Extinction by asteroid a rarity: 'Sick Earth' extinctions more likely

In geology as in cancer research, the silver bullet theory always gets the headlines and nearly always turns out to be wrong.

For geologists who study mass extinctions, the silver bullet is a giant asteroid plunging to earth.

But an asteroid is the prime suspect only in the most recent of five mass extinctions, said USC earth scientist David Bottjer. The cataclysm 65 million years ago wiped out the dinosaurs.

"The other four have not been resolvable to a rock falling out of the sky," Bottjer said.

For example, Bottjer and many others have published studies suggesting that the end-Permian extinction 250 million years ago happened in essence because "the earth got sick."

The latest research from Bottjer's group suggests a similar slow dying during the extinction 200 million years ago at the boundary of the Triassic and Jurassic eras.

At the 2008 Joint Annual Meeting of the Geological Society of America, USC doctoral student Sarah Greene drew similarities between ocean conditions at the Triassic-Jurassic boundary and after the end-Permian extinction.

At both those times, bouquet-like structures of aragonite crystals formed on the ocean floor. Such structures are extremely rare in Earth's history, Greene said.

"The fact that these deposits have only been found at these two specific times that are associated with mass extinction suggests at the very least that maybe there's some shared ocean geochemistry … that could be related to the cause of the extinctions," Greene said.

"The Triassic-Jurassic extinction cause is totally up for grabs at the moment," she added.

Also at the meeting, USC doctoral student Rowan Martindale presented results from her studies of coral reefs during the Triassic-Jurassic extinction.

"The coral reefs look actually very similar to modern coral reefs," she said. "At the end-Triassic mass extinction, you lose all your reef systems. And nobody's figured out why that is."

Martindale identified two distinct types of ancient reefs: one dominated by coral and another consisting mainly of mud and debris, possibly held together by bacteria.

A theory for the end-Triassic extinction needs to explain how both types of reefs could have been killed off, Martindale said.

Any knowledge about end-Triassic reef death could be useful in understanding the current reef crisis, widely attributed to climate change.

"We're looking at it as a model to give us any insight that we might have for today's decline for coral reefs," Bottjer said.

Source: University of Southern California

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Oct 07, 2008
There have been what, five or six great extinctions events? We know with certainty that one, the K-T event, was caused by an Impact.

One in five (or six) is not "rare".

Oct 07, 2008
Something can be rare even though it has occured. For example, if there is only, say, a 1 in a million chance that you will find 1000 dollars on the ground, and then you do, that just means you got lucky, not that it isn't rare to find 1000 dollars on the ground.

In this case perhaps the dinosaurs were just unlucky... that doesn't mean asteroid related extinctions can't also be rare.

Please consider what you are saying a bit more before posting so we can keep the comments as relevant as possible, instead of detracting from the articles with comments that question the obvious.

Oct 09, 2008
There are still impact hypothesis problems for the K-T extinction. Timing of event is not quite right.

Oct 26, 2008
The idea that a cataclysmic event killed off larger animals may not be entirely wrong but I think the impact may have come just a little too late to have been responsible.
Also, I am afraid I am at odds with the current view that the atmosphere we know today was largely in place in the pre-Cambrian.

Please allow me to present a contrary theory.

Carbon in the Atmosphere
My interest is in the longitudinal development of the production and then the removal of carbon into and from the atmosphere. Its production would mainly be through volcanic activity releasing CO2 into the atmosphere and its removal would mainly be via photosynthesis. I developed a theory that looks at how the longitudinal development and removal of atmospheric carbon has taken place following a well known pathway in chemistry. Looking at the atmosphere as a result of a continuous chemical reaction, which looks "stable" at any point in time, I felt that over time it must follow a growth curve. To prove this I used CO2 and O2 as the two primary indicators in the %u201CEarth%u201D chemical reaction. In addition, the atmosphere being the main reservoir of those two active gasses it is also where their respective partial pressures, in my theory, are the limiter and promoter respectively of metabolic rate. I used certain events in the geological fossil record as markers to indicate the points of change along the developmental curve. The relationship between the two gasses, and hence the metabolic rate supportable, is expressed by their relative partial pressures at any particular point in time.
To make this all work I researched the origin in the animal fossil record of where qualitatively different metabolic rates occurred. My assumption here was that animal life would evolve into new areas of opportunity sooner rather than later by taking advantage of the ability to move faster, be stronger and utilize new environments previously inaccessible, in a purely opportunistic manner. This would happen only when a constraining factors on the metabolic rate were relaxed.
The minimum necessary and sufficient conditions to allow and maintain a higher metabolic rate would be a low enough CO2 partial pressure AND a high enough O2 partial pressure. This is the reason why the growth curve is an appropriate tool to describe the development of animal life - it describes the results of the %u201CEarth%u201D chemical reaction, it also happens, coincidentally, to chart, at any given point in geological time, where the leading edge of the metabolic rate for animal life on the planet would be.
It is now possible to specify the parameters of the growth curve. The curve must extend the whole length of Earth's history. It must show a lag phase where the ratio between the two gasses would show little or no change. Here the creation of O2 would be more than compensated for by the production of CO2 due to massive volcanic activity. A log phase where there was a rapid change in the two gasses, and if it developed that far, a stationary phase then a decline or death phase. The stationary phase would return to a situation where there would be little change in the relationship between the two gasses, but at a different level to that in the lag phase. The decline or death phase would reflect a reversal in the relationship between the partial pressure of the two gasses to a dangerous level, where the metabolic gains made during the log phase would start to unravel.
The lag phase is the pre-Cambrian, extending way back to 4 billion years ago, with plant life coming onto the scene about 2 billion years ago. Animal life existed during some of this time but it is difficult to say when it started. All the various plant forms utilised the CO2 producing, among other things, O2. Animal life used O2 to make its metabolism of food work. That metabolism creates CO2 and in some animals, to rid themselves of the gas, which otherwise would build up in their tissue, they use Ca cations plus another oxygen atom, along with the CO2 all combining to produce a particle of calcium carbonate. Sufficient particles would provide an irritation within the animal's tissue and be pushed to the margins. The particles fall to the sea floor and form limestone, or, in deeper water disassociate into its component parts.
As the level of atmospheric CO2 falls and O2 increases the amount of CO2 that can be liberated by the animals' tissue increases, causing more calcium carbonate to be produced. However, the animals' ability to remove calcium remaining the same, eventually a build up of calcium in and around the animal is inevitable. These build ups reflected the shape of the animal and became the first calcium based fossils, and the start of the Cambrian. They also mark the end of the lag phase of my growth curve.
At the beginning of my log phase of the development curve, animal metabolism sees advances with the appearance of fish type animals, and at its end, the appearance of birds. The whole of the log phase of the development of metabolic rates happens over a 250 to 300 million year stretch. What supporting evidence is there in the geological record for such a change?
The evidence is in the amount of carbon taken out of the atmosphere over that 250 million year. Over this period there is massive deposition of CO2 in many forms. Its cumulative reduction and the cumulative increase of O2 via photosynthesis, drives the rapid change in metabolism over this period by reducing the partial pressure of CO2 and increasing the partial pressure of O2. Thus, a) allowing the release of greater amounts of CO2 from animal tissue, and, b) providing the O2 necessary to support the faster rates of metabolism. These cumulative changes in the gasses are reflected in the line of the growth curve during the log phase. The log phase line does not represent a smooth progression. In my opinion, reversals in the general trend happen as a result of increased volcanic activity and/or the loss of plant life. Reversals result in the partial pressures, particularly of CO2, changing to the detriment of removal of the gas from animal tissue. In these periods of reversals, those animals that have evolved to be the most advanced in terms of their metabolic rate OR bulk are the most at risk. They can no longer eliminate CO2 from their tissue at the rate demanded by their metabolism.
The result is an increase in acidity due to the trapped tissue CO2, possibly of pharmacological proportions. This lowering of the pH causes the genetic coding found in the cells in this particular tissue to become more unstable than the bodies waste control systems can cope with. The end comes from genetic disorders. This theory suggests that mass extinctions happen in the risk groups described, by the method described. Extinction events mark reversals on the growth curve, but progress later recovers with changes in the partial pressures of the two gasses getting back on course, leading to environments where even higher metabolic rates are possible and further qualitative changes to animal life happen.
The log phase ends when changes in the partial pressures of the two gasses slow and there no further qualitative changes to animal life are recorded. The stationary phase has begun.
In my terms the stationary phase started about 100 million years ago. This phase is marked purely by progress within animal life forms currently existing rather than any qualitatively new animal life forms, in terms of higher metabolic rates, evolving.
Recent human activity has forced an end to the stationary phase and the decline or death phase has begun. While the CO2 levels were decreasing, all previous life forms were able to enjoy an atmosphere which permitted the necessary removal of the waste product from their tissue. However, in the last 500,000 years, humans have had a growing effect on the %u201CEarth%u201D reaction by producing more and more CO2 at the expense of the mechanisms which balance that reaction.
By increasing the partial pressure of CO2 over this period, Man has brought %u201CEarth%u201D to a point where it can no longer continue to support the highest metabolic rates in animal life, or lower metabolic rates where the mass of tissue has evolved without the mechanism to remove CO2 as efficiently as required in the changed circumstances. The result is the lowering of tissue pH. This higher acidic environment allows a higher error rate during DNA replication. The body%u2019s mechanisms for clearing damaged protein have not evolved quickly enough to eliminate this damage. Genetic flaws abound in tissue and will kill more and more, possibly to the extinction of whole groups of animals, leaving only those with very low metabolic rates untouched.
This theory is testable. It can be considered from either the geological or biological bases of its evidence. I am asking that it is tested to destruction, and quickly because of its inescapable conclusion that life as we know it on this planet will change faster than we believe possible due to Global Warming, which is bad enough, but if increased partial pressure of CO2 is endangering higher metabolic rates, then we might already be too late to save ourselves.

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