Periodically there have been planet-wide mass extinction events that have wiped out many plant and animal species. A prime example would be the asteroid impact of some sixty-five million years ago that ended the Cretaceous and, apparently, brought an end to the dinosaurs (although they may have already been in decline or had been wiped out by the Deccan Traps flood basalt eruptions and climatic change caused by those eruptions). The much earlier Permian extinctions were even greater and were apparently triggered by the Siberian Traps eruptions. In one sense, these mass extinctions are no mystery: both plant and animal species, on land and in the sea, can be stressed by rapid climate changes and by truly catastrophic events, such as the aforementioned mega volcano eruptions, or major comet or asteroid impacts. Moreover, such eruptions and/or impacts may also trigger sudden climate change.
Nor is the selectivity of extinctions always a mystery. Large things, including the largest animals and plants, are less common than small things. There are many grains of dust but far fewer planets. There are many tiny crustaceans and few whales. Also, predators are less common than their prey species. We can easily visualize how, at the end of the Cretaceous, the largest dinosaurs and marine reptiles might be so reduced in number that survivors would have trouble in finding mates, causing a further decline, and also a reduction in genetic diversity leading to inbreeding. Meanwhile, smaller animals, including mammals, would, though greatly reduced in number, still be numerous enough to avoid these problems and to rebuild their populations. It is also quite possible that the smallest dinosaurs would be unable to compete with the mammals.
Catastrophic climate change is, in some respects, not mysterious at all. It’s not, for example, that we don’t know what causes ice ages—we know of too many causes. The sun’s energy output is not constant, and the Earth’s orbit periodically becomes more, and then less, elliptical; and its axis of rotation wobbles in a complete circle every 25,920 years. The present positions of the continents, which slowly drift about over the eons, separating and then recombining, allow the buildup of continental ice sheets in the Antarctic and sea ice in the partially enclosed Arctic Ocean; and the ice, reflecting the sun’s energy, causes further cooling. Massive volcanic eruptions can inject dust and sulfur compounds into the upper atmosphere, reflecting yet more energy. The variables are so complex that we cannot predict the outcome.
But what of mass extinctions that affect one region of the earth more than others? What of mass extinctions that wipe out some predators but not others, or some relatively small and numerous herbivores but not others? The extinctions at the end of the last major glaciation, though far less extreme than the Cretaceous or Permian extinctions, were very, very selective.
This last glaciation is believed to have begun roughly 76,000 years BP, possibly triggered by a mega volcano in Indonesia, although some researchers believe that the buildup of massive ice sheets, primarily in Europe and North America, somehow triggered the vulcanism. Researchers differ in their opinions, and there is much uncertainty; but the climate varied back and forth, and the ice sheets advanced and, at times, even retreated a bit. The peak of the cold period was perhaps 17,000 BP, and then the climate generally warmed until about 12,800 BP, when the climate abruptly became, once again, much colder, until about 11,600 BP, when the Earth suddenly warmed; the glaciers mostly melted; and sea levels rose rather abruptly. The event of 12,800 BP is generally referred to as the Younger Dryas; the name comes from a flowering tundra plant, Dryas octopetalia, an indicator of a colder climate. Two less extreme colder periods were the Older Dryas and the Oldest Dryas. It is important to understand how rapidly these climate changes occurred; the last major glaciation took only a few decades to cover much of the Northern Hemisphere and parts of the Southern in ice sheets, although it took centuries for them to reach their full depth. As an example, recently Mt. St. Helens rebuilt some of its glaciers that were vaporized by the 1980 eruption. It is estimated that the Younger Dryas took only 10 years or so to restore melting ice sheets and drastically reduce the temperature.
For many animals, plants, and people in the Southeast Asia region, the eruption of 76,000 BP would have been immediately fatal. Then animals in colder regions would have died off in large numbers before they could migrate to warmer regions, and many in warmer regions would have been exterminated by drought and desertification, for, when the earth cools, there is less evaporation of seawater, fewer clouds, and generally less precipitation. Faster than most species could adapt, the climate got colder and, at times, a bit warmer. Prior to the Younger Dryas, many animals would have migrated back into colder regions or former deserts, only to be trapped by sudden cold and drought. Then, when the climate warmed and the glaciation finally ended, many would have been wiped out by massive floods of glacial melt water released by the collapse of ice dams. Reaching the sea, these floods would have triggered mega tsunamis, destroying plants, animals, and people on the coasts. This is almost certainly the source of flood legends and the story of the sinking of Atlantis.
So climate change is not really a mystery, nor is mass extinction. The problem is the selectivity. Our understanding of it is compounded by the fact that there is no way to determine with absolute certainty the date of species’ demise. It used to be assumed that the massive elephants known as mammoths completely died out by the end of the last glaciation, despite stories of encounters in Siberia as late as the nineteenth century. But now we know that a few survived in North America until about 10,000 BP and a tiny number still lived on St. Paul Island, off the coast of Alaska, until at least 5750 BP, and on Wrangel Island north of Siberia until about 4500 BP. Giant ground sloths survived on some Caribbean islands until about 4,700 BP. No one seems to have asked how they got there to begin with. Yet other kinds of elephants still live in the wild in Asia and Africa; only in America were they completely wiped out, and this includes mastodons, elephants that lived only in North America.
Most people have heard of saber-toothed cats, but few know that lions—more or less identical to modern lions—lived all across what is now the United States during the last glaciation and then disappeared completely. Yet lions still live in the wild in Africa and in western India; they survived in Iraq until about 2600 BP and in southern Europe, including southern France, until about AD 100, and in Morocco until 1922. But they were selectively wiped out in North America. Fragmentary remains from South America may have been lions, or possibly very large jaguars.
Short-faced bears lived only in North America, and probably weighed as much as a ton. They seem to have been very numerous, especially in California. They also vanished from the fossil record after about 11,000 BP.
Camels originated in North America, and still live in South America (llamas and alpacas and their wild relatives). They survive today in Africa and Asia, but camelops hesterus became extinct in North America by about 11,000 BP, or perhaps as late as 10,000 BP. Few of the many fossil remains show signs of having been butchered by humans.
In fact, the old theory that human hunters decimated the North American mega fauna has always been inadequate. It had been believed by anthropologists that human hunters only arrived in the Americas at the end of the last major glaciation, but now it is generally recognized that they were here much, much earlier, and even some mainstream researchers are now willing to admit that some of the original inhabitants may have come here by sea, from Europe. And why would human hunters have wiped out lions and elephants in North America but not in Eurasia and Africa?
If the Younger Dryas event was triggered by some cataclysm that primarily affected North America, that might at least explain why the mass extinctions were greater here, though not why certain species died out and others survived. There are currently three main theories.
One is that a massive supervolcano eruption caused the cooling. Some have even suggested that the culprit might be the Laacher Sea volcano in, of all places, Germany. It is a two-kilometer-wide crater or caldera lake in the Rhineland, which still vents volcanic gases and is in the Vulkarleifel region, the volcanic part of the Eifel Mountains. It last erupted at about the right time, some 12,900 BP, but it is estimated that the eruption was no larger than that of the recent Mt. Pinatubo explosion. Since that eruption produced no major climatic change, it is unreasonable to assume that the one in Germany would have done so; and its direct effects would have been on Europe, not North America. There is no evidence of a supervolcano erupting in North America during this period.
The second theory concerns the thermohaline circulation in the oceans. Warm surface water in the tropics is made saltier (and more dense) by evaporation and is carried by ocean currents to colder latitudes, where it cools, sinks, and then, very slowly, flows back toward warmer regions. As the glaciers in North America, melted they formed Lake Agassiz, northwest of the present Great Lakes, and larger than all of them combined. There is considerable uncertainty, but many researchers believe that it may have drained slowly down the Mississippi until ice sheets retreated enough for it to rush in a massive flood down the St. Lawrence Valley where its fresh water diluted the salt water of the northern Gulf Stream, making it less dense, preventing it from sinking, and thereby shutting down the whole process so that the Gulf Stream ceased to flow and bring warmth to northern regions. There are several serious problems with this theory. For one thing, it is not certain that this would really stop the flow of warm surface waters from the tropics, and, if it did, it would likely take too long to explain the Younger Dryas. In any event, the Gulf Stream primarily warms Northwestern Europe, not North America. Also, some researchers believe that Lake Agassiz actually drained abruptly up the Mackenzie River into the Arctic Ocean, not into the North Atlantic. It is not clear how that would have caused a climate disaster.
The third theory suggests that a massive comet or asteroid struck North America, killing huge numbers of animals with its blast and heat, and killing more by causing widespread forest fires. Those who survived the fires would have starved because much of the vegetation would have been destroyed. Proponents of this theory point to charred carbon from some 50 sites in North America, microscopic diamonds, iridium, and magnetic microspheres. Melted rock spherules and microdiamonds have also been found around Lake Cuitzeo in Mexico and dated to around 12,900 BP. Critics of the impact theory, however, argue that there is not enough charred carbon to indicate truly extensive fires in North America during that period, and have even questioned the microdiamonds and the iridium. Yet, of all the current theories, only the impact theory can at least explain why the mass extinctions seem to have been more extreme in North America than elsewhere; and this may also explain the sudden disappearance of the Clovis culture, whose distinctive stone points are not found in later sediments.
The absence of a crater is really no problem. A tight swarm of objects composed of loosely compacted material, typical of comets and many asteroids, could have caused multiple air bursts over a large area, principally over what is now the United States and Canada. Possibly only small craters were formed, and many fragments would not have penetrated the thick ice sheets. Animal populations would have been decimated, and the descendants of the survivors could have mostly been finished off by the next sudden change…the final melting of the glaciers around 11,600 BP.
Yet even this theory, aside from the fact that its critics question the evidence for the impacts, does not fully explain the selectivity. Why did grizzlies survive, but not the short-faced bears? Why did lions in North America die out, while cougars (and, further south, jaguars) survive? Why did North American horses become extinct when moose, elk, deer, and pronghorn antelopes still live here? In the end, we have to admit that there are some mysteries we may never be able to solve.