Dryas is the genus name of a cute little yellow flowering plant that likes cold weather. In certain pollen cores in Europe, the Dryas indicates glacial or near glacial conditions. When the last Ice Age waned, we see the Dryas pollen going away. But then, it comes back and is visible in the pollen record for about 500 years or so. This is the return of glacial conditions we now know as the “Younger Dryas.”
The Younger Dryas is visible in all sorts of records, not just in pollen cores, but as in the visible remains of the re-advance of previously retreating glacial ice.
At one point, it was theorized that the Younger Dryas occurred because the North Atlantic Conveyor shut down for a while. The North Atlantic Conveyor is a current that moves warm water from, ultimately, the Indian Ocean, around Africa, and up to the North Atlantic. In these northern regions, the warm water cools and evaporates. This causes what was warm water to become cold (so it sinks) and saline (so it sinks). The cold, hypersaline (relative to the rest of the ocean) sinking water pulls the warm current north, thus serving as a sort of engine for this conveyor.
Global warming could turn off the conveyor by reducing the amount of evaporation that occurs, which would then trigger an ice age, if other conditions are right. Or, it melting of Arctic ice could add too much fresh water to the surface of the North Atlantic, reducing the effects of hypersalinity, thus turning off the conveyor. This is the premise of of that movie, The Day After Tomorrow: Global warming starts the next ice age.
This all comes from the idea that the Atlantic Conveyor, which had turned on and which was contributing to the end of the last ice age by warming the Northern Hemisphere, turned off and thus caused the Younger Dryas to occur. What actually caused the turning off, this story goes, is the release of huge amounts of cool fresh water down the Saint Lawrence Seaway after the breaking of a hypothesized ice dam behind which was a huge fresh water inland sea. It’s a cool story.
Unfortunately for this beautiful hypothesis, the exact timing of the movement of fresh water into the North Atlantic (that did happen) and the onset of the Younger Dryas (that also did happen) was not, in the end, close enough for this scenareo to have been possible. But maybe some other scenario, possibly involving fresh water or melting ice or whatever, could explain the turning off of the conveyor.
Yet another hypothesis was recently advanced that the real controlling factor on Northern Hemispheric warming is not the Atlantic Conveyor or other sea currents, but rather, the nature of waves of air (like the Jet Stream) over North America. Long term shifts in this continental wave could increase or decrease overall temperature in the Northern Hemisphere, turning off or on glacial cycles.
(All of this has to fit with the Milankovitch Cycle phenomenon, in which the liklihood of glaciation is tied to the orbital geometry of the Earth in relation to the Sun … but that’s another story)
But there is yet another, newer, hypothesis that has bee presented to explain the Younger Dryas, and this one is a real blast. No, really, I mean it. A blast:
New scientific findings suggest that a large, extraterrestrial rock may have exploded over North America 13,000 years ago, explaining riddles that scientists have wrestled with for decades, including an abrupt cooling of the atmosphere and the extinction of large mammals….
The time period in question is called the “Younger Dryas,” a time of abrupt cooling that lasted for about 1,000 years and occurred during an inter-glacial warm period. Evidence for the temperature change is recorded in ice cores.
According to the scientists, the extraterrestrial rock must have been about five kilometers across, and either exploded in the atmosphere or directly hit the Laurentide ice sheet located in the Northeastern section of North America. Wildfires across the continent would have resulted from the fiery impact, killing off the vegetation that was the food supply of many of the larger mammals like the woolly mammoths, causing them to go extinct. Since the Clovis people of North America hunted the mammoths as a major source of their food, they too were affected by the impact and their culture died out, explained Becker.
The scientific team visited over a dozen archaeological sites in North America where they found high concentrations of iridium, an element that is rare on Earth, and is almost exclusively associated with meteors. They found microspherules of glass-like carbon, which form at high temperatures and are thought to be a result of the impact blast. Also present were another type of impact tracer – carbon molecules called fullerenes with gases trapped inside.
The team concluded that the impact of the space rock melted a large portion of the Laurentide ice sheet, causing enormous amounts of cool, fresh water to flow into the Atlantic and Arctic Oceans. This would have caused a major disruption of the circulation of warm and cold water in these oceans, leading to a cooler atmosphere and the glaciation of the Younger Dryas period.
The scientists found evidence for the impact as far west as the Santa Barbara Channel Islands. Kennett said that the best examples from the West Coast were found at a site on Santa Rosa Island.
The Paleoclimate Program of the National Science Foundation and NASA funded this research.
Maybe. Maybe not.
Has the inflow of fresh water from Lake Agassiz ruled been conclusively ruled out as a cause for the Younger Dryas? Last I heard, it was still in the running, but I’m behind on my scientific reading. Is there a source on this I could access without the top-secret clearance you big-time university profs have?
Doyle: I would not say conclusive across the discipline, but the ugly facts are pretty convincing. This does not mean that fresh water influx has no influence or interest, but it is hard to account for the Y.D.
In the context of a global-warming-induced slowdown of the conveyor belt, ‘ice age’ is an exaggeration, as the conveyor belt only accounts for 3-6 C of extra-latitudinal warmth see here or here. Furthermore, it is unlikely that global warming could ‘turn off’ the convey-er belt completely, as ice sheets in the modern arctic are far smaller than at beginning of the 8.2 kyr event, and so there is far less ice available to be melted into fresh water. Finally, the rapid warming of the Arctic, which is already far warmer today than it was just before the 8.2 kyr event, would further reduce the effect of a conveyor belt slowdown. In the end, all Europe can expect from a conveyor belt slowdown is a temporary stall in the warming. More here . Note I refer to the 8.2 kyr event because some papers show that the catastrophic draining of Agassiz and Ojibway is nearer to the more recent 8.2 kyr event than to the Younger Dryas.
LL:
I deleted your second (corrected) post and corrected your first post.
It is not known that the conveyor shutting down would not induce an ice age. This is up for grabs. It is not entirely clear that shutting down the conveyor would not have other, cascading effects. Also, the relative importance of other forcing factors is not established. There is still a correlation between activity of the conveyor and, yes, actual ice age conditions.
What is important here, probably, is that during a period when orbital conditions favor ice age conditions, ice age conditions typically occur (or at least, they have done so since the closing of the Panamanian Land Bridge to an increasingly severe degree without fail). But I don’t think anyone believes that orbital geometry by itself is sufficient. There is a lanudry list of possible forcing mechanisms, and in the big picture, we probably can’t expect one of them to be “the” mechanism, nor can we necessarily expect a particular mechanism to behave the same way each cycle. A volcano here, a volcano there, an enormous lake empties into the ocean vs. a ginormous lake empties into the ocean …
Even the relevance of the conveyor may not be stable.
I totally agree that melting of the smaller amount of ice around today might not turn off the conveyor. But as far as I know, that particular model was only proposed in that movie, which gets a lot of things wrong!
There was more than one draining of these interior lakes, most likely, through the Gulf of Mexico and the Gulf of St. Lawrence. I have not seen an 8.2 ky date for either, however, but I don’t religiously read that literature … more in fits and starts.
Meteor impacts are the most likely reason for all the major extinctions visible over the geologic ages. The possibility of a meteor impacting the ice sheet of North America approximately 13,000 years ago, becomes more real when one looks at the Carolina Bays - 500,000 craters in a variety of soft sediments - all of the same age, all with a lifeless layer of blue grey-mud post impact. No meteoric material is found in these craters. Judging by the dunes pushed up, they were all created by a force coming from the North West. If it wasn’t a watery comet, then how about bullets of water from melted ice from a meteor impact on the ice sheet in Western Canada?
When comparing the geologic time scale with the age of 5-300km dia meteor craters, it is noticed that many coincide with mass extinctions on the planet seen in the Geologic time scale. This has not stopped during the time hominids, who have lived on the planet for the last 3 million years. The earth dimming effect of volcanic eruptions precipitated by meteor impacts would surely have played a major part in ice ages. The periodic nature of comets orbiting the sun in highly eccentric orbits are the most likely cause of many of the cyclic catastrophic climate changes we see in the geologic past. The 10.5km Bosumtwi crater in Ghana 800,000 years ago may even be what changed the Earths wobble from a 41,000year cycle of ice ages to a 100,000 year ice age cycle. Looking at meteor impacts greater than 20km dia there appears to be clusters of them approximately every 35myrs, possibly as the Solar system passes through the plane of the ecliptic of the Milky Way. Why are people reluctant to acknowledge meteors in the shaping of our planet when moon craters are testimony to the quantity of objects that have impacted us? When one looks at the frequency of Earth craters over the ages, there has been no reduction through time. There have been just as many 20km craters ~35myr, ~70myr and ~140myrs ago. luckily for us, the last 170km crater was 65myr ago.
Peter: Do meteor impacts cause volcanic eruptions? Are there any actual known examples of this?
Greg asked; Do meteor impacts cause volcanic eruptions? Are there any actual known examples of this?
Yes, ice core samples in the Antarctic that contain volcanic ash commonly contain Iridium - meteoric material. - (go to Google for refs).
The Deccan of India is a 5,000ft thick lava flow 65 million years old. Taking plate tectonics into consideration, it would have been directly opposite the 170km Chicxulub crater on the Yucatan peninsula. Reverberations around the planet would have caused a massive rupturing of the crust in a geologically stable area.
A simple analogy of this process is in car accidents - when the head impacts with the windscreen, brain damage is caused at the back of the head, not at the point of impact.
There is another 480km crater in Wilkes land, Antarctica dated at 248 million years old - the end of the Permian period and the Age of Amphibians. Directly opposite this impact site would have been Siberia where there is a 12,000 foot thick lava flow dated at 248 million years. The Atherton Tablelands is another lava flow in a geologically stable area - 2,000feet thick, 1 million years old - possibly from an ocean impact in the north Atlantic.
What are your reactions to the notion that the 500,000 Carolina bays were created by ice and water debris from a meteor striking the Laurentian ice sheet?
I think the chronology of the lifless blue clay layer of the Carolina Bays is consistent with the Younger Dryas extinction of Megafauna.
Peter:
Thanks for that useful information. I’d heard/read about the Chiczulub as a source of the Deccan basalt flow, and the Siberian as well, now that you mention it. I was wondering mainly if there were more recent examples. More to the point, this would be cool if we could see it happen now!
One of the problems with the late Pleistocene in Laurentia is that I think much of the basic research is not really done. The surficial/glacial geology is more complex (and interesting) than the attention afforded to it. Unless cobbles become a really important economic resource, that may remain the situation for some time. Under these conditions, it may be easy to fit a number of different hypotheses to various views of the data.
That is of course why the Impact theory is interesting … This is very exploratory and exciting.
I am not sure there was a younger dryas extinction, however.
I am just a layman here, but this just may be the blast structure from thie YD impact event.
http://theholocenecomet.spaces.live.com
Take a look.