Jan 12 2015

Another Salvo in the Mammoth Extinction Controversy

There are many enduring controversies within science, and they are fun to follow. There are raging debates about the so-called Hobbit or Homo floresiensis, is it a new species or a diseased human? You may be surprised to hear that there is still a controversy over whether or not the dinosaur extinction was due to a meteor impact or other terrestrial factors (although I think this one is heavily tilting toward the impact theory).

One controversy I have been following, here and on the SGU, concerns the Young Dryas and whether or not the cooling characteristic of that period was due to melting glaciers or a local comet impact.

Such controversies always raise a few general issues for me. The first is how the mainstream media covers them, which I always find disappointing. Properly covering genuine scientific controversies is challenging, but that is what science journalists are supposed to do. What I find is that they tend to present each new study in the debate as if it is definitive and has ended the debate, rather than putting it into the proper context of the ongoing controversy.

Another common mistake is to rely on one expert rather than getting a reasonable sample. They tend to weight the story toward the side of the expert on which they relied, and maybe provide only token coverage of other views. There is also, of course, the issue of proper balance. Reporting should reflect the balance of opinion in the scientific community. It’s OK to present minority opinions but they should be presented as such.

Another issue I find interesting is the relationship between different types of explanatory systems. In this case we are seeing a specific example of the greater debate between gradualism and catastrophism – are current geological features explained by gradual processes over millions of years or by sudden dramatic events? The history of geology started out far to the catastrophe end of the spectrum, influenced by stories of the great flood. But then the pendulum swung far to the gradualism end of the spectrum, which became dogma for many decades.

Now, as if often the case, it is recognized that both views are correct – there are gradual processes that are punctuated by occasional catastrophes.

With all that in mind, let’s take a look at the Younger Dryas. The last glacial maximum ended about 20,000 years ago. That glacial period was followed by interstadial (warm) periods and stadial (cold) periods. The term Dryas refers to the indicator genus (Dryas octopetali) which is a tundra flower that was much more widely distributed during cold periods. Its pollen in core samples is therefore a good indicator of a stadial period.

Following the last glacial period there were three Dryas mini-cold periods, the Oldest Dryas from 18,000-15,000 bp, the Older Dryas from 14,000 to 13,700 bp, and the Younger Dryas from 12,800 to 11,500 bp.

The Younger Dryas also coincides with the extinction of much of the North American megafauna (like the mammoths) and the Clovis culture. The Clovis, named after Clovis, New Mexico, are a paleoindian culture defined by their distinctive stone spear points. They were big game hunters, so it’s not surprising that the Clovis industry went away with the big game (it’s not clear if the people went away, or just their hunting culture, which had to be replaced with a small game culture).

The Younger Dryas was not only cold, it turned North America into a dry, windy, and inhospitable place. The controversy is over what triggered the Younger Dryas.  There are two main theories. The first is that the melting of the North American ice sheet at one point opened a river of fresh water into the north Atlantic ocean. The sudden dumping of massive amounts of fresh water into the Atlantic reduced its salinity and shut down the ocean currents that bring warm tropical waters north, warming the continent. This lasted until the ice sheets melted and the fresh water river stopped.

The more recent rival theory is that a comet impact in present day Canada threw up dust and triggered extensive continental cooling.  In 2013 a major study in support of this theory was published. They found:

Now, in one of the most comprehensive related investigations ever, the group has documented a wide distribution of microspherules widely distributed in a layer over 50 million square kilometers on four continents, including North America, including Arlington Canyon on Santa Rosa Island in the Channel Islands. This layer –– the Younger Dryas Boundary (YDB) layer –– also contains peak abundances of other exotic materials, including nanodiamonds and other unusual forms of carbon such as fullerenes, as well as melt-glass and iridium.

Critics of the impact theory, however, has recently replied with yet another study, this one looking at the Younger Dryas Boundary layer in Syria. The authors write:

Siliceous scoria droplets, measuring from 1 to 10 mm, from one late Pleistocene and four early Holocene archaeological sites in northern Syria are compared to similar droplets previously suggested to be the result of a cosmic impact at the onset of the Younger Dryas global cooling event. The findings demonstrate that the presence of siliceous scoria droplets are independent of age and thus are not specific to the beginning of the Younger Dryas. Occurrences have not been reported from natural deposits, but are instead associated with buildings destroyed by fire and thus appear to be restricted to archaeological sites. We therefore conclude that melting of building earth in ancient settlements can occur during fires reaching modest temperatures. There is no evidence to suggest that siliceous scoria droplets result from very high temperature melting of soil and are the result of a cosmic event.

Therefore, at least in Syria, some of the evidence used to support the impact theory may instead come from ancient house fires. The authors further point out that the droplets were formed of local dirt, not dirt from another continent, that the droplets were most likely formed from modest heat (rather than the intense heat of an impact), and that they were spread out over 3,000 years (not one moment in geological time as would be expected from an impact).

Impact theorists, however, have other lines of evidence on which to stake their claims, and this is certainly not going to end the controversy.


The reasons for the ongoing controversy include that fact that there are many independent lines of evidence, no one of which is definitive, and they can point toward different conclusions. There are also different interpretations of these lines of evidence.

It is also possible that the history scientists are trying to reconstruct is in reality more complex than either theory currently states. Perhaps, for example, there was a meteor impact around the time of the Younger Dryas, but it was small and didn’t trigger the Younger Dryas.

In any case, I love reading about scientists engaging in such narrow debates. They are passionate, often brutal, but in the end focus on the evidence. All sides may have picked a pony, but ultimately they want to discover the truth and will have no choice but to listen to the evidence. Such debates also show how clever scientists can be, as they search for alternative explanations and subtle ways of interrogating the evidence.

Such controversies also reveal the caricature of science and scientists often presented by science deniers and anti-scientists. They try to portray scientists as dogmatic, closed minded, and lacking in imagination. Nothing could be further from the truth. Following these narrow science controversies, ones that do not have huge political or ideological implications, in a way is like observing scientists “in the wild.” We get to see how they think and operate when they are not acutely aware that anyone is really paying attention.

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