How Did It Happen? A Guide to the Downfall of the Dinosaurs

1. How Did It Happen?A Guide to the Downfall of the Dinosaurs Susan Bratek http://news.nationalgeographic.com/news/bigphotos/images/060905-dinosaurs_big.jpg

2. Abstract The Cretaceous-Tertiary extinction is a very important event in the geologic time scale. This boundary shows a mass extinction on Earth and what caused it is very uncertain. Evidence on Earth shows that it could have been caused by an impact, volcanism, both, or neither. The Chicxulub crater in Mexico may very well be the site of the impact, while the Deccan volcano traps in India could be the area of volcanism that took place. There may have been multiple impacts, or some other event may have occurred, making the Earth unlivable. Either way it is looked at, there was a massive loss of life on the Earth and no one is really sure how it happened. Scientists can only look at the Earth today and try to figure out what happened 65 million years ago.

3. Introduction There are many different theories as to what caused the Cretaceous-Tertiary Extinction (Fig. 1). Since it happened around 65 million years ago, there is little evidence on the Earth as we see it today to show us just what happened (Chapman, 1989). Some scientists believe an impact from an asteroid or comet caused the extinction. Others claim it was flood basalt volcanism. Still others suppose it was the combination of these two events, while others do not think it was either. Some think it was due to climate and sea level change on the Earth and other scientists believe changes in the Earth’s orbit and magnetic field had to do with the extinction. The timing of these events also comes into play. These are all theories and while there is evidence to support each idea, nothing is certain. Because it happened so long ago, it is very debatable as to what really happened.

4. Fig. 1. Mass extinctions shown on the geologic time scale, with the K-T boundary at around 65 million years ago.http://www.lpl.arizona.edu/SIC/impact_cratering/Chicxulub/Chicx_title.html

5. Evidence Evidence that is available of this extinction is the Chicxulub crater on the Yucatan Peninsula (Fig. 2). It is said that this crater represents the impact of a meteor that caused the extinction of the dinosaurs. Another mechanism studied is the Deccan volcano traps in India, which are said to have caused flood basalt volcanism which killed the species. There is also the Iridium anomaly, which points to an impact. Any way it is looked at, there are many hypotheses and no definite conclusions. Meteor Impact: Mexico The hypothesis that mass extinction at the K-T boundary was caused by an impact can be confirmed by the Chicxulub crater in Mexico (Kring, 2007). See Fig. 3. The presence of Iridium (Fig. 4) in the sediments found at the K-T boundary supports this, as well as the presence of shocked quartz, which is only known to be produced by impact cratering (Kring, 2007). Shocked quartz (Fig. 5) is found mostly in western North America, so the location of the crater makes sense. The crater was suggested to be around 180 km in diameter (Kring, 2007). This size confirms that the impact was large enough to display the specific types of sediments found around the world at the K-T boundary. The structure of the rock found at the site of the crater shows that it had to be formed by the melting of the Earth’s crust due to an impact.

6. Fig. 2. This is a map of the location of the Chicxulub crater in Mexico (Keller et al., 2003).

7. Fig. 3. Chicxulub impact crater trough and sinkholes (Sullivant, 2003).

8. Fig. 4. The rock sequence found near the Chicxulub crater in Mexico. The lower part (a) consists of impact melt spherules. The upper part (b) consists of sandstones, siltstones, shales, and Iridium near the very top (Kring, 2007).

9. Fig. 5. Shocked quartz from Haiti (a) and the Chicxulub crater (b) (Kring, 2007).

10. Effects on Earth This impact would have affected the Earth in several ways. The main cause of change would be due to the debris from the impact combining with the atmosphere. One effect would be acid rain, which could have lasted from a few months to several years. Debris from the impact was distributed worldwide which caused atmospheric heating, raising the temperature several hundred degrees (Kring, 2007). There is evidence of wildfires due to these increased ground temperatures, which would have greatly increased the amount of carbon on Earth. Dust and aerosols from the impact, along with soot from the wildfires, would eventually cause temperatures to cool down. They also stopped sunlight from reaching the ground, inhibiting photosynthesis. This severely affected the food chain, as the base of the marine food chain, plankton, no longer could survive (Kring, 2007). This loss of sunlight may have also led to a period of glaciation (Twitchett, 2006). Ozone was also depleted, and there was increased greenhouse warming after the dust and aerosols settled.

11. Geological Disasters Two geological disasters were abundant right after impact: tsunamis and earthquakes. Tsunamis covered the Gulf of Mexico and traveled to Caribbean and Atlantic basins. The waves were 100 to 300m high and disturbed sea floor sediments to depths of 500m (Kring, 2007). There were also magnitude 10 earthquakes which caused slumping of coastal sediments. Shock waves and air blasts covered the radius of 900 to 1800km (Kring, 2007). All of these events, along with high winds over 1000 km/h and intense heat, destroyed all vegetation and any animal in its path. The effects of this impact transformed the Earth for a period of around 1,000 years.

12. Differing Views Despite this evidence, there are objections. Some scientists argue that this impact happened well before the K-T boundary and that there was a second impact which caused the mass extinction. Twitchett suggests that the Chicxulub impact occurred 300 kyr before the K-T boundary (2006). There is a problem with this hypothesis as the evidence for only one impact at the boundary is so great. Also, if a different impact was responsible, it is unknown where. Due to plate tectonics, oceanic crust is constantly renewed, thus erasing any sign of an impact crater if it hit in the ocean (White and Saunders, 2005). Another objection to the theory of one impact is that the impact that did occur was not great enough to cause extinction and that other factors on the Earth were the cause. Still another objection is that there were multiple impacts that occurred right around the K-T boundary.

13. Volcanism Another cause of extinction at the K-T boundary that has been greatly researched is intense volcanic activity. Evidence of this can be found in India, where the Deccan traps (Fig. 6), which cover around one-sixth of India, erupted around 65 million years ago (Negi et al., 1993). Some say this was due to an impact site around the area. However, flood basalts limit any inspection of the area for Iridium or shocked quartz, which could prove this impact hypothesis. This area of India is characterized by a thin crust, thermal springs, up-warping, and seismically active areas (Negi et al., 1993). Some scientists believe that with the event of an impact, this thin area of up-warped crust would have huge eruptions of lava. It is believed that some of these lava flows were under water as well (Keller et al., 2008). See Fig. 7. The suggested area of impact, near Bombay, is related to high heat flow and temperature gradients, which point toward a magma chamber at a shallow depth (Negi et al., 1993). The effects on Earth would be similar to the first impact hypothesis discussed. The large amounts of basaltic lava and toxic gases would stop photosynthesis very quickly. There would be acid rain along with tsunamis.

14. Fig. 6. Area of Deccan flood basalt and supposed impact site in India (Negi et al., 1993).

15. Fig. 7. Three lava flows of the Deccan volcanism. 1. Upper (a) and middle (b) lava flows. 2. Upper (a), middle (b) and lower (c) lava flows. 3. Sediments are thin, discontinuous, recrystallized and do not have any fossils. 4. Pillow-like structures of (c) show that some lava erupted under water (Keller et al., 2008).

16. One of the fundamental principles of stratigraphy, superposition, can be very helpful in determining the order of the events which have occurred. Since the mass extinction, Chicxulub impact, Deccan volcanism, and climate change all happened over a couple hundred thousand years, the best way to determine the sequence of events is to look at the sediments and fossils in the rock units of the K-T boundary. According to Keller et al., by doing this the end of the Deccan volcanism coincides with the K-T boundary, making it a very possible cause for the mass extinction (2008). It has also been suggested that seismic waves from an impact could trigger volcanism on the opposite side of the Earth of the impact site. While this would make sense, the Deccan flood basalts were around fifty degrees away from the opposite point on the Earth to the Chicxulub crater at the time of the K-T boundary (Alvarez, 2003), making this hypothesis less credible.

17. Coincidence or Not? There are also scientists who look at the fact that an impact and basaltic eruptions are not connected in any way. They do not think that an impact triggered volcanism. They believe that these two events just so happened to occur at the same time, and cause a mass extinction. One possible hypothesis is that while Deccan volcanism was occurring, it would have created high stress conditions on Earth, high toxicity and low levels of oxygen, and then a meteor impact just wiped out the already dying life (White and Saunders, 2005). Another point is that basaltic volcanism can be seen at other times in the geologic time scale where mass extinctions did not occur, thus suggesting that volcanism alone would not be the only cause. More than One Impact? There are strong arguments for multiple impacts that caused the mass extinction at the K-T boundary. There are three impact craters that have been found to be close in age to the K-T boundary. There is a 24 km wide crater in Ukraine around 65.2 Ma, a 12 km wide crater in the North Sea around 65 Ma, and the Chicxulub crater in Mexico which is around 180 km wide with ages from 65.0 to 65.4 Ma (Keller et al., 2003). While the first two are much smaller in size than Chicxulub, the combination of these impacts would have increased the greenhouse effect, increased climate temperatures, and eventually lead to mass extinction.

18. The End of the Dinosaurs However the mass extinction at the K-T boundary happened, it was devastating to about seventy percent of living things on the Earth (Sullivant, 2003). The dinosaurs, flying reptiles, large swimming reptiles, and many other marine animals were all killed (Litwin et al., 2001). See Fig. 8. The fossil record does confirm the dinosaur decline. While any living thing in the proximity of the impact was immediately destroyed, everything else just could not live in the condition the Earth was put in. The food chain was disrupted due to the acid rain, wildfires, and temperature variations. This put a lot of stress on the animals and they just could not survive. Another effect related to this is that reproductive failure due to the impact or volcanism, or both, would also have led to extinction (Milner, 1998).

19. Fig. 8. K-T boundary survival and extinction of species (Milner, 1998).

20. Discussion While there is much evidence for all of these theories on the mass extinction at the K-T boundary, it is hard to find an absolute solution. Due to natural processes on the Earth such as erosion, plate tectonics, and volcanism, much of the evidence is gone. Scientists can only look at what the Earth provides now and draw their own conclusions from it. “The present is a key to the past” has never been truer. Whether it was an impact or multiple impacts, volcanism, or something entirely different, the fact remains that nearly all life on Earth was obliterated. Conditions became completely unlivable for most species due to lack of food, climate change, acid rain, tsunamis, and earthquakes. And this is something that has happened many times in the geologic time scale. Sometimes it is absolutely amazing to think about how much has happened on the Earth and realize that humans are just a tiny part of it.

21. References Alvarez, Walter, 2003. Comparing the Evidence Relevant to Impact and Flood Basalt at Times of Major Mass Extinctions, Astrobiology 3, pp. 153-161. Chapman, Clark R., 1989. Scientists Sort Out Differing Evidence of the Cause of the Dinosaur Extinction, Earth in Space 2 pp. 7-9. Keller, G., W. Stinnesbeck, T. Adatte and D. Stuben, 2003. Multiple impact across the Cretaceous-Tertiary boundary, Earth-Science Reviews 62 pp. 327-363. Keller, G., T. Adatte, S. Gardin, A. Bartolini and S. Bajpai, 2008. Main Deccan volcanism phase ends near the K-T boundary: Evidence from the Krihna-Godavari Basin, EIndia, Earth and Planetary Science Letters 268 pp. 293-311.

22. Kring, David A., 2007. The Chicxulub impact event and its environmental consequences at the Cretaceous-Tertiary boundary, Palaeogeography, Palaeoclimatology, Palaeoecology 255 pp. 4-21. Litwin, Ronald J., Robert E. Weems, and Thomas R. Holtz, Jr., 2001. Dinosaurs: Facts and Fiction, http://pubs.usgs.gov/gip/dinosaurs/. Milner, A.C., 1998. Timing and causes of vertebrate extinction across the Cretaceous-Tertiary boundary, Meteorites: flux with time and impact effects 140 pp. 247-257. Negi, J.G., P.K. Agrawal, O.P. Pandey and A.P. Singh, 1993. A possible K-T boundary bolide impact site offshore near Bombay and triggering of rapid Deccan volcanism, Physics of the Earth and Planetary Interiors 76 pp. 189-197. Sullivant, Rosemary 2003. A ‘Smoking Gun’ for Dinosaur Extinction, Jet Propulsion Laboratory, http://www-b.jpl.nasa.gov/news/features-print.cfm?feature=8.

23. Twitchett, Richard J., 2006. The palaeoclimatology, palaeoecology and palaeoenvironmental analysis of mass extinction events, Palaeogeography, Palaeoclimatology, Palaeoecology 232 pp. 190-213. White, Rosalind V. and Andrew D. Saunders, 2005. Volcanism, impact and mass extinctions: incredible or credible coincidences?, Lithos 79 (2005) pp. 299-316.