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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. 

Matt Staples,
2nd year geology with oceanography, 
Southamton university
mss196@soton.ac.uk



Discuss the competing volcanic and Impact theories used to explain 
the K/T boundary

Introduction

        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

Impact 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 
to Earth.
        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. 

Volcano-Greenhouse Theory

        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., 
1994)
        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.


Conclusion

        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.












References


BURKE, K., CAMARGO-ZANOGUERA, A., HALL, S. A., MARIN, L. E., QUEZADA-MUNETON, 
J. M., SCOTT LEE, D., SHARPTON, V. L., SUAREZ-REYNOSO, G., SPUDIS, P. D., 
URRUTIA-FUCUGAUCHI, J. 1993. Chicxulub multriring imact basin: Size and other 
characteristics derived from gravity Analysis. Science, 261, 1564-1566

BROTHERS, B. 1996. Concerning mass extinction across the K-T Boundary 
http://www.cen.uiuc.edu/~bbrother/kt.html

HOTLZ, T. R. 1994. (No subject) 
http://www.cmnh.org/fun/dinosaur-archive/1994Nov/0312.html 

HOTLZ, T. R. 1995. Re: Extinction at the K/T boundary.
http://www.cmnh.org/fun/dinosaur-archive/1995Nov/0628.html 

KOEBERL, C. 1989. Iridium Enrichment in volcanic dust from the blue ice fields, 
Antartica, and possible relevance to the K/T boundary event.  Earth amd 
planetary science letters, 92, 317-321

KOPPESCHAAR, C. 1997. Environmental damage caused by Meteor Crater impact.
http://www.xs4all.nl/~carlkop/meteorcr.html

MCLEAN, D. M. 1997. Dinosaur Extinction: The Volcano-Greenhouse Theory.
http://www.vt.edu:10021/artsci/geology/mclean/Dinosaur_Volcano_Extinction/

MORELL, V. 1993. How lethal was the K-T impact. Science, 261, 1518-1519

PAMPINO, M. R., VOLK, T. 1988. Mass extinctions, atmospheric sulphur and 
climatic warning at the K/T boundary. Nature, 332, 63-65

SUTHERLAND, F. L. 1994. Volcanism around the K/T boundary time - Its role in an 
impact scenario for the K/T extinction events. Earth Science reviews, 32, 1-26

San Diego Natural History Museum.1997. Who survived this mass extinction?
http://www.sdnhm.org/kids/ponder/ponder1b.html


----------------------
Matt
mss196@soton.ac.uk