Sunday, June 18, 2006

The Ordovician Extnctions

I have long promised a post on the Ordovician Extinction. I had planned on doing this prior to the family trip to Ukraine. I finally have a bit of time to do the write up, but it is still a lesser write up since, frankly, this is one of the extinctions I am not that interested in. Terrrestrial environments have long held my attention and there's almost zlich to report here on them. This is strictly an extinction that took place out in the sea. Gosh. To makes matters worse, it seems to be completely uncontested in any of the books I have read as to what happened.

Like many of the extinctions there was actually more than one event that took place. In this case, like the Permian Extinction there were two. They had a single related cause. They were both relatively quick. The first event is termed the 'Hirnantian' after the locale of the fossils. The second one I have seen termed the 'Ashgillian''. I had best make a quick description though of the situation that the world's oceans were in prior to the extinctions.

The world was relatively warm. More so than now, or so I gather. Dr Scotese has a nice diagram of temperature of time that helps show this. The oceans themselves where stratified. That is to say that the different layers didn't mix very much. The top was oxygeniated like ours are today. There was a middle layer the was low in oxygen and a bottom that anoxic in a big, big way. You might say that the oceans were the Black Sea write large. The layers of the biosphere were very stratified as well.

What happened in the first extinction? The ocean levels dropped. This trigered the Hirnantian extinction. All that shallow water habitat went bye-bye. The oceans mixed at the same time losing their stratification. All layers became oxygenated. Nowadays, that makes our oceans very productive, but back then, for all the species that had very specific levels of oxygen that they lived at, that was a disaster. If you lived especially in the shallows only, this was equally a disaster, obviously.

The second extinction took off when the oceans suddenly rose and covered back up to where they were before. In the process the oceans had restratified and the anoxic portion ate most of the habitat where life had taken refuge. This specifically hurt those that lived in the Hirnantian habitats.

So, what caused this? It seems that Gondwanaland glaciated. Swallowing the water in ice on land as glaciers lowered the sea levels everywhere. The evidence for this is actually in the oxygen isotope ratios. They indicate a drop of 10 C in the tropical seas! Then when the cold snap passed, the oceans rose fast, in geological timescales, and killed those that had been happy in the cold environment. The world froze. It's the opposite of what happened in the Permian (it cooked then). The next post on Extinctions will be about the Devonian...or as I have seen it called 'One Damned Thing After Another'.

So what's your poison? Global cooking? Global freezing? Oceanic anoxia? Asteroids? Or an intelligent species? Choose, but choose wisely. ;)

No comments: