The Ordovician Mass Extinction

Finally it is time to leave behind the Precambrian and start with our current eon, the Phanerozoic! Bear in mind that it started 541 million years ago so we still have a long way to go before we make it to today. Besides, we haven't even talked about dinosaurs (or at least when they died out)!  

Here, have a dino pic anyway. Source: Smithsonian.com

Speaking of dying out... we are now entering a time of mass extinctions. So far there have been 5 mass extinctions (the Big Five!) and as you may have seen in the news, there is talk of whether we are going through the sixth mass extinction or not.  

Wait a minute, this blog deals with climate, not extinctions! True, but life and climate are closely connected. So if there is a great extinction, it is likely there was a change in the environment at the time, though the exact causes may vary.

Trilobite. Source: Wikipedia.com

The first of the Big Five mass extinctions happened at the end of the Ordovician (hence its name, "Late Ordovician Mass Extinction"), about 443 million years ago. By this time life had evolved to more complex forms and taken over the oceans. There were echinoderms, sponges, the all famous trilobites, among many others (Harper, 2006). It was a generally long warm period with sea levels that were the highest of the Paleozoic (Munnecke et al., 2010). The continents were distributed from the south pole to low latitudes and most of the northern hemisphere was covered by an ocean. Gondwana was the largest of these continents.

What happened?

There was another glaciation. This time, due to the configuration of the continents, the glaciation was limited to the southern hemisphere (Sheehan, 2001) and not as extreme as the Snowball Earth events, but enough to lower the sea level and reduce the area of available habitats for marine life. In addition it directly affected the taxa that were not adapted to cold temperatures. As a result, 61% of the marine genera went extinct (Finnegan et al., 2012).

Box 1. Source: Munnecke et al., 2010 

Fig. 1 shows reconstructions of atmospheric CO2 and O2 during the Paleozoic, with the end of the Ordovician marked in red. There is no certainty of these, with several reconstructions being shown in the graphs, but it seems to be clear that the O2 were not yet as high as present day (<21%) and CO2 was possibly up to 10 times present day values (for the latest value, check out the side bar!), around 4000 ppm (Munnecke et al., 2010).

Fig. 1: Atmospheric CO2 and O2 during the Paleozoic.
Munnecke et al., 2010  

The mechanism proposed to explain the decrease in CO2 levels during the late Ordovician was an increased silicate weathering caused by a mountain building period (Sheehan, 2001; Munnecke et al., 2010). This weathering consumed atmospheric CO2 helping reduce it until it reached a critical threshold that allowed the growth of ice sheets and lowering of the sea level. This threshold is debated, but generally accepted as around 8 times present day levels, with Munnecke et al. (2010) giving a value of around 3000 ppm. It is not comparable to current conditions (see Box 1) and is MUCH MUCH higher than what would be required today to melt the ice caps.

Once again the CO2 levels decreased, allowing the temperatures to drop and ice to grow. Then back to the usual suspect: as the ice expanded it provided a positive ice albedo feedback, reinforcing the cooling and further growth of ice. Lower sea levels also exposed more land subject to weathering that contributed to continue reducing atmospheric CO2. However once the ice covered the land, the weathering that had been the mechanism to drawdown CO2 was reduced, leading to a gradual build up of CO2 in the atmosphere once again.