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

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?

4 comments:

Chuck said...

Um, the formula unit doesn't matter. If you like, you can write dolomite as Ca0.5Mg0.5CO3.

In the modern ocean, Mg stays in solution, as MgSO4 is very soluble. Reduce the sulphate, and then maybe you can form dolomite.

Einme said...

ah, I knew I was full of it. Thanks.

Mel said...

My question is - what is the picture below. Looks like it's from a cave. Is the green coloring on the wall microbial? It's a nice picture. :-)

Einme said...

It is a travertine spring between Los Alamos and White Rock. It isn't a big cavern, in fact that is the whole cavern. I had to duck to take this shot. I have a photo of the outside of the spring, it is really small.

I do remember the green as being some sort of microbial activity.

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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