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.

A frozen planet - II

Planet vitals:

- Average temperature: -20 to -50°C

- Pack ice thickness: 500-1500 m
(Kirschvink, et al., 2000)

Source: BBC

How did the Earth fall into such extreme conditions?

First of all, the young Sun was weaker than in the present day (Tziperman et al., 2011; Young, 2012). Perhaps the only reason the Earth was not under permanent icehouse conditions in the past was the high concentration of greenhouse gases (mainly CO2 and CH4) in the atmosphere. This means that with a reduction in greenhouse gases, the Earth would cool down (Young, 2012 and Tang and Chen, 2013).

CH4 levels fell with the rise of oxygen during the Great Oxidation Event, eliminating one of the greenhouse gases. CO2 reduction was also related with this event, since microorganisms would consume it during photosynthesis (Tang and Chen, 2013). However, weathering is also an important mechanism to drawdown CO2. This is where the supercontinents come in. Young (2012) notes that the Snowball events at the beginning and end of the Proterozoic coincided with the presence of supercontinents. These are very thick and rise higher with respect to sea level than smaller continents, exposing more surface subject to subaerial weathering, so more CO2 can be "buried", effectively reducing atmospheric CO2. Fig. A is a simplified diagram that shows the conditions that made the Snowball Earth events (marked with red arrows) possible. For the first event there was low solar luminosity and a drop in CO2, caused by increased weathering due to the Kenorland supercontinent. Once it broke up, the CO2 drawdown was reduced, but maintained a downward trend (the atmospheric O2 was still rising). Solar luminosity also progressively increased, preventing icehouse conditions despite the lowering CO2. However, once Rodinia was formed, the CO2 weathering increased once again, breaking the balance and plunging the Earth into another glaciation.

Fig. A: Relation between glaciations and supercontinental cycle. Young, 2012

Once the ice began to expand across the planet, there was no stopping it; a runaway ice-albedo effect would be triggered when sea ice crossed between 30° to 40° latitude, causing the whole planet to be covered in ice (Tziperman et al., 2011).

How did it end?

It seems like there is no escape. With the runaway ice-albedo effect, the cooling effect is reinforced endlessly. However, as shown in fig. B, weathering is prevented by the ice cover, allowing CO2 levels to build up again, warming the planet and allowing the ice to melt (Young, 2012). Volcanoes would be the source of CO2 (Hoffman et al., 2002) and Kirschvink et al. (2000) estimated it would take roughly 35 million years to end a Snowball event.

Fig. B: Feedback loop causing alternation between icehouse and greenhouse conditions. Young, 2012.