is enough effect for an interruption, or a slowdown, what happens to the heat energy in the topics that used to be carried away… how is it redistributed?
Pete I agree, it would be neat if ALL the observations could be explained by a single event, this may be one of those “tipping point” events, like Krakatau is associated with the “Year without Summer” in 1815-1816 (SO2 aerosols).
Much as the possible cometry impact is an interesting hypothesis, I’m not sure it’s getting us any further. For example, for possible analogies of the heat required, and the likely resulting water melt, I suppose we might be able to use the larger jökulhlaups as analogies, although the heating in this instance is from below, rather than above, and takes place over days or weeks. The last big one I recall was in 1996, with peak flows of around 40,000 cumecs (e.g. see http://www.hi.is/~mmh/gos/photos3.html), so this is still ‘small’ compared with the rates inferred for the meltwater pulses around the last deglaciation, and I think also around the 8.2 ka event. Nevertheless, it shows just how large, endogenetic processes can produce major melting i.e. we don’t need to invoke cometry or other events to get them. It also shows that the melting itself doesn’t have to be sudden for the meltwater pulse to be sudden. Glacial outbursts are common enough anyway (geomorphologically, at least); for example, there was a major burst in the early 1940s in the Ferpecle valley, in Switzerland, which occurred without warning and took out all of the bridges down to Sion. The meltwater could well have been building up for years, over decades or indeed much longer, so again, there’s no necessity to invoke a catastrophic causation, even though the event was catastrophic. My gut feeling, therefore, is let’s not latch on to the cometry idea unless we really have to i.e. the simpler, inherent explanations just don’t stack up.
Hope this makes sense; comments welcome …
Re 40 . . . like Krakatau is associated with the “Year without Summer” in 1815-1816 (SO2 aerosols).
That would be Tambora. Krakatau was 1883 I think.
Re # 39 dean: “I just find it interesting/curious that 30 years ago, no one thought it was probable.”
That’s the amazing thing about science – our knowledge always increases with time. Scientists are continually learning things they didn’t know before. Sometimes, things once thought improbable are discovered to be highly probable. And some things thought to be highly probable turn out to be improbable.
Thanks Gavin for a nice review.
Like many I doubt that the traditional explanations are in such a mess that a one-off cometary explanation is needed. What about the sequence Oldest Dryas/Bolling/Older Dryas/Alleroed/Younger Dryas. Does each get a comet? What about other Heinrich events?
Unless the evidence for ejecta starts popping up all over the place, I suspect that this idea will fade.
Nick O. (#41) is correct that jökulhlaups are not that uncommon, although the Gjalp eruption in 1996 clearly did not do much to the ocean circulation in the North Atlantic. There was a larger similar event in 1918 about 100km west of Gjalp (associated with the Katla eruption) and the only oceanic influences I know of is a possible mini-tsunami (probably as a result of a landslide on the continental slope).
These events were, however, shortlived and initiated by a subglacial eruption.
For the late glacial period (and the PBO event in the early Holocene) the chief suspects are the rerouting of meltwater and the sudden drainage of pro-glacial lakes. There is ample evidence that such events occurred. There is also some evidence for coincidences in timing, which raise the interesting possibility that “Younger Dryas”-like events were driven by more than one source.
There is a review on this in:
“Were abrupt Lateglacial and early-Holocene climatic changes in northwest Europe linked to freshwater outbursts to the North Atlantic and Arctic Oceans?”
By Nesje A, Dahl SO, Bakke J (HOLOCENE 14 (2): 299-310 MAR 2004)
re: 41, 42, 32
Mateo, thank you, I accept the correction. Krakatau was fairly impressive in terms of its cooling effect, as I recall, but not up to the 1815 effect.
Nick, thank you for the jokulhlaup reminder. Lake Agassiz was contemporaneous. I don’t think the two hypotheses are incompatible. I was curious to see whether the energy of a hypothetical impactor was anywhere near what might be required for a good-sized melt in the ice sheet. Assuming a Great-Lakes sized (2.26×10^4 km3) discharge in a year (3×10^7 sec) gives a flow rate of 0.75 x 10^6 m3/sec, compared to a still quite impressive 4×10^4 m3/sec from your reference.
The multiple discharges of glacial Lake Missoula on the western side of the continent would be tough to rationalize with comets.
I am impressed with the reports of widespread coincidence of the carbon layer with the Younger Dryas timing, the spatial distribution of meteorite finds along the southeast coast of the US, and the peculiarly oriented “Carolina Bays” which look like splash marks.
Andre (#32), The criticism of the timing by carbon dating (Andre, #32) sounds like “close but no cigar”, will have to wait for experts to sort out. That’s science.
Thanks to all,
Re #45 Jerry,
If you would take a few moments and work yourself through some oh.. three to four hundred publications pertaining Younger Dryas and glaciation then the more prevailing trend is to date glacier advances around 11,000 +/-50 years carbon dated, which obviously denotes the start of the Younger Dryas. Indeed close, but no sigar. 10,000 radiocarbon years BP equals 12,915 calendar years BP, while just about all high resolution proxies, except for GISP2, have the Younger Dryas started just after 12,700 calendar years BP. That gap is just too big, and given the widespread aridness of the Younger Dryas, where would the snow come from to get all the glacier advances?
It seems that the 200 years between 12,9 and 12,7 ka is the true mystery interval, advancing glaciers (caused by ETE?) while the water isotopes in all proxies suggest a warm period, the last Allerod spike.
How this discrepancy could form is explained here
[[Just an idea – couldn’t a comet of such dimension influence the earth’s orbit around the sun?]]
Not enought to be easily measurable. To estimate the difference, try computing the mass of a sphere of ice 500 meters in radius. Then compare it to Earth’s mass: 5.9736 x 1024 kilograms (NASA 1998).
Jerry posts:
[[Pete I agree, it would be neat if ALL the observations could be explained by a single event, this may be one of those “tipping point” events, like Krakatau is associated with the “Year without Summer” in 1815-1816 (SO2 aerosols).]]
Tambora. Krakatau was in the 1880s, if I remember correctly.
Interesting article. I haven’t read all the comments, but I do remember hearing a few months ago about someone studying the extinction of the Clovis culture and claiming that it was several smaller meteor impacts and not one big one.
And, as someone else already said, impacts from space seem to be a lot more frequent than we’d like to admit.
Interesting.
I have something to add to the discussion, with the caveat that I am merely passing it on, although on the face of it it seems plausible.
For entertainment I read books on the topic of aliens and alternative prehistory stuff. Think like Von DAniken etc.
So, this book called “Uriels machine” by Christopher Knight and RObert Lomas has a chapter on a putative comet impact about 9600 years ago.
Now, this is not the same date as 13,000 years ago, but anyway, some of the evidence they assemble is probably relevant. They attribute researchers Edith and Alexander Tollman with compiling information regarding a comet strike 9,600 years ago. One point is that there is a nitric acid peak in ice cores from this period, the nitric acid being formed from atmospheric nitrogen by great heat.
They also claim a variety of tektites have been found from around the world from the period in question. Also, there is a blip in the radiocarbon dating curve from that period, which would probably be caused by destruction of the ozone layer by incoming meteors.
They also claim that various deposits of sand etc have been found far above sea level, where they have been deposited by tsunamis.
So that is the kind of evidence to look for when trying to work out whether something hit the earth 13,000 years ago.
However, the data given in Knight and Lomas’s book does not cover the period 13,000 years ago. They also suggest that their impacts nearly 10,000 years ago caused a warming of the planet, although they do not suggest a mechanism.