The end of the dinosaurs

Last week we saw the largest of the mass extinctions. This week we'll cover the most famous one: the Cretaceous-Paleogene extinction event! This case is different from the other ones we've seen*: the trigger came from outer space  The killer asteroid left behind the Chixculub crater in the peninsula of Yucatan, that has 100 km in diameter  (Schulte et al., 2010). It was identified and dated in 1992, confirming its age of ~65 million years, coinciding with the extinction (Kring, 2007).

Some drama before you read on...

This extinction event killed not only the dinosaurs, but 70-76% of all species (Jablonski, 1994)! But how exactly did the asteroid accomplish it?

Well, once the asteroid hit the Earth, the initial consequences were incredibly strong earthquakes (models suggest magnitudes over 11!) and tsunamis. (Schulte et al., 2010). However, since this blog is oriented towards climate changes and environmental effects, let's center on the massive amounts of material ejected around the planet.

First, the impact site had anhydrite (CaSO₄), so the ejected material  contained sulfates, that once in the stratosphere would reflect the sunlight, causing cooling of the Earth's surface and limiting photosynthesis. The sulfates, dust and soot that reached the stratosphere may have remained there for a year (Kring, 2007). Schulte et al., (2010) however says that the cooling caused by the sulfate aerosols may have lasted for decades, lowering the temperature by 10°C.

Another effect was acid rain, caused by "shock-heating" of the atmosphere during the impact and mainly by the raining down of the ejecta. This heating produced NOx, which in addition to the sulfates in the debris, contributed to acid rain falling up to a few years after the impact (Kring, 2007).

The impact also caused wildfires. Though the extent is still not known, there is evidence from the soot recovered that ~10⁴ GT of CO₂ and ~10² GT CH₄ were released from these wildfires. The impact itself added CO₂, CH₄ and H₂O to the atmosphere. These greenhouse gases can remain more time in the atmosphere than the sulfates and dust, so a warmer period may have followed after the initial cooling.  (Kring, 2007).

The impact as cause of the extinction is the most widely accepted, but it is important to note that at the time of this event there was something else going on. Just like at the end Permian, there was massive volcanic activity that went on for about 1 million years. The Deccan flood basalt eruptions were located in present day India. However, as Schulte et al., (2010) mentions, the impact event and the volcanic event were magnitudes apart - the former injected up to 500 Gt of sulfur to the atmosphere almost instantaneously, while the latter contributed with up to 0.5 Gt of sulfur per year.

*Some studies suggest that this also caused other extinctions, like the end Permian event, with Becker (2004) presenting the Bedout crater as possible evidence, but it is still disputed.

End Permian Mass Extinction

Finally, the big one. Around 90% of all species went extinct: 70% of land vertebrates and 80-96% marine animals (Chen & Benton, 2012). After millions of years of living beings surviving the challenges of the changing Earth, what happened that almost wiped them out?

Fig. 1: A marine ecosystem before and after the mass extinction. Chen & Benton, 2012. © John Sibbick

The end Permian extinction event was 252 million years ago and it happened "quickly", in less than 100,000 years (Shen & Bowring, 2014). The exact causes are still debated, though there is one that stands out. This time the culprit seems to be volcanism (Payne & Clapham, 2012) - though not a lone volcano erupting, but massive eruptions, flooding extensive areas with over 5 million km³ of basalt. The eruption of the Siberian traps, located in central Russia, went on over a period of 1 to 2 million years (Shen & Bowring, 2014). The volcanic activity released huge amounts of CO₂ to the atmosphere; this could have even been increased due to interactions with organic deposits. Svensen et al. (as cited by Payne & Clapham, 2012) calculated that over 30,000 GT of carbon was released to the atmosphere! As you probably expect, there is evidence for a rise in global temperatures of around 8°C (Shen & Bowring, 2014). 

However, not only did the CO₂ influence the temperature; it likely caused ocean acidification and carbonate saturation. This would affect the marine animals, specially those with shells, leaving them without refugia (Payne & Clapham, 2012). 

Another mechanism involved in the extinction was marine hypoxia/anoxia, perhaps caused by the excessive chemical weathering of the Siberian traps, releasing phosphorous to the oceans (Payne & Clapham, 2012). The excess nutrients would have promoted eutrophication and hypoxia.

The emissions from the eruptions would have included not only CO_2, but sulfur, HCl and CH₃Cl, among others. Black et al. (2014) used a model to assess the effect of these emissions on the ozone layer and acid rain with different eruption scenarios. They found that the ozone layer could have been depleted up to 30% due to HCL and up to 67% due to CH₃Cl when these emissions reached the atmosphere. The effects would have been felt worldwide - increased UV B radiation with mutagenic effects. For the acid rain they considered a CO₂ concentration 10 times higher than present. This effect alone would have meant acid rain with a pH of around 4, and it would have been present worldwide. Combined with sulfate, for a 1 year eruption of 2400 km³, the pH may have dropped as low as 2, with the effects mainly in the Northern hemisphere (the location of the traps).

It took about 8-9 million years for life to make a full recovery (Chen & Benton, 2012), after almost being wiped out in 100,00 years. But this mass extinction paved the way for a new revolution of life, and the soon to begin age of the reptiles.

RIP
520 - 252 million years ago
Source: Wikipedia.com

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.