The Marvelous Speed of Deglaciation
Cartoon illustrating estuarine circulation at a fjord. The fresh water, moving south from the melting glacier, carries some saltwater with it, which must then replaced by saltwater from the south. This process could have carried warmer water from the south to the north during the Boelling-Alleroed
The Potential Energy of Ice
The fact that ice age cycles have a "sawtooth" shape, featuring slow buildup and rapid decay, begs the question: Why should deglaciation be so fast? From a standpoint of physics, we should perhaps expect the reverse: to make ice requires getting rid of energy but it takes lots of energy to melt ice. Hence wasting energy into space would seem to be more easily done than bringing energy to the ice and making it do work.
What we must not forget is that the ice itself has lots of potential energy stored within it. If we can get an ice sheet to start moving, internal friction will heat it from the inside, it will waste into the sea, and its water carried to warmer regions. From this point of view, deglaciation can be fast because the ice is inherently unstable, especially after it has had time to depress the earth’s crust upon which it sits. At elevations below sea level, ice can erode the bedrock very deeply. This crustal depression happens each time the ice builds up, but the erosion is cumulative over the entire period of the northern ice ages, beginning about 3 million years ago. Once there is a deeply carved portion of the Earth, like Hudson Bay and Baltic Sea, there is then a potential heating system right within the center of the ice sheets, which can be activated when seawater is admitted. The same is true all around the margins, for example in the spectacular fjords of western Norway and Alaska.
If this idea about instability is correct, then once deglaciation starts we should see faster and faster decay of ice sheets, and, on the whole, this is true. The largest number of fast deglaciations is found within the last quarter of the entire "Pleistocene," the period of northern ice ages. Also, it seems, the more ice there is, the faster the deglaciations.
We might also expect another feature if this concept of unstable ice is correct: those glaciers which have their base below sea level should be extremely unstable. And there should be other glaciers that are not so unstable, whose bases are firmly grounded on solid bedrock. If that is so, we should see a two-step deglaciation: first the unstable ice is destroyed and then, later, the stable one. The detailed record of the last deglaciation suggests that this is indeed so. The best record for what happened is seen in the Greenland ice itself. According to the data, around 14,500 years ago there was extremely rapid warming, with conditions changing from fully glacial to fully interglacial conditions in one person's (or one mammoth's) life time. Unstable glaciers responded by collapsing at a furious pace. Within a thousand years or so, sea level rose by 100 to 200 feet! Within one century there would have been people witnessing the flooding of vast areas of low-lying coastal plains.
Graph of mean sea surface temperature
versus relative abundance of N. pachy
and a microphotograph of N. pachy
(sin.). Note how the abundance of this foraminfer increases with colder waters. [From: NOAA
The period of warmth associated with the first step of deglaciation lasted about 1500 years; it is called the "Boelling-Alleroed," after places in Denmark where the layers of sediment were first described that established this warm period. The warmest conditions are concentrated at the beginning of this period, the reason for which not clear. Perhaps it had to do with the import of heat from the south by a process associated with the filling of the northern seas with meltwater. The meltwater would have moved south on the top layer of the ocean (since its lack of salt makes it light). This meltwater would have entrained saltwater in this movement through mixing. The saltwater is then replaced by influx of ocean waters from the south. This well-known process is known as "estuarine circulation" and is commonplace wherever rivers enter the sea. The subsurface flow from the south could have brought heat to the north. Another possible mechanism for early warming is ocean circulation changes. The ocean is intimately involved in what is going on during deglaciation, and this is seen in the abundance of a cold-water planktonic foraminifer Neogloboquadrina pachyderma (sin.) (The "sin." refers the chambers in the protist that make a leftward spiral when looking at the earliest chamber and drawing a line to the later ones.). In glacial conditions, the ocean off Greenland is rich in this cold-water species, but in warmer conditions, this species rapidly disappears. In fact, this plankton indicates whether the place where the sea floor core was north of the polar front or south of it.
Dryas octopetala, after which the “Younger Dryas” was named.
The Younger Dryas
After the initial warm spell, between 14,500 and 13,100 years ago, there was a drop of temperature toward colder conditions that briefly reversed, but then remained long-lasting after 12,800 years ago. In fact, this drop near 12,800 years ago was precipitous and resulted in extremely harsh conditions. After much balmy weather in the British Isles and northwest Europe, permafrost was re-forming in the lowlands of Holland and northern Germany, and an icy wind blew (again) from the east. Chances are this was a major time of stress for mammoths, other great mammals, and people: the “Great Ice Giant” in Scandinavia was winning the fight. The sea level rise stopped entirely, and a great wall of moraines was built by the remaining Scandinavian ice cover during the time that followed, reaching from southern Norway through southern Sweden into southern Finland. This period of cold, the last attempt of the ice age to hold on, is called the "Younger Dryas," with the term "Dryas" being the name for the genus of an arctic flower growing on fields close to glaciers, found on top of the "Alleroed" layers of sediment in Denmark.The Younger Dryas ended as abruptly as it began. All through this grim period, the sun had stood as high as ever in the summer, and was larger than average, as Earth was now closest to its star when the days were longest. The additional summer heat was soaked up by the glaciers and perhaps by the oceans, until a threshold was reached. As before, when jumping into the Boelling-Alleroed from glacial conditions, the climate now jumped into the second step of deglaciation and again, the change in one person's (or one mammoth's) life time was truly extraordinary. The polar front in the North Atlantic rapidly receded to its present position, and all of Europe and much of the rest of the northern hemisphere benefited from this, as the Iceland Low took over to move heat from south to north.
Warming improved conditions for the growth of plants and any mammals that had made it through the grim climatic fluctuations of the Younger Dryas were now ready to expand. Alas, there may not have been many left to take advantage of this boon. The few animals that were present apparently were rapidly decimated by skilled hunters using increasingly sophisticated weapons and methods. This was a time for the expansion of people and reduction of everything else. It was the first great mass extinction of the last million years, and it started the domination of humankind over everything else on the planet. Sea level rose quickly, very rapidly at first and then slowing as the easy-to-melt ice was gone. Again, vast areas of low-lying coastal plains were flooded. River mouths drowned and estuaries such as Chesapeake Bay were created. Shorelines expanded and hunting and fishing were good.
Forests all over the planet expanded greatly, taking carbon dioxide out of the atmosphere. However, other processes kept pumping carbon dioxide into the atmosphere from the ocean because as sea level reached its maximum it began to flood great regions of carbonate shelves, and shell-making organisms responded vigorously. They extracted calcium carbonate from seawater to make their skeletons — corals and coralline algae, mollusks and foraminifers — and, in doing so, they changed the chemistry of the seawater, which could now hold less carbon dioxide in solution than before and gave it off to the atmosphere. Hence, corals helped transfer carbon from the sea into the forests. The speed of deglaciation helped in the transfer since things went too fast for the ocean to mix deeply while the change in chemistry was taking place. This prevented the effect of degalciation from being neutralized by reactions at depth, such as carbonate dissolution on the deep sea floor.