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K/T Extinctions. Volcanic-Greenhouse or Impact?
I recently had to write an 2000 word essay on the following topic. I
got very interested in the K/T boundary and have decided to put my
post here where you scientist lot can rip it shreds for me and point
out all my over assuptions.
2nd year geology with oceanography,
Discuss the competing volcanic and Impact theories used to explain
the K/T boundary
About 65 million years ago something big and nasty occurred.
The event marked the end of the Cretacious period and the beginning
of the tertiary. Exactly what happened is still a problem that
confronts scientists. However, the results have become fairly easily
discernable.Due to the incident 47% of the genera on the planet
packed their bags and quitely became extinct, taking with them 76% of
the species around at the time Including 96% of all marine species.
This mass extinction also removed 90% of the marine calcareous
nannoplankton which probably had an influence on the outcome of the
event, and will be discussed in the conclusion of this essay. Some
evidence shows the extinctions was stepped, and other evidence shows
that many species were in decline in the few million years before the
event. There are two popular theories of what happened, these are
1.) Impact theory
2.) Volcano-Greenhouse theory
Simply stated, the theories is that a meteor hit the Earth
and cause the deaths of many species, quite notable the famous
dinosaurs are included on the casualties list. However, from this
simple theory arise many problems. The first is that there is a great
debate over weather,in fact, the impactor was a meteor, a large comet
or a small asteroid. A point that many have used to try and help
settle the argument is that at the K/T boundary there is an iridium
anomaly. But sometimes one scientists belief is unshakable, despite
the evidence shown to him. Some believe that you are only likely to
get such an iridium peak from a metallic asteroid, (Holtz, T. R.,
1995) but most people insist the impactor was a nickel-iron meteor
with other heave metals and including iridium. Others say that the
impactor was probably a comet on the statistical grounds that there
was plenty around. Good examples of the iridium anolalies appear in
Italy, (That area happens to anso have been volcanically active, and
the volcano must be considered as a source for the iridium) and in
Antarctica on the Blue Ice fields. In addition, evidence for an
impact at this time occurs as vitreous tektites that have been found
world wide in the K/T boundary layer, as well as shocked quartz,
feldspar grains and soot from global woldfire, a proposed consequence
of the impact as molten material thrown out by the impact fell back
So where did the impactor impact? Good question, and many
scientists went out looking for evidence of the actual impact site at
some point along the line. The crater for this impact was found in at
Chicxulub in Maexico. It lay 300 to 1100 metres beneath Tertiary
carbonate rocks in the Northern Yacatan platform. It was initially
found by oil prospectors drilling and finding unexpected breccias.
Geophysical methods were used over this area, specifically gravity
anomaly data. The data revealed two concentric rings with a central
high. The diameter of these rings was about 180km. In 1993 new data
revealed two faint extra rings, further from the center. This gives
the crater a new total diameter of about 300km. This is believed to
have been caused by an impactor 6km across. By way of comparison, a
meteor the size of half a football pitch collided flat plain of the
southern Colorado Plateau near what is now Winslow, Arizona. Winds
travelling 2000km per hour rushed from the impact, killing everything
in the first 4km and seriously crippling anything upto 24km away. The
winds would have been hurricane force 40km away and vegetation would
have been destroyed up to 1500km away. If you imagine scaling that up
to an impactor 6km across, you begin to see how easy it is to believe
in this theory. Too improve on the kill ratio of this theory, some
scientists state that these was more than one impactor, maybe due to
a ?cloud? of meteorites.
The impact would have destroyed all living things in North
and South America for a large radius, and detroyed a large amount of
marine life due to the shock of the impact alone, this idea if
colaberated by a thick fossil fish bed a smale distance ouside the
expected marine part of the impact, preserved by the large amount of
dead orgamic matter falling after them, and also the dust and larger
particulated raining down through the ocean. The impact would have
thrown vast quantities of CO2 into the atmosphere, and the impactor
would itself volatolize significantly, adding consider quantities of
other gases, like SO2, H2S, CO2, etc. Particulates might include
things like heavy metal oxides of iron, nickel, copper and Zinc, but
also significant quantities of As, Se, Hg, not to mention the iridium.
The vast amount of gases and dust thrown into the atmosphere
would have acted to block out the sun and increase the albedo levels.
Global cooling is supposed to have occured for a proposed period of
between three months to several years, (It is likely to be at the
longer end of the scale.) This global cooling and global blackout
triggered extinctions among the plant kingdom, and then among
herbivores that depended upon plants for food, and so on. then caused
the death of many species including our friends from the films, the
dinosaurs. Another problem, the K/T boundary layer shows no evidence
of global blackout and refrigeration, (McLean, D. M. 1996) The global
wildfires and CO2 are believed to be responsible for the death of 96%
of marine animals, and for a long period the sea was almost empty in
a state known as the ?Stangelove ocean.? The CO would produce a major
increase in the solubility of calcite and quite likely of silica too.
Organisms like nonnoplankton require calcite or silica to grow their
shells, and if they are having to work much harder to get these
shells, there could be a massive die-off.
Yet another problem.
Why is it that this ?global cooling? killed off a small, cold
adapted dinosaur called Hypilophodants, while the 85% of the climate
sensitive turtle species survived? So that?s no evidence for cooling,
and the wrong type of creatures dying.
Dewey M. McLean coupled the Deccan Traps volcanism to the K-T
extinctions in 1979 originating the volcano-greenhouse extinction
theory. The idea behind this theory is that CO2 degassing from the
mantle associated with the Deccan Traps added vast amounts of extra
carbon into the carbon cycle. The cycle would not have been able to
remove this extra CO2 and it would remain in the atmosphere. CO2 is a
green-house gas. The Deccan Traps main eruptions occured 65 million
years ago and at the around the same time other volcanoes and sources
had become active, such as the Cameroon volcanic line, (66 million
years ago.) The Coral Sea-North Tasman spreading rift (Began opening
by sea floor spreading 65 million years ago) and the initial
Greenland Basalt floods erupted 64-62 million years ago, as well as
the pre-existing hotspots like the Hawaiian and a dozen others, thus
the added CO2 from the Deccan traps coincided with the build up of
other volcanic carbon dioxide in the atmosphere from different
scources, triggering greenhouse warming.
The extra CO2 in the atmosphere would also directly result in
the death of some species. For example, humans live in an environment
with an amtorperic CO2 content of 0.01%, if the CO2 percentage rose
to 0.03% parts of the human brain would cease to function properly
death would soon follow.
McLean also isolated another physical mechanism that would
cause reproductive problems for reptiles, birds and some mammels,
(90% of marsupials,) and including the dinosaurs. Global warming
raises the temperture worldwide, (Though it is important to not that
global warming can actually lead to some areas becomeing cooler due
to shifting wind patterns.) Eggs and vunerable embryos (such as those
of the marsupials) could easily become over heated and die or have
the blood flow to the uterine tract diminished, damaging and killing
embryos. In addition, looking again at Human males, we see that the
male testicles remain outside the body to keep their temperature as
low as possible to aviod infertility. For some species, the
dramatically increased temperatures may result in a reduced
fertility, and this may cause a problem in maintaining the species
population. This will destroy the population dynamics, evolution
rates and could lead to extinctions. This will alter the food chains
and creatures further up the chain (assuming that they too are not
effected by the warming) will begin to place demand on a new source,
which could lead to over hunting and another population failure.
Eventually, any top carnivores are going to find themselves in direct
competition for the last few victims and they too would have soon
died out, this time of starvation.
There is also the idea that for many creaures, the increased
heat would make it too hord to try and hunt for prey.
The iridium peaks found around the world in the K/T boundary
can also be accounted for be the scientists that follow this course
of belief , as Iridium is present in present volcanoes (Hawaii and
Reunion) and in volcanic ash found in the Blue Ice fields in the
Antarctic. In the Italian section of the K/T boundary there is a
sharp iridium anomaly that peaks on an elevated hump, suggesting a
protracted supply of iridium supply. It is suggested there might be a
mixed extra-terrestrial and volcanic process that explained this.
(Sutherland, F. L., 1994)
The K/T clays are basaltic in origin, as would be most ash
from volcanos, though even an impactor could create this if it hit
the right type of rock. The impactor, (if it existed) however is
believed to hit a carbonate bed to add all that CO2 into the
atmosphere, it is an intrinsic part of that theory.
Again, the ?strangelove? ocean is proposed after a die off
caused by the increase solubility of calcite and silica due to the
increase in CO2, and organisms like nannoplanktons having to work
much harder to precipitate their shells. A similar effect this extra
CO2 may have had would be to cause a decrease in the thickness of egg
shells, (an shown in evidence of dinosaur shells, Sutherland, F. L.,
Tests show that shoch quartz can be formed experimentally
from sea floor basaltic and gabbroic rocks in addition to
quartz-feldspar target rocks.
A possible interaction between the two theories?
It is proposed (Sutherland, F. L., 1994) that an impact
occured a few million years before a large amount of volcnic
activity. A major part of the evidence for this theory is that there
is such strong evidence for both occuring. Some of the reasons for
the order chosen is the melt ejecta layer with silica signitures
(Most likely from a meteor) can be found beneath a widerspread
?fireball layer? incorporating soot with basaltic dust and most of
the Ir anomaly. The paper by Sutherland indicates a possible double
impact that triggered volcanism in addition to high hotspot vocanism
levels, especially in the southern hemisphere. The second impact is
believed to have been the impact that caused the Manson crater.
This effects of this theory would have a lot of CO2 thown
into the atmosphere, and for 1-3 million years hotspot activity and
the volcanic activity mentioned above would have kept the levels high
and the carbon cycle out of equilibrium. The effects of the CO2 would
be the same as the theory above.
The proposed volcanic-greenhouse theory proposed by McLean,
1979 does seem very feasible, and much of the evidence can be
accounted for using this theory. The theory also can take into acount
much of the ?selective? extinctions caused. Heat could well have
caused embryonic damage and species decay.
However, there is a great amount of evidence that says at
least one impact occured, specifically large impact did occur at the
chicxulab site about 65 million year ago. The tektites are best
explained by the impact theory.
It is important to note that many of the species that became
extinct at the K/T boundary had been growing smaller in numbers up to
the point of the mass extinction. Dinosaurs (though the fossil record
for them is not large enough to say for definate) seemed to have
stabile population for two million years before the impact, then
after the impact they were gone. The extinctions are, in fact,
staggered before the K/T boundary in several steps, not all occuring
at once at the boundary itself.
Either way, it is important to realise that neither theory
(or combination of both) directly caused the extinctions, but were
the cause of the killing mechanism. If the ecosystem was stressed
(Many species were in decline imeadiately before the K-T boundary)
when the disaster occured, it would add insult to injury and maybe
pushed it all to far, causing species so become extinct and the food
webs to collapse.
The ?Stangelove? ocean talked about above may have had an
important effect in maintaining the CO2 levels in the atmosphere.
Because of the 90% extinction of the calcareous nannoplankton caused
a severe reduction in the dimethyl sulphide (DMS) released into the
atmosphere by phytoplankton. DMS acts as a precurser for most cloud
condensation nuclei, thus marine cloud albedo cover fell by 80% and
resulting in higher sea temperatures. These high temperatures could
have been a factor in the mainenance of low productivity. The
decreased albedo and increased sea temperatures would have maintained
high temperatures. Mammels in higher lattidudes probably survived by
hunting at night and resting during the day, birds were able to
migrate to cooler climatic regions.
The Dinosaurs, after 180 million years of survival and
evolution had probably evolved themselves into their own neiches, and
then change came they were unable to cope.
The two theories are both fairly workable. The problem is,
each scientist may find ways of interpreting data to his or her own
uses or theories, this is likely to be true even for this study.
However, it is the personal opinion of this author, that the combined
theory is most plausible, though of the idea that volcanism was/could
be produced by impact he is unsure. It could be possible that it was
simply a coincidence of timing. The theory about the strangelove
ocean had been included in this essay as it does seem to help explain
certain parts of the selective extinctions.
Most strongly of all, this author is sure that global cooling
did not occur, this is due to the evidence of vast amounts of CO2 and
SO2, (along with others,) both greenhouse gases and the drop in
marine cloud cover would oppose any effects of dust in the atmosphere.
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