Ramblings about what I encounter within the realm of the geosciences, as well as the occasional rant about nonsense.

29 February 2008

Light plates subduct faster than dense plates...

This is a neat idea I came across, it explains (at least it clears up to me) why certain boundaries move considerably faster than others. Think Nazca vs. Pacific. It sounds feasible, but I would like to read the Nature paper for more information.

Click here for the story

If you want to read the actual article here is the citation (and I just found the link [subscription required]):
Saskia Goes, Fabio A. Capitanio and Gabriele Morra. "Evidence of lower mantle slab penetration phases in plate motions." Nature, 21 February 2008.

27 February 2008

What's the mineral that won't give out when there is heat all about?

Recently, in journal club, we came across a "slush-ball" earth paper. A couple of us got hung up on how and increase in atmospheric CO2 could result in rapid precipitation of CO3. Those of you familiar with Mel have probably already read about what I think may be the mechanism, so skip ahead if you heard the "can of coke" analogy already.

Essentially, what (as I understand it) may have happened was during a snowball (or possibly even a slushball) earth, the atmosphere was not able to freely (at least not efficiently in places) mix with the oceanic water. One of the tenets the paper discussed was the importance of underwater volcanics to providing a source of energy for chemosynthesis. These would also have supplied an amount of CO2. Without anywhere in the overall system for the CO2 to go (assuming for the moment that there was restricted mixing of the atmosphere and ocean), the CO2 would be dissolved into solution. This would force the pH to drop. This moves the system out of that heady goal of equilibrium (dynamic ain't it). Enhancing the dissolution of any CO3 (carbonates) present [thank you Le Chatlier's principle] would move the system back towards equilibrium. This could (?) result is super-saturating the oceans with CO2.

However, once it becomes possible for the atmosphere to mix with the oceans, the oceans would begin degassing. Like opening a can of coke. This would result in the atmosphere increasing its CO2 concentration, while simultaneously dropping the relative concentration of CO2 in the oceans. Now, all of that CO3 that has been dissolving as a result of Le Chatlier's principle are once again out of equilibrium. So, the only way to shift back to equilibrium will be resume precipitation of CO3. This would also result in a positive feedback loop, since the increase in atmospheric CO2 will enhance warming, which will in turn melt more of the ice, which will in turn allow more efficient mixing, which will in turn increase atmospheric CO2....

Now, those of you that skipped ahead, you can pay attention again
As I recall, the paper we read placed the snowball/slushball time as late proterozoic (right before the Cambrian explosion, in fact the snowball/slushball may have had a causal relationship). As I also recall (if I find where I heard this I will post it), this is also seedy period of earth's past where the preferred CO3 that was deposited wasn't Calcite or Aragonite (as it is in most locations throughout the Phanerozoic as I recall). The main CO3 was Dolomite CaMg(CO3)2. Dolomite takes 2 CO3 molecules into its crystalline structure. In my physical chemistry class, we learned that the size of a molecule is less important to dissolution problems than the number of molecules you have (for example, you can dissolve twice as much sugar in a volume of water as you can salt, because salt dissociates into 2 ions, while sugar remains a single molecule). So, even though dolomite is a more complex molecule, the fact that it can take twice as much CO3 into its structure as other carbonates could (???) preferentially result in dolomite precipitation (if the system is rapidly trying to precipitate CO3). Or, I could be talking complete nonsense. As I have said, I was just thinking about this, I haven't even done the basic chemistry to see if it is possible.

Now, my questions for consideration:
Could a degassing of the oceans, as I described, be possible?If so, could it result in a precipitation of dissolved CO3 (as laid out by Le Chatlier's principle)?
Could a rapid shift out of equilibrium (as described) preferentially precipitate dolomite?
Answer: Not as I have stated it, oops (see comments). (edited on Feb, 28th 8:45 pm MST)And finally, does this make any sense (whatsoever) or should I trade in my rock hammer for a paper hat?

We will start with the end...

Well, here I am. I figure a good opener would be to discuss (rant) about a side feature of my ongoing and nearly finished (hopefully) thesis. I am working with the Cretaceous-Tertiary (KT) Boundary within the Williston Basin in central MT. Initially it was broader in scope, but has essentially been whittled down to one aspect.

I am focusing on comparing lithostratigraphy (stratigraphy based upon rock characteristics) and chronostratigraphy (stratigraphy based upon time). Essentially, I am comparing the two methods as it applies to the terrestrial end Cretaceous (end-K) event. Unbelievably straight forward, I know, but that is it. Lack of any significant funding (the bank of the parents were my primary benefactors) forced me into a project which I now jokingly refer to as a 4th grade science project on steroids.

But the state of my thesis is not necessarily what I wish to discuss. I am curious about the whole obsession with the end-K event itself. I was routinely turned down for funding for not addressing causal mechanisms for the extinction, despite only wanting to evaluate lithostratigraphy and chronostratigraphy. I have read up on extinction of the dinosaurs as it applies to the KT boundary, but I am always amazed at the "jump" that individuals make from showing an event happened to saying that it was what caused the extinction event. Aside from the problems of obtaining an age estimate of the boundary (I have seen error bars as small as ± 40,000 years), people have yet to acknowledge problems related to timing the actual extinction of the dinosaurs.

Signor and Lipps addressed this problem in the 80s (if I find a link I will add it, like this). They demonstrate that it will be generally impossible to distinguish a gradual extinction event from a catastrophic extinction event. This was published in the early days of the Bolide impact hypothesis, but it didn't stop anyone from jumping to conclusions. Alvarez et al. 1980 (subscription required for the full text .pdf) do a superb job of showing a bolide collided with the earth at approximately the time of the dinosaurs extinction. However, they never show any evidence to link the two events except through a variant of the Post hoc ergo propter hoc argument. They happened around the same time, therefore one caused the other.
note: They also make a passing remark about this sort of event causing mass extinctions in general (an unfalsifialble hypothesis as they state it, because they indicate if there is no evidence of an impact, it would be the result of an ice-rich comet being the impactor).

As Signor and Lipps' paper demonstrates, we don't know exactly when the two events occurred in relation to each other. For all we can gather from the fossils, we will never be able to know which fossil is the last dinosaur. We will never even be able to know if the last dinosaur was preserved. Without knowing this information, attempting to determine the sequence of events (even if we know a specific date for an impact event) is meaningless.

Another problem facing the whole extinction concept is a lack of definition. Despite talking reason and making perfect sense, Signor and Lipps have overlooked a key point. As has the rest of the extinction centered community. Nobody has ever bothered to define a gradual extinction, and nobody has bothered to define a catastrophic extinction. I have been involved in discussions among the Paleo grad students where half the group want to define it as a rate and the other half think it should be magnitude. Personally, I think of it as rate. Because, by definition, every extinction has a magnitude of 100% for that given species. But as for how quick or slow an extinction is.... I don't think anyone would be able to make an argument as to where to put the boundary between gradual and catastrophic that is based on anything other than opinion.

So I am forced to conclude that until someone can address and surmount the problems addressed by Signor and Lipps, the mechanism of every mass extinction event (including the dinosaurs) remains an intractable question. We simply cannot know when a species went extinct in relation to a supposed cause. Unfortunately, a good many people are so focused on finding "the answer", they aren't stopping to ask if the question (as we currently understand it) even makes sense.

Disclaimer

All the Latin on this page is from my vague recollections from High School. There are mistakes in the text. I just was trying to get the point across

Between Los Alamos,NM and White Rock, NM

Between Los Alamos,NM and White Rock, NM
The photo of the travertine spring was taken in the small opening in the center of the image.

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