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

How Did It Happen?
A Guide to the Downfall of the
Dinosaurs
Susan Bratek
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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.
Introduction
There are many different theories as to what caused the CretaceousTertiary 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.
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/C
hicx_title.html
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.
Fig. 2. This is a map of the location of the Chicxulub crater in Mexico (Keller
et al., 2003).
Fig. 3. Chicxulub impact crater trough and sinkholes (Sullivant, 2003).
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).
Fig. 5. Shocked quartz from Haiti (a) and the Chicxulub crater (b) (Kring,
2007).
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.
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.
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.
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.
Fig. 6. Area of Deccan flood basalt and supposed impact site in India (Negi
et al., 1993).
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).
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.
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.
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).
Fig. 8. K-T boundary survival and extinction of species (Milner, 1998).
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.
References
Alvarez, Walter, 2003. Comparing the Evidence Relevant to Impact and
Flood Basalt at Times of Major Mass Extinctions, Astrobiology 3, pp. 153161.
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.
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. 189197.
Sullivant, Rosemary 2003. A ‘Smoking Gun’ for Dinosaur Extinction, Jet
Propulsion Laboratory, http://www-b.jpl.nasa.gov/news/featuresprint.cfm?feature=8.
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.