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