May 23 2013

The Younger Dryas

I love raging scientific controversies. I am not talking about vaccines and autism, global warming, evolution, or homeopathy – these are not actual scientific controversies. They are political controversies intruding onto science.

I prefer nerdy scientific debates that have insignificant political implications. I like to see two groups of scientists arguing about the evidence over some narrow scientific question.  That way you get pure science without all the distortion and nonsense of politics and ideology. That is when you see how science really works.

Take for example 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 an 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 is of particular interest because it coincides with not only a cold period but a major drought, and with it 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).

There are lots of interesting questions about the Dryas and the Clovis, but the raging controversy I have been building up to is this – 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.

A more recent rival theory, however, has been making headway. A recent study, in fact (and the trigger for today’s post), adds additional support to the theory that a meteor impact triggered the Younger Dryas.

For completeness I will mention that there is also a theory that a massive eruption of the Laacher See volcano in Germany threw up enough ash which spread over North America to cause the sudden cooling. This is a minority theory, however – the main dispute is over glacial melting vs meteor or comet  impact.

Much of the controversy surrounds the presence of microspherules and other markers of a large impact. Several research teams claimed to find such spherules in the Younger Dryas boundary layer, while two teams did not, disputing the original results. Yet another team therefore independently and blindly examined the relevant samples and did find microspherules. The evidence seemed to be heading in the direction of an impact.

Now a research team led by James Kennett, who is part of the team who proposed the impact theory, recently published a thorough analysis of the Younger Dryas boundary layer:

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.

They had previously reported microspherules and nanodiamonds, but their results and interpretation were disputed. Volcanic eruptions, lightning strikes, and forest fires can also produce many of the same findings. In the new study, however, they address these alternate sources of the boundary layer findings.

Examinations of the YDB spherules revealed that while they are consistent with the type of sediment found on the surface of the earth in their areas at the time of impact, they are geochemically dissimilar from volcanic materials. Tests on their remanent magnetism –– the remaining magnetism after the removal of an electric or magnetic influence –– also demonstrated that the spherules could not have formed naturally during lightning strikes.

The study involved 28 colleagues from 24 institutions, so this is no longer one isolated research team making claims.

Don’t think this ends the controversy, however. Just a few months ago another team published a paper in which they claim to have ruled out the impact theory.

Researchers argue that no appropriately sized impact craters from that time period have been discovered, and no shocked material or any other features of impact have been found in sediments.

They also found that samples presented in support of the impact hypothesis were contaminated with modern material and that no physics model can support the theory.

“The theory has reached zombie status,” said Professor Andrew Scott, from the Department of Earth Sciences at Royal Holloway. “Whenever we are able to show flaws and think it is dead, it reappears with new, equally unsatisfactory, arguments.

Impact theory proponents argue that a large airburst by a comet could have done it without leaving a crater.  It remains to be seen if this newest analysis will answer some of the criticisms of opponents of the impact theory.

Let the controversy rage. These are always the best science stories, because we get to watch from the sidelines as scientists duke it out, using logic and evidence to make their case. It shows how complex and subtle science can be, and how disputes within science are resolved.

Eventually we will get to a consensus. The various sides will figure out what evidence will be convincing, they will gather the evidence and we’ll see what happens. Sometimes the minority view clings on longer than it should, but the majority consensus generally follows the evidence wherever it leads.

Personally I don’t really have a preference. Both the impact and the ice sheet melting theories are interesting in their own way. For now, I’m just enjoying the show.

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