One of the most interesting and exciting stories in science is that of the Younger Dryas. The Younger Dryas was a climate event that had important effects on human history, and that has been reasonably linked to some of our most important cultural changes, and ultimately some evolutionary changes as well. That is one reason why it is interesting. In addition, the Younger Dryas was a pretty big deal … a climate change or something like a climate change that caused massive changes all around the earth, and fairly recently. But the cause of the Younger Dryas is at present unknown, although a series of explanations have been advanced, each as convincing as the next depending on one’s point of view. The Younger Dryas itself is interesting, and the story of how scientists have studied it and the changing explanations emerging from that research is just as interesting.

The latest science is beginning to suggest that it is all even more interesting and exciting (and scary) than previously thought.

The Younger Dryas is named after a cute little yellow flower called the Dryas. There were two periods during the last “Ice Age” Where Dryas pollen shows up as a kind of marker in ancient sediments (like in the muck on the bottom of a pond or lake, for instance). The younger layer is called the Younger Dryas, and it has been known for many decades.

Meanwhile, around the world in regions previously covered by the last glacier there was recognized a phenomenon called a “readvance.” When glaciers stop expanding, there is a lot of melting at the leading edge of the ice. Here form features called moraines, consisting of cobbles, gravel, silt, etc. … the dirty crap that the glacier has chewed up as it grew that is now being spewed out the edge like the icky sandy stuff you see all over parking lots in the spring when the winter snow melts (this will mean nothing to those of you living in warm environments, of course).

The moraines can be very large and impressive features. For example, Long Island. The whole freakin’ thing is a giant Moraine. Or Cape Cod.

These moraines can be dated using a variety of technique, and they are then understood to be the terminal feature of one particular glaciation. Often, because history tends to repeat itself under certain circumstances, we find moraines on top of moraines, with the deeper ones representing earlier glacial periods. The study of moraines was key to the initial understanding that there even was an Ice Age, and in working out the basic chronology of glaciations.

So, a “readvance” is when you have a moraine indicating that a glacial cycle is ending, and then somewhere stratigraphically above the moraine and in a somewhat (but usually not too much) different geographical location, you find another mini-moraine, indicating that the glacial ice temporarily expanded again. The glacier came back. Advanced again. Readvanced.

So there are a number of long-ago identified “readvances” in North America and Europe. It turns out that all of the latest “readvances” linked to the last ice age date to the same time period as each other, and to the same time periods at the Younger Dryas.

So the last glacial cycle happened, then abated, and by the way, the retreat of the glaciers that occurred then was linked to very very warm conditions. I once found a subtropical species of clam in the late glacial sediments near Boston, and that was pretty run of the mill evidence … one bit among many … indicating that the end of the last glacial cycle was accompanied by very warm conditions in the north Atlantic and elsewhere.

But then that retreat stopped and, apparently, full blown glacial conditions returned.

This is very interesting for many reasons, but mostly because it is a tiny, recent, and thus quite manageable and observable using good techniques, example of the onset of glacial conditions It is very hard to understand and study the onset of glacial conditions in the distant past, because subsequent glacial activity may damage much of the relevant evidence, and because it is a long time ago. But the Younger Dryas is smaller, less destructive of evidence of its own origin, and recent. So we can perhaps use it as an example of glacial onset and address the question: What causes an Ice Age to happen?

All I want to do here is to point you to a new article in Science that provides further evidence for a theory that originally was viewed with great skepticism but that has recently started to show some promise as it passes trough the gamut of professional obstinacy we know of as the Scientific Method. But first, a quick review of what we thought caused the Younger Dryas.

Back in the 1980s, we were all very excited to have a very cool theory for the start of the Younger Dryas based on then novel ways of looking at ocean currents together with extremely high resolution climate data from an increasingly large number of ocean and lake cores. This was the Atlantic Conveyor theory.

The Atlantic Conveyer is a current that starts as a poorly organized surface current (really a bunch of enormous blobs) of warm water exiting from the Indian Ocean into the southern Atlantic. This water then moved north to the North Atlantic where it cooled and evaporated. The evaporation exacerbated the cooling and made the water more saline. Thus, warm water became denser and colder, and thus sunk to the bottom of the ocean. What goes down must come up, etc., so this sunk cold water would the run in a deep water current …. a kind of giant sub-oceanic river greater in volume than all the world’s fresh water rivers combined times some not too small number …. back to the Indian Ocean where it was heated up again.

The loss of heat from this water in the North Atlantic kept the northern continetns relatively ice-age free. The idea is that every now and then the conveyor turns off and an ice age happens. In the case of the Younger Dryas, the thing that turned it off, according to this model, was the influx of great amounts of fresh water from an inland sea that flooded the North Atlantic.

There is no doubt that this great inland sea (and other like it) existed, and there is no doubt that there were great influxes of fresh water into the North Atlantic (and elsewhere, such as the Gulf of Mexico, etc.) at various times. The problem, as was discovered with even greater resolution of the climate data, that the 1,300 year long Younger Dryas began at a time centuries away from the most likely time of the flood.

A second idea that had been around for some time then came to prominence. This is the idea that the great Mountain Wave simply moves up and down. It works like this. There is a wave of air moving around the earth at any given latitude. As you go up and down the longitudes, the direction of the wave shifts back and forth and changes in it’s character. The junction between two of these waves is the jet stream, and there are several such features. The details are way more complicated than I can address here. The point of this idea is that there is a big wave of air movement over the North American continent, and it moves seasonally (winter vs. summer), but it also moves at a larger scale, such that for some centuries, or for thousands of years, or even longer periods, it has one pattern, and for other periods, it has a different pattern. One pattern gives you ice ages, a different pattern does not.

The third and most recent theory is that some kind of enormous extraterrestrial but near earth explosion happened … like a swarm of comets hitting the atmosphere all at once and exploding, maybe even running into the earth but not leaving much of a crater. This theory gets pretty zany. In one region of North America, researchers claim that there are little tiny bits of shrapnel in almost every large extinct mammal bones that they find. In other words, the North American megafauna got wiped out in a matter of seconds or minutes because they literally got fragged.

This event would have also involved climate change of the type we see with the Younger Dryas, and the timing of the event is quite possibly perfect for explaining the onset of the Younger Dryas glaciation.

The latest study is from Science, and is a follow up on criticism leveled at an earlier paper published in PNAS. From the Science paper:

We report abundant nanodiamonds in sediments dating to 12.9 … thousand calendar years before the present at multiple locations across North America. [there are two types of ] diamond … in this boundary layer but not above or below that interval. Cubic diamonds form under high temperature-pressure regimes, and n-diamonds also require extraordinary conditions, well outside the range of Earth’s typical surficial processes but common to cosmic impacts. …. These diamonds provide strong evidence for Earth’s collision with a rare swarm of carbonaceous chondrites or comets at the onset of the Younger Dryas cool interval, producing multiple airbursts and possible surface impacts, with severe repercussions for plants, animals, and humans in North America.

This study is reviewed in a recent blog post at Real Climate

I venture to guess that this impact theory will pan out and be demonstrated as very likely. All of this must be understood in the context of Milankovitch Cycles, of course. These are the orbital cycles that more or less (more more than less, as a matter of fact) map onto the comings and goings of ice ages over the last two million years. Within the context of these orbital geometric cycles, events such as running into a herd of comets or great fresh water inland seas catastrophically dumping into the ocean or sudden metastable changes in air currents could turn on, or off, a particular climate pattern.

We have not seen the end of this discussion, no doubt.

D. J. Kennett, J. P. Kennett, A. West, C. Mercer, S. S. Q. Hee, L. Bement, T. E. Bunch, M. Sellers, W. S. Wolbach (2009). Nanodiamonds in the Younger Dryas Boundary Sediment Layer Science, 323 (5910), 94-94 DOI: 10.1126/science.1162819


  1. #1 Ethan
    January 11, 2009

    I normally ignore typos, but this one is too good. “A swarm of comments” indeed!

  2. #2 Greg Laden
    January 11, 2009

    If Freud was a blogger, that would have been a Freudian slip.

  3. #3 Lilian Nattel
    January 11, 2009

    Thanks for explaining this step by step and in regular English. Much appreciated.

  4. #4 Pyre
    January 11, 2009

    As is well known, very small grayish “industrial” diamonds make an excellent abrasive for grinding hard materials, when attached to a grinder surface.

    Consider the possibility that such a surface, perhaps a hemisphere-sized piece of “sandpaper” (coated with nanodiamonds rather than sand) was the source of this layer — when the paper eroded after grinding the ground into moraines, and incidentally causing terrible cold by blocking the sunlight.

    Call this the Sun/Sky-Shielding Super-Sander Small-Stones Scenario, or the Great Gray Clay-Inlaid Frayed Decayed Abrader Hypothesis (all set about with fever-trees). Future research might be funded by Ground-Ground Grants.

  5. #5 yogi-one
    January 12, 2009

    We’re all gonna die! A swarm of comets! Everyone, stop what you are doing, and run into the streets, screaming!

    Actually, it would be kind of cool to get vaporized by a comet. What a way to go!

  6. #6 clinteas
    January 12, 2009

    That is way cool ! The megafauna got fragged by diamond shrapnel from outer space….

  7. #7 Graculus
    January 12, 2009


    OK, I’m just an interested amateur, but…

    They are claiming that hardly any megafauna died in under conditions conducive to fossilization before the supposed event.



    Does this smell funny to you?

  8. #8 kai
    January 12, 2009

    While we’re picking on spelling, I don’t think you really meant to say “The evaporation exasperated the cooling…” Did you perhaps intend “exacerbate”? (Or just “increased”?)

  9. #9 Greg Laden
    January 12, 2009

    Graculus: The northern region have almost no Pleistocene fossils dating to prior to the glacial terminus because those areas were wiped out by the most recent glaciation, so this is not a difficult claim to support at all.

    Kai: Woah!

  10. #10 Ian
    January 12, 2009

    And I thought the Younger Dryass was the name of a new super-absorbent diaper! I guess you live and learn!

  11. #11 Cal Harth
    January 12, 2009

    The comet swarm is a very interesting explanation for the onset of the Younger Dryas. I live about 60 miles north of you. The evidence here has been hard to explain for a long time. The forests that had invaded here prior to that event were buried in till when the readvance occurred. The glacial ice here was about a thousand feet deep at times.
    The Des Moines lobe raced across Minnesota, with sublobes that dammed up periglacial lakes of huge size. You live on or near the Anoka Sand Plain which was from shoreline deposits of Glacial Lake Grantsburg. Our house is about 8 miles from the former north shore of that lake.
    I guess it makes a difference in interest level if one lives in a glacially sculpted region.
    If you have been to permafrost regions you may have heard someone say “Dry on Dryas.” It is much easier to walk where Dryas grows than on soggy tussock tundra.

  12. #12 SteveF
    January 12, 2009

    A couple of comments…..


    The YD cooling was probably more of a northern hemisphere thing really. Accurate dating is essential. See this summary in Science:


    Well not really. The Younger Dryas wasn’t an ice age. It was a stadial event. This isn’t a semantic difference because the causes of these climate events are fundamentally different.

  13. #13 Pyre
    January 12, 2009

    Greg, you need to turn of commenting, for all our sakes. Some internet traffic may go through microwave relays, and many ‘netters have wireless routers. Heavy blog traffic means a swarm of comments hitting the atmosphere, with deadly results.

  14. #14 Pyre
    January 12, 2009

    turn of –> turn off

  15. #15 Blind Squirrel FCD
    January 13, 2009

    I’m missing something here. These Pleistocene fossils have tiny fragments embedded in them.
    Take a grain of sand and give it any high initial velocity you care to. What is the effective range of this particle at sea level? 20 yards or so? Imagine trying to swat a ping pong ball under water. Do you see what I am driving at?

  16. #16 Greg Laden
    January 13, 2009

    Blind squirrel: Good point, and I’m sure worth pursuing. But, as Hercule Perot would say, we are not swatting zee ping pong ball sunder zee zee, are we? The energy levels involved converted terrestrial carbon into diamonds. These impact forces are the kinds of things that form tektites (we assume) and can thus get a small object moving at escape velocity long enough of them moving, that the effluence of such an event emanating from one planets surface can spray particles into the orbital zone of an adjoining planet (thus we find these little bits of Mars or the Moon here on earth).

    Larger bits are presumably moving and exploding at the same time. Also, imagine your ping pong balls moving at nine times the speed of sound. The atmosphere in front of the ping pong ball is burned off.

  17. #17 Blind Squirrel FCD
    January 13, 2009

    I am willing to believe that meteors formed diamonds and tektites, that escape velocity can be reached and material can travel from one body to another in the solar system to another. None of this has anything to do with my original objection that it is impossible to kill so much as a squirrel with a grain of sand at any significant distance.


    I am unable to find any meaning in this statement. Atmosphere does not burn. The grain of sand burns.

  18. #18 Greg Laden
    January 13, 2009

    Blind: OK, I get it.

    (as an aside, burning totally involved the atmosphere, certainly, but that s probably not important here)

    Your objection is to the lethality of a grain of sand traveling at, say, 2,000 kph entering a mammoth.

    I’m going to have to go back and read (and blog) the original paper. I’m not sure if the cause of death was always or even ever this particular effect … its just tha the effect links the death to the event closely enough. Even if these are bones laying around on the landscape, that’s pretty good considering there is a lot of other putative evidence for the event and its consequences.

  19. #19 Blind Squirrel FCD
    January 13, 2009


    No, my objection is that the grain of sand cannot travel at 2000KPH in our atmosphere for more than a few feet before it is stopped by friction or vaporizes from heat.

  20. #20 Nathan Myers
    January 27, 2009

    Squirrel: 1. A grain of sand can travel at any speed, for any distance, if the air it is traveling in is moving at the same speed. 2. As I read it, the grains involved are not blamed for killing anything; they are found embedded in one side of exposed mammoth tusks.

  21. #21 Blind Squirrel FCD
    February 4, 2009

    Nathan: Your point no. 1 makes a bit more sense. As far as no. 2 goes, yes Richard Firestone, the team leader claims that mammoths were killed by these fragments.

    Firestone said he believes meteorites exploded over parts of Beringia — the Ice Age-era region that includes parts of modern-day Alaska, Yukon and Siberia — creating showers of fragments, or “micrometeorites,” that struck and killed many of the prehistoric creatures that roamed the area.


  22. #22 Blind Squirrel FCD
    February 4, 2009

    another quote from Thompson

    “They probably came in under very high velocities, hundreds of kilometres (sic) a second almost. It must have exploded and just embedded these things in the tusks in large quantities, as many as 100 or more in one tusk

    The highest theoretical speed for an asteroid entering the atmosphere is 72.9 KPS. This would be a very rare event. I can’t help wondering what else this guy is wrong about.

  23. #23 Greg Laden
    February 4, 2009

    Is 72 kilometers per second not fast enough for you?

    Also, there is an element being ignored here. These diamonds are not object entering the atmosphere. They are objects forming in the atmosphere as products of a very high energy impact and subsequent stuff that happens. It seems to me that they only have to be going at the speed of sound or maybe twice the speed of sound to get the effects seen in the bones, and I’ve read impact reconstructions in which gas clouds are moving this quickly along with shock waves. The gas cloud provides the raw material, the shock wave the extra compressive energy, and the whole thing is moving at the speed of the average rifle bullet.

  24. #24 Blind Squirrel FCD
    February 4, 2009

    Greg, that is the best explanation I have heard yet. I can’t wait to hear how this all comes out.

  25. #25 Dennis Cox, Fresno, Ca.
    March 13, 2009

    A “swarm” is a large group of annoying insects. Pretty small word for a comet cluster. a group of comets is a Catastrophe of Comets.


    Some ejecta I found that had been tossed around
    was waiting for someone to assess.
    That ‘twas up in the air before it got there.
    What tossed it, I had not a guess.
    Strange, and quite queer, no volcano was near,
    or round crater to account for the mess.
    How rude, and sore cruel; to break every rule,
    then lie in plain sight n’er the less?

    In northern Mexico you will find ten thousand square miles of ejecta, breccias, melt basins, and pyroclastic flows as fresh and pristine as the day they first cooled. And if you follow the flows of those materials back to their respective sources you find no impact crater, or volcanic vent. Just bare, burned, smoothly melted, stone. Or strange dragon spine shaped mountains without a trace of alluvial material. And you will find yourself knowing beyond all shadow of doubt that the source of all the heat and destruction came from above.
    If you aren’t looking down there you aren’t doing impact science. And you aren’t going to unravel the mystery of the Younger Dryas cooling. Or the Megafaunal extinctions of the early Holocene. You are just chasing butterflies in the playground. C’mon you guy’s catch up.

  26. #26 Rod Chilton
    July 28, 2009

    Hi Greg: I just noticed your interesting site and your discussion of the Younger Dryas. I think that you covered this topic very well. The Younger Dryas is something I have been researching over the past eight years. Very early on I noted many problems with the North Atlantic Ocean circulation theory. This sent me along the road to investigating alternative ideas, one of which Dr. Kennett and others have derived quite recently, and that is a comet impact. Or as they indicate more specifically, an air bursting comet, as occurring in the skies over North America. Well, all this and I believe much more is the conclusion I have reached and presented in a new book I call Sudden Cold. If you are so inclined check it out at my website Thank-you for this forum, Yours truly Rod Chilton