Reef Chemistry Question of the Day #196 What happens to carbon dioxide in the ocean?

[Randy Holmes-Farley]

Reef Chemistry Question of the Day [HASHTAG]#196[/HASHTAG]

Here's a thought experiment. Suppose you put a bunch of CO2 molecules into the ocean (randomly across all of the oceans). Come back 1000 years later, where are they now?

You can just pick the biggest one, or you can rank order all of the following choices

A. In the air
B. In the ocean as bicarbonate
C. In the ocean as carbonate
D. In the ocean as organics (including organisms)
E. On the land as organics (including organisms)
F. In sediments or organisms as calcium carbonate

So a rank order answer might look like A>B>C>D>E>F

Good luck!

















































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[DamianOZ]

A

 

[beaslbob]

doesn't is depend on how deep you put the co2 in the ocean?

Assuming anywhere near the surface:

a>d>F>b>c>e

 

[Myka]

I'm going on a limb and saying that D would be the largest portion, then F, then maybe A?? I don't really know...

The "1000 years" I think is a random number, and the answer would be the same if the number was 1 year, wouldn't it?

 

[Randy Holmes-Farley]

I'm going on a limb and saying that D would be the largest portion, then F, then maybe A?? I don't really know...

The "1000 years" I think is a random number, and the answer would be the same if the number was 1 year, wouldn't it?

The 1000 years was taken from a chemical oceanography book as a random time, but long enough to allow good mixing between various locations.

 

[Randy Holmes-Farley]

And the answer is...nearly all stays in the water (which means that is where most of the planet's mobile carbon is)

87.5% B. In the ocean as bicarbonate
10.5% C. In the ocean as carbonate
0.5% E. On the land as organics (including organisms)
0.4% D. In the ocean as organics (including organisms)
0.5% E. On the land as organics (including organisms)
0.15% A. In the air
Almost none F. In sediments or organisms as calcium carbonate

The exact numbers likely vary from study to study, but these were taken from Chemical Oceanography my Millero.

 

[DamianOZ]

So why aren't tanks using CaCO3 rectors using CO2 not elevating Alk. greater than 3:1 Alk:Ca?

 

[Randy Holmes-Farley]

So why aren't tanks using CaCO3 rectors using CO2 not elevating Alk. greater than 3:1 Alk:Ca?

I'm not sure I understand the question, but a CaCO3/Co2 reactor breaks CaCO3 into Ca++ and 2 HCO3-, and that is 2.8 dKH of alkalinity for each 20 ppm of calcium. That is also the proportion that corals use it.

If there is some magnesium in the media, that reduces the calcium a little bit as magnesium swaps in in its place.

 

[DamianOZ]

Yes, but the CO2 added to the water isn't staying in the system as Bicarbonate and carbonate, which is what your suggesting would happen if we added CO2 into the ocean isn't it?

 

[Randy Holmes-Farley]

Yes, but the CO2 added to the water isn't staying in the system as Bicarbonate and carbonate, which is what your suggesting would happen if we added CO2 into the ocean isn't it?

I wouldn't suggest the numbers I quoted for the ocean and added CO2 relate quantitatively to a reef tank.

The CO2 added in a CaCO3/CO2 reactor may make its way back to the air, or it may get incorporated by photosynthesizing organisms, or it may stay as part of the HCO3- and CO3-- present in the seawater.

 

[DamianOZ]

If the concentration of CO2 gases in the water has not exceeded solubility, then I agree. CO2 + H2O > H2CO3 > H + HCO3 > H + CO3
But in photosynthetically active water O2 will evolve and CO2 will absorb during the day, at night the reverse is true. The water breaths with the Air, changing O2 and CO2 back and forth, day and night aiming for equilibrium.
Adding extra CO2 to the water, will see a net change in the Air CO2. If added during the day, it will convert to >H2CH3>.... but less absorption of CO2 from the air will take place. At night CO2 is already exceeding saturation in the water, so it is off gassing.

Anyway, thats my understanding and how I come to A being the most appropriate answer.

 

[Randy Holmes-Farley]

OK, I see what you are saying. You are suggesting that if I add excess CO2 to the water, it will come off to the air.

It is true that some CO2 will come off, but not necessarily the same CO2 molecules as were added. It will equilibrate with the huge amount of bicarbonate and carbonate very quickly, and then some excess will come off. So unless you add a large amount relative to the total present in seawater, that won't change things appreciably.

Even in a reef tank, the amount of bicarbonate and carbonate present is large. In 100 liters of seawater, there is more than 5 L of pure CO2, or the amount in 16,000 liters of air. So adding a little bit (say, 1 mL of CO2 gas) to that will just mix in and then some will come off.

In my question, I am not assuming that you are adding a large amount of CO2 relative to CO2 capacity of the ocean.

 

[Myka]

In my question, I am not assuming that you are adding a large amount of CO2 relative to CO2 capacity of the ocean.

That would be quite the feat!

 

[JimWelsh]

In 100 liters of seawater, there is more than 5 L of pure CO2, or the amount in 16,000 liters of air.

Well, once that CO2 escapes and becomes gaseous, sure, but it's only about 10.4 grams of CO2 in solution, or about 0.01% of the mass. The way you phrased it, it almost sounds as though seawater is 5% CO2!

 

[Randy Holmes-Farley]

Well, once that CO2 escapes and becomes gaseous, sure, but it's only about 10.4 grams of CO2 in solution, or about 0.01% of the mass. The way you phrased it, it almost sounds as though seawater is 5% CO2!

Gas, yes, I can't help it if you fixate on liquid CO2.

 

[DamianOZ]

I think the difference is, your following the process of individual CO2 molecules added.
I'm talking about the over all CO2 volume.

I'm suggesting the sea (or any body of water with photosynthetic organisms) will regulate CO2, adding CO2 may see those particular molecules change to alkalinity, but doing so will mean that other CO2 molecules will not be taken from the air or if you added them at night when the water is already saturated with CO2, more CO2 will leave the water than would have other wise.
Over time, the waters CO2 content or the waters Alkalinity will not have changed, the CO2 content in the air will have increased and intern the atmosphere.

 

[Randy Holmes-Farley]

I think the difference is, your following the process of individual CO2 molecules added.
I'm talking about the over all CO2 volume.

I'm suggesting the sea (or any body of water with photosynthetic organisms) will regulate CO2, adding CO2 may see those particular molecules change to alkalinity, but doing so will mean that other CO2 molecules will not be taken from the air or if you added them at night when the water is already saturated with CO2, more CO2 will leave the water than would have other wise.
Over time, the waters CO2 content or the waters Alkalinity will not have changed, the CO2 content in the air will have increased and intern the atmosphere.

Yes, that's true. If you add a substantial amount of CO2 to seawater, some small portion will leave to attain a new equilibrium with the air. It is not a large fraction.

The number I quoted reflects the fact that the very great majority of available CO2 on earth is in the ocean, and when some leaves the ocean, there is actually more in plants/soils/animals/organics on land than there is in the air. There is really not that much in the air, by comparison to the ocean.

So I stand by the idea that adding some CO2 to the ocean leaves nearly all of it in the ocean, and some comes out.

 

[TbyZ]

So if 98% of co2 entering the ocean (quickly) converts to bicarbonate/carbonate, & the more carbonate there is the higher the pH, how would ocean acidification occur?

 

[Randy Holmes-Farley]

So if 98% of co2 entering the ocean (quickly) converts to bicarbonate/carbonate, & the more carbonate there is the higher the pH, how would ocean acidification occur?

Conversion of CO2 to bicarbonate and carbonate releases H+, which lowers pH (pH is a measure of H+)

CO2 ---> H2CO3 --> H+ + HCO3- --> 2H+ + CO3--