Wednesday, November 21, 2007

Of Impacts, Eruptions, and Extinctions

There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact. ~Mark Twain

Sometimes I think scientists just don't like admitting they don't really know. That would explain the tendencies toward contrarianism on the one hand and bandwagons on the other. Take the Cretaceous Era extinction event that wiped out our buddies, the dinosaurs.

Mass extinctions have taken place irregularly throughout our planet's history. Because that sort of thing could happen to us, scientists have more than just casual curiosity in what causes these catastrophic events. The Cretaceous event has always been near the top of the list for their attention, partly because it's the most recent big event (although there have been some subtle extinction events since) and because it took out the dinosaurs, who, beyond being an object of fascination for all of us, were wildly successful for about 165 million years.

All sorts of theories were tossed around for years, including disease and climate change, when in 1980 Luis Alvarez, a physicist, and his son Walter, a geologist, came up with the wild idea that a meteor impact was the culprit. Frankly, their theory was not met with universal acclaim, mostly, I suspect, because they were not paleontologists (although Walter Alvarez's expertise in geology should have counted for something).

Flash forward to the mid-1990's and the discovery of the Chixulub crater. Suddenly, we were looking at a massive crater that was formed at about the right time. In other words, we had the proverbial smoking gun. Alvarez and son were transformed from fringe-theory pushers to mainstream geniuses.

It's not that everyone bought in to the impact theory; Bob Bakker, for instance, wasn't having any of it, sticking to the disease theory. For the most part, though, a meteor as the cause of the dinosaur die-off became candidate number one.

Then along comes Gerta Keller. By the time Ms. Keller got into the act, paleontologists were beginning to consider that more than one factor was involved in killing off the saurians. The Deccan traps, a monstrous outflow of lava, had been going on for an incredible period of time. The effect of volcanic eruptions on the climate are fairly well-known, so it was beginning to be felt that the dinosaurs were in serious decline by the time that the meteor came calling. Chixulub simply finished them off.

For some reason, this wasn't good enough for Ms. Keller. She decided that Chixulub was small potatoes and that there must have been another meteor or two. The fact that her evidence was shaky and that no one could find the other impact craters didn't dissuade her much.

Now she's decided that meteor impacts, especially Chixulub, had nothing to do with the extinction at all. It was all the Deccan lava flows. Again, she's playing a little fast and loose with the data, picking and choosing which lava flows are involved. She seems to be in that class of scientist who, having taken a contrarian stand, must defend it with any piece of loosely-related data that comes along.

At the other end of the spectrum are the growing number of scientists who want to use impacts to account for everything that's ever happened. Remember the lesser extinction events I mentioned? One of those took place around 13,000 years ago, when the mammoths disappeared and humanity itself was hanging on for dear life. Generally, disease and/or climate change have been considered prime candidates in this event. Recently though, a new theory was tossed out that involved -- you guessed it -- an extraterrestrial object smashing into the planet.

Of late, even the great Permian extinction event was being bandied about as being related to an impact or impacts by meteors or comets, at least by the popular science media. So, in the spirit of Gerta Kellar, we have doctoral student Catherine Powers who has published a paper blaming the whole thing on -- wait for it-- the lava flows of the Siberian traps.

So, volcanism is the hot candidate (if you'll pardon the pun) in the extinction racket these days. In Ms. Powers case, though, I'd have to question just how original her research is. It seems that Dr.
Gerald K. Czamanske of the U.S. Geological Survey has also suggested the Siberian traps as a prime cause of the Permian kill-off. Except he did so in 1992.

I suppose Ms. Powers had some new data to expound, but it would have been nice if the article about her had made some mention of Dr. Czamanske's earlier work.

It would appear, then, that impacts are going out of style in favor of volcanic catastrophes when it comes to mass die-offs. In the case of the Permian, it never seemed like a likely cause since there was no tell-tale iridium layer like the one found at the end of the Cretaceous, although that never stopped anyone from suggesting the theory.

At some point, it will occur to scientists that they may have to admit that mass extinction is more complicated than any single-cause explanation will allow.